Finish for stainless steel to prevent galvanic action with aluminum
Q. 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.
- St. Paul, Minnesota
A. I have had success using high phosphorus electroless nickel on the stainless steel screws in contact with bare aluminum 6061.
Consultant - Poulsbo, Washington
(Don is co-author of the
book "Plating on Plastics")
A. 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
A. 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
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
Hi Bill; hi Phil. Everyone is encouraged to offer their opinion and we thank Bill for his. However, I don't really agree that magnetism or carbon content has much to do with galvanic reaction. Stainless and aluminum are fairly far apart galvanically, so, under bad & salty environmental conditions galvanic corrosion can occur. But stainless steel hardware is commonly used on anodized aluminum balcony railings and similar situations without it causing a major problem in many cases. Galvanic corrosion can often be a subtle issue rather than a go/no-go one :-)
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Q. 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^aluminum. 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.
A. Hi Bernard. That's probably not a good idea. I don't think you should have metal-to-metal contact if you are mixing galvanizing, aluminum, and stainless steel and want a 20+ year outdoor life.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
A. 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
A. 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
A. 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
A. Would a fiber/rubber washer work in this application?Jeff Friesen
- Winnipeg, MB, Canada
February 21, 2008
A. 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.
- Sydney, NSW, Australia
May 19, 2008
A. 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 scratched (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.
Engineering Consultancy - Luton, UK
May 7, 2009
Aluminum will sacrifice to 18-8 stainless steel fasteners unless you put an aluminum topcoat on them.
- Atlanta, Georgia
March 29, 2010
A. If you use Silicone rubber sealant [linked by editor to product info at Amazon]
or Loctite 243 [linked by editor to product info at Amazon] 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 aluminum housing.
We have used this technique and seen over 12 years of service if the anode is changed out periodically.
- Falmouth , Massachusetts
April 22, 2010
A. 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.)
- Ottawa, Ontario, Canada
June 11, 2010
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.
Metal Fabricator - QLD AUSTRALIA
August 21, 2010
A. 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 or anodized before powder coating?
Ted Mooney, P.E. RET
Pine Beach, New Jersey
February 14, 2015
Q. Thank you for your mentions above. I recently try to use stainless screw to fix and connect aluminum truss. The aluminum is A5052, A6061 and A6063. I'm not sure whether 300 series stainless steel (8-18 stainless steel) has no electric reaction with aluminum.
As I know aluminum has higher galvanic corrosion tendency than steel and stainless steel. Also, I found some information about this problem at www.ehow.com/info_8135383_screws-compatible-aluminum.html
"Stainless Steel Screws
Stainless steel is an alloy of carbon steel that is, itself, resistant to corrosion. However, stainless steel is reactive with aluminum, and when a stainless steel screw is in contact with an aluminum base metal, the aluminum is likely to corrode. As is the case with carbon steel screws, a plated stainless steel screw is less likely to corrode aluminum; screws treated with a high-quality coating consisting of zinc and aluminum flakes are especially resistant to corrosion."
- Seoul, Korea