Finish for stainless steel to prevent galvanic action with aluminum

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

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!

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.

A. 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 :-)

Regards,

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.

Regards,

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.

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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.

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 sealing compound between the hardware. Makes a much better seal than just the anodization.

A. Hi Jeff. If the washer prevents metal-to-metal contact, it will prevent galvanic corrosion. Galvanic corrosion involves electrons flowing from one metal to the other and they can't flow through fiber/rubber.

Luck & Regards,

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A. Just came across your problem while addressing an identical problem of my own. Hard anodising (not chromic, but thick sulfuric 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.

A. Steve,

Aluminum will sacrifice to 18-8 stainless steel fasteners unless you put an aluminum topcoat on them.

A. If you use Silicone rubber sealant [affil links] or threadlocker ⇦this on eBay or Amazon [affil links] 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.

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.)

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?

Regards,

A. Thank you for your mentions above. I recently triedto 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."

adv.
We have recently developed special hard anodizing for SS (400-series) that is electrically isolated. This will help on contact with aluminum. If you are concerned on SS300-series, like 316 or similar, contact us, and we will investigate this for you.

A. Hi Michael. Although it is possible to isolate bolts and screws from the metals they are connecting, it's more complicated than a washer under the screw head and involves bushings (tubes) as well.

I think I would make sure all of the steel is well painted, and all of the aluminum is well painted, and call it done.

If you wanted to truly isolate the steel from the aluminum you would need to put a thin non-conductive gasket, like visqueen poly sheeting between the steel and aluminum, and add a bushing along with the washer so the stainless screws don't touch the aluminum.

Regards,