Crevice Corrosion on Swim Platform Brackets

[SBriggi]

Hi folks!

I'm a new member to this forum but I've found great information in your various threads over the last year as I become comfortable with my 2001 29.9 Bruckmann Bluestar.

As I prepare to launch this spring, I've noticed some crevice corrosion (pitting) on the underwater portion of the stainless brackets supporting the teak swim platform. The previous owner had the underwater section of the brackets coated in a black paint - bottom paint, maybe...propeller paint, maybe...maybe something else.

I have removed the coating to inspect the pitting and the brackets appear solid. The brackets are not bonded and there are no zincs attached. Now I'm wondering what is the proper protection, if one exists, for stainless brackets that are partially submerged in salt water...

Let me know your thoughts. Thanks, Steve

 

[DDW]

Painting may have been the problem. SS wants to be in oxygenated water to develop the oxide coating that keeps it from corroding. If deprived of oxygen, it will corrode nearly as fast as ordinary steel. Zincs wouldn't hurt either. I know they guys doing the stainless work for Bruckmann, it will be 316L so high quality material.

 

[syjos]

Connect all the brackets electrically to the transom zinc on the interior side of the transom with wire attached to each bracket mounting bolt then to the zinc.

 

[SBriggi]

Thanks for the comments DDW. It's great to hear 316L stainless was used for the brackets; short of bronze, this is the right material. I'll take a few photos before launching this season and take another look when I haul in the fall. It will be interesting to see if simply removing the paint slows/resolves this issue.

She also had a corrosion problem on the aluminum backing plates for the struts. Over time, the corrosion increased to the point water was very slowly leaking into the bilge, keeping the backing plates well wet. This winter I replaced the backing plates with 1/2" G10 plate (McMaster Carr) and the stainless fasteners with bronze fasteners (Fair Wind Fasteners).

If the platform brackets show further deterioration, I'll look into zinc protection as Syjos suggests.

Thanks for the quick responses!
Steve

 

[DDW]

Are the brackets in any way connected to the electrical system or ground? I'd measure that carefully with an ohmmeter. If they are, there are many other things that could be going on.

 

[Nick F]

Hi Steve,

I worked for many years on design of underwater equipment (for the oil industry). The splash zone is the toughest environment around saltwater. Alternate wetting and air exposure make for a corrosive environment. Also, as seawater dries it does so by the water evaporating and the salt content of the remaining water increases, making it even more corrosive.

Technically, crevice corrosion occurs in a crevice - usually where a SS piece is clamped against something else. What you are experiencing may indeed be crevice corrosion, but may be surface pitting.

As DDW said, SS gets its protection from the surface oxide layer which it develops. Once breached, corrosion occurs and drills little "holes", showing as pitting. 304 SS is very susceptible to surface pitting and 316 SS, with its higher chrome content is much more resistant.

The fix is to re-establish a smooth external surface with its oxide layer. I recommend mechanical polishing, followed by a chemical process known as passivating (an acid treatment), or by electro-polishing. The passivating can, in fact, be done in place using a gelled product known as pickling paste. A marine store product called Naval Jelly is a weak version of this. Industrial grade pickling paste is more effective but is a seriously nasty product with health risks. A safer alternative to pickling paste is electro-polishing. Take the pieces to a shop that does this.

As regards crevice type corrosion, protect against this by putting sealant (caulk, butyl tape) between the clamped surfaces. This will prevent seawater sitting in the crevice.

A note about using zincs for protection. This is only effective for metal that is totally immersed. As your brackets are partly in air, those parts will continue unprotected.

Good luck!
Nick

 

[SBriggi]

Thanks Nick. It's great to have input from people with your level of experience as well as the willingness to help! I've looked into pickling and passivation and will put this on next winter's maintenance list.

Thanks again,
Steve