Corrosion can be defined as the deterioration of materials under the influence of an environment. Without exception, the corrosion of metals and alloys (majority of materials used in industry) in aqueous environments (the most often encountered environment) is an electrochemical reaction.
Though uniform corrosion is an idealized form of corrosion and because less damage than the other forms of corrosion, it is more appropriate to understand this form of corrosion. This leads to uniform thinning of the structures. The attack is measured in terms of penetration. They have the units mpy (mills per year) or mm per year. This can be determined by any gravimetric method.
One or more of the following methods can be adopted to prevent uniform corrosion.
- Cathodic Protection
- Protective Coatings
- Selecting Proper Materials
When dissimilar metals or alloys differing in their galvanic or corrosion potential are employed and if they are electrically shorted they induce this type of corrosion. The corrosion rate of the alloy with lower corrosion potential will be accelerated by that of higher corrosion potential.
- The active metal is corroded
- Grooving of the interface
- Noble metal deposits from the stream
- Graphite lining or bricks
- Provide electrical insulation between the metal
- Choose alloys closer in the galvanic series
- Provide design in structure so as to make anodic to cathodic ratio extremely large.
- Coat both anode and cathodic areas. Otherwise coat only the cathode.
- Protect the corroding metal with a sacrificial anode, which is anodic to the corroding metal.
Accelerated corrosion occurs if differential aeration exists due crevice, metal joining (lap joints, flanges etc.)or any deposits. Interestingly the location starving for oxygen is forced to become anodic and the region having free access to oxygen becomes cathode.
- Rivets, flanges, lap joints are attacked internally.
- Deposits such as corrosion products, organic deposits, growth of organisms etc. cause corrosion.
- Improper drainage of vessels, pipelines cause accelerated attack.
- Avoid riveting, go in for welding
- Design for proper drainage
- For stainless steels high Mo content (316,317 and Haste alloys) reduces crevice corrosion
- Remove the deposits
- Use solid non-absorbent gaskets
Alloys in presence of certain ions (such as halides) are prone to pitting. The rate of penetration within the pit can be as high as one million times as compared to the surroundings.
- Normally grow in the direction of gravity
- The alloy environment combination is likely to promote pitting
- Pitting has taken place along inclusion
- Eliminate the specific ions responsible for pitting (say halides in the case of SS)
- Choose alloy resistant to pitting. In stainless steels high Mo promotes resistance (haste alloys, duplex stainless steels)
- Mild steels serve better in chloride environment than SS if certain amount of uniform corrosion is tolerated. Monel has more resistance in this environment.
SELECTIVE LEACHING (DEZINCIFICATION):
When noble and active elements form an alloy it results in selective removal of the latter. As a consequence the alloy loses its strength and fails prematurely. Cu-Zn alloys are well known where in dezincification occurs if Zn content exceeds beyond 15 wl. Similarly there is denickelification, desiliconation, decobaltification.
- They give rise to plug and layered types of attack.
- Change in colour (from yellow to brown in the cases of brasses)
- X-ray diffraction can sometimes reveal selective removal of one element
- There can be a change in density in some cases.
- Addition of any one of the elements namely Sn, As, Sb and P
- Al addition reduces overall corrosion and to some extent dezincification.
This type of corrosion occurs as a result of selective attack of the grain boundaries when either grain boundary becomes highly active or phases prone to selective attack are formed. Stainless steels, which are normally resistant to inter-granular attack, when subjected to an heat treatment between 400-900 C become sensitive to inter-granular corrosion (IGC). This range can vary depending on the composition of the alloy. This treatment is called sensitization treatment and alloy is said to be sensitized. This is mainly due to the formation of Cr23C6 and the consequent grain boundary depletion. Welding, a common practice in fabrication causes such an IGC attack.
- Attack of the alloy away from the weldment called heat affected zone.
- Clear ditch type of attack along the grain boundary and consequent weakening seen at higher magnification.
- Choose low carbon and extra low carbon stainless steels (such ss are 3041,3161,3171)
- Choose Ti or Ta and Nb containing alloys (321,347)
- Provide a solutions treatment to redissolve the carbides (1050 °C, 30 m)
When there is a relative movement of the corrosive environment with respect to the alloy it can lead to erosion corrosion. Pipelines and heat exchangers are subjected to such a kind of failure.
- Attack at the bends in pipelines
- Grooves in the direction of liquid flow.
- Reduce the velocity of the medium
- Choose hard materials
- Avoid sharp turns
- Provide hard coatings.
Some variation in erosion corrosion is cavitation damage. Here there is damage due to bubble formation and collapse when there is hydrodynamic variation in pressure difference along the line. At low pressure water/liquid vaporizes. When the same is subjected to higher pressure bubble forms and subsequently implodes. This leads to plastic deformation and formation of cavities as brought out in.
Moving/vibrating interfaces under load causes damage akin to wear called fretting damage. Here the relative movement is relatively small in angstroms. Typical failed surface under this process is brought out in.
STRESS CORROSION CRACKING:
When there is a conjoint action of stress and environment. Stress corrosion cracking occurs (SCC). However SCC is specific to environment. The alloys are susceptible to SCC only when specific ions are present akin to pitting corrosion. In addition the alloys fail only if the stress exceeds a threshold level below which they are safe.
- SCC in austenitic stainless steels are predominantly trans-granular in nature.
- Failure occurs by brittle mode.
- Ions promoting SCC of that particular alloy must be present. Say Cl and O2 for austenitic SS and ammoniacal solution for Cu base alloys.
- If the alloy is sensitized it can promote inter-granular mode of cracking.
- Select the alloy that is not susceptible to the environment.
- In the case of SS, control either Cr or O2 one can keep either one of them low.
- Apply load lower than the threshold stress.
- Provide compressive stresses by sand blasting or shot blasting.
- Avoid stress concentration.