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Finish for stainless steel to prevent galvanic action with aluminum

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I am submerging an instrument we manufacture in non-salt water, but the water may be hard, and it stays there for weeks or months. I am using a black hard anodize coating with Teflon seal for the aluminum. I use stainless steel screws for assembly of the instrument. We still seem to be seeing some corrosion of the aluminum where the screws are.

It could be that the screw scrapes off part of the anodize/Teflon and then we see corrosion of the aluminum.

It could be that some galvanic reaction between the screw and aluminum takes place that promotes corrosion. I think I have the best aluminum treatment for my application with the black anodize with a Teflon coating. Does anyone know of a treatment for stainless steel that is dielectric, i.e. it inhibits electrical conductivity between the aluminum 'housing' and the stainless screws, which would, I believe, reduce the galvanic action and any corrosion it promotes. Really I'm trying to do anything which helps and minimizes the corrosion; it doesn't have to be a complete solution.

Thanks,

Steve Kerrigan
- St. Paul, Minnesota


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I have had success using high phosphorus electroless nickel on the stainless steel screws in contact with bare aluminum 6061.

Don Baudrand
Don Baudrand, Consultant
 
Poulsbo, Washington



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An approach you might try would be to add a third metal that is more active than either the stainless or aluminum (like zinc or magnesium). As long as all three metals are in electrical contact with each other the most active metal will corrode first. One thing to be careful of is when you couple dissimilar metals is the ratio of surface areas of the joined two metals. For example, if instead you did not anodize the aluminum you might predict that the resulting corrosion would be slow and uniform over the entire surface of the aluminum. However, by anodizing the aluminum you have protected the surface of the aluminum - now when you get a scratch in the anodize,the resulting corrosion will be greatly accelerated.

Hope this helps!

John Bauchat
- Milford, New Hampshire


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18-8 Stainless steel has no reaction with aluminum. Make sure that your screws are 18-8 type material. Also 300 series stainless will work . Just don't use any 400 series stainless steel as it is magnetic (has higher carbon content) which is the reaction problem.

Bill Trobaugh
- Richmond, Virginia


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While searching for info on galvanic reaction between stainless steel and galvanized steel I found Bill Trobough's comment that 18-8 stainless does not react with galvanized steel. Does anyone know how 400 series stainless reacts with galvanized steel and how they both react with 30 micron anodized aluminium. They will be used outdoors and we would like a life expectancy of 20+ years.

Bernard Warren
- UK


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Go to www.mcnallyinstitute.com/04-html/4-1.html and look at the section an GALVANIC CORROSION. It also has a list of materials and which would be the anodic end and which would be the cathodic end.

Ray, Chem Eng
- Louisville, Kentucky


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I am interested by Bill Trobaugh comment regarding 18-8 stainless steel reaction to aluminum. Does the 18-8 stainless steel still not react with aluminum in a sea water environment.

Phil Chambers
- EASTBOURNE, East Sussex, England


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We fabricate and install powder coated aluminum rails on ocean front homes for 20 years. Corrosion is a constant problem in this area. we have had success with powder coating over unsealed clear anodizing, however failures do sometime occur mostly in the cast flanges. we have been running a side by side real world test on one project chromic acid conversion coat vs anodizing and we have found that after 1 year the only noticeable failure is not in the test area (drilled hole and broken surface) but where the 18-8 st. st. screw (probably broke the finish)mounts the flange to the deck. this is occurring on the chromate flanges only. this leads me to believe that the problem is more likely the galvanic reaction between the st. st. and the aluminum than the aluminum oxidation undermining the powder/conversion coat because the exposed test area shows no sign of failure.

Richard Morrone
- Port Reading, New Jersey


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I have worked on plenty of marine hardware and the big problem is using two dissimilar metals. That will eat you up every time. The best I've been able to do to keep it down when there is no other choice is use a marine seaming compound between the hardware. Makes a much better seal than just the anodization.

Randall Dannemann
- Huntsville, Alabama


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Would a fiber/rubber washer work in this application?

Jeff Friesen
- Winnipeg, MB, Canada


February 21, 2008

1. The box for our aluminium foil contains the warning: "Do not cover stainless steel plates or dishes with the foil" along with the one about nor using it in a microwave.
2. My wife steams an egg custard (leche flan). The custard is in an aluminium cake tin which she covers with alfoil and places in a stainless steel pan. The inner bottom of this pan has become extremely dull and discoloured.

Richard Dobie
- Sydney, NSW, Australia


May 19, 2008

Just came across your problem while addressing an identical problem of my own. Hard anodising (not chromic) but thick sulphuric anodising, around 100 microns, will provide electrical insulation between the bolt and aluminium and prevent electrolytic activity. Put a thin preferably soft washer in there to prevent the anodising being scratch (though it's pretty tough).

Other precautions you can adopt are sealing of the connection with a sealant =>
or paint, or using bolts with plating nearer the electrochemical series no. stainless.

David Simpson
Engineering Consultancy - Luton, UK

3M Marine Sealant


May 7, 2009

Steve,

Aluminum will sacrifice to 18-8 stainless steel fasteners unless you put an aluminum topcoat on them. Contact Elco Construction Products for technical information.

Ron Stokes
- Atlanta, Georgia


March 29, 2010

If you use Silicone rubber sealantamazoninfo or Loctite 243amazoninfo on the 18-8 screw threads it will stop some local corrosion as well be sure to use a Magnesium anode that has a proper electrical connection to the alum. housing.

We have used this technique and seen over 12 years of service if the anode is changed out periodically

Christopher Nicholson
- Falmouth , Massachusetts


April 22, 2010

My response to the initial question of SS screws used on an anodized aluminum box: Is it possible that you paint or seal the bolt/screw heads to prevent moisture entering the contact area?

The other option which has already been mentioned is by "wet-installing" your bolts with some sort of paint, primer or sealer. Cover the bolts and install them before the paint dries.

I've seen both of these techniques used in the aviation industry which has strict regulations with regard to salt spray testing and failures.

(I can't guarantee success and I am not allowed to provide a professional opinion on behalf of my company.)

Paul Mak
- Ottawa, Ontario, Canada


June 11, 2010

Hi
I install Aluminium Balustrade in a coastal area and have had problems
with corrosion.
I have powdercoated aluminium bracket with internal thread that takes a 316 S/S
bolt. After approx. 3 years corrosion has started.
Is there a product that can insulate the S/S from the aluminium with success.
thanks for your input.

15625

Mark Hatton
Metal Fabricator - QLD AUSTRALIA


August 21, 2010

Hi, Mark

If the problem is that the internal threads of the aluminum casting are corroding, then the materials previously mentioned may be of some help in reducing the contact and protecting the internal threads.

But not all corrosion is galvanic corrosion due to dissimilar metals; if the problem is general corrosion of the outside of the item, I personally doubt that the stainless steel screw is a major contributor, rather that the problem is inadequate pretreatment or powder coating. Are the components chromate conversion coated before powder coating?

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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