INTRODUCTION

GENERAL ASPECTS OF CORROSION, CORROSION CONTROL AND CORROSION PREVENTION

1. Definition of corrosion

The generally accepted definition of corrosion reads as follows:

Corrosion is thr undesired atrack of o material as a result of chemical or electrochemical reacfions with components from the ewironment.

Attack, decay or becoming unserviceable by physical, mechanical or biological causes are therefore excluded. Examples of such phenomena are superheating wear, erosion, cavitation, swelling of plastics, decay of wood, etc. On the other hand, corrosion is considered as includ- ing combined forms of attack in which the simultaneous occurrence of corrosion by chemical or electrochemical attack and the effect of the other above-mentioned causes leads to an item becoming unserviceable at an above-nor- mal rate. Examples of these phenomena are stress corro- sion, erosion corrosion and fatigue corrosion.

The definition distinguishes between chemical and electrochemical reactions, for corrosion can he subdi- vided into electrochemical corrosion and chemical corro- sion. Electrochemical corrosion is the more common. By electrochemical corrosion is meant corrosion resulting from reaction between a metal surface and an ion-con- ducting environment. Such corrosion can occur if the metal comes into contact with a liquid in which an electrolyte (a substance which, when dissolved in water or another solvent, is separated into ions) is dissolved or with a melted electrolyte (for example, salts or oxides). hlost cases of electrochemical corrosion occur in aqueous environments such as mains water. boiler water, cooling water, sea water, etc., solutions of salts, acids and bases, a humid atmosphere and a wet soil. Melted salts and organic solutions of electrolytes are used to a far lesser extent, and therefore occur far less frequently as environ- ment in which electrochemical corrosion can occur.

By chemical corrosion is meant corrosion resulting from reactions between a metal surface and gases (gase- ous elements or gaseous componnds). Because this form of corrosion usually occurs at temperatures substantially higher than ambient temperature it is also referred to as high-temperature corrosion. An cxample of this is sulphidation.

2. The consequences of corrosion

The coniequences of corrosion are both technical and economic in nature. Corrosion shortens the life of pro- duction facilities and transport systems by causing their premature collapse, sometimes accompanied by acci-

dents. Repairs required to remedy leakages and blockages push up the cost of maintenance. Malfunctions in pro- duction facilities resulting from corrosion give rise to loss of output. Contamination of products by metal oxides and contamination of the environment can also occur as a result of corrosion. Of the world's iron production. 10% is lost annually by rust formation alone. The total direct and indirect costs associated with corrosion amount to some 4% of gross uational product, which means about $500 per year per head of the population in the in- dustrialized countries. Of this loss, approximately one quarter can potentially be saved by taking appropriate corrosion prevention measures in good time.

3. The theory of electrochemical corrosion

In the electrochemical corrosion process, there are always at least two reactions involved, namely an anodic or osidarion reaction, during which a metal dissolves while releasing electrons, for example:

and a carhodrc or reduction reactron, in which the elec- trons formed are removed. for example.

in acidic solution:

in neutral aerated solution:

Whether a metal uill corrode, however. depends on two factors, namely: (i) the level of the equilibrium potential of the cathodic

and anodic reactions in relation to one another. in which case that of the anodic rsaction should always be lower than that of the cathodic reaction;

(ii) the speed of the corrosion reaction (that is, the summed reactions at the anode and cathode), which in turn depends on various factors, su+ as nature of the reactions taking place, the concentration and supply of the reacting substances, and the form of the corrosion products.

In addition to the occurrence of oxidation and reduction reactions, other factors also play a role, such as tempera- ture, the velocity of elerrolyte movement and the resis- tance of rhe electrolyte.

In the laboratory, the corrosion potential and the speed of the corrosion reaction can he accurately de- termined for any metal in any environment, by measur- ing polnrisation curves with a potentiostat.

1988 CAE 0-A001 1991 1-4001 1991 2-A001

4. Forms of corrosion Group I

Figures 1-3 give a survey of the most commou forms of corrosion, which have been classified according to the degree of identifiab'ility and the aids which are required for their identification. Group I (Fig. 1) covers the forms of corrosion which are readily identifiable wirh the naked eye, whilst the Group I1 (Fig. 2) forms of corrosion may sometimes reqnirz supplemeutary analysis, and verifica- tion by means of microscopic examination is often needed for the corrosion forms in Group IT1 (F ig 3). A number of comments on the various forms of corrosion are made below, although their causes are left out of consideration.

Uniform attack; also knou-n as general corrosion, takes place evenly over the entire surface of the metal. As a result of this regular character and the usually well-de- finable regular rare of corrosion, predictions of life can be made u~ithm reasonable limits of accuracy. Accord- ingly; general corrosion does not prcsent any particular technical problems, and the application of a so-called corrosion allowance is possible.

Localized corrosioii is far more treacllcroua in nature and far less readily predictable, for example localized corrosion undzmeath deposits caused by differences in

(a) Uniform attack orio>noi surfnce

(b) Localircd attack

Severe local corrosion (e.g. underneath deposits)

Crevice corrosion (often in stainless steel)

Fig. 1. Group 1: Forms of damage which are indmtifiable by visual cxaminarion

1988 CAE 0-.4002 1991 1-A002 1991 2-A002

aeration. and pitting attack caused by the presznce of oxygen (for example in boilzrs) or as a result of the presence of chlorides (in the case of stainless steel). The maxitnum pit depth is the decisive factor determining the life of a piece of equipment.

A particularly notorious form is crevice corrosion in stainless steel which, like corrosion underneath slndge

(a) Velocit~ phenomena

Erosion corrosion Attack in a distinct flow pattern 1e.g irnpingemcnt attack at coppe or coppzr alloys)

Localized artack downstream of irregularity (eg, welded joint, conrtnc~ion.

Localized groovcd attack (e.8. m bends)

Erosion and cavitation ernsmn occurs due to the presence of solid particulate matter. droplets or azr bubbles in the m d i u m and causes uniform wear, with a distinct flow pattern, 3s well as local erosion of matctial cavitation occurs due to the implosion of cavities or steam huhhies and causes locslizcd pittiug allack with sharp edges (see figure)

Fretlmg Atrnck due ro rubbir.g contact

l b ) In l e rgmnu l~ irattzck Iniergranular corrosion. dong the grain boundaries leg. weld decay: locnked attack nt about 1/2 cm from the weld in stainless rteeli

(c) Exfoliation Layer corrosior, layered swelling of the rualciial (e.g. in damp atmosphere. in moist wil, in stagnanr hater)

~ d ) De-alloying attack Selective attack to alloys, one cornponenr of which is dissolved ( e g dcri~rcificarmn of brass)

deposits, must be attributed to differences in aeration of the metal surface. In fact; crevice corrosion may be considered as a special form of pitting corrosion.

Galvanic corrosion, which occurs when there is electri- cal contact hetween different metals which arc inkrcon- nected both n~etaU'ically, and via an elzctrolyte, belongs to this group. The pitting attack aa n result of deposition

f low direction

Layer

Fig. 2. Group 11: Forms of damage which may require additional analycs for identification

1988 CAE 0-A003 1991 1-A003 1991 2-A003

of noble metal particles on less noble metal is also a form of galvanic corrosion.

Group I I

Very often a distinct flow pattern can be identified in the corrosiotl picture. This ma? be the case in erosion corrosion (for example at high velocity flow) and in erosion pure and simple (for example, where a gas is contaminated with droplets or solid particles). However, the effect of velocity on the attack is not always associ- ated with a distinct flow pattern, for example in the case of erosion, or cavitation in pump impellers, or highly localized erosion corrosion downstream of an irregularity in a pipe.

As was already stated above in the definition of corro- sion, the phenomena of erosion and cavitation cannot be considered as f o r m of corrosion. Fretting, mechanical disturbance of various metals due to under-lubrication, although leading to attack. cannot strictly be classed as corrosion either.

The phenomenon of weld decay is a form of localized corrosion occurring about 1 cm from the weld in stainless steel. Microscopic examination clearly reveals the inter- granular picture (attack along the grain boundaries) of this corrosion.

Exfoliation is usually an advanced stage of general corrosion. as occurs in virtually motionless aggressive environmenrs.

Selective dissolution occurs in alloys, where one of the metals enters into solution. This may occur layer-wise but also plug-wise, and then leads to pitting corrosion. The best-known example of this is dezincification of brass.

(a) Stress conorion cracking Stress corrosion (e.g. srainless s l ed usually tranrganular)

(b) Fatlgue corrosion Mechanical cracking due to fluctuating stress (material fatigue)

( c ) High temperature attack Scaling, formation of a thick oxide layer (e.g. superheater rubes in a boiler1

Group IT1

If static stresses are present in a metal; for example as a result of construction and/or use, and the metal is situated in an electrolyte. this may lead to stress corro- sion. In stainless steel, stress corrosion commonly pro- ceeds in a transgranular fashion (cracking right through the crystals) but in other metals, such as aluminium, copper and unalloyed steel, in an intergranular fashion. Hence, identification requires microscopic examination.

A constantly fluctuating load may lead to cracking caused by metal fatigue, a purely mechanical phenome- non, but if there is contact with an electrolyte, corrosion may cause that cracking to accelerate (fatigue corrosion).

Chemical corrosion, in which gases at high tempera- ture act on material (high-temperature corrosion). can lead to scaling, the formation of thick oxide layers, or in the case of hydrogen gas, to cracking as a result of decarburization.

5. Analysis and diagnosis of damage

A certain systematic approach should be observed in hunting the causes of corrosion. For this purpose a number of stages can be distinguished. as indicated be- low. The procedure may be simplified, depending entirely on the nature and scale of the problem. The overall result should be a report in which recommendations are made as to how the corrosion can be prevented in future. Sometimes the measures to be tc&en xi11 demand a high investment, and it will not be possible to implement them at short notice. In such a case, recommendations should be made for both short-term and long-term measures.

Internal attack (e : . hydrogen attack of steel)

Fig. 3. Group Ill: Forms of damage which usually require verificaion by microscopic analyus; sometimes rhey arc also clearly visihle to thc naked eye.

1988 CAE @A004 1991 1-A004 1991 2-A004

Sysrernmics for dlagnosing the causes of corrosion

(A) Gather data on the corroded part (the system) (a) typc of construclion (descript~on, drawing.

sketch) and purpose; (b) composition of material (type code of metal or

alloy); (c) any pre-treatment and/or surface treatment

(coatings); (d) contact with other n~etals.

(Bj Gather data on the corroded surface hy means of inspection (a) appearance (pitting uniform, cri ichg, flow pat-

tern. depth. etc.) to he recorded by visual report, photographs. drawings, etc.;

(h) corrosion products present on the surface (chcm- ical 'inalysis):

(c) other dcposits present on the surface. originating from elsewhere or from the environment (chem- ical analysis);

id) appearance of coating (intact, pitting, blistering, exfoliation. under-rusting, etc..).

(C) Recording the operating conditions (a) nnnturc of the cmironment (gas, liquid, solid or

mixture); (b) composition of environment (cl~emical analysis.

aggressiveness in relation to the present material, contaminations), constant or variable (average, ninimum/maximum);

(c) physical data (temperature, pressure, flow veloc- ity);

(d) mechanical aspects: load (staticjdynamic, mag- nitude, tensile/pressure. frequency of load fluc- tuations);

(e) operation (cnnrinuous, intermittent, standard/ nonstandard conditions, repairs. maintenance, inspeclions).

(D) Supplementary data (a) age, time to failure of the corroded part; (hj damage occurred before, measures taken (which

checks?); (cj is the corrosion specific or more general in the

plant or the system'?; (dj what checks on surface treatment (coatings)?

(E) Additional examination (a) material: identification by chemical analysis and

mechanical resting; (b) corrosion appearance: simple microscopic exam-

ination, with application of replica technique if necessary, non-destructive te3ting:

(c) cause of corrosion: metallographic examination, scanning electron nicroscopy, X-ray diffraction. etc.:

(d) corrosion measurements on-site andjor long- term tests and electrochemical analysis in the laboratorv;

(e) litcrature rcstarch.

(b) conclusions and recommendations for short-term u ~ d long-term measures (to be drawn up in consultation with the user, and taking into account the scale of the damage).

6. Corrosion conhol

For control of the corrosion. a selection may bc made from among the following measures:

(ij Choice of a different material. Application of another more corrosion-resistant material. Hoxever, the choice of material is determined not only by the corro- sion resistance but also by the mechanical properties and economic considerations.

(ii) Design modifications. By adapting the desizn, treatment and construction, the part of the systcm can be rendered less vulnerable to corrosion.

iiii) Applicatiou oT coatings. In this nay material can be separated from the environment (metallic. inorganic non-metallic or organic coatings to be applied after proper pre-treatment).

(iv) Change of environment, for example removal of oxygen and/or raising the pH dosing of inhibitors

in combination with proper checks: change/control of temperature, flow velocity, stray currents.

(v) Intervention in the reactions, in particular with electrochemical methods such as cathodic protection (with sacrificial anodr or with inert anodes with impressed current) and anodic protection (for example to stainless stcel).

(vi) Changing the procedures. Introduction or modifi- cation of procedures for start-up, shut-down, operation and stoppage; recommendatioms concerning inspection, corrosion monitoring, maintenance.

of environmeni and 1 conditions

3. gathering and appiisatmr of corrosion data 1

b. Inspectmn and approval

(F) Reporting (a) analysis of the results and statement of the diag-

nosis; Fig. 4.

1988 CAE 0-A005 1991 1-A005 1991 2-A005

7. Corrosion prevention

The analysis and diagnosis of damage as discussed in Section 5 is required in the case of corrosion occurring in use. The corrosion problems are solved by the trouble- shooting method. It is, of course, far better not to allow matters to reach that stage. and initiate the corrosion control much earlier, namely at the design stage of the systzm. This is known as corrosion prevention.

Figure 4 indicates the general aspects of corrosion control and prevention in the various stages. with the

necessary icedbacks. 11 goes withou~ saying that proper consultation with a corrosion specialist remains of the greatest importance.

In practice, unfortunately, we see all too often that corrosion control is only thought of when the design is already cut and dried. This means that by then, it i q

virtually nevzr possible to obtain a solution which is optimal from the corrosion angle. That can only be done by integrating corrosion control in the form of corrosion prevention as a basic part of the design process.

GLOSSARY OF TERMS USED 1N THIS WORK

Alkalinity reduction

4naerobic

Anode

Anodic protection

Anodic reaction

Atmospheric corrosion

Atlemperator

Bimetallic corrosion

Rlooming

Boiler feedwater

Carbonation of concrete

(part) removal of the carbo- nate and bicarbonate alkalinity From water, mostly by ion ex- changers.

free of air or uncon~bined oxy- gen.

electrode at which anodic reac- tion predominates.

electrochemical protection by increasing the corrosion poten- tial to a value corresponding to the passive state. Nore: This technique needs very little current flow but it leads to intensified corrosion in case of power breakdown.

clcctrode reaction equivalent to a transfer of positive charge from the electronic to the ionic conductor. Note: An anodic reaction is an oxidation process: Me + Ment+ ne-.

corrosion with the earth's atmosphere at ambient temper- ature as the corrosive environ- ment.

a heat exchanger in which the superheated steam is cooled by boilcr water at the outside.

contact corrosion (deprecated): Galvanic corrosion, where the electrodes are formed by dis- similar metals or other elcc- tronic conductors.

corrosion of anodi~ed durnini- um by acid residues due to in- sufficient rinsing.

mixture of steam condensate and make-up water.

the attack of concrete by carbon dioxide. The calcium hydroxide is transformed to calcium bi- carbonate that is flushed hy water.

1988 CAE 0-A007 1991 1-A007 1991 2-A007

Carbonic acid corrosion

Carburization

Cathode

Cathodic protection

Cathodic reaction

Caustic embrittlemznt

Cavitation

corrosion by water that has be- come aggressive by dissolved carbon dioxide (CO, t H 2 0 -t H2C0, * COf- + 2Ht) (see also sweet corrosion). Note: This corrosion causes uniform attack in condensate return lincs.

the absorption of carbon into a metal surface at high tempera- tures. by which carbides are formed as a result of which the metal becomes brittle. hbte: Distinguish from the term carburizing as used in metal- lurgy.

electrode at which cathodic re- action predominates.

electrochemical protection hy decreasing the corrosion poten- tial. (See galvanic protection and impressed current protec- tion).

electrode reaction equi~alent to a transfer of negatwe charge from the electronic to the ~ o n x conductor. Nore: A cathodic reactlon is a reduction process: Ox + n r - - Red

that form of stress corrosion cracking occurring in carhon steel exposed to alkaline solu- tions.

damage of a material associ- ated with collapse of cavities (or vapour bubbles) in the liquid at a solid-liquid inter- face, in the high pressure re- gions of an unit. Nore: Damage being worse st the outlet sidz of pumps or across control valves or orifice plates. Minilnum internal pres- sure within the unit must al- ways bc greatcr than the "bub- ble paint" of the liquid to avoid this t?-pe of attack.

Cavitation corrosion a process involving conjoint corrosion and cavitation. Note: Cavitation corrosion may occur, for example, in rotary pumps and on ships' propellers.

Cementation the plating out action of a no- ble metal on a less noble metal (e.g. copper on zinc or alunli- nium).

Chelant corrosion corrosion by a chelating agent.

Concentration corrosion corrosion cell in which the cell

Corrosion

Corrosion cell

Corrosion fatigue

Creep

Crevice corrosion

Deaerator

potential difference arises from a difference in concentration of the corrosive agent(s) near its electrodes.

physicochemical interaction be- tween a metal and its environ- ment which results in changes in the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system, of which these form a part. Note: This interaction is usu- ally of an electrochemical na- ture.

short-circuited galvanic cell in a corrosion system, the corrod- ing metal forming one of its electrodes (see galvanic cell).

a process involving conjoint corrosion and alternating straining of the metal. Note: Corrosion fatigue may occur when a metal is subjected to cyclic straining in a corrosive environment. Corrosion fatigue may lead to cracking.

the continuous plastic elonga- tion of a metal under an ap- plied stress.

corrosion associated with, and taking place in, or immediately around, a narrow aperture or clearance. Note: The formation of a dif- ferential aeration cell. is the cause of this corrosion.

a mostly horizontal tank in which the boiler feedwater is freed of dissolved oxygen (and other gases) by intensive con-

Dealkalization

Dealloying

Decarburization

Demineralization

Dezincification

Differential aeration cell

Economizer

Electrolyte

Electroplating

Embrittlement

End grain

tact with fresh steam, according the laws of Henry and Raoult.

see Alkalinity reduction,

the selective corrosion (re- moval) of a metallic constituent from an alloy, usually in the form of ions (e.g. dezincifica- lion). Note: This corrosion is also known as parting or selective leaching.

see Hydrogen attack.

removal of all the dissolved mineral matter from the water by ion exchangers.

selective corrosion of brass re- sulting in preferential removal of zinc. Dedncification is a form of dealloying. Note: This corrosion causes mechanically weak copper-rich areas in the form of plugs or layers; sometimes both zinc and copper corrode, but copper is redeposited.

corrosion cell, in which the potential difference arises from a difference in the concentra- tion of oxygen near its elec- trodes. Note: The location with the lowest oxygen content, e.g. in a crevice or underneath deposit, becomes anodic and will be at- tacked.

a heat exchanger, placed in the flue gases, for heat recovery by the boiler feed water.

an ionic conductor, usually in aqueous solution.

electrodeposition of a t h ~ ad- herent layer of a metal or alloy on an object serving as a cathode. Nore: This process is also known as electrogalvanizing.

severe loss of ductility of a metal (or alloy).

The surface of the metal which is cut perpendicular to the di- rection of rolling

1988 CAE 0-A008 1991 1-A008 1991 2-A008 <

End grain attack

Environment

Erosion

Erosion corrosion

Exfoliation

False brinelling

Fatigue

Filiform corrosion

Fretting corrosion

Selective corrosion of the e i~d grain. During the rolling of steel, nonmetallic inclusions are rolled out and elongated into long "stringers" all in the di- rection of rolling. When the end grain is exposed to a strong corrodent, such as boiling nitric acid, end graiu attack starrs at areas where stringers are ex- posed on sheet or plate ends and can aggressively proceed down into the steel. Austenitic stainless steels but carbon steels as well may be susceptible to end grain attack.

the surroundings or conditions (physical. chemical, mechani- cal) in which a material exists.

deterioration of a surface by the abrasive action of solid par- ticles in a liquid or gas, gas bubbles in a hquid, liquid drop- lets in a gas or due to (local) high flow velocities.

a process involving conjoint corrosion and erosion. Nvre: The corrosion products are continuously removed by erosion, as a result of which the corrosion is accelerated.

a thick layer-like growth of loose corrosion products (see Layer wrrosion). AV~te : Exfoliation is generally oriented in the direction of roll- ing, extrusion or principal de- formation.

damage caused b y vibration be- tween metal parts.

a process lcading to fracture re- sulting from repeated stress cycles wc1I beloa the normal tensile strength. Such failures start as tiny cracks which grow to cause total failure.

corrosion that occurs under some coatings in the form of hairs or filaments progressing across the metal surface.

a process involving conjoint corrosion and oscillatory slip between two surfaces in con- tact.

Galvanic cell

Galvanic corrosion

Galvanic protection

Galvanizing (hot dip)

General corrosion

Grain

Graphitic corrosion

Heat affected zone W A C

Nore: Fretting corrosion may occur, for example, at mechani- cal joints in x'ibrating struc- tures.

Combination of different elec- trodes connected in series with an ionic conductor. Note: The galvanic ccll is an electrochemical source of elec- trical current and will produce current when the electrodes are connected by an external con- ductor.

corrosion due to the action of a corrosion cell. Note: The term has often been restricted to the action of bi- metallic corrosion cells, i.e. bi- metallic corrosion.

clectrochen~icd~ protection in which the protecting current is obtained from a corrosion cell formed by connecting an aux- iliary electrode to the metal to be protected.

coating of iron and steel with zinc using a bath of molten zinc.

corrosion proceeding at almost the same rate over the whole surfacc of the metal exposed to the corrosive environment. .Wore: This corrosion is also known as overall or uniform corrosion.

a portion of a solid mela1 in which the atoms are arranged in an orderly pattern. The irregular junction of two ad- jacent grains is known as the grain boundary.

selective corrosion of grey cast iron. resulting in preferential removal of metallic constitu- ents, leaving graphite. Graphitic corrosion is a form of dealloy- ing. Note: This corrosion is alsn known as graphitization but this term is not recommended because of its use in metal- lurgy.

refers to the area adjacent to a weld where the thermal cycle has caused microstructural

1988 CAE 0-A009 1991 C.4009 1991 ?-A009

Hydride embrittlement

Hydrogen attack (high temperature)

Hydrogen blistering

Hydrogen damage

Hydrogen embrittlement

Hydrogen grooving

changes which generally affect corrosion behaviour.

embrittlement due to the pre - cipitation of metal hqdride phases formed by the diffusion of hydrogen into the metals magnesium; zirconium, titani- um, niobium, vanadium and tantalum.

The occurrence of cracks or fis- sures owing to pressure build- up by methane gas, formed by the reaction of atomic hydro- gen diffused into the steel above roughly 260" C and metal car- bides or dissolved carbon. Nore: This corrosion is also known as decarburization or methane blistering.

the formation of blisters on or below a metal surface from ex- cessive internal hydrogen pres- sure. Note: Hydrogen may be formed during cleaning, plating, corro- sion, and so forth. The atomic hydrogen diffuses into the steel. The diffusion stops at con- taminations in the steel, where atomic hydrogen is converted into molecular hydrogen. Pres- sure build-up then causes the steel to fracture.

a general term for the embritt- lement. cracking, blistering and hydride formation that can oc- cur when hydrogen is present in some metals.

a process resulting in a de- crease of the toughness or ductility of a metal due to ab- sorption of hydrogen. Note: Hydrogen embrittlement often accompanies hydrogen formation, for example by cor- rosion or electrolysis, and may lead to cracking.

A high local attack m the form of grooving in carbon steel used for storage and handling of strong sulphuric acid. The cor- rosion takes place when strong acid is not moving, by disturb- ing the ferrous sulphate layer as a result of the steady passage of

Impingement attack

Impressed current protection

Inhibitor

Intergranular corrosion

lnterphak.oa

Knife line attack (KLA)

Langelier index

hydrogen bubbles, evolved in the corrosion of steel in sul- phunc acid on a preferred path. The normal steady state of low corrosion (0.1 to 0.5 mm/yr at ambient temperatures) will he disrupted and grooving will re- sult with high local corrosion rates of 3 mm/ yr or more.

localized erosion corrosion caused by turbulence or im- pinging flow at certain points.

electrochemical protection in which the protecting current is supplied by an external source of electric energy.

chemical substance which de- creases the corrosion rate when present in the corrosion system at a suitable concentration, without significantly changing the concentration of any other corrosive agent. Note: A corrosion inhibitor is generally effective in a small concentration. In commercial applications additives are sometimes named as inhibitors.

corrosion in or adjacent to the grain boundaries of a metal.

a contrast technique in which both phase contrast and inter- ference contrast are used to- gether for contrasting and mea- suring, both for transmitted and incident light (see detailed ex- planation in Background Infor- mation on the Interphakoa Technique, pp. A13-A14).

corrosion resulting in a narrow slit in or adjacent to the filler, parent boundary of a welded or brazed joint. Note: This corrosion is a form of weld decay sometimes ob- sened on stabilized stainless steel.

the Langelier index (Li) or saturation index is a theoreti- cally derived quantity for the judgement of corrosive or scal- ing qualities of water. The in- dex is deduced from the real pH, and the calculated satura- tion pH (pH,) of the water:

Layer corrosion

Li = pH - pH,, and is interpre- Pitting corrosion ted as follows: Li Tendency of rhe

water Rehar corrosion i 2.0 srrong scaling

tendency +0.5 slight scaling tend-

ency, not corrosive Ryznar index 0.0 in balance, pitting

possiblc - 0.5 slightly corrosive,

not scaling < - 2.0 strongly corrosive

corrosion of internal layers of wrought metal, occasionally re- sulting in exfoliation, i.e. de- tachmcnt of mattacked laqers.

Liquid metal embrittle- emhrittlement of a metal meit (LME)

Magnetite

m-Alkalinity

Metal dusting

hlicrobiologically induced corrosion WIG)

Nirriding

Oxygen scavenger

caused by penetration oI ot lm molten metals.

Fe,O,. black iron oxide which is formed in water or steam in the case of deficiency of ox- ygen. It is magnetically attrac- table.

the acid consumption in m e d l on titration of water up to pH 4.2.

accelerated deterioration of metals m carbonaceous gases a1 elevated temperatures to form a dustlike corrosion product.

corrosion associated with the action of nlicroorganisms prc- sent in the corrosion system. .Vote: This corrosion is also known as biological or n1ic1-o- bial corrosion.

thc absorption of nitrogen atoms by a metal; it may re- main dissolved. or form metal nitrides.

a chemical substance which liberates the boiler feed water from oxygen by means of a chemical reaction. An oxygcn scavenger is used instead of or in addition to a deaerator.

the acid consumption in meq/l on titration of alkaline (boiler) water up to pH 8.2.

corrosion resulting in pits, i.e. cavities cxtcnding from Lhe surface into the metal.

corrosion of concrete rzin- forcement bars, also known as reinforcement corrosion.

The Ryznar index (Ri) or sta- bility index is an empirically derived quantity for the judge- ment of corrosive or scaling qualities of the water. Tlus in- dex can also be calculated from the pH and pH, (see Langelier index): Ri = 2 pH,-pH, and is interpreted as fouows: Ri Tende~tcy of the

water < 5.0 strong scaling

tendency 5.0-6.0 slight scaling ten-

dency 6.0-6.5 in balance, pitting

possible 6.5-7.5 slightly corrosive 7.5-9.0 strongly corrosive , 9.0 very strongly cor-

rosive

Sandelin phenomenon this phenomenon was discov- ered by Mr. Sandelin and refers to the growth on of the zeta alloy layer during the galvaniz- ing process of silicon-killed steel s i th Si content of 0.2-0.458.

Season cracking a term usually applied to stress corrosion cracking of brass, in ammoniacal environments.

Selective corrosion corrosion of an alloy whereby the components react in pro- portions which differ from their proportions in the alloy (see also dealloying).

Shcr ardinng the coating of iron and steel with zinc by heating iin contact with zinc powder at a tcmpcra- ture below the melting point of zinc.

Sigma phase a hard, brittle. nonmagnetic in- termediate phase with a te- tragonal crystal structure? con- taining 30 atoms per unit cell, space group P42/mnm, occur- ring in many binary and ternary alloys of the transition ele- ments. The composition of this

Sigma-phase emhrittlement

phase in the various systems is not the same and the phase usually exhibits a wide range in homogeneity. Alloying with a third transition element usually enlarges the field of homogene- ity and extends it deep into the ternary section.

embrittlement of iron-chro- mium alloys (most notably austenitic stainless steels) caus- ed by precipitation at grain boundaries of the hard, brittle intermetallic signla phase dur- ing long periods of exposure to temperatures between ap- proximately 560 and 980°C (1050 and 1800°F). Sigma- phase emhrittlement results in severe loss in toughness and ductility, and can make the em- brittled material susceptible to intergranular corrosion.

Stainless steel all steels with a chromium con- tent of at least 13%.

Steam hlanket~ng reduction of wall thickness of the upper part of the inclined water tubes of a steam boiler due to magnetite (Fe,O,) flak- ing off. caused b) insufficient cooling.

Stray-current current flowing through paths other than the intended cir- cuits.

Stray-current corrosion due to stray-current. corrosion Note: The metal is attacked at

the spot where the current leaves.

Stress corrosion a process involving conjoint corrosion and straining of the metal due to residual or applied stresses. Note: Stress corrosion is spe- cific to particular metals in par- ticular solutions. Carbon steels in hot alkali solutions, austen- itic steels in hot chloride solu- tions and copper based alloys in solutions containing ammo- nia can be susceptible.

Stress corrosion cracking due to stress corro- cracking (SCC) sion.

Sulphate-reducing these bacteria, of which the bacteria genus Desulphoribrio is the

most common, reduce sul- phates to sulphides in anaerobic conditions (e.g. in nou-aerated well water or under deposits in aerated water). The sulphide at- tacks steel and other metals. The presence of hydrogen or organic matter accelerates this corrosion.

Sulphidation the reaction at high tempera- tures of a metal or alloy with a sulphur-containing species to produce a sulphur compound that forms on or beneath the surface of the metal or alloy.

Sweet corrosion this term is used in oil and gas production for carbon dioxide corrosion. which often causes pitting and localized attack, in oil or gas-producing systems.

Tin plague recr)stallization of tin due to the allotropic transformation from /3 to a tin that occurs below the temperature of 18" C. The tin becomes brittle and dis- integrates into powder.

Transgranular cracking through the grains or cracking crystals of a metal.

Under-deposit corrosion corrosion associated with, and taking place under, or im- mediately around. a deposit of corrosion products or other substance. Noze: The corrosion is the re- sult of the formation of a dif- ferential aeration cell.

Weld decay a term applied to areas ad - jaceut to welds of certain alloys which have been subjected to intergranular corrosion because of metallurgical changes in the alloy (commonly applied to certaln grades of stainless steel).