DIFFERENT FORMS OF CORROSION

As we have discussed in our previous post about the “Corrosion Engineering” where we have seen the basic of corrosion mechanism, we have also discussed the engineering materials that may suffer from corrosion in our previous post “corrosion failure examples” and “corrosion measurement techniques”.

Let us go ahead with new post in respect of corrosion engineering, where we will see the basic Problems caused by corrosion.

Let us consider the various forms of corrosion that we normally analyze in our industrial life during analysis of breakdown of equipment. We have prepared a set of corrosion forms as mentioned below.

General corrosion or uniform corrosion

General corrosion or uniform corrosion is exemplified by the rusting of material i.e. steel. Examples of general corrosion or uniform corrosion are tarnishing of silver and the green patina correlated with corrosion of copper.

General corrosion or uniform corrosion is rather expectable. The service life span of components could be predicted based on comparably simple prepossession test results. Allowance for general corrosion or uniform corrosion is comparably simple and normally employed when we go for designing a component for a predefined environment.

Figure 1: General corrosion 

There are few processes for reducing the general corrosion or uniform corrosion and these are as mentioned below.

1. Cathodic protection
2. Coatings
3. Suitable material selection
4. Inhibitors

Galvanic corrosion

Galvanic corrosion is generally assigned for corrosion between dissimilar metal. Galvanic corrosion will be occurred when two dissimilar materials such as aluminium and mild steel will be coupled with each other in a corrosive environment. Let us consider one example of galvanic corrosion, consider the joining of two materials those are not similar with each other such as electrical contact in seawater.

Figure 2: Galvanic corrosion 

Two materials, those are not similar with each other, are coupled with each other in a corrosive environment then one out of both materials will act as the anode and second one metal will act as cathode. The less noble material will act as anode and this metal will be corroded rapidly than it would all by itself and on other hand The cathodic metal corrodes gradually than it would all by itself.

Many boaters apply this information to secure the advantage. Sacrificial zinc anodes are normally used in order to protect the materials of boat components. The zinc anode corrodes conversely thereby securing the boat component. 

The zinc anodes should be maintained and must be replaced after a set of time or when required in order to secure the continuous protection. Other alloys could also be used as sacrificial anodes. Magnesium and aluminum sacrificial anodes, in some situations, will provide better protection.

Pitting Corrosion

Pitting corrosion is basically referred as localized type of corrosive attack. Pitting is exemplified by the construction of pits or holes on the surface of metal. Pitting corrosion may cause failure as discontinuity while the complete corrosion, as measured in terms of weight loss, might be comparatively minimal.

We must understand that rate of penetration in case of pitting corrosion might be the ten to hundred times that by in case of uniform corrosion or general corrosion.

Pits or holes might be comparatively small and yes much difficult to detect and secure the position. In some situation holes or pits might be masked just because of general corrosion or uniform corrosion. Pitting corrosion may take little time to commence and propagate to develop up to a size which could be seen easily.

Figure 3: Pitting corrosion

Pitting types of corrosion normally occurs more rapidly in an inactive atmosphere. The combativeness of the corrodent will alter the rate of corrosion i.e. pitting corrosion.

Let us think few methods in order to reduce the effects of pitting corrosion and these are mentioned below.

1. Apply quite pitting corrosion resistant types of materials
2. Reduce the combativeness of the atmosphere / environment
3. Select and prepare the best design in order to reduce the pitting corrosion

Crevice corrosion

Crevice corrosion is basically referred as localized type of corrosive attack. Crevice type of corrosion will be occurred at narrow spaces or openings in between two metal surfaces or also in between surfaces of non metal and metal. A concentration cell will form with crevice being depreciated of oxygen. This differential aeration in between the crevice i.e. micro-environment and external surface i.e. bulk environment will provide crevice an anodic character. It will contribute to a deeply corrosive situation in crevice for example washers, flanges, rolled tube ends, deposits, threaded joints, gaskets O-rings and Lap joints.

Figure 4: crevice corrosion

Let us think few methods in order to reduce the effects of crevice corrosion and these are mentioned below.

1. Apply quite crevice corrosion resistant types of materials
2. Reduce the crevice from the design
3. Reduce the combativeness of the atmosphere / environment

Stress corrosion cracking

Stress corrosion cracking is basically a type of failure mechanism which is caused due to tensile stress, environment and susceptible material. We must understand that temperature is a key and very important environmental factor that affects cracking. Stainless steel and aluminium are quite accepted for stress corrosion cracking issues. Though, all metals are impressionable to stress corrosion cracking in right atmosphere.

Figure 5: stress corrosion cracking

Stress corrosion cracking is basically dangerous types of failure as it could be occurred without loads significantly below yield stress pr an externally applied load. Therefore, catastrophic failure may occur without accessible depreciation or serious deformation of the component. Pitting corrosion is basically correlated along with phenomena of stress corrosion cracking.

Let us think few methods in order to reduce the effects of stress corrosion cracking and these are mentioned below.

1. Apply quite stress corrosion cracking resistant types of materials
2. Remove the chemical species as it promotes the stress corrosion cracking
3. Change the process of manufacturing in order to reduce the tensile stress
4. Reduce the combativeness of the atmosphere / environment

Other various forms of corrosion

Let us see the few more forms of corrosion

1. Intergranular corrosion
2. Atmospheric corrosion
3. Selective leaching
4. Corrosion fatigue
5. Erosion corrosion
6. Microbiological corrosion
7. Cavitation and impingement
8. Stray current corrosion
9. High temperature corrosion

Factors accelerating the corrosion

1. Temperature
2. Humidity
3. Ph
4. Pressure
5. Oxygen
6. Aggressive ions
7. Thermal and mechanical stresses
8. Velocity of fluid

Image courtesy : Google 

Let us go to the next post in respect of corrosion engineering

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