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Corrosion Control and Prevention by Design 2014 Compiled from literature by Dr. Balakrishna Palanki

palankibalakrishna@yahoo.com

1 The Principles of Protection

Removal of any one of the four – anode, cathode, electrical contact and electrolyte

forms the basis of protection. Chemistry control in the form of removal of corrosive

agents from a system is a widely used method. One method is using de-aerators to

remove dissolved oxygen and to a lesser extent carbon dioxide. Treating the water by

softening and demineralization removes the dissolved solids and reduces the

conductivity.

Thermodynamic: If the E is raised so that the metal is in the passivity region of EH-

pH diagrams, it is anodic passivation. If it is lowered so that it is in the immunity

region, it is cathodic protection. Changing the pH may bring the point into the

passivity or immunity zone.

Kinetic:

1. To make the anodic reaction more difficult i.e., to change the slope of the E

versus i line so that i decreases.

2. To increase the IR drop by increasing R by decreasing the conductivity of the

electrolyte or introducing insulating separators between dissimilar metals.

Corrosion can be controlled by (1) Materials selection, (2) Proper design, (3)

Electrochemical protection (anodic protection and cathodic protection), (4)

Inhibitors, (5) Paints/Coatings

2 Materials Selection

In alloy selection, there are several “natural” metal-corrosive combinations. These

combinations of metal and corrosive usually represent the maximum amount of

corrosion resistance for the least amount of money. Some of these natural

combinations are as follows:

1. Stainless steels – nitric acid

2. Nickel and nickel alloys – caustic

3. Monel-hydrofluoric acid

4. Hastelloys (Chlorimets) – hot hydrochloric acid

5. Lead – dilute sulfuric acid

6. Aluminium – nonstaining atmosphere exposure

7. Tin-distilled water

8. Titanium – hot strong oxidizing solutions

9. Tantalum – ultimate resistance

10. Steel – concentrated sulfuric acid

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Stainless steels have widespread application in resisting corrosion, but it should be

remembered that they do not resist all corrosives. In fact, under certain conditions,

such as chloride containing mediums and stressed structures, stainless steels are less

resistant than ordinary structural steels. Stainless alloy are more susceptible to

localized corrosion such as Intergranular corrosion, stress corrosion cracking and

pitting attack than ordinary structural steels.

Ordinary carbon steel is widely used for sulphuric acid in concentrations over

70%. Storage tanks, pipelines, tank cans, and shipping drums made of steel

commonly handle 78%, 93%, 98% acids. Lead takes over below 70% acid where

steel is attacked and vice versa.

Corrosivity in fresh water varies depending on oxygen content, hardness, chloride

content, sulphur content, and many other factors. For example, a steel hot-water tank

in home may last 20 years in one area but only a year or two in other areas. In hard

water, carbonates often deposit on the metal surface and protect it, but pitting may

occur if the coating is not incomplete. Soft waters are usually more corrosive because

protective deposits do not form, cast iron, steel and galvanized steel are most widely

used for fresh water.

3 Protection by Design: A proper selection of the material for any particular

corrosive environment and a sound engineering design are the best means of corrosion

control. The use of dissimilar contacts should be avoided where the presence of an

electrolyte may result in galvanic corrosion. If two different metals have to be used,

they should be as close as possible in the galvanic series. The anodic material should

have as large an area as possible relative to the cathodic material. If practical,

dissimilar metals should be insulated.

A proper design should avoid the presence of crevices between adjacent parts of a

structure; it should also avoid such conditions as rapidly moving water, the

accumulation of solids and pockets of stagnant liquids. These conditions can give rise

to oxygen concentration or metal ion concentration cells resulting in severe pitting.

Whenever possible, the equipment should be annealed to reduce residual stresses.

Equipment should be supported on legs to allow free circulation of air and prevent the

formation of damp areas.

4 Design Rules

1. Weld rather than rivet tanks and other containers. Riveted joints provide sites

for crevice corrosion.

2. Design tanks and other containers for easy draining and easy cleaning. Tank

bottoms should be sloped toward drain holes so that liquids cannot collect

after the tank if emptied. Concentrated sulphuric acid is only negligibly

corrosive toward steel. However, if a steel sulphuric acid tank is incompletely

drained and the remaining liquid is exposed to the air, the acid tends to absorb

moisture, resulting in dilution, and rapid attack occurs.

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3. Design systems for the easy replacement of components that are expected to

fail rapidly in service. Frequently, pumps in chemical plants are designed so

that they can be readily removed from a piping system.

4. Avoid excessive mechanical stresses and stress concentrations in components

exposed to corrosive mediums. Mechanical or residual stresses are one of the

requirements for stress-corrosion cracking. Follow this rule especially when

using materials susceptible to stress-corrosion cracking.

5. Make sure materials are properly selected. If possible, use similar materials

throughout the entire structure, or insulate different materials from one

another.

6. Avoid electrical contact between dissimilar metals to prevent galvanic

corrosion.

7. Avoid sharp bends in piping systems when high velocities and/or solids in

suspension are involved (erosion corrosion).

8. Provide thicker structures to take care of impingement effects.

9. List complete specifications for all materials of construction and provide

instructions to be sure the specs are followed all the way through to final

inspection. Specify quality control procedures if relevant. Be sure all relevant

codes and standards are met.

10. Set realistic and scheduled dates for delivery of equipment, so that fabricator

does not adopt short cuts.

11. Specify procedures for testing and storage of parts and equipment. For

example, after hydraulic testing do not let the equipment sit full or partially

full of water for any extended period of time. This could result in microbial

corrosion, pitting, and stress corrosion. With regard to storage, spare stainless

steel tubing showed stress-corrosion cracking when stored near the seacoast.

12. Properly design against excessive vibration, not only for rotating parts but

also, for example, for heat exchanger tubes. Specify surface finish to minimise

fretting corrosion.

13. Poor quality or rough polished finishes can lead to discolouration and reduced

corrosion resistance in some environments. This is generally due to rough

surface valleys allowing trapping of contaminants and moisture. Avoid vague

or missing information about surface finish, in particular, use of terms like

‘brushed’, ‘satin polished’, ‘dull polished’. Specify surface finish

quantitatively.

14. Provide for “blanketing” with dry air or inert gas if vessels “inhale” moist

marine atmosphere while being emptied. Select Plant site upwind from other

“polluting” plants or atmosphere if relevant and/or feasible.

15. Avoid hot spots during heat-transfer operations. Design heat exchangers and

other heat-transfer devices to ensure uniform temperature gradients. Uneven

temperature distribution leads to local heating and high corrosion rates.

Further, hot spots tend to produce stresses that may produce stress-corrosion

cracking failures.

16. Design to exclude air. Oxygen reduction is one of the most common cathodic

reactions during corrosion, and if oxygen is eliminated, corrosion can often be

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reduced or prevented. In designing chemical plant equipment, particular

attention should be paid to agitators, liquid inlets, and other points where air

entrainment is a possibility. Exceptions to this rule are active-passive metals

and alloys. Titanium and stainless steels are more resistant to acids containing

dissolved air or other oxidizers.

17. The most general rule for design is: avoid heterogeneity, dissimilar metals,

vapour spaces, uneven heat and stress distributions, and other differences

between points in the system, which would otherwise lead to corrosion

damage. Hence in design, attempt to make all conditions as uniform as

possible throughout the entire system.

5 Area Effect

From the fact that nearly the entire voltage drop in the galvanic cell occurs at the

water cathode interface, the effects of changing the geometry of the cell may be

predicted. If the anode and cathode are moved closer to each other, there is little

increase in current. As the cathode is withdrawn from the solution, the current is

found to be proportional to the cathode area that remains submerged. If the anode area

is reduced, there is only a small decrease in current.

The practical consequences of these area effects are important. In nearly all

commercial gate valves there are brass rings in the cast iron body, but because the

brass cathode area is very small compared to the iron anode, galvanic corrosion is

negligible. Note that a protective coating placed on the anode must be perfect, or very

severe pitting will occur at any pinholes. On the other hand, if the cathode is coated

with an insulating film, galvanic corrosion can be greatly reduced even with a

relatively poor coating.

6 Cathodic Protection

Although cathodic protection is thermodynamic in principle, the kinetics of a system

will usually determine whether or not cathodic protection is economically worthwhile.

In a natural corrosion system, i.e. one having no external supply of current, there is

balance between the anodic and cathodic reactions on the metal surface at the rest

potential Ecorr. If the corrosion current, Icorr is reduced to zero by shifting the potential

to, or below Eo, then an increased cathodic reaction will exist on the metal surface, at

least equal to Icath. This reaction has to be satisfied by the applied cathodic protection

system. In other words, the cathodic protection installation must be able to provide a

balancing anodic reaction at least equivalent to Icath. Sacrificial anode cathodic

protection is the constructive use of the galvanic corrosion.

Anodic Protection

Anodic protection is based on the formation of a protective film on metals by

externally applied anodic currents.

7 Inhibition

Anodic or Chemical passivation: Within certain pH limits the only requirement for

passivation is that the potential of the metal should be brought to and held within a

region exhibiting a low current density. This requires the provision of a cathodic i.e.,

an electron consuming reaction whose E/I characteristics are such that it is capable of

sustaining or balancing an anodic reaction greater than Icrit at Ecrit. This cathodic

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reaction requirement can be fulfilled by an external electron sink (dc source +

auxiliary electrode) or reduction of a chemical oxidizing agent (nitric acid).

Adsorptive Inhibitors: The protective action of these materials is due to their

blanketing effect over the entire surface i.e. both anodic and cathodic areas.

Chemical Addition: Chemical additions to a system that alter the chemical reaction or

tie up a particular corrodant is a common method of control. Filming amines (organic

compounds that are derivatives of ammonia) accomplish protection by forming

adhering organic films on metal surfaces to prevent contact between corrosive species

in the condensate and the metal surface. Phosphates and sodium hydroxide are used to

adjust the system pH and remove hardness.

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