MS - SS Corrosion

[SAinSCT]

I need some advise on a problem i have encountered........
We are in the process of refurbishing 2 off dive bells, but once the internals were blasted and MPI tested we discovered several areas where severe local corrosion has taken place.

I took impressions of the the worse affected areas and corrosion was up to 9mm deep. The bells are manufactured from mild steel and has stainless steel floor decking inside. The affected areas coincide with the areas where the SS was in contact with the MS. Now I am aware of Galvanic corrosion and its affects, but would like to know if there could be any other foactors that could affect the rate of corrosion. As the bells had a major refit in 2005, the rate of corrosion must have been around 4mm / year.

Given that this area is flooded with seawater more often than not, is there something else that could affect the rate of corrosion?

Your comments are much appreciated.

 

[EdStainless]

The presence of any crevices would accelerate the corrosion. Such as where the two metals overlapped. Any other crevice formers such as mud or poorly adhering paint would also contribute to the problem.
It could also be that this area had prior corrosion damage that wasn't cleaned up enough, the old 'it isn't very bad' situation.

One other thing comes to mind, if the entire SS floor was wet and the water only came up a little way onto the carbon steel then the anode/cathode ratio would be very unfavorable and this would accelerate the corrosion.

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Plymouth Tube

 

[gr2vessels]

Paint thick the stainless steel bit with a good quality of marine grade of paint, perhaps discontinue the stainless steel contact place with some insulating plastic material, resistant to seawater. It might work to slow down (not prevent!) the generic corrosion and in particular the galvanic corrosion.
cheers,
gr2vessels

 

[SAinSCT]

Thanks alot for the advise guys, it is much appreciated.