corrosion

CORROSION CorrosionRate

Pressure Vessel Engineering

CORROSION RATE

CORROSION RATE MEASUREMENT WEIGHT LOSS METHOD

The rate of uniform corrosion can be measured using corrosion coupon testing by weight loss measurement. Coupon corrosion testing is predominantly designed to investigate uniform corrosion. ASTM Designation: G31 gives a definite guideline for carrying out such an experiment. This practice describes accepted procedures, which includes specimen preparation, apparatus, test conditions, method of cleaning specimens, evaluation of results, calculation and reporting of corrosion rates. A good corrosion rate expression should involve (i) familiar units, (ii) easy calculation with minimum opportunity for error, (iii) ready conversion to life in years, (iv) penetration and (v) whole numbers without cumbersome decimals.

Corrosion rates have been expressed in a variety of ways in the literature; such as percent weight loss, milligram per square centimeter per day, and grams per square inch per hour. These do not express corrosion resistance in terms of penetration. The expression mils per year is the most desirable way of expressing corrosion rate. This expression is readily calculated from weight loss of the metal or the alloy specimen during the corrosion test. The conversion from other units to obtain mils per year is given in Table 2. As per ASTM

G31 calculating corrosion rates requires several pieces of information and several assumptions; (i) the use of corrosion rates implies that all mass loss has been due to uniform corrosion and not due to localized corrosion, (ii) the use of corrosion rates also implies that the material has not been internally attacked as by dezincification or intergranular corrosion and, (iii) internal attack can be expressed as corrosion rates if desired. However, in such a case the calculation must not be based on weight loss (except in qualification test such as practice A 262), which is usually small but on microsections, which show depth of attack.

Assuming that localized or internal corrosion is not present, the average corrosion rate can be calculated by the following equations:

Corrosion rate = (K x W) / (A x T x D)

Where K is a constant; T the time of exposure in hours to the nearest 0.01 h; A the area in cm2 to nearest 0.01 cm2; W the mass loss in g to nearest 1 mg (corrected for any loss during cleaning) and D the density in g/cm3.

Many different units are used to express corrosion rates. Using the units for T, A, W and D from Table 3, the corrosion rate can be calculated in variety of units with appropriate value of K given in Table 3.

Table 2: Conversion from other corrosion rate units to obtain mills per year

Unit to be Converted Multiplier

Inches per year 1000

Inches per month 12.1000



Table 3: Corrosion rate units with appropriate value of K

Corrosion Rate Units Desired Constant (K) in corrosion Rate Equation

Mils per year (mpy) 3.45 x 106

Inches per year (ipy) 3.45 x 103

Inches per months (imp) 2.87 x 102

Millimeters per year (mm/y 8.76 x 104

Micrometers per year (m/y) 8.76 x 107

Picometers per second (pm/s) 2.78 x 106

Grams per square per hour (g/m2h) 1.00 x 104 x D4

Milligrams per square Decimeter per day (mdd) 2.40 x 106 x D4

Micrograms per square meter 2.78 x 106 x D4

Per second (g/m2 s)

ADensity is not needed to calculate the corrosion rate in these units. The Density in the constant K cancels out the density in the corrosion rate equation.

Millimeters per year 39.4

Micrometers per year 0.039

Milligrams per square Decimeters per day (mdd) 1.44/density

Grams per square meter per day 14.4/density



Monograph for mpy, ipy, ipm and mdd A rapid and ready conversion for several corrosion rates can be made by means of a monograph shown above. Mathematical computations are not necessary, and the accuracy is good. The monograph is particularly helpful when data in milligrams per square decimeter per day are encountered. This permits conversion of mills per year, inches per year, inches per month and milligrams per square decimeter per day (mdd) from one to another. The first three names are directly converted on the scale A. These are then converted to mdd by means of C scale and the B scale for density. The mdd does not consider or include the density or type of material

involved. Density is given as grams per cubic centimeter. CORROSION RATE MEASUREMENT ELECTRICAL RESISTANCE METHOD Weight-loss measurements indicate the average corrosion rate over a period of time. Electrical resistance measurement is comparatively a better technique that the weight loss method. In this method, a coupon of material, identical to the alloy whose corrosion rate to be measured is exposed to the corrodent and periodically withdrawn to measure its loss of weight, which directly relates to corrosion rate [17]. Its operation is based on the increase in electrical resistance of an exposed



corrosion coupon material. A metallic conductor sensing probe, generally a thick wire, strip, or tube of the same material as the equipment under test, is exposed to the process stream. The electrical resistance of this probe is compared with that of an identical reference probe that is shielded from the corrodent. As the exposed probe corrodes, its electrical resistance increase, and this change is related to the extent of corrosion. This method is fast and sensitive. It is quite similar to the weight-

loss coupon method, but enjoys the great advantage of permitting continuous monitoring without removing the coupon. It is also superior to weight-loss method because errors caused by removal of the corrosion products is eliminated and continuous monitoring can indicate the effect of process variable on corrosion rate. Electrical resistance probes can serve as an accurate measure of the corrosion rate only when the corrosion is uniform.

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Source: Scraped from Corrosion of Austenitic Stainless Steels: Mechanisms, Mitigation

and Monitoring edited by H.S.Khatak, Baldev Rai. Chapter Uniform Corrosion of Austenitic Stainless Steel by Nisgshen and Mudali.

CORROSION IGSCC&TGSCC


INTERGRANULAR AND TRANSGRANULAR SCC

Stress corrosion cracking can proceed in one of two ways:

1. Intergranular Stress Corrosion Cracking (IGSCC) - Cracks propagate along the grain boundaries.

2. Transgranular Stress Corrosion Cracking (TGSCC) - Cracks run through the individual grains.

Transgranular corrosion is also known as intragranular corrosion and transcrystalline corrosion.

INTERGRANULAR CORROSION

Intergranular corrosion (IGC), also known as Intergranular Attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides.

This situation can happen in otherwise corrosion-resistant alloys, when the grain boundaries are depleted, known as grain boundary depletion, of the corrosion-inhibiting elements such as Chromium by some mechanism. In nickel alloys and austenitic stainless steels, where chromium is added for corrosion resistance, the mechanism involved is precipitation of chromium carbide at the grain boundaries; resulting in the formation of chromium-depleted zones adjacent to the grain boundaries (this process is called sensitization). Around 12% chromium is minimally required to ensure passivation, a mechanism by which an ultra thin invisible film, known as passive film, forms on the surface of stainless steels. This passive film protects the metal from corrosive environments. The self-healing property of the passive film make the steel stainless. Selective leaching often involves grain boundary depletion mechanisms.

These zones also act as local galvanic couples, causing local galvanic corrosion. This condition happens when the material is heated to temperature around 700 C for too long time, and often happens during welding or an improper heat treatment. When zones of such material form due to welding, the resulting corrosion is termed weld decay. Stainless steels can be stabilized against this behavior by addition of titanium, niobium, or tantalum, which form titanium carbide, niobium carbide and tantalum carbide preferentially to chromium carbide, by lowering the content of carbon in the steel and in case of welding also in the filler metal under 0.02%, or by heating the entire part above 1000C and quenching it in water, leading to dissolution of the chromium carbide in the grains and then preventing its precipitation. Another possibility is to keep the welded parts thin enough so that, upon cooling, the metal dissipates heat too quickly for chromium carbide to precipitate.

The ASTM A923, ASTM A262, and other similar tests are often used to determine when stainless steels are susceptible to intergranular corrosion. The tests require etching with chemicals that reveal the presence of intermetallic particles, sometimes combined with Charpy V-Notch and other mechanical testing.



KNIFELINE ATTACK (KLA)

Another related kind of intergranular corrosion is termed knifeline attack (KLA). Knifeline attack impacts steels stabilized by niobium, such as 347 stainless steel. Titanium, niobium, and their carbides dissolve in steel at very high temperatures. At some cooling regimes (depending on the rate of cooling), niobium carbide does not precipitate and the steel then behaves like unstabilized steel, forming chromium carbide instead. This affects only a thin zone several millimetres wide in the very vicinity of the weld, making it difficult to spot and increasing the corrosion speed. Structures made of such steels have to be heated in a whole to about 1950F, when the chromium carbide dissolves and niobium carbide forms. The cooling rate after this treatment is not important, as the carbon that would otherwise pose risk of formation of chromium carbide is already sequestered as niobium carbide.

EXFOLIATION CORROSION / LAMELLAR CORROSION

Aluminium-based alloys may be sensitive to intergranular corrosion if there are layers of materials acting as anodes between the aluminium-rich crystals. High strength aluminium alloys, especially when extruded or otherwise subjected to high degree of working, can undergo exfoliation corrosion, where the corrosion products build up between the flat, elongated grains and separate them, resulting in lifting or leafing effect and often propagating from edges of the material through its entire structure. Intergranular corrosion is a concern especially for alloys with high content of copper.

Other kinds of alloys can undergo exfoliation as well; the sensitivity of cupronickel increases together with its nickel content. A broader term for this class of corrosion is lamellar corrosion. Alloys of iron are susceptible to lamellar corrosion, as the volume of iron oxides is about seven times higher than the volume of original metal, leading to formation of internal tensile stresses tearing the material apart. Similar effect leads to formation of lamellae in stainless steels, due to the difference of thermal expansion of the oxides and the metal.

Copper-based alloys become sensitive when depletion of copper content in the grain boundaries occurs.

Anisotropic alloys, where extrusion or heavy working leads to formation of long, flat grains, are especially prone to intergranular corrosion.



Intergranular corrosion induced by environmental stresses is termed stress corrosion cracking. Inter granular corrosion can be detected by ultrasonic and eddy current methods.

TRANSGRANULAR CORROSION

Transgranular corrosion is a type of localized corrosion which occurs along cracks or faults across the crystals in metals and alloys. It follows the pattern of grains in the individual lattices of the material. Stress corrosion of austenitic steels is usually transgranular.

Transgranular corrosion occurs through or across a crystal or grain. In transgranular corrosion, the fracture travels through the grain of the material. The fracture changes direction from grain to grain due to the different lattice

orientation of atoms in each grain. In other words, when the fracture or crack reaches a new grain, it may have to find a new path or plane of atoms to travel on because it is easier to change direction for the crack than it is to rip through. Corrosion chooses the path of least resistance. Since the corrosion proceeds preferentially within the grain, the grain boundary material is retained.

In this type of corrosion, a small volume of metal is removed in preferential paths that proceed across or through the grains. It sometimes accelerated by tensile stress. In extreme cases, the cracks proceed through the entire metal, causing rupture or perforation. Chloride is the leading cause of transgranular cracking.

Transgranular attack has a very characteristic branching habit which is easily recognizable in scanning electron microscopy (SEM), fractographic and metallographic section examination. Transgranular stress corrosion cracking occurs mainly in chloride cracking of austenitic steels.

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Source: ScrapedfromWikipedia(Mar.2015)

CORROSION CommonStressCorrosionCrackingMechanisms


COMMON STRESS CORROSION CRACKING SYSTEMS

Material Environment Environment Concentration Temp. Mode

CARBON STEEL

Hydroxides High High Intergranular

Nitrates Moderate Moderate Intergranular

Carbonates / Bicarbonates Low Moderate Intergranular

Liquid Ammonia - Low Transgranular

CO/CO2/H2O - Low Transgranular

Aerated water - Very High Transgranular

LOW ALLOY STEELS (e.g. Cr-Mo, Cr-Mo-V)

Water - Moderate Transgranular

STRONG STEELS

Water (Y.S > 1200 MPa) - Low Mixed

Chloride (Y.S > 800 MPa ) - Low Mixed

Sulphide (Y.S > 600 MPa ) - Low Mixed

AUSTENITIC STAINLESS STEEL (INCLUDING SENSITISED)

Chloride High High Transgranular

Hydroxide High Very High Mixed

SENSITISED AUSTENITIC SS Areated Water -

Very High Intergranular

STAINLESS STEELS

Thiosulphate or Polythionate Low Low Intergranular



DUPLEX STAINLESS STEEL

Chloride Very High High Transgranular

MARTENSITIC STAINLESS STEEL

Chloride + H2S High Moderate Transgranular

Chloride (usually + H2S) Moderate Low Transgranular

HIGH STRENGTH STEELS

Water Vapour - Low Transgranular

ALUMINIUM ALLOYS Chlorides Low Low Intergranular

TITANIUM ALLOYS

Chlorides High Low Transgranular

Methonol - Low Transgranular

COPPER ALLOYS (excluding Cu-Ni)

N2O4 High Transgranular

Ammoniacal Solutions and other Nitrogenous Low Low Intergranular

This Table presents the systems for which SCC problems are well established and of practical importance. The absence of a metal-environment combination from this Table does not mean that SCC has not been observed. There are rarely well-defined temperatures or concentration limits for SCC, and the ratings given here are indicative only. As an approximate guide the terms used equate to the following ranges of values:

Environment Concentration Temperature

LOW Up to 10-2 M Ambient

MODERATE Up to 1 M Below 100 0C

HIGH Around 1 M Around Boiling

VERY HIGH Near Saturation Above Boiling



Note that significantly increased local concentrations may be obtained under the influence of local boiling or evaporation, or by accumulation in pits and crevices, and cracking is often obtained for nominal concentrations that are much lower than is indicated here.

The fracture mode is classified as intergranular (I) where cracks go along the grain boundaries, transgranular (T) where cracks go across the grains, or mixed (M) where there is a combination of the two modes, or where the mode can vary depending on the conditions. There are often circumstances that can cause the fracture mode to change (e.g. chloride SCC of sensitised austenitic stainless steel may give intergranular cracking).

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Source:

1. Scraped from Stress Corrosion Cracking National Physical Laboratory

CORROSION IntergranularCorrosionEffectofSensitization


INTERGRANULAR CORROSION

(Effect of Sensitization)

Intergranular corrosion (IGC), also known as Intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides. Like other common materials, metals have a visible grain structure when they are viewed under magnification. Rapid corrosive attack of immediately adjacent grain boundaries with little or no attack of the grains is called Intergranular Corrosion.

This situation can happen in otherwise corrosion-resistant alloys, when the grain boundaries are depleted, known as grain boundary depletion, of the corrosion-inhibiting elements such as chromium by some mechanism. In nickel alloys and austenitic stainless steels, where chromium is added for corrosion resistance, the mechanism involved is precipitation of chromium carbide at the grain boundaries, resulting in the formation of chromium-depleted zones adjacent to the grain boundaries (this process is called sensitization). Around 12% chromium is minimally required to ensure passivation, a mechanism by which an ultra thin invisible film, known as passive film, forms on the surface of stainless steels. This passive film protects the metal from corrosive environments. The self-healing property of the passive film makes the steel stainless. Selective leaching often involves grain boundary depletion mechanisms.

These zones also act as local galvanic couples, causing local galvanic corrosion. This condition happens when the material is heated to temperature around 700C for too long time, and often happens during welding

or an improper heat treatment. When zones of such material form due to welding, the resulting corrosion is termed weld decay. Stainless steels can be stabilized against this behaviour by addition of titanium, niobium, or tantalum, which form titanium carbide, niobium carbide and tantalum carbide preferentially to chromium carbide, by lowering the content of carbon in the steel and in case of welding also in the filler metal under 0.02%, or by heating the entire part above 1000C and quenching it in water, leading to dissolution of the chromium carbide in the grains and then preventing its precipitation. Another possibility is to keep the welded parts thin enough so that, upon cooling, the metal dissipates heat too quickly for chromium carbide to precipitate. The ASTM A923, ASTM A262, and other similar tests are often used to determine when stainless steels are susceptible to intergranular corrosion. The tests require etching with chemicals that reveal the presence of intermetallic particles, sometimes combined with Charpy V-Notch and other mechanical testing.

Another related kind of intergranular corrosion is termed Knifeline attack (KLA). Knifeline attack impacts steels stabilized by niobium, such as 347 stainless steel. Titanium, niobium, and their carbides dissolve in steel at very high temperatures. At some cooling regimes (depending on the rate of cooling), niobium carbide does not precipitate and the steel then behaves like unstabilized steel, forming chromium carbide instead. This affects only a thin zone several millimeters wide in the very vicinity of the weld, making it difficult to spot and increasing the corrosion speed. Structures



made of such steels have to be heated in a whole to about 1950F, when the chromium carbide dissolves and niobium carbide forms. The cooling rate after this treatment is not important, as the carbon that would otherwise pose risk of formation of chromium carbide is already sequestered as niobium carbide.

Aluminium-based alloys may be sensitive to intergranular corrosion if there are layers of materials acting as anodes between the aluminium-rich crystals. High strength aluminium alloys, especially when extruded or otherwise subjected to high degree of working, can undergo Exfoliation corrosion, where the corrosion products build up between the flat, elongated grains and separate them, resulting in lifting or leafing effect and often propagating from edges of the material through its entire structure. Intergranular corrosion is a concern especially for alloys with high content of copper.

Other kinds of alloys can undergo exfoliation as well; the sensitivity of cupronickel increases together with its nickel content. A broader term for this class of corrosion is lamellar corrosion. Alloys of iron are susceptible to lamellar corrosion, as the volume of iron oxides is about seven times higher than the volume of original metal, leading to formation of internal tensile stresses tearing the material apart. Similar effect leads to formation of lamellae in stainless steels, due to the difference of thermal expansion of the oxides and the metal.

Copper-based alloys become sensitive when depletion of copper content in the grain boundaries occurs.

Anisotropic alloys, where extrusion or heavy working leads to formation of long, flat

grains, are especially prone to intergranular corrosion.

Intergranular corrosion induced by environmental stresses is termed stress corrosion cracking. Inter granular corrosion can be detected by ultrasonic and eddy current methods.

Rapid attack at the grain boundaries can result in grains dropping or falling out of the metal surface resulting in the disintegration of the steel. Figure 1 shows the appearance of a surface where this is occurring. In practical application, the loss of cross section thickness and the introduction of cracks can have severe consequences for applications like pressure containment.

Figure 1: Intergranular corrosion grain boundary attack and dropped grains

Photo Courtesy TMR Consulting

Sensitization

Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the steel or alloy to be susceptible to intergranular corrosion or intergranular stress corrosion cracking.



Unsensitized microstructure

Heavily sensitized microstructure

Certain alloys when exposed to a temperature characterized as a sensitizing temperature become particularly susceptible to intergranular corrosion. In a corrosive atmosphere, the grain interfaces of these sensitized alloys become very reactive and intergranular corrosion results. This is characterized by a localized attack at and adjacent to grain boundaries with relatively little corrosion of the grains themselves. The alloy disintegrates (grains fall out) and/or loses its strength.

The photos show the typical microstructure of a normalized (unsensitized) type 304 stainless steel and heavily sensitized steel. The samples have been polished and etched before taking the photos, and the sensitized areas show as wide, dark lines where the etching fluid has caused corrosion. The dark

lines consist of carbides and corrosion products.

Intergranular corrosion is generally considered to be caused by the segregation of impurities at the grain boundaries or by enrichment or depletion of one of the alloying elements in the grain boundary areas. Thus in certain aluminium alloys, small amounts of iron have been shown to segregate in the grain boundaries and cause intergranular corrosion. Also, it has been shown that the zinc content of a brass is higher at the grain boundaries and subject to such corrosion. High-strength aluminium alloys such as the Duralumin-type alloys (Al-Cu) which depend upon precipitated phases for strengthening are susceptible to intergranular corrosion following sensitization at temperatures of about 120C. Nickel-rich alloys such as Inconel 600 and Incoloy 800 show similar susceptibility. Die-cast zinc alloys containing aluminum exhibit intergranular corrosion by steam in a marine atmosphere. Cr-Mn and Cr-Mn-Ni steels are also susceptible to intergranular corrosion following sensitization in the temperature range of 420C - 850C. In the case of the austenitic stainless steels, when these steels are sensitized by being heated in the temperature range of about 520C to 800 C, depletion of chromium in the grain boundary region occurs, resulting in susceptibility to intergranular corrosion. Such sensitization of austenitic stainless steels can readily occur because of temperature service requirements, as in steam generators, or as a result of subsequent welding of the formed structure.

Several methods have been used to control or minimize the intergranular corrosion of susceptible alloys, particularly of the austenitic stainless steels. For example, a high-temperature solution heat treatment,



commonly termed solution-annealing, quench-annealing or solution-quenching, has been used. The alloy is heated to a temperature of about 1,060C to 1,120C and then water quenched. This method is generally unsuitable for treating large assemblies and also ineffective where welding is subsequently used for making repairs or for attaching other structures.

Another control technique for preventing intergranular corrosion involves incorporating strong carbide formers or stabilizing elements such as niobium or titanium in the stainless steels. Such elements have a much greater affinity for carbon than does chromium; carbide formation with these elements reduces the carbon available in the alloy for formation of chromium carbides. Or the stainless steel may initially be reduced in carbon content below 0.03 percent so that insufficient carbon is provided for carbide formation. These techniques are expensive and only partially effective since sensitization may occur with time. The low-carbon steels also frequently exhibit lower strengths at high temperatures.

Intergranular Attack of Austenitic Stainless Steels:

With austenitic stainless steels, intergranular attack is usually the result of chromium carbide precipitation (Cr23C6) at grain boundaries, which produces a narrow zone of chromium depletion at the grain boundary. This condition is termed sensitization and it is shown schematically Figure 2. Sensitization involves the precipitation of chromium carbides at grain boundaries, which results in a narrow zone of chromium depletion at the grain boundary.

Figure 2: Chromium depletion at the grain boundaries or sensitization

Because the chromium is the primary alloying element that makes stainless steel corrosion resistant, the chromium-depleted regions are susceptible to preferential corrosion attack. It is believed that this occurs because the chromium content immediately adjacent to the carbide may be below that required for the stainless steel alloy. If the carbides form a continuous network on the grain boundary, then corrosion can produce a separation or gap at the boundary and possible grain dropping or loss.

Chromium Carbide Precipitation

The chromium carbides tend to precipitate at the grain boundaries of austenitic stainless steels in the 5100C to 787.80C (9500F to 1450F) temperature range. Any exposure or thermal excursion into this temperature range during metal manufacture, fabrication, or service could potentially sensitize the steel.

Common practices such as welding, stress relief, and hot forming can expose the steel to the sensitizing temperature range. The formation of chromium carbides is readily reversed by a solution anneal heat treatment.



The test methods outlined in ASTM A262 have been developed to detect susceptibility to intergranular attack in austenitic stainless steels.

The time and temperature required to produce susceptibility to intergranular attack (IGA) is dependent on alloy composition, particularly the carbon content. Figure 3 shows the time-temperature-sensitization curves for Type 304 alloys with varying amounts of carbon content.

Figure 3: Time-temperature-sensitization curves for Type 304 alloys as a function of carbon content

Image courtesy of the Nickel Institute

Three approaches have been used with the austenitic stainless steels to minimize to the effects of IGA. Material that has been sensitized can be solution annealed by heating to a temperature where the carbides dissolved and the chromium-depleted regions are eliminated. The carbon is then kept in solution by rapid cooling through the sensitizing temperature range. The recommended solution anneal temperature depends on the alloy and is typically done in the range of 1037.8 to 1176.70C (19000F to 2150F) followed by rapid cooling.

Resistance to IGA can also be achieved by reducing the carbon content to below 0.030% level. As shown in Figure 3, lower carbon contents move the nose of the time-temperature-sensitization curve to longer times. The low carbon grades such as Types 304L, 316L, and 317L have been designed to resist sensitization during typical welding operations, but they do not resist sensitization by long term exposure in the critical temperature range in service. The higher alloyed, more corrosion resistant stainless steels such as the 904L and 6Mo alloys have very low carbon contents and susceptibility to IGA is typically not a concern.

The addition of stabilizing elements such as Ti, Nb (Cb), and Ta can also provide increased resistance to sensitization, especially for long-term exposures in the critical range in service. These stabilizing elements tend to form carbides that are more stable than chromium carbide in the temperature range of 1232.20C to 787.80C (22500F to 1450F). So as the alloy cools from high temperatures, the carbon combines with the stabilizing elements and is unavailable for chromium carbide precipitation at the lower sensitizing temperature range of 5100C to 787.80C (9500F to 1450 F). Common stabilized austenitic grades include Type 321, 347, 20-Cb3, and 316Ti. Figure 4 summarizes the carbide precipitation reactions that occur in type 304 and 347 stainless steels.



Temperature Range

Precipitation Reactions

Melting point 1232.20C (2250F)

Niobium (Columbium) carbide dissolves Chromium carbides dissolves

1232.20C to 787.80C (2250 to 1450 F)

Columbium carbide precipitates Chromium carbides dissolves

5100C to 787.80C (14500F to 950F)

Chromium carbides precipitates

5100C to 21.10C (9500F to 70F) No reaction

Figure 4: Precipitation Reactions in Type 304 and 347 Stainless Steel

With the stabilized grades, standard solution annealing treatments generally do not tie up all the available carbon. So when the stabilized grades in the solution-annealed condition have long time exposures to the sensitizing temperature range (14500F to 950F), chromium carbide precipitation and sensitization can occur. Stabilizing heat treatments can be used to more effectively tie up carbon by completing the precipitation reactions. These treatments consist of holding the alloy for several hours in the 815.60C 871.10C (15000F to 1600F) temperature range. [See ASTM A403, supplemental S10, p 301 volume 1.01.]

Intergranular Attack of Ferritic Stainless Steels:

Although intergranular attack of ferritic stainless steels is similar to that found in austenitic stainless steels, there are some important differences. Because the solubility of nitrogen is low in the ferritic crystal structure, the precipitates that cause sensitization in ferritic grades include both chromium carbides (Cr23C6) and chromium nitrides (Cr2N).

With the ferritic grades, sensitization occurs during cooling from higher temperatures > 926.70C (>1700F). At these high temperatures the carbides and nitrides are put into solution and during cooling they can precipitate at grain boundaries resulting in chromium depletion. The very high diffusion rates in the ferrite structure make it impossible to cool the steel fast enough to avoid precipitation of carbides and nitrides at grain boundaries. For this reason, most commercial ferritic grades avoid sensitization by restricting the level of C and N and requiring the addition of stabilizing elements such as Ti, Ta, or Nb.

If sensitization has occurred in a ferritic stainless steel, the condition can be healed by back diffusing chromium into the depleted regions. Healing can be achieved by holding the material at 593.30C 648.80C (11000F 1200F) for several hours.

The test methods outlined in ASTM A763 have been developed to detect susceptibility to intergranular attack in ferritic stainless steels.

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Source: Main content scraped from Wikipedia.

CORROSION TestingforIntergranularCorrosion


SENSITIZATION EVALUATION TESTS FOR STAINLESS STEELS

(Testing for Intergranular Corrosion as per ASTM A262)

Austenitic stainless steels work. unless incorrectly heat treated

In general, intergranular corrosion occurs as a result of precipitation of nitrides, carbides, and other intermetallic phases, such as sigma phase, that occurs along the grain boundaries. If materials with incorrect heat treatment enter service, they are liable to crack or fail by intergranular corrosion much more rapidly than properly treated materials.

In everyday applications, corrosion varies by materials and solutions. For example, in highly oxidizing solutions, intergranular attack can occur due to intermetallic phases, while attack of carbides may occur in somewhat less oxidizing solutions. Due to the variance of attack in different materials, numerous methods (Practices B-F) have been developed to assess intergranular corrosion.

For most of the applications of austenitic stainless steels it is required to assess whether a fabricated component is sensitized and has become susceptible to IGC. ASTM has standardized the test procedure and the specifications are detailed in ASTM A262 (practice A-F) and G108 (45).

Muraleedharan has reviewed these tests and has compared the various test procedures for both conventional and electrochemical tests (46). These standard tests are commonly used as qualification / acceptance tests during purchase / fabrication stages. However non-inclusion of acceptance limits in these standards leaves the interpretation of the results open to the users. The salient

features of these procedures are discussed briefly.

ASTM A 262 Practice A test Oxalic Acid Test Oxalic Acid Etch

ASTM A262 is a screening test to help find batches that are incorrectly processed. Classification of the structure after A262 Practice A analysis will determine if the material is acceptable or if additional testing is required. Practice A, the oxalic acid etch test is used as a rapid technique to screen samples of certain stainless steel grades to ensure they are free of susceptibility to intergranular attack. The test is generally performed for acceptance of materials, but not sufficient for rejection of materials.

This test consists of electrolytically etching a polished specimen in 10 wt % oxalic acid solution at room temperature at a current density of 1 A/cm2 for 1.5 minutes. The etched structure is then examined at 200 X. In this test, chromium carbide is dissolved preferentially and the microstructure gives an idea of chromium depletion which is responsible for IGC. The different microstructures which can be obtained are presented in Fig. 7 (a-f). If there is no carbide precipitation step structure (a) is obtained, because of the differences in the rate of etching of variously oriented grains. Dual structure (b) is obtained, if chromium carbide precipitation is discontinuous. Ditch structure (c) is obtained if grain boundaries



are completely surrounded by chromium carbide. Even if one grain is completely surrounded by ditch, it is characterized as ditch structure. Step and dual structure are acceptable but if the structure is ditch the material may or may not be sensitized and hence it has to be further tested by any one of the ASTM tests (B to F). This test is only a qualitative test but is very useful as screening test. This cannot detect sigma phase in molybdenum bearing alloys. Since titanium and niobium carbides do not dissolve appreciably in this test, this can be used to detect chromium carbide precipitation even in stabilized stainless steels. ASTM further recommends a heat treatment at 950 K for 1 h and water quenching for low carbon SS, for weld simulation before varying out this test. This test also characterizes the microstructure with inclusions as end grains. End grain pitting (II) (transverse section) (Fig. 7g) is not considered to be acceptable because although these steels do not contain chromium carbide precipitates at grain boundaries, active inclusions in the form of stringers undergo IGC in oxidizing environments such as nitric acid. For instance, (Fe, Mn) sulphide and oxide inclusions stringers lead to catastrophic IGC in HNO3 medium but not in other environment.

In metallography, intergranular corrosion shows up under the optical microscope as black lines around the grain boundaries (with the proper etchant).

In SEM analysis, intergranular corrosion is clear by the dark lines where the grain boundaries are eaten away.



Fig. A: Step Structure

Fig. B: Dual Structure

Fig. C: Ditch Structure

Fig. D: Isolated Ferrite Pools

Fig. E: Inter-dendretic ditches

Fig. F: End Grain Pitting I

Fig. G: End Grain Pitting II

Classification of etch structure after Oxalic acid etching

(ASTM A262 practice A)



ASTM A 262 Practice B test - The Streicher Test - Ferric Sulfate-Sulfuric Acid

Practice B, also known as the Streicher test, uses weight loss analysis to provide a quantitative measure of the materials performance. This practice includes boiling the sample for 24 to 120 hours in Ferric Sulfate Sulfuric Acid solution, and measures the materials performance quantitatively. It is typically used for stainless alloys such as 321 and 347, Cr-Ni-Mo stainless alloys, and nickel alloys to evaluate the intergranular attack associated with the precipitation of chromium carbides at grain boundaries.

In this test a sample of surface area 5 - 20 cm2 is exposed for a period of 120 h to boiling solution of 50% H2SO4 + 2.5 % Fe2 (SO4)3. Corrosion rate is calculated from weight loss measurements. ASTM practice does not specify the criterion to judge the susceptibility of the material to IGC. Normally accepted limit for 304 SS is 48 mpy. Streicher has reported (47) that if the ratio of the weight losses of sensitized to annealed material is greater than 1.5 to 2.0, the material is considered as susceptible. This test is applicable to austenitic stainless steels and it detects the IGC associated with chromium carbide precipitation and chromium depletion. Sigma phase in 321 and 347 SS are attacked whereas that in Mo bearing 316 SS is not attacked.

ASTM A 262 Practice C test - The Huey Test Nitric Acid

In this test, a sample of 20-30 cm2 area is exposed to 65 wt% HNO3 for five 48 h period. After every 48 h, the solution is changed and the sample is weighed. The corrosion rate for each period and the average for the five periods are determined.

ASTM does not state the acceptance criteria. Experience has shown that corrosion rate < 18 mpy for 304 SS and < 24 mpy for 304 L does not lead to IGC. The material is not acceptable, if the corrosion rate is increasing rapidly for the successive periods. Besides chromium depleted zone, carbides and sigma phase in molybdenum bearing alloys are attacked in this test. These alloys can give high corrosion rates even when they are immune to IGC in other tests, which reveal sensitization caused by chromium depleted zones. Submicroscopic sigma may also form in stabilized grades of 321 and 347 and show susceptibility to IGA in HNO3 test. This test has to be followed only when the alloy is intended to be used for nitric acid service.

The maximum allowable corrosion rate and any available data on the sensitizing heat treatment performed needs to be specified by the end user.

ASTM A 262 Practice D test

In this test samples are tested in 10% HNO3 - 3% HF solution at 343K for two, two h periods (fresh solution is used for each period). If the ratio of the weight loss of the sensitized to annealed material is greater than 1.5, the sample is considered to be susceptible to IGC. This detects only chromium depletion from carbide precipitation and not submicroscopic sigma.

ASTM A 262 Practice E test - The Strauss Test Copper-Copper Sulphate-16% Sulphuric Acid

In this test, austenitic stainless steel specimen is embedded in metallic copper chips and then exposed to boiling 16% H2SO4 + 10% CuSO4 for 24 h. After the



test, the specimen is bent through 1800 over a mandrel of diameter equal to the thickness of the specimen. The bent specimen is examined under low magnification. If cracks are seen, the material is considered to be sensitized. Although this is not a quantitative test, ASTM gives acceptance criterion for this test. Electrical resistivity and tensile properties are changed considerably by the IGC. These can be used for quantifying the degree of sensitization (DOS). Muraleedharan et al have suggested a modified version to determine the DOS quantitatively (48). Flat tensile specimens can be exposed to the test solution and can be pulled to fracture at a strain rate of 6.6 x 10-4S-1 and DOS can be correlated to % loss in strength as follows:

DOS = % loss in strength =

[1 UTSexp/ UTSsterp] x 100.

ASTM A 262 Practice F test - Copper-Copper Sulphate-50% Sulphuric Acid

Practice F, is a weight-loss based analysis that provides a quantitative measure of the

materials performance, and is commonly used to analyzed as-received stainless steels.

This test is useful for Mo bearing SS for which practice B and D have been used so far. Since practice B shown corrosion rates due to the presence of molybdenum associated phase in SS, IGC arising exclusively due to chromium can be obtained from this test. This test may also be used to evaluate resistance of extra low carbon grades to sensitization and IGC caused by welding or heat treatment. It involves exposing the specimen to boiling Cu - CuSO4 - 50% H2SO4 for 120h and measuring the weight loss. Similar to the other tests it does not indicate the rejection criteria.

From the above details, it is clear that the ASTM standard practices have three draw backs: (i) they are only qualitative, (ii) destructive and (iii) time-consuming (except practice A). Hence several electrochemical techniques were developed to determine the susceptibility of a material to IGC which are fast, non-destructive and quantitative.

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Source: Scraped from:-

1. Corrosion of Austenitic Stainless Steels: Mechanisms, Mitigation and Monitoring edited by H. S. Khatak, Baldev Raj.

2. www.g2mtlabs.com.

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"

Behavior of Austenitic Stainless Steels in Evaluation Tests for the Detection of

Susceptibility to Intergranular Corrosion* MARSHALL H. BROWN

Abstract

The c:Nrac:1erlnia, advInUlIJIH. loci IImitellons of \/Irioul metl'lOch lor theOelKlion oi susceptibility to innrllranulef corrosion In .usumldc mlnlllSS SUM!II dUI to carbld, prw:ipiQltion .re dru:ussed. InforlTllltlon on corrosion ,.-eft and aecapUineilimlu in nlttic: Kid Ind lenic lulfale-sullurlc Kid lestl it: pre18nted end lIIu51rl1ld by n.1lnleal tt.UI from the evauiUlon of Ipproxinwtely 10.000 amplel represen1ing (ommettRiI A/SI 300 Jl!rill$ ttllol"" neels.

ASTM A-262 1 describes recommended practices for several reliable methods fOf detecting susceptibility to inter-granular atlack In stlUllless steels. There have been repealed requests for the inclusion in A262 of tahles of accepTable limits or fJ'Pico/ roltS (or guidance of those not familiar in detail wilh Ihe application of these proct'dures. ASTM Subcommittee AIO.04 (now A0I.14) decided that the inclusion of such tables was not appropriate for a methods spjfiCOlton and thai the desired information could be best supplied by paper in which an adequate background on the objectives of the lest and interpretalion of test results could be included . Thls paper WitS prepared to meet that objtctive. The author has been intimately associated since 1936 with evalualion lests on specimen.s representing stainless steels purchased by E. I. du Pont de Nemours and Co . Wilmington , Delaware (hereafter referred to as Du Pont). and with the developmen.t and comparison of test methods and conelalion of test results with service experi. ence.

Purpose of Evaluation Test.s It should first be emphasized that the purpose of these

evaluation tests is solely to detect susuptibility to Inter-granular corrosion a:s influenced by variations in processing and/or composition. Material shown to be susceptible mDY or ItJJlY 1101 be intetgranularly attacked in another environ-ment ; this must be established independently by specific tests or by service experience. The rt'sulu of long time service exposures which provide useful guidelines as to the types of environmenls which are capable of intergranularly attacking 300 series stainless steels of varying degrees of susceptibility have been presented by Auld] and by the High Alloys Commitlee of the Welding Research Council.'

The mosl damaging environmenu are hot solutions containing certlin inorganic and organic acids, including nitric, sulfuric. phosphoric. formic. acetic, and lactic acids. Severe.ly sensitized material as determined by evaluation tesu will be intergnnularly au acked at a more rapid rate

SuhmiUed for publiution February. 1973. 0E. I. dll Ponl de Nemoun and CO . Int . Wilmmaton. DE

Vol. JO, No.1, January, 1974

and over a wider range of temperatures and concenuations than mildly sensitized material.

It should also be understood thai evaluation lests for detection of suscepu'bility to int.t'lgnlllular attack arc of no value for !.he prediction of resistance to general conosion , pitting, or slress COtTogon cracking (SCC) in othel environ ments. Such information can only be obtained by appro-priate tests or service experience under the applicable exposure conditions.

8y rar the major proportion of !.he stainless steels manufactured are used under conditions which are nOI rufficiently corrosive to intergranularly attack even sus-ceptible material. Large tonnages which go into such end uses as automotive, general and indusltial construction equipment, aircraft, electrical equipment. appliances. utensils. sanitary and office equipment, and architectural applications. art seldom, If ever, used in environments where intergranutar corrosion is a ractor. Even in chemical processing. the bulk of the sl1'ljn]ess s1eei equipment is specified to mini.mize product contamination under such mildly corrosive conditions thllt inlergranullJ corrogon is not a problem. For such purposes, Type 304 is' satisfactory in the aswelded condition and there is no need for evaluation teSts. h is estimated that in Do Pont current practice evaluation tests are spedfied for only abou l 15%of the stainless steel purchased. For services where intts' granular corrosion of susceplibl~ material would be expected, evaluation tests are required and, if welding is involved, the extra low carbon (usually) or stabiUzed grades are specified.

For uns1abiliud alloys containing more than O.OJI,l. carbon, evaluation tests. when specified. are made on as-receivcd material to check the erfectiveness of final heal treatment. For the stabillud or extra low carbon grades. evaluation tests are performed after a sensitizing treatment (ie., one hour al 671 C (1250 F) to determine whether susceptibility to intergranular attack might be developed in a welding operation. Results from long time seqic:e tesu1 3 indicate that such sensitizing treat.menu inIJoduct' a con-siderably h igher degree of susceptibility than wou ld result

rrom good welding practice; however, it can be "sued that safety ractor iJ need~ an order to aHow ror abuses such as excessive heat input or extensive puddJing, which rrequent-Iy occur tn welding opera lions.

In seneral, but With certlin exceptions as noted tater, susceptibility to inlergranular corrosion in austenitic stain leu steel is believed to be due 10 the precipitation or Mlle, type carbides al the grain boundaries. ThJs results in localized impoverishmmt in chromium In the immediately adjacent areas to proVide. path ror interzranulllr penetra-tion by those corroSives which are clpable or DllDcklng susceptible material. The primary purpose or aU evaluation tellS discussed here u to detect susceptibility to inter granular corrosion due 10 harmrul carbide preeipitation.

Types or Eva lu3 lion Tests

CcppoCappe SuIIQt~Sullu'ic Acid T~l (ASTM A262, Practice EJ

The first widely used lest ror detecting susceptibility to inlergranular corrosion wu an early version or Ihe boiling acidified copper sulfate test commonly known as the Strauss Test and descnbed in ASTM A393 ," Originally JOme uwestigators rollowed Intergranular penetration by electricaJ resistance measure men IS berore and after expo-sure to the testing solution, but assessment of damage IS usually made by bendmg the exposed specimen throug.h 1800 and examtnlng the outside surface ror crtck$. The usual exposure time IS 72 hours. With the high carbon contents of the early days, differentiation bttwn solulion Qnn~laJ-'l4ia{e, qumchtd moten"Ql and ~nlitlud ma(e,iJll was quite satisfactory. However, with the much lower carbon contenlS or the stainless steels as now produced, a more senSItive method IS needed to follow less extreme changes in susceptibility , which would require much longer boiling time5 than 72 hours in the Stnus.s solution to delec,-! It was shown by Scharfstein and Ei5tnbrown' that a Type 304 heat conlaining 0.068% cubon would pus the 72 how Strauss test even after a sensiliz.ing treatment of up to 4 hOUri at 617 C.

An Improved modification or the Strauss test, employ InB the same solution composition, but with !.he test sample In contact with metallic copper has been developed.' ,1 This ttst , known as the copper-copper sulrate,sulfunc acid test, was thoroughly investipted by ASTM Subcommittee AIO.04 and was incorporated into A-262 as Prtclict E in 1968, It is or comparable senSItivity to the other methods In A-262 and requires only 24 hours boiling. like the StrlUSS test, interpretation i5 ba~d on visual examination or bend test specimens .so that the specimens Ire classified only as acctptable or nonacceptable. Since sufficient lime has been allowed for all to become familiar with the Improved methOd, the ASTM balloted In 1912 to dis-contmue A-J93 in rOlvor of Practice E of A262. The copper-copper sulrate-sulruric acid lest is also being incorpomed into the new intemational ISOrrC-17/sc-7 Standards for Intergranular Corrosion Tests fot Austenitic Stainless St. els instead or the older acidified copper sulrate test described in A-J93.

2

NI"ic Acid Tell (ASTM A-262, Ptaclic~ C)

The boiling 65% nitric acid ttsl, of len rererred to as the Huey test, was first described by W_ R. Huey in 1930.a " Over the yean, it has had the most widtspread use in American practice or any or the evaluation testl, Interpre-tation is based on corr05ion rate calculated from weight loss, supplemented in some instances by visual or micro-scopic examination ror grain dropping. A quantitative measure of the degree or sensitization iJ Ihereby providtd for comparative purposes However, the nitric test requires 240 hours boiling. It abo hi! certain other limitations. as described below, which must be recogmzed and guarded against for proper interpretation or resullS,

The test solution volume to specimen surface area rat io should be at least 125 mJllnl . If the corrosion products in the testing solution are allowed to build up to an excessive level, severe intergranular corrosion can occur on even solution annealed sUIflIess rree of prtcipitated carbides, Ttus il due to the presence or hexavalent chromIUm ions formed through the oxidation or Crl to Cr" by the boiUng 65% nitric acid. It was shown by Delong lO that acceleration in con0510n tate in the nitnc acid test begms when the chromium content of the tesung solution exceeds about 0.004% and IOcreues rapidly with higher chromium concentrations, Assuming a specimen surface area or 3 sq 10, a test solution volume of 600 ml and a chromium content of 18 ,5~ 10 the alloy wllh an equal proportiOn lfl the corrosion products, the test solution .... ould reach the 0_004~ chromium level after 48 hours bolllni With 1 corrosion rate of about 0.0071 m/ mo, or after 96 hours boiling with a corroSion rate of about 0.OOJ61O/mo. For a specimen of maximum surrace area to meel the 125 mllin' ratio in 600 ml of teS! solution (4 8 sq in), the cOltespond Ing corrosion rate for 48 hours bOlhng would be 0.0044 in/mo. These rates are well beyond either typical rutes or specified maximum permISSible rates liS will be shown later. Thus, with normal testing pTl'lctice, the chromium content or the solution never reaches the level where acceleration lR rate begins unless the sample IS In the stflSlUled cond.lion . In rOUline evaluatton (estmg, the- selr ilcl:eleralJon or severely senSitized materia! may even be considered an advantage Since it increases the spread between good Hnd bad samples.

The nUQ)lQlenl chromIum iol'/ effect in nuric acid testing can be avoided by the use of mullisample testing equipment such as thlll originally descnbed by DeLpng. ' o This Involves exposu.e or tesl samples "in the lOner ~r two concentric vessels, tach contaming boiling acid, with a common condenser so atTlnged thai all of the condensate is retumed to the inner \'essel or sptcimen ttSt cup, By overflow rrom me test cup, corrosion products are con-tinuously removed so thai the chrom ium conlent of the test solution never reaches the critical level. This arrange ment permits teStinB many specimens simultaneously in the SlIme container. As would be expected, corrosion rates in the multisample tester are essentially the same as in nask tests ror annealed material bUI are substantially Itlwer ror

CORROSION-NACE

.

I ,

.

sev~,ely sensitized specimens. The major portion of Du Pont nitric ICld evaJultion tests in retent years has been in mullisamplt lesler, usuaDy with three lest periods of12, 96, and 72 hours (not necessarily in that sequence) in order to avoid weekend shutdowns without loss of testing Time. In n.sk tests, three periods lIt also used. with 600 ml of terting solulion, specimens exposed in individual flasks, and specimen surface ireD seldom exceeding 2.5 sq in

Cross jeetiena' &rell in bat. wire, and tubuJar products arc somct'mes subject to end grain mack (also ca1led pin-hOling). The probable mechanism is localized attack On the exposed ends of inclUSion stringen to produce crevices where CT' can accumulate. When Ihls is the else, the proportion of the lOll] arca represented by the expo$ed CTOSS secllon may innutnce the results It is stipulated in A262 that specimens for Mdc (em should be propor-tioned so that the area of the expostd cross secuon shall not exceed half the total exposed area of Ihe specimen. Microscopic examination of the exposed specimen at low power magnification lS a requirement for the InterpretaTion of results when end p-ain Ittaa is suspected of bemg an appreciable factor.

The nmic aCId test was successfully used for many yellS 10 detect harmful carbide precipitation in existing lades of austemtic stainless steels. However. when the utra low carbon (0.03% max) grades became commercially available, it was soon found lhat for Types JI6land 3171,. the mlric lest IS also sensitive to lOme other phase or condition frequently resu111ng from the reqUired sens.ltizing heat Ireatment, whiCh experience has shown dot$ not cause intergranulu attack In other environments. This phost has bn generally assumed to be submicroscopic sigma, II - I] allhough pOSJlive proof for such a mechanism has never been demonstrated III p~senee is postulated from the flet that longer exposure time m the senSitizing temperature Bnge does produce readily identifIable sigma phase par-ticles, but in that form, they have relallvely Jinle erfect on corroSion Iale. Since the other available methods described In A-262 are not sensitive to the presence of subm-icroscopIC sigma in Types 316land 3171., the nitric teSI should not be used for evaluation of these grlde! unless the mllenal represented Is actually iOlended for nitric ,tid service. There IS 1150 some poSSibllilY of encountering submicroscopic sigma 10 Type! 316 and 317, bUI these grldes are tuted In the commerciaUy annealed condition and with the higher carbon content. the effect of carbide precipitation is almost Ilways predominant.

FDTIC Sulfott-Sulfuric Acid T~st (ASTM A-262, Practice B)

The ferric sulfatesulfuric acid lest. often referred 10 IS the Streicher test or the ferric sulfate test, was originally described by M A Slreicher in 1958. 1 It is of comparlble stnsitivity to the nitric test but requires only half the boiling time. while retaining the advantage of a numerical corrosion rate for comparison of the relative performance of the specimens evaluated. It is not sensitive to the presence of submicroscopic sigma in Types 316l and 317L as produced by the usual sensitizing treatment (J hour al

Vol. 30, No. I, January , 1974

677 C). and in Du Pont practic:r:. the ferne sulfate lest has long been used for the evaluation of these grades. Also it is not sensitive to end grain aUacle. Accumulation of corro.. lion products does nOI accderale the corrosion rale as It does in the mtric lest, so that several specimens may be tested in tht same nask. However, there are a few preC1lulions referred to in A-262 which must be rigidly observed. The solution must be brought to I boillnd all of Ihe ferric sulfale dissolved before the test specimen is Immersed. Additional ferric sulfate inhibitor may have to be Idded (or the solution chang~) if there I! e.xcessive common of a very severely Ktlsilized specimen as evi-denced by I color change f,om amber to dark green_ The inhibitor ongmally added would be enlmly consumed If the totll weight loss of the speCimen, or specimens, exceedt Ibout 2 grams, In which case, the solulion would act like boiling SIR sulfuric .cid and the specimen would be dissolved_ Also. care must be liken that all scale formtd during heat lIeatrnent is completely removed before start-ing the test. If. for instance, small patch of scale Is not removed from a stamped number durinS polishing, IClIVl-don due 10 galvanic action may occur. as evidenced by gas evolution and IOCIJ color change immediately after immer-lion of the specimen. In this case. Ihe specimen should be immediately removed .nd repolished, or the scale removed by immersion in concentrated nitne acid 8t about 93 C (200 Fl, and the Itsl restlfted. Nitric-Hydrofluoric Acid Test (ASTM A-162. Pttzcli DJ

The omit.hydrofluorlc acid lest IS adopted fOT eVllua-tlOn purposes was first deSCribed by O. Warren in 1958.1S It consists of IWO 2 hour periods in 1($ IINO,-3* !iF solution al 70 C (ISS F) and thus reqwru leu testing lime thin any of the other methods. It was devtloped specifi-cally to differentiate between precipitated carbides and submicroscopic ligml In the molybdenum belring grldes. As described In A-262 . it applies only to the evaluation of Types 316. 316L. 317, and 317l, although il probably could be readiJy adapted to other gradrs. if desired The bue corrosion nit of stlinless sleels in HNO,HF solulJons is quite high and varies substanllally with relatively slight differences in composition Within the same grade. It IS.. therefore. neceuary 10 run IWO tesu and compare the corrosion nle of the specimen to be evaluated (JH~iltd for Types 3161nd 317 and 5eIlSiliztd fOf Types 116lloo 317l) Wilh another specimen of the same material which is free from precipitated carbides as. shown by I step structure in the oxalic ac.id etch test. If the al-feceivtd material docs not show a step structure, a portion of it must be laboratory annealed 10 provide Ihe required good sample for comparison_ A ratio of less than 1.5 between Ihese IWO rates indicatts the malerial represented to be S3lisfICtOry. This test SI\'es consistent and reliable results but hIS nOI had widespread we for routine evaluations. presumably because of the ncassity of using a ratio of two ttst raTes for interprelation. the rnconvenirnce of handling solutions conllinins hydrofluoric acid, and the :availability of the fenic sulfate test whkh was dt\'eloped a' :tbolll the same lime.

]

OXl1lic Acid Etch T~t (ASTM A-262, hactlce A)

The oxalic acid etch ttst is not actually an evaluation test bUI a formalized classification of etch structures which is used to sc~en OUI those specimens which would unquestionably pass the applicable evaluation test as listed in A-262. It was firsl described by M. A. Slrckher in 195316 and the results of an ASTM CoopenliYc tesllng program were published in 19S4.1'J Details of procedure and classification arl! given in A-262. Seep structures (essentially free of carbides) and ditch structures (many completely encircled grains) are easily clauified. DUDl structures. which may ha ... e many partially encircled grains, require more extensive microscopic examination to deter-mane if any arc completely encircled. StriCt imcrprcllI.tion as specified in A26] stipulates thai a sample In which any completely encircled grams arl! found cannot be accepted without an evaluation test. Experience hI! shown that most samples wowing only an occasional random encircled grain will also pass any of the evaluation tem but appreciable compromise in interpretation cannot be safely made with-out some risk of accepting specimens which might fail In an evaluation test.

As indicated above, no male rial is rejected on the basis of the oxalic etch test alone; those specimens which cannot be aa:epted are rubjected to the appropriate cnluation te.$1. Most of the .speCimens which are not scrtened OUI by the oxalic elch test will still meet evaluation lest require. ments. However, since the etch test can be performed quickly, il is quite advantageous to use it for incoming spctlmcm so long u a considerable proportiOn can be screened OUi.

Early Du Pont experience with the routme use of the oxalic ereh tesl soon disclosed thaI for Type 304l. only a dis.3ppoiOtingly low percentage could be screened after a sensHizing Ireatment of I hour al 677 C. The screening percemage was fanly good for plate, sheet, and strip Ilnd forgmgs, but much lower for rod, bar, and hex and miscellaneous (mostly fittings such as elbows, tees, con-centric R!ducers. and Hanges). For tube and pipe, especially small diameter tublOg where both interior carburizalion anc! end grain attack Ire frequently found, the proportion screened was so small as to make the procedure hardly wonhwhile. Since more evaluat Ions are made on Type 304L than any other grade, further studies were con-ducted. 11 It was found thai the ~reenjng ptn:enlage could be substantially Improved , while slill retaimng assurance th31 the specimens would meet evaluation test require. ments. if a sensitizing lime of 20 minutes (instead of I bour) were used. The 20 minute sensil izlng trealments are carned oul In a neutral salt bath since varillion in time 10 reach 677 C In an au furnace Is 100 greal for consistent results. The use of 20 minute sensitizing treatmentS substantially increased the screening percentage for all forms except rod, bar, and hex, which showed only a slight difference, possibly due to the greater mtidence of end gram allack.

DtJ POnft data on oxalic etch test screening percentages comparing I 'hour and 20 minute senSitIzing (Jeatments on

4

Type 304L are Vlewn in Table I; abo included are data for Types 304 and 316 (as commercially annealed) and Type 316l (after 20 minutes at 677 C). It is probable that the oxalic etch leS! could also be applied to other grades such 31 Types 3095 and 341, but no studies to supply the necessary background have been reported so far.

A limited amount of dllta on oxalic etch test screening is also available from other sources. For bar and strip material, one source reponed 251 of 276 samples of Type: 304 and 223 of 230 samples of Type 316 were screened in the commerCially annealed condition, but only 2 of 49 samples of Type 304L after I hour at 617 C_ Another source reported S3 of70 sampJes(aU forms) of Type 304L were screened and of the 17 samples subjected to nitric tesl,8 failed_

Experience with Evalu.ation Tests The nitric acid lest WitS the first test to be applied on a

major scale for Ihe routine evaluation of specimem. repre senting purchased maierial. Du POnt specifications calling for a maximum ptrmiSSJble nitric lest lale of 0.0015 in/mo (18 mils per year) for /8.[J-S and /8-8SMo (0.07% carbon max) appeared III me early 193O's_ This limiting rale was selected on the basis of available data as low enough 10 guard against mlergranular failures In services for which the alloys were otherwise suitable, and high enough to be a practical requirement for producers and fabricalOrs to meet. Subsequent correlation with service ex.pcrience proved to be so good that the originally selected value of 0.0015 In/mo has never been ch3nged. At that lime, water quenching of fubricated equipmenl after a solution anneal was required for austenitic stainless sleels, lltld welded specimens representing these /illuf lIeollTe1rmellls wen: also evaluated. When the stabilized grades appeared. the practice of exposing incoming specimens at a temperature which would sensllile unslabilized mlilerial$ was adppted in order 10 establish that the material represented could .be safely used in the as-weld~d condition (ie .. without hell.l tre:u-ment after fabrication). A maximum permissible rate or 0.0020 in/rna after exposure for ] hour al 617 C was specified for Type 341. and the same rate was speCIfied for Type 304L when It became available. However, as pre viously discussed, the behavior of Sl!l/SlIIzed Type 316L in nitnc tests was found to be complicated by tht- presence of rubmicroscoplc sigmD. consequently the nllnc 1,,1 is not recommended for Type 316L unless the matenal is actually to be used fOI nitric aCid service, which lS seldom. The ferric test is the one most generally used with a fIlIximum permissible rate of 0.0040 tn/mo.' ~

It should also be ~cognized that enluation tests are a

service environments. Likewise, tht sensitizing Ire&lmenIJ used for the extra low carbon and stabilized grades are more severe than would be experienced with good welding practice and thereby provide a margin of safety for abusei in welding. On the other hand. there are I few environ-menlS which Irt capable of mtergranularl)' attacking even solutiOn , annealed st!inless su:els without precipitated cubides land which will pass all of the evaluation tests discussed. These include hot nilric acid iueJf under con-ditions where the concentration OrCr 46 can build up above the critical level, rulric acid solutions to which mong oxidizing Igconls such IS chromates, vlnadales, and cerales have betn added. and hot chromjc acid solutions. Recent work. by Armijo, AU51. and othersl t-] I has resulted in the proposal of a schae segr~ation mechanism to explain the mtergranular attack on solution annealed materia] in these highly OXidIZing media. Their results indicate that concen-tration of certain mmor elements (particularly silicon and phosphorus) is the primary controlling factor for this type of attack.

In current Du POOl practice, eviJUltion tests are speCified on(y whVl il is known or su1pud IhDt nlllttriDl shown to be susptibte 10 mterrranuJtu CQrrorion would be 1II1ugranularly attacked irz the intended Jervi~ enlJiron-nu:nt. SlOtt the L gudes Ire usually specified for such ItTYlctS (except when no welding is im'oived). the numbe, of Type 304 .nd 316 specimens evaluated has decreased to smaJl propanaon of the whole. Incoming specimens of Types 304 Ind 316 (also cast CF8 .nd CF8M) in the as-received condition. and of Types 304L and 316l aOer I sensitizing treatment, are first screened by the oxalic etch lest Those which cannot be accepted by the etch test "e subjected to the appropriate evaluation test. This m.y be enher the fernc sulfate or nitric add test fot Types 304, 3041., 316, CFg, .nd CF8M (depending upon plant preference), but alwlYs the ferric sulfate test for Type 316L unless the material represented IS to be used in service mvolving nitric acid (very seldom). Other grades are subjected direc:tly to evaluation lests since Insumdent InfarmBtlon has been developed to classify oxalic etch Structures. The maximum acceptable corrOSlQn rates for the most common lus-temllC wades as currently listed in Du Pont specifications are $hown in Table 2.

Comparison of Eyaluation Tesl Data From lime to Lime, tabulations of Du Pont evaluation

test data hive been made ovef some period for some speCIfic purpost. Such tabulation covering approximately 10,000 tests an Clst 18S (now CF-S), 18..g Mo (CF8M), and FA-20 (eN.7M) which ilIustrllted the effect o f compo-51110n variables on nitric test rates for Slaul1ess caslings was published in 1949.5 A recent summary of nuric and ferric test data on wrought minim Illoys broken down into corrosion rate incremen ts is available for use in thls paper. Thts includes 7732 incom ing sa mples of which 5466 were Type 304 l and 1454 were Type 3 16L Of the 10111 sampes, 3 139 were Icceptd a n the basis of the: oxalic elch tW, 2624 were subjected to nildc tests. and 1969 to ferric tests. The lime periods represented were January, 1964

Vol. 30, No. I , Jan'*Y, 197.

through MlY, 1965; AuguSt, 1966 through April, 1967; January, 1968 throuah December, 1969; plus 1970 and 197 1 data for grades olhCT than Type 304L.

Since Ou Pont data are from specimens representing purchased material specified to meet evaluat ion test reqUirements, It an be ugued that they are biased to .1 least some extent through special handling actarded the material by the producers. Consequently, producers were Isked to supply evaluation teSI data from their own mill or laboratory records for comparison. Such information was submitted by the AUeghney Ludlum Steel Corp., Armco Steel Corp., C. O. CarlSon, Inc., Carpenter Technology Corp., Crucible Stamleu Steel Division. EaSiern Stainless Slee] Co., and the United States Steel Corp. The combmed data from these sources are lhown under the he.dmg Producers. They include 2296 nitric tesls (1613 on Type 304L) and 179 ferric sulfate tests. In eomparing Du Pont and Producers data, II should be underslood thll there are severallmpolwn differences In the makeup of the teSlS.

For Types 304, 304L. and 316 in both teStS and for Types 316l and 3171111 the ferric sulflle test, Du Pont test results include only samples which were nat ac:cept.ble on the basis of the oxahc etch test . I.e., I lalge propaJ1ion of the specimens which would ihow the lowest corrosion rates were screened out and not tested.

For a considerable propOrtion Df Du Pont nitric tests on Type 304L .nd for aU ferric rulflle lests on Types 3041., 3l6L, and 317L. I "scns.ili7inl treatment of 20 minutes at 611 C in a salt bath (inSle-ad of I hour in 9n air furnace) was used In order to increue Ihe proportion which could be screened by the oxaJic elch test. The resulls from 20 minutes and 1 hour 5enSitiZUlg IreatmenlS are shown separately. Where sensititing treatme:nU arc lllvolved, Pro-ducers data are ,II for I hour treatments.

The specimen! represented b~ PrOducers data all faJl within the classificatiOns of plate, sheet, and strip, tube and pipe, and rod, bar Ind hex. Ou Pont data fOf all fanns also includes I large miscellaneous category consistmg princi pally of vanous types of nltjngs and forgings. Test resulls for castings and all weJd me tal pads lie not included In some cases whtre the numbers repre~nted are sufficiently large to have Slatistical significance, dati for the different forms are shawn separately.

A major proportion of incoming Du Pant specimens are transmi tted by fabricators or ~lehouses rather than directly from the producers so thai the passibUlly of mixup In material or heat treatment is increued. As In extreme example, in one lot of seven Type 316 rod specimens of different sizeS. five failed the evalualion test and one WIS found by rou tine spot lest to (onlain no molybdenum.

As an aid in inlerprel'tion of the paphs of corroslon dati which faDow, Table 3 presen ts PJodUCfrs and Du Pont nitric acid tesl data for Type 304L (after 1 hour.t 677 C) in numerical farm as the result s were orilinally tabulated. DistributiOn o f corrosion rates is shown IS Ihe number of samples falling wi thin sdected rale increments. Producers data inclucX ttst rates for aJl sample., bu' .,. previously sl.led . Du Panl tcsled only specuMns ... Mch wele not

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TABLE 1 - ou Pont Dati on Proportion of Simples Screened (ACCt!pted) By the Oxalic Etch Tnt

Type 304l Iher 1 Hr .ll 677 C

"'11, "'eet. end strip Tube Ind pipit Rod, blr,.nd hll( Fofgingl MJ_Il.n8O\.l1 All forml

Type 304L .trer 20 Min It tr77 C Pllt., shHl,.net striP Tube Ind P'PI Rod, blr,.oo h,1I FMglngl Misc.lI."'01Jl All Fotml

Type 316L 1ft. 20 Min II m c Platl, th.II, Ind strip All Forms

Typtl304" Commltfl:llUy An.-c Plllte, shet!t, 100 strip All Forms

Ty~ 316 H CommlttCially Ann.led P1Ite, 1hHI, Ind IIlr ,p All Forms

TABLE 2 - Maximum Acceptable Evalu. tion Tut Rates Specified by Do Pont for Services Where Susceptible

Material Would Be Intergranulatly Attacked

T,,,

"" 3D

"

The nitric test data given in Table 3 for 01/ forms of Type 304L tested after a sensitizing treatment of I hour at 677 C are also shown In graphic IQrm in Figure 1. in which the percentage of samples above the indicated corrosion rate IS ploued fOT the selected rate increments.. The number of samples represented m each case Is shown in parentheses following the symbol Identification. In view of the situation describeQ In the preceding paragraph. Producers data are shown b~th for lotal-samples and excluding Source A. In order to provide II betfer basis of comparison on the graphs, Du Pont data ale shown both as: (1) peJ'cent of rem above the indicated corrosion rate, and (2) percent of total SlImpfes above the indjeated corrosion rate. For this purpose, it is assumed thnt the samples SCreened by the oxalic acid elch test will not show nitric test rates above 0.0010 in/mo (or ferric test rales above 0.0030 in/mo). Experience has indicated these limits to be approximately correct, and that most screened samplel will give consider ably lower conosion rales. As shown in Figure 1. the percentage of samples above the selected rate incremenTS up to and iJlcluding the 0.0020 in/rna limit is considerably higher for Producen total samples than for Du POnt total samples or even Du Pont tests only. However, when the results from Source A are excluded, the data are in quite reasonable agreement. Du Pont data show a larger (bUI still small) proportion of specimens with rates above 0.0050 In/ rna, which is at least partially due to mixups in material (Le.. Type 304 instead of Type 304L).

Figure 2 shows Du Pont nitric test dlla for aU forms and plate, sheet. and strip of Type 304L tested after sensilizallon fOI 20 minutes al 671 C. Differences between the tests only and tott.! sample basis are greater with the shorter sens.itizing lime, since the proportion screened by the oxalic etch lesl is almost doubled . The specified maximum ule ror specimens sensitized 20 minutes 01' 671 C was originally set at 0.0010 in/rna and has not been changed. It can be argued that a value of 0.0015 in/ mo as allowed ror Type. 304 wouJd be more oonsisu!nI . The dala indicate that the effeCI of such a change would be sliiht. With 1he 0.1)010 mlmo limit. Ihe proportion of total specimens (all forms) above the specified maximum was slighll) lower than wilh the 0.0020 in/ rna rale for I hour sensiliz.alions (7.5% vs 8.6%); with a 0.0015 inlmo limit for 20 minute sensi.til.Jtion. the 7.5% would have been reduced only to 6.5%. The rdative behavior of various forms is also quile similar to thllt With the longer senSItizing time. No Producers dllta art available for 20 minUle sensitizing treatments.

Figure 3 presents nhoc lest dalll (or all forms ofTypt 304 as commercially annealed. The number of samples represented is much smaller than for Type 304L but is sufficient to be ItatlsticaJly significant. The proportion screened by Du Ponl with the oxalic etch tcst by Du Ponl is much highe.r (64~) than ror Type 304L Agreement between Producers and Du POllt d:lIa on the tOlal $lImples baSIS is quite good.

Figure 4 covers nitric test dilla for all forms of Type 316 as commercially annealed. Here, the differem'ts

Vol. 3D, No. 1, January. 1974

___ --~::_==-"T!'m '... 0 ...... _ ... ~

7. ___ _ II ___ ~ 4 ____ _

-

J 1: I. i. j"

FIGURE 1 _ Nitric: Kid tat diU ,,1"1 Type 304L ;111 .. one hour ;It 677 C taU formsj.

FIGURE 2 _ Ou p"I"I' l"I;ul(: Kid lest cYU "" Type 304l 1ft. 20 mlnUIU It f'i17 C.

belween Producers and Du Ponl data on the total samples baSIS are large and if plale. sheet, a.nd strip are considered separately. they are extreme. For plale. sheel. and strip. two of four SOUTces reponed that 52 of 100 samples gave rate! above 0.0015 in/mo. In contrast. another source reported no samples of 30 tesled abo .. e 0.0015 in/mo. while Du Ponl had 4 of 21 teSIS leprestn1ing 63 samples of which 42 wele passed by the oxalic etch teSt . As previously explained, Du Pont currently runs very- few nitric lem on Type 316: for services where intergranular corrolion is 10 be feared, Type 316l is normally specified and tested by the ferric sulfate lest. However, Du POni mn thousands of nitric tem on 18-8-S-Mo (specified as 0.07% carbon ma.""

TABLE 3 - Nitric Add Test Resulu on Type 304L After One Hour at 677 C IASTM A-262, Practice C)

PI.II. snMI_ lOCI Sulp PToducer.(I} 0... Porol

TuM .nd PIp'- Rod. e .... nd Hu AJI Form. Ou Pont(2) P.odu-utl Du Pont Proch.IC .... Du Ponl Produo:.,.(1)

Totill s.mpl" 770 893 ". .,. 220 258 1116 22 "

Nunber O)r.lic s.c:r-..:l try ASTM 1.262 Pl'ktice A >n 29 .. 57'

Number of T"a 770 '"

126 497 220 190 1116 1637

O,nributron of Comuion R.la In 'nJMo 0.00050 .nd below 13 21 40 30

., B2 0.00051 to 0.00075 145 200 54 20' .. 53 285 .14 0.00076100.0010 203 180 17 117 54 306 '64 0.0011 10 0.0015 167 8' 13

" 2. 32 205 233

0.0016100.0020 .. "

10 18 "

8 122 54 0.0021 100.0030 59 9 1 9 72 48 0.0031 to 0.0060 36 8 3 17 3

" 42 53

o.oosl to 0.010 12 17 20 .. 0.011 end.,.,.." 7 0 ,. 3 10 7 ..

,. of Tem Above 0.00050 99.2 97.5 813 92.0 86' 91.9 .... 949 ,,"of Tftb Abo .... 0.0010 48.8 24.6 21.0 274 26.4 415 "9 292 "of Tut. Above 0.0015 21.0 85 16.6 17.0 15.0 247 23.1 ". "of T.rtI AboVtl 0.0020 14.4 5.8 87 113 8. 205 ". ". 'I. 0' Toa.1 Simp/II:!. AboYI 0.0020 14.4 3.' 87 12.5 8 . 15.1 12.6 B.

(I)()QIU 001 indudo: 4117 pbte iheet .nd s trip amp ... which were brohn dow" onl,. inlo < 0.0010 (1'9). 0.0011 10 0.0010 (152), .nd > 0.0010 (56) eoffO,lon .. ft: ,ncr"m91",

(l)lndudlU OIlier form. JUeh u fillinp .nd forJInp..

involved longer holding times and rugher temperatules than often used today. It IS possible that variations in annealing pr.Ct1ce may be an appreciable factor in the erratic nitric telt resuits reponed. It is known that higher annealing temperatures and longer holding limes can substantially reduce (but not eliminate) the submicroscopic sigm4 ~ff~Cl in rutric tests on sensitlled Type 316L.2l However, as previously stated, the mtnc test is not recommended for the evaluation of Type 316l urness the material Is actuaUy 10 be used in scrvice Involving nitric aCid.

Figure S shows nitric test data on all forms of Type 347 after .sensitlLing one hour at 617 C and rlgUTe 6 for all forms of Type 309S. For Type 347; Producers and Du Pont data are m good agreement. The pro port ion of specimens excu:dmg the specined maximum nte is higher for Type 347 than for Type 304L. and it is probable that secondary phases are sometimes involved, although to a much lesser extent than with Type 316L For 3095, the Producers data consiSlS of only five tests. In Ihis grade, more thin 80':Jt of the nitric test rates are below 0.00050 in/mg.

InsuffiCient data are available on Type 321 to Justify plotting_ A limiting rate of 0.oe)30 m/mo In the nitric acid test afler one hour at 677 C has sometimes been specified In Ihe past . The cunent tabula tions include only 13 tests, with Du Pont reporting two of seven samplu Ind Producers

8

four of six samples above this rate, Streicher}) has shown that sensitizing treatments tn some cues resulted In the formation of secondary phases which caused increased rates m the mtric acid test and also, to a lesstr degree, in the ferric sulfate test. These were nOI reVelled by the Quilc etch test , the HNO,HF test, or the copper sulfatesulfulle aad test. i1 has also been shown that TIC and Ti(C,N) particles are subject to direct a1lack by boiling 65% HNOJ.H,lS For plITuciesSCIuered throughout the matrix, Uus results in only superficial surface .Itu.:k. lIowever, m the fUsion zone of welds, it is poSSible to develop a continuous gram boundluy network of TiC, which Is soluble in hot HNO] and causes a speCialized form of inter granular or interdendrilic corrosion referred 10 as fi1SUr~ Quack_ l ',11

Figures 7 and 8 present Du Pom ferric test dtra on Types 304L and 317L, respectively, iested after :liscnsi-tWng tIeatment of 20 minutes at 677 C Differences between dats for alJ forms and plate, sheet, and strip lire less than in the nllnc test, rince the ferric test is not sensitive to end grain attack The limited Producers ferric test data submitted for both Types 304L and 316l are shown in Figure 9. The specimens were all plate, sheet, and strip and were lested after a senSitizing treatmcnt of one hour at 677 C_ For Type 304L. the resuJ ts differ only slightly frum the Do Pont data based on 10 minute

CORROSION- NACE

.,.

I

-

-

J 1 : J j. l .

-

0-.., __

- -

0"_ ,_",. ... _--..-a __ .............

-,--.. ~ ...

FIGURE 3 - Nitric: acid lest dar. on TVI"I 304 .. eomm.,cl.Uv .nnM'" '_II lornul.

J 1 J j j

- ..... ~ ..

FIGUFIE 5 - Nitric: Kid IWI data on TVPlc 347 _her on. hoUr.l 671 C 1.tllolfNI).

sensitizing IrealmenlS. For Type 3161... the Producers raftS are considerably higher, bUI only 17 lests were reporttd. Figures 10 and II cover Ihe limited ferric leSI data available fOI Types 304 Ind 316. respectively. In the commercially anneale-d condition. The Produceu data show a subSlan. tlaiJy higher propoTllon of the gmples tested above the sdecled rate tnCJcmenls .t the lower levels, bUI only slightly more above the 0.0040 in/mo maximum (10.0% 'IS 6.2% for Type 304 and 3.0% 'IS 2.4~ (0' Type 316).

Figure 12 compares Du Pont results on Type 304L as obtained by: (I) nitric lUIS after one hour at 677 C. (2) mtric Itsls aru~r 20 minutes al 617 C, and (3) f~nic sulfat~ tuls afler 20 minutes at 677 C. Th~ differ~nl forms are shown Stparalely IS well 115 the combined totals (ir .. all forms). The dall 8re ploued on the total umples basis (i.e.. samples screened by the oxalic etch test are Included as OJ)()IO infmo maximum). Since no rale Incrtmenn below 0.0010 infmo are included, the scale isupanded 10 enable benet separation of the curves for the diHerent fo rms. The proportion of samples above Ihe specified maximum rales

Vol. 30. No. I . January. 1974

J

r----,~~""""T""...., . .. -_ ...... .... ---,.. ._ .. _-

.u J 1 : I I j l"

;:" :-:':;::~~~~~:;~::::::~:;~ .. ~ ...... ~- '.-~-~--- - - -

- c-- ..... ~ ....

FIGURE _ Nonie .cod teJ1 dlt' on Type 316 n comm.ttllllly .n,....ltd 1"1 form.l.

I: J. j.

- -'-_ .....

FIGURE 6 - NlIIie Kid I", dille on TyPl' 309S .. comrMl'd.Uy .n'*lltd 1.11 form.' .

-

by the Ihree test methods are quue coniislenf fOf plate. sheet, and strip (3.4%, 3.4%. and 3.n. respecti\'e1y) where the complicating effects of end grain allade ;md localized carburiz8ILon Ire rarely encounter~d. Also, tht chance for mixup in malerial Is minimized as compared. for instanct. to small Jots of rod, bar, and ht'( which frequently COJTl(' from warehouse or fabricator stocks. On the all forms basis. tht correspondmg values for Ih~ thrtt leSt mtlhods art 8.6%,7.6%, and 4.B':l. rtsptClively. The Iowtr v31ue for lhe felne sulfate lest rC'Sull$ is to be exptcted sinct the ftrrie tesl is not SC'nullvt to end grain attack \\hich Is sonltllmes found on bar and tubul:u products in nilde 3cid. The proportion of lube and pipe specimens abm"e the specified maximuhl rlllt apptars to be appreciably higher for nitric ttsts with one hour stnsitillitions Ihan for nitric tesll wnh 20 minute sensitiulion&. This may be :11 least partially explamed on the bas.is that interior carbunz:llion is frt quently found in small bort tubing (tspeclally ~ ITlch 00 or less), and thai one plant using larg~ qU3n1 it its of such tubmg preftrs thlll n:IlUlliiolis bt madt by nil l"ic add tUIS with one hour stnslIizations: ,:onSt'~uently. Ihe dala are

9

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TABLE 4 - Average Corrosion Ralet for Passing Samples

"od~ Du Pont ..... F~m Condition T .... "iMo T_. ,,-

Nitrk Acid THII (ASTM A262, PJltC1iee CI ,

""L Plall. ShMt, andSUlP I hr. 617 C 254 0.00093 491 0.00084

"'" Tube IIOd Pipe lhr,611C 115 0.00081 '31 0.00Il05 300' Rod, Blr, .nd Hex lhr,677C

'" 0.00015 IS' 0.00092

300' M,_llan)ul lhr,617C 374 0.00084 300' All Forms

(Total) 1 hr, 617 C ... 0.000B6 '047 0.00085

"'" p,,~, ShlNt, .nd Strip 20 min, fS77 C 58 0.00011

"'" Tubt.nd P,pe 20 min, 617 C 25' 000064

"'" Rod, Bar, nd Hlx 20 min, a17 C .. 0.00072

30" M,tce:llanlfOUl 20 min, 677 C ,.7 0.00073 "'"

AU Frm ITlltall 2Om,n, 677 C 558 000068

300 All Forms Comm, An,..I" .,

corrosion

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