Corrosion of s/steel 316

[elv]

Hello,
we have a motor housing made of s/steel aisi 316. It is a thin metal body about 2mm thick and cold formed (deep drawn and spun). Two holes are also cold worked (punched)for the electrical cables to go through. Recently we've seen pitting around these holes (where the rubber cable gland sits) forming an o-blong shaped pitting area where the seal contacts the housing. Close to the holes we also spotweld 2 studs. The studs have also corroded and fallen off.
The motor which is part of a pump operating in sea water, has its cable coming out of these 2 holes around which corrosion was seen.
Can anybody explain what initiated the corrosion?
thanks for your help!
ELv

 

[Metalguy]

Sounds like typical crevice corrosion. 316 is very susceptible to Cl attack, and crevices concentrate the Cl.

Either eliminate them or use a more Cl resistant SS, such as 254SMO.

 

[mcguire]

You have a couple things going on here. It sounds like the rubber seal forms a crevice and, as metalguy correctly says, that will allow chlorides and Fe ions to concentrate, lowering pH and causing corrosion.
The punched holes create surfaces perpendicular to the rolling plane of the metal. This exposes inclusions which make such a surface pit more readily. Lastly the welds which hold the studs are the least corrosion resistant even if properly cleaned. Unless they are laser welds they will have a crirical pitting temperature 10C lower than the wrought metal. The cold work on the housing has a negligible effect.
316 is marginal in seawater. You could move up to 2205 to eliminate the problem or try 316Ti if you can locate any.

 

[TEV]

I agree with Metalguy and McGuire with one exception: In experiments I performed several years ago, I found significantly increased susceptability to chloride corrosion in heavily cold worked 316 stainless steel, particularly in lots with compositions near the minimums allowed of the austeniitic phase stabilizers Nickel, Carbon and Nitrogen. The cold working can cause martensite formation, which increases corrosion susceptability. Martensite is magnetic, whereas austenite is not. See if your housing is attracted to a magnet.

btw - This phenomenon is not well known outside of the metallurgical world. I set up an experiment for my son to do for his high school science project on this. It won the state! We used 316L with 10.1% Ni content, solution annealed. It had no attraction to a magnet in that condition, but he showed that increasing deformation induced increasing response to a magnet.

 

[rolledalloy]

Agree with responses 2205 would be beneficial. 2205 has been used for pumps in desalination since conditions are not stagnant and it offers better resistance than 316L. 6% moly stainlesses such as AL-6XN or 254SMO mentioned previously would offer even greater resistance to seawater.

 

[elv]

thank you all for your replies!
really helpful!
cheers.

 

[mcguire]

TEV, your observation on martensite being preferentially corroded in line with observations that chromium depleted areas are found within stainless and these regions are the first to transform to martensite and would also be the least corrosion resistent.