Corrosion- Chemical deterioration

Corrosion- Chemical deterioration

  • Conventional engineering materials are not able to serve at their full potential for long periods of time i.e. they gets deteriorated during the service.
  • They are many reasons for deterioration of engineering materials, like physical, chemical, mechanical, weather, etc. 
  • Chemical deterioration is most common in metals, and also . observed to some extent in ceramics. It is known as corrosion. 
  • Physio-chemical deterioration of polymers is known as degradation of polymers.
  • Corrosion of metals is mainly due to electro-chemical reactions. 
  • It is also most influenced by temperature and concentration gradients. 
  • Relative tendency of a metal to get corroded is presented in terms of its electro-motive force (EMF). 
  • Standard EMF series is suitable for easy recognition of basic elements for their relative tendency towards corrosion. 
  •  Another usefulness of the series is to pick material for cathodic protection.
  • Corrosion of metals is classified based on the manner in which it is manifest into eight forms . 

Uniform corrosion 

As name suggests, corrosion occurs over entire exposed surface. Less of detrimental. Easy to monitor and control. Is it also most common form. Painting is best counter measure for it. 

Galvanic corrosion

Occurs when two metal with different EMFare electrically connected, of which one gets corroded. Counter measures include: insulation; pairing metals with less difference between their EMF; altering the corrosive environment; tailoring the design of components w.r.t. their EMF.

Crevice corrosion

This occurs as a result of concentration gradient within a component. Corrosion occurs at the site of lower concentration. Counter measures include: welding instead of riveting; removing scales, etc. 
 Pitting : Other localized corrosion. Difficult monitor and very dangerous. Material removal is minimal, but occurs in normal to surfaces. Counter measures: polishing of surface, and other common methods. 

Inter-granular corrosion

This is due to concentration difference at micro-level. Grain boundaries are inferior to grains, and more prone to corrosion. Counter measures: suitable heat treatment; addition of alloying elements; low %Cinsteels.

Selective leaching

Occurs in selective metals. It is selective removal of a particular metal from the component. Example–dezincification of brass. Counter measures: change of material; cathodic protection. 
 Erosion-Corrosion : It is acceleration of corrosion due to mechanical actions. All metals and alloys are prone to this form of corrosion. More damaging in case of metals with passive scales. Counter measures: change of component design; clearing the fluids for particultes.

 Stress concentration 

 Also known as stress corrosion cracking (SCC). Result of combined action of tensile stresses and corrosive environment. Counter measures: change of environment; cathodic protection; addition of inhibitors.


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