Cathodic Protection of Stainless Bowsprit

[Lojanica]

Hi all. I just repaired a large heavy stainless steel bowsprit. It was not a new fab but a rebuild. I am interested in some protection beyond simple passivation. At present there are teak boards used as floor boards on top of the heavy stainless superstructure. Would using aluminum under the boards in contact with the stainless afford some added protection? The idea would be that the mass of aluminum (say five strips of aluminum 1/4" thick by 5 ft long by 2 inch wide) under the teak boards would act as sacrificial anodes to the stainless.

Comments?

Thanks

[thinwater]

The aluminum plate answer sounds like a mess. Could you please describe any prior corrosion issues? Generally with stainless they can be solved with proper bedding and isolation from dissimilar metals and electrical circuits. With details, we might troubleshot.

[Lojanica]

Old heavy ss bowsprit. Railing cut off and new tubing welded to main heavy wall 2" ss. The problem is stress cracking and then crevice corrosion. Not structural now that it has been repaired but there are spots near some of the weld patches that are starting to corrode again. There is no dissimilar metal issues only weather and stress cracking then crevice corrosion. The area that is affected can be easily cleaned and passivated but the crevices are gonna continue to corrode away. Short of building a whole new bowsprit (which isn't needed at least for a long time) and electropolishing I was wondering why couldn't I just add some aluminum under the teak and let that corrode at a much faster rate. I know cathodic protection works under water but would it add protection at all to these areas.

[transmitterdan]

It doesn't sound like you have a problem with dissimilar metal corrosion. Aluminum can't stop crevice corrosion. Welds in SS need to be passivated if exposed to corrosive environment like salt water. If the welds are corroding then it's probably only a matter of time before it eats through the metal. How much time is anyone's guess. Very hard to measure the rate of corrosion.

[Lojanica]

Was passivated x 2 prior to install. It seems the corrosion is occuring where the teak traps moisture against the stainless. Thats why even though the wood is not a disimilar metal I thought that applying a strip to the back of teak might resolve the problem. Moisture is an electrolyte and the wood ss interface is a crevice. Stupid or worth the effort. It's hard to passivate the area well without taking the teak strips off so I thought why not just have aluminum sheared and when the teak gets bolted down just put the aluminum strips down first?

[colemj]

You are not experiencing galvanic corrosion, which is what anodic protection helps prevent.

You are experiencing poultice corrosion, where the lack of oxygen and a continually wet environment is causing the problem.

Aluminum strips will not help you here, and will probably make the situation worse as they corrode and cover the stainless with sticky oxidized paste.

You need to seal the interface between the teak and the stainless. This will probably involve removing the teak. Installing a thin plastic isolator sheet between the wood and the SS may help some.

Mark

[transmitterdan]

I agree with Mark. Stainless steel needs oxygen to oxidize the chromium that is part of the alloy. This is what passivation is in reality This oxidized surface layer is what makes stainless stain less. Without this chromium oxide layer it's just like any other steel. It will rust when exposed to chlorides such as salt water. If something is scratching the chromium oxide surface and there is no oxygen to oxidize the chromium then the iron will rust.

One possible solution would be what Mark suggested. Maybe you can raise the teak by placing plastic washers between the bowsprit and the teak planks where they are bolted. That would allow air to circulate around the stainless. Also, a freshwater rinse would have an easier time removing salts. McMaster Carr sells all kinds of plastic washers. Try to find some that are high density polyethylene or Delrin.

[transmitterdan]

Oh, and one more thing. Try to seal the fasteners so no water is trapped where the threads of the fastener engage. Tef-Gel is good but any sealant that keeps water away from the threads will help.

[Lojanica]

Thanks. Good tips. Will take teak off repassivate and add spacers with plastic washers.