7 forms of corrosion i

8/12/2019 7 Forms of Corrosion I

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FORMS (TYPES)

OF

CORROSION:

Metals – Environments- Conditions



The Eight Forms of Corrosion

1. General corrosion or uniform attack2. Galvanic corrosion

3. Crevice corrosion

4. Pitting

5. Intergranular attack

6. Selective leaching or dealloying

7. Stress corrosion cracking

8. Erosion or velocity-accelerated

corrosion

...………

.L o c al i z e d C or r o si on….....



EVIRONMENTS

1) Dry atmosphere.2) Damp (humid) atmosphere.

3) Wet atmosphere (rain, snow) .

4) Acids (HCl, HCIO4, H3PO4). RUSTING

5) Atmospheric contaminants.

6) Process water containing hydrogen sulfide

7) Brine solutions.

8) Industrial atmosphere.

9) Hydrocarbon containing wet hydrogen sulfide



• In atmospheric corrosion which also exemplifies uniformcorrosion, a very thin layer of electrolyte is present. It is

probably best demonstrated by putting a small drop ofseawater on a piece of steel.

• On comparing the atmospheric corrosion with aqueouscorrosion, the following differences are observed:

On a metal surface exposed to atmosphere, only a limitedquantity of water and dissolved ions are present, whereas theaccess to oxygen present in the air is unlimited.

Corrosion products are formed close to the metal surface, unlikethe case in aqueous corrosion, and they may prevent furthercorrosion by acting as a physical barrier between the metalsurface and environment, particularly if they are insoluble as inthe case of copper or lead.

• The following is a simplified mechanism of aqueous corrosion of iron(Fig. 4.2):

Mechanism of General Corrosion



Figure 4.2 Aqueous corrosion of iron



EXAMPLES OF UNIFORM CORROSION

Tarnishing of silver ware.

Tarnishing of electrical contacts.

Rusting of steels in open air.

Corrosion of offshore drilling platforms.

Corrosion of galvanized steel stairways. Failure of distillation columns.

Corrosion of electronic components.

Corrosion of underground pipes.

Corrosion of automobile bodies. Corrosion of heat exchanger tubes.

Corrosion of structural steels.



Prevention of Uniform Corrosion

• Galvanizing (zinc coating)

• Painting

• Electroplating• Cathodic protection



2.-8. LOCALIZED CORROSION

• A range of corrosion processes

• Selective attack (part of surface), not all

• Environment (CO2, Cl-, pH, Flow rate)

• Material: Occurs at heterogeneous site of the metal(segregation, inclusions, different phases, grainboundaries)….. i.e. structure non-uniformity or

discontinuity.

• Mechanical

– Stress (static or fluctuating)



2. Galvanic Corrosion

• Occur when two different metals are in electricalcontact

• Noble metal provides sites for cathodic reaction

• Less noble metal act as the anode

• Important factors in galvanic corrosion

a) relative areas of anode and cathode

b) difference in potential between anode and

cathodec) effect of anodic polarization on anode (some

may passivate)



Important Parameters to consider:

• Relative areas of anode and cathode.

At Icorr : Ia=Ic , Aa ia=Ac ic

• Difference in potential between anode and cathode

– EMF and Galvanic series

• Anode may passivate

• Effects of environment (e.g. electrolyte temp.)

– Zinc and steel (inversion of anode and cathode at 100°C)

• Distance effects

– Depends on the conductivity of the solution



MECHANISM OF GALVANIC CORROSION

• Consider Fig. 4.4. For the formation of a galvanic cell (Fe and Cu),the following components are required:

A cathode.

An anode.

An electrolyte.

A metallic path for the electron current.

• In the case of copper and steel, copper has a more positive potentialaccording to the emf series, hence, it acts as a cathode.

• On the other hand, iron has a negative potential in the emf series

(-0.44V), hence, it is the anode. As a matter of principle, in agalvanic cell, the more noble metal always becomes the cathodeand the less noble always the anode.

• Moisture acts as an electrolyte and the metal surface provides ametallic path for the electron current to travel. Thus, when a piece ofcopper is joined to iron, all qualifications required for the formation ofa galvanic cell are fulfilled and galvanic corrosion proceeds.



FACTORS AFFECTING GALVANIC CORROSION

• The following factors significantly affect the magnitude of galvaniccorrosion:

1. Position of metals in the galvanic series.

2. The nature of the environment.

3. Area, distance and geometric effects.

1. Position of Metals in the Galvanic Series

As mentioned earlier, the further apart the metals are in thegalvanic series, the greater is the chance for galvanic corrosion.

The magnitude of galvanic corrosion primarily depends on howmuch potential difference exists between two metals. For a

particular environment, the metals selected should be close to eachother in the galvanic series to minimize galvanic corrosion. Activemetals should not be joined with passive metals. Thus, aluminumshould not be joined to steel, as aluminum being more active wouldtend to corrode.



2. The Nature of Environment

Due consideration must be given to the environment that surroundsthe metal.

- For instance, water containing copper ions, like seawater, is likelyto form galvanic cells on a steel surface of the tank.

- If the water in contact with steel is either acidic or contains salt, thegalvanic reaction is accelerated because of the increased ionizationof the electrolyte.

- In marine environments, galvanic corrosion may be accelerateddue to increased conductivity of the electrolyte.

- In cold climates, galvanic corrosion of buried material is reducedbecause of the increased resistivity of soil.

- In warm climates, on the other hand, it is the reverse because ofthe decreased resistivity of the soil



3. Area, Distance and Geometric Effects

• The anode to cathode area ratio is extremely important as themagnitude of galvanic corrosion is seriously affected by it. The area

ratio can be unfavourable as well as favourable.

• The area ratio of the anode to cathode plays a dominant role ingalvanic corrosion. As a given amount of current flows in a galvaniccouple, the current density at the anode or cathode controls the rateof corrosion.

• For a given amount of current, the metal with the smallest area hasthe largest current density and, hence, is more damaged if corrosionoccurs at it.

• In Figure 4.6, the current density on aluminum bolt is very large(small area) and serious galvanic corrosion of aluminum takesplace. This shows the end result of an unfavourable anode/cathoderatio.



EXAMPLE OF GALVANIC CORROSION

– Galvanic corrosion of aluminum shielding in buried

telephone cables. – Galvanic corrosion of steel pipe with brass fittings.

– Galvanic corrosion of the body of the ship in contact

with brass or bronze propellers.

– Galvanic corrosion between the tubes and the tubesheet in heat exchangers.

– Aluminium conduit buried in steel reinforced concrete

– Galvanic corrosion of steel coated with copper due to

the defects in copper coating. – Galvanic corrosion inside horizontal stabilizers in

aircrafts.



METHODS OF PREVENTION OF GALVANIC CORROSION

1) Select metals, close together , as close as possible, in the galvanicseries.

2) Do not have the area of the more active metal smaller than the areaof the less active metal.

3) If dissimilar metals are to be used, insulate them.

4) Use inhibitors in aqueous systems whenever applicable andeliminate cathodic depolarizers.

5) Apply coatings with judgment. Do not coat the anodic member of thecouple as it would reduce the anodic area and severe attack

would occur at the inevitable defect points in the coating.

Therefore, if coating is to he done, coat the more noble of thetwo metals in the couple which prevents electrons beingconsumed in a cathodic reaction which is likely to be corrosion-rate controlling.



METHODS OF PREVENTION OF GALVANIC CORROSION

6) Avoid joining materials by threaded joints.

7) Use a third metal active to both the metals in the couple

8) Sacrificial material, such as zinc or magnesium, may he introducedinto this assembly.

For instance, zinc anodes are used in cast-iron water boxes ofcopper alloy water-cooled heat exchangers.

9) In designing the components, use replaceable parts so that only the

corroded parts could be replaced instead of the whole assembly.

Above all, understand materials compatibility which is the key tocontrol galvanic corrosion.



Galvanic corrosion in alloys

• Galvanic corrosion can occur between different phases in analloy

• This is especially important when one of the phases is much

more active as a cathode.

• Examples:

Copper-containing precipitates in aluminium alloys:

initiate pitting corrosion

Fe and Cu impurities in commercial zinc:

cause a large increase in corrosion rate compared to pure

zinc



3. Crevice Corrosion

• Intensive corrosion frequently occurs with crevices and

other shielded areas of metal surface exposed toelectrolyte.

• Can occur even under non metallic contacts, e.g.rubber gaskets

• Differential aeration cell is developed due to depletionof O2 under crevice.

• This phenomenon limits the use (particularly of steels)in marine environment, chemical and petrochemicalindustries.

• Main example: the case of passive SS alloys.



CAUSES of CREVICE CORROSION

a) Presence of narrow spaces between metal-to-metalor non-metal-to-metal components

b) Presence of cracks, cavities and other defects onmetals.

c) Deposition of bio-fouling organisms and slimes.

d) Deposition of dirt, mud, dust, etc.. on a metal surface.

e) Existence of voids, gaps and cavities betweenadjoining surfaces.



MATERIALS AND ENVIRONMENT

Conventional steels, e.g. SS 304 and SS

316, can be subject to crevice corrosion

in chloride containing environments,

such as brackish water and seawater.

Water chemistry plays a very important

role.



Joints



MECHANISM OF CREVICE CORROSION A unified mechanism which is rather over-simplified is given

below (Fig. 4.17):



The four stages of crevice corrosion suggested are shown in Fig. 4.18

(1) Depletion of oxygen in crevice due to consumption in the cathodic reaction.

(2) Increase of acidity in the pit by the process of hydrolysis.

(3) Breakdown of the passive film on the surface at a critical value of pH.(4) Propagation of crevice corrosion with further hydrolysis and production of

acidity.



PREVENTION OF CREVICE CORROSION

1) Use welded joints in preference to bolted or riveted joints.

2) Seal crevices by using non-corrosive materials.

3) Eliminate or minimize crevice corrosion at the design stage.

4) Minimize contact between metals and plastic, fabrics and debris.

5) Avoid contact with hygroscopic materials.

6) Avoid sharp corners, edges and pockets where dirt or debris could

be collected.

7) Use alloys resistant to crevice corrosion, such as titanium or Inconel.

Increased Mo contents (up to 4.5%) in austenitic stainless steels

reduce the susceptibility to crevice corrosion.



8) Apply cathodic protection to stainless steels by connecting toadjacent mild steel structure.

9) For better performance of steels in seawater, allow intermittentexposure to air to allow the removal of protective films.

10) Use inhibiting paste, wherever possible.

11) Paint the cathodic surface (not the anode).

12) Remove deposits from time to time… i.e. Clean!

13) Take precautions against microbial corrosion, which creates

crevices and is damaging to low Mo stainless steels

PREVENTION OF CREVICE CORROSION (Cont’d)



4. PITTING CORROSION DEFINITION

• It is a form of localized corrosion of a metal surface wheresmall areas corrode preferentially leading to the formationof cavities or pits while the bulk of the surface remains un-attacked.

• Metals which form passive films, such as aluminium andsteels, are more susceptible to this form of corrosion.

• It causes failure by penetration with only a small percent

weight-loss of the entire structure.

• It is a major type of failure in chemical processing industry(CPI). The destructive nature of pitting is illustrated by thefact that usually the entire system must be replaced!



Pitting Corrosion

• Localized corrosion selectively attacks an area of ametal surface where there is:

A surface scratch

An emerging dislocation A compositional heterogeneity

• The result is the formation of a hole

• One of the most insidious form of corrosion

Weight loss is very small but grave consequences



ENVIRONMENT

• Generally, the most conducive environment for pitting is the

marine environment.

• lons such as Cl-, Br - and I-, in appreciable concentrationstend to cause pitting of steel. Thiosulfate ions also inducepitting of steels.

• Aluminum also pits in an environment that cause the pittingof steel.

• Oxidizing metal ions (e.g. cupric, ferric and mercuric) withchloride cause severe pitting.

• Presence of dust or dirt particles in water may also lead topitting corrosion in copper pipes transporting seawater.



M n+

M

O2 OH- O2 OH-

1- Inside the growing pit the hydrolysis of Mn+ lowers the pH and breaks

down the passive film. The cathodic oxygen reduction reaction continues

outside the pit.

2- The presence of chloride is important, as it allows a pH of about 1 to beachieved (HCl is a strong acid, and does not associate) and the metal

chlorides are very soluble (e.g. FeCl2).

3- Other anions (e.g. OH- and SO42-) can inhibit pitting, either by buffering

the pH in the pit or by causing the precipitation of a salt film.

Mechanism



PREVENTION OF PITTING CORROSION

1) Use materials with appropriate alloying elementsdesigned to minimize pitting susceptibility, e.g.molybdenum (Mo) in SS.

2) Provide a uniform surface through proper cleaning, heattreating and surface finishing.

3) Reduce the concentration of aggressive species in thetest medium, particularly chlorides.

4) Use inhibitors to minimize the effect of pitting, whereverpossible.

5) Make the surface of the specimen smooth and shiny anddo not allow any impurities to deposit on the surface.



PREVENTION OF PITTING CORROSION (Cont’d)

6) Minimize the effect of external factors on those designfeatures that lead to the localized attack, such as thepresence of crevices, sharp corners, etc.

7) Apply cathodic protection, wherever possible.

8) Coat the metals to avoid the risk of pitting.

10) Add anions, such as OH- to chloride environment.

11) Operate at a lower temperature, if service conditionspermit.

7 forms of corrosion i

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