What are the factors that contribute to crevice corrosion in stainless steel?

[jra12222]

Hi all,
I am trying to understand how the process of either crevice or pitting corrosion works (I believe they are quite the same)
I understood the part of how they start and what enhances them or how they can be prevented, but I can't understand an actual scenario.
Say for example we have stainless steel in a water solution;

1. From which elements and to which elements do the electrons travel?
2. With which elements does the water react? -> and does it always change to hydrogen and hydroxide?
3. How does hydrogen increase the acidity?
4. Why does the steel where the corrosion take place always become the anode?

I would really appreciate it if someone could explain this to me!

Thanks a lot!

 

[anorlunda]

It would be helpful to us to say what level you are starting at, and what research you did before posting. It would be frustrating to write a long answer and then have you say, "I already know that." or "That's too advanced for me."

For example, these two Wikipedia articles answer some of your questions.
https://en.wikipedia.org/wiki/Pitting_corrosion
https://en.wikipedia.org/wiki/Crevice_corrosion

 

[jra12222]

I am actually in my final year of the degree (mechanical engineering) however materials was never my strong point
I did read that information before and I also watched a number of videos on pitting/crevice corrosion, however I couldn't find a suitable example explanation.

A summary of what I know (will focus on Crevice):

• the fact that the crevice needs to be big enough for the corrodent to enter but small enough to remain stagnant,
• the fact that crevices trap pollutants, moisture, and excludes oxygen (not entirely sure about the oxygen however to be honest, since it is required for corrosion to take place)
• the oxide layer formed between the metal and oxygen to stop electron flow
• this layer can be broken by mechanical means like scratching
• and in order for crevice corrosion to take place 2 factors are important:
  1. drop in potential into the crevice
  2. and the chemical composition of electrolyte

As you can see what I know (assuming it is correct) is only the theoretical aspect, I don't know how it would actually happen.