Question

write short note on pitting corrosion waterline corrosion crevice corrosion ​

Answer 1

Answer:

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Explanation:

Crevice corrosion explained

What is crevice corrosion? Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material.

What causes crevice corrosion? Crevice corrosion is initiated by a difference in concentration of some chemical constituents, usually oxygen, which set up an electrochemical concentration cell (differential aeration cell in the case of oxygen). Outside of the crevice (the cathode), the oxygen content and the pH are higher - but chlorides are lower.

 

Chlorides concentrate inside the crevice (the anode), worsening the situation. Ferrous ions form ferric chloride and attack the stainless steel rapidly. The pH and the oxygen content are lower in the crevice than in the bulk water solution, just as they are inside a pit. The pH inside the crevice may be as low as 2 in a neutral solution. Once a crevice has formed, the propagation mechanism for crevice corrosion is the same as for pitting corrosion.

 

critical crevice corrosion temperature

The major factors influencing crevice corrosion are:

1.  crevice type: metal-to-metal, metal-to-non-metal

2.  crevice geometry: gap size, depth, surface roughness

3.  material: alloy composition (e.g. Cr, Mo), structure

4.  environment: pH, temperature, halide ions, oxygen

What materials are susceptible to crevice corrosion? All film-forming alloys whose corrosion resistance depend on the stability of passive films are particularly susceptible to crevice corrosion. Examples are all grades of stainless steels and aluminium alloys.

 

How to evaluate a material's resistance to crevice corrosion?

A material's resistance to crevice corrosion is usually evaluated and ranked using the critical crevice temperature (CCT) in accordance  with the ASTM Standard G48-03: Standard Test Methods for Pitting and Crevice Corrosion of Stainless Steels and Alloys by Use of FeCl3. The critical crevice temperature is the minimum temperature (°C) to produce crevice attack and CCT is usually lower than the critical pitting temperature (CPT). Computer software such as CRA-Compass can be used to assess the crevice corrosion resistance of an alloy for a given temperature and chloride concentration.