Question

81) You are a engineer in a shipping company and you have identified some corrosion on one of your ocean freighters that is occurring below the water line, around rivets that have some kind of green slime covering them. What’s happening and what can you do to mitigate the problem?

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Please answer in clear words NO guessing!!! Thanks

Answer 1

Corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited is called Crevice corrosion

Stage 1: Corrosion occurs as normal both inside and outside the crevice:

Anodic reaction: M→Mⁿ⁺ +ne ; Cathodic reaction: O₂+2H₂O+4e=4OH^-

The positively charged metallic ions are electrostatically counter balanced by OH-

Stage 2: at this stage, the cathodic reaction inside the crevice consumed most of the oxygen available

Stage 3: Cl- and OH- diffuse into the crevice to maintain a minimum potential energy. Metal chloride is formed. Hydrolysis of metal chloride lowers pH and MCl_n + nH₂O = M(OH)_n + nHCl.

Stage 4: More Mⁿ⁺ ions attack more Cl^- leads to lower pH inside crevice, metal dissolution accelerates and more Mⁿ⁺ ions will be produced that will lower pH.

The simplest method for preventing crevice corrosion around rivets are

1. Reducing crevices in the design of the structure

2. By painting

3. Sealing the faying surfaces with a polysulfide is known to be an effective method for preventing corrosion.

4. Cathodic protection could be an effective method, but anodic protection is often improper.

5. Another common protection method is using alloys which are less vulnerable.

6. Application of passivating compounds such as chromate and nitrate is well practiced to prevent corrosion.

7. Overlaying susceptible areas with an alloy which is more resistant to crevice corrosion is another protective measure.

8. Reblast and repaint with suitable paint which can withstand in sea water environment.