Stainless Steel Corrosion

A letter from Steve D'Antoniok, published here with permission from Steve.

June 17, 2014

On Steve's website he makes it clear that both the lack of oxygen and the presents of moisture are needed to create corrosion.

I have issues with a stainless steel propeller shaft, possible problems with stainless steel shaft log, and a leaky stainless steel freshwater tank.  I posed specific questions to Steve about stainless steel shaft logs and fresh water tanks, and whether epoxy would be a good enough moisture barrier to prevent the corrosion.

Here is his response.....

 

 

Chris:

You’ve posed some very good corrosion questions related to stainless steel corrosion.  I have covered this subject on many occasions in articles for various magazines, as well as on this website, the latter is available at http://www.stevedmarineconsulting.com/ezine/index.php?p=18   You might wish to review that column. 

Stainless steel, when used in a constantly submerged environment, can be problematic.  In order for it to maintain its tough, corrosion oxide corrosion barrier, it must be exposed to the atmosphere, as it would be above the waterline, in the cabin, on deck or, or dissolved oxygen in flowing sea or fresh water.  When exposed to stagnant water, which is oxygen depleted, the oxide barrier breaks down, at which point the stainless steel becomes, as we say in the world of corrosion, active rather than passive, it is affected by a process known as crevice corrosion.  The electro-chemistry of this process, while interesting, isn’t as important as the practicalities of how this alloy is used. 

Ideally, avoid using stainless steel in these environments, and this would include embedding hardware in the hull, including fasteners, through hull fittings and shaft logs.  The above-mentioned column opens with a photo of a severely corroded, “wasp-waisted” strut fastener removed from the fiberglass hull of a twenty year old vessel.

The preferred alloy for these scenarios is zinc-free bronze, or bronze alloys whose zinc content does not exceed 15% (less is better).  Fiberglass, particularly for shaft logs,  is even better in that it is corrosion-proof.  Alternatively, if stainless steel must be used, it should be 316 series (not 304) for underwater hardware.  Shaft alloys should be proprietary AQ 22, rather than AQ 17, or high corrosion resistance duplex stainless steel in Europe, and never ordinary stainless steel, regardless of alloy.  If it is to be welded, it must also carry an L suffix, such as 316L, denoting its low carbon content, making it suitable for welding.  Failing to use low carbon stainless steel in welded applications will often lead to a phenomenon known as carbide precipitation, weld decay or weld migration, wherein the chrome content of the alloy adjacent to the weld is depleted, making it, and I’m simplifying, essentially ordinary steel, which rusts.  The rusting can occur very quickly after welding, above the water line, regardless of access to oxygen.  It’s a common problem with welded stainless steel tanks, the welds discolor quickly and begin to turn brown with surface oxidation or rust.

Creating a barrier between the stainless steel and oxygen depleted water, using an epoxy coating or primer, may stave off this problem for a while, however, it can also have the opposite effect.  If water manages to migrate under the coating, in an area where it’s cracked or chipped, the lack of oxygen in this region will promote corrosion.  While better than nothing, relying on a barrier coat to prevent corrosion of stainless steel is not a long term solution.  Grease will not promote corrosion on a stainless steel alloy shaft as it excludes both air and water, the latter is needed for crevice corrosion to occur. 

Finally, you can attempt a repair of a stainless steel water tank using epoxy (it should be of the food-grade variety).  The leak could be a case of weld decay.  Thorough cleaning and abrading of the surface will be necessary.  Any flexing will likely lead to failure of the patch, so make sure it is not exposed to pressure of any sort.

Sincerely,

Steve D'Antonio

Steve D'Antonio Marine Consulting, Inc.

PO Box 111

Wake, Virginia  23176-0111

 

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