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Dissimilar metal aluminum/304 stainless galvanic corrosion

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Q. I have an application were I have a 1/4" thick plate of aluminum in direct contact with a sheet of 304 Stainless Steel. Also I want to use an all aluminum pop rivet (both body and mandrel made out of aluminum)which will fasten the 1/4" thick plate with stainless steel. This application will see sea coast environment (but not an underwater application). Is there a chance of galvanic corrosion especially the pop rivets weakening out? I am looking for a quick solution. I looked for similar stainless steel pop rivets, but they are not available in the time frame I need to work with.

Saibal S deleted
Operations Manager - Douglasville, Georgia


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A. I think there's a 99 percent chance, Saibal, but maybe someone will offer encouragement.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey
     
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A. Ted is correct, check out the EMF series, you'll see that aluminum is more active than the stainless (iron) so it will corrode preferentially.

Paul D. Stransky, CEF
- Putnam, Connecticut


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A. Dead certain for corrosion. You may be able to prevent it by putting an insulator between the two. Try neoprene or something similar. It must not adsorb any moisture however.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


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A. Epoxy paint the faying surfaces and pull the rivets wet (soaked, including inside) with chromate primer. Not ideal but good enough for government work.

James H deleted
- Jacksonville, Florida


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A. Aluminum in a seacoast environment is not a long term proposition. However, you may want to consider using stainless steel fasteners. The rate of galvanic corrosion is related directly to the ratio of cathode area (stainless) to anode area (aluminum). In other words a relatively small anode area like an aluminum pop rivet will be attacked much faster than a large one like the 1/4" plate. Good luck.

Fred D deleted
high performance architectural metals - Cranberry Twp., Pennsylvania


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A. Try contacting High Energy Metals. They make metallurgically bonded structures of dissimilar metals, specifically for sea coast environments where corrosion due to dissimilar metals in contact is a threat.

Jim P deleted
connector development - Wenatchee, Washington


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The explosive bonding sounds like a good method to bond dissimilar metals in some cases, Jim, but how does the fabrication method have any impact on galvanic corrosion? I thought it depended on factors (the emf difference between the two metals, a metallic connection, an ionic connection, and the relative surface areas) that have little to do with the connection method. What am I missing? What material would you explosively bond onto the aluminum pop rivets?

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


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Explosion bonding creates a covalent (metallurgical) bond between the dissimilar metals. There is no GALVANIC reaction because the metals behave as one. This does not mean that there will be no breakdown of, say, the aluminum in a corrosive environment. Secondary zincate or chromate should help.

Jim Petri
- Melbourne, Florida


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I think we're on different wavelengths, Jim: if Mr. Sengupta pop rivets an aluminum sheet to a stainless sheet with aluminum pop rivets it's not going to work :-)

Electroplating and galvanizing and some other metal finishing processes also can alloy metals together in the same way. The galvanic corrosion issue arises not in the alloy or the coating or the substrate, but if the substrate becomes exposed due to incomplete coverage, or a scratch through to the base metal because you then have a galvanic battery between two metals.

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


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Q. My question pertains to the external swaging technique of fittings on tubing in aerospace applications. It is considered acceptable to swage 21-6-9 steel fittings on 6061-T6 aluminum tubing. Yet, this would appear to be a clear case of dissimilar metal contact, resulting in galvanic corrosion over time. I would like an explanation as to why this is, nevertheless, acceptable.

Zvi Burg
aviation - Tel Aviv, Israel


January 20, 2008

Q. Along the same theme - I am currently restoring my Land Rover Defender, which has an Aluminum body. I wanted to replace all of my external fasteners (which do touch the body) with Stainless Steel.

The previous OEM fasteners - either zinc or galvanized - rusted very quickly and the rust bled onto the painted aluminum surface. Can I still use the Stainless steel fasteners and create a barrier between the SS and the aluminum with Zinc or Galvanized washers? Any guidance would be appreciated.

Kevin Hall
- Delray Beach, Florida


January 24, 2008

A. Why can't you put a piece of rubber or fiberglass as a insulator so they don't touch.Use a sleeve for the bolt.

Tim Callanan
- Belize City, Belize, Central America


February 19, 2008

Q. Ted, can you explain to my man with the land rover defender why he had the problem with the rivets.
Love your comments.

Fred Demshick
architectural metal and glass - Bensalem, Pennsylvania


February 19, 2008

A. I'll certainly be happy to try, Fred.

Picture a conventional dry cell battery still sitting in it's original packaging. The shell is made of zinc while the center rod is carbon/graphite. The two dissimilar materials are not touching each other, but there is an electrolyte, a salty liquid path between them (the black glop). Nothing is happening so far, and the battery will have a long shelf life.

Now take the battery out of the package and, with two pieces of wire, connect a flashlight bulb to the battery. We know that the bulb will light. Here is what is happening: electrons flow out of the negative pole of the battery (the zinc side), through the wire and bulb, causing it to light, and back to the positive pole (graphite) of the battery. Inside the battery, the zinc metal at the surface has become positively charged zinc ions (zinc atoms from which one or more electrons have been removed) and they dissolve and start traveling through the electrolyte glop and meet up with their electrons once again at the graphite rod and become metallic zinc once again, depositing on the carbon. There is a balance here that allows current to flow: the wire connection provides a path for the electrons, the salty glop provides a route for the positively charged zinc ions.

Note what was required for the battery to function: the two different materials for the two electrodes (zinc and graphite), an electrical path (the wiring) and an ionic path (the salty glop). You can also make a battery using aluminum for the negative pole, stainless steel for the positive pole, rainwater contaminated with road salt as the electrolyte through which ions can travel, and a physical connection between the aluminum and stainless for the electrons to travel. If you have all of those conditions you have a battery or a 'galvanic corrosion cell'.

If you can keep the aluminum from touching the stainless, as Tim C suggested, you have no path for the electrons and the corrosion stops, similarly to removing the wires from the battery. If you can keep the area totally dry and absolutely free of salt, you have no ionic path and the corrosion stops. But what usually happens with the stainless rivets in aluminum is you have everything you need for the aluminum to corrode away into the salt water as ions.

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


February 27, 2008

Q. If I am using stainless bolts threaded into aluminum parts can I prevent corrosion by putting Loctite on the threads and the heads of countersunk screws?

Ronald L Richardson
- Conyers, Georgia


February 2008

A. I would doubt that Loctite would give you a reliable isolation of the two metals because the threads may break through and touch, even if you are able to apply a pinhole-free layer. However, I have read sales claims from manufacturers of marine sealants that this approach works. I guess I'd say if it is for a personal item it is worth trying but if it's an engineering question for a product line, then no. Just an opinion.

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

3M Marine Sealant


April 3, 2008

A. I would suggest a product called ECK. This would be a perfect application for this product.


Richard Nay
- South Elgin, Illinois


April 4, 2008

Hi, Richard. My experience has been that not even gold plating has made aluminum pop rivets into satisfactory fasteners for stainless steel, even in more general applications let alone in the "seacoast environment" that Saibal S is speaking of. If you were referring to Ronald R's posting instead, coatings of that sort sound more promising.

But should you sell your coating process to an aluminum pop rivet supplier and they start advertising it for use on stainless steel panels, I'd love to track how it works. Thanks.

Regards,

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


March 1, 2009

thumbsup2Gentlemen, many thanks for sharing your knowledge and experience. You solved my little problem, ss fixing to alloy mast on my chums yacht.

regards

Andrew

Andrew Ford
Just retired - Plymouth, UK


May 6, 2009

Q. Do you see issues with using threaded bronze valve on stainless steel pipe? They are closer on the potential scale than aluminum and stainless, but still are different enough that I am concerned. These are going to be in a harsh/humid environment. thanks!

Stephanie Cook
- Newport News, Virginia


April 6, 2010

Q. I'm an inspection engineer in a petroleum refinery company in Egypt
I want to ask about the possibility of galvanic corrosion inside jet fuel tank for internal parts assemblies in the following 2 cases:
1- aluminium flanges with stainless steel bolts & nuts
2- stainless steel cable welded on external surface of aluminium pipe

Shereen Sami
refinery - Egypt


April , 2010

A. Hi, Sami. The general principals discussed here are that aluminum and stainless steel are not actually galvanically compatible because they have significantly different potentials, but the passivity of both can reduce the problem to non-problematical for non-critical applications if the area of the aluminum is greater than the area of the stainless steel (stainless fasteners on aluminum architectural features usually aren't a significant problem, but aluminum pop rivets on stainless steel sheets would be).

So I would expect that stainless fasteners on aluminum flanges would not be a significant issue. Stainless wire welded to aluminum pipes, however, sounds a bit problematical to me because there is no opportunity at all for electrical insulation, and the welding process will probably generate several different types of areas that lack any passivation.

Regards,

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


June 29, 2010

Q. Many thanks to all the contributions, they have been most informative, however back to me, I have a situation in roof location on a major London train terminus where I have a few 100 no 3 x 50 mm wide aluminium trims to install as glass retainers, these to be fixed with a stainless steel screw, we are currently proposing the use of an aluminium washer with EPDM seal to weatherproof the fixings. What are your thoughts re corrosion and long term aesthetics. Design life is 10 years min.

Martyn Smith
- Dunstable, Bedfordshire, United Kingdom


August 31, 2010

Thanks James H. This is a great thread. I have been trying to answer this question for a long time for aluminum spars and stainless pop rivets that are heavy load bearing (application is sailing). The epoxy paint is a great idea. However, I am going to use plastic from a milk jug as a spacer, then soak the pop rivets as you said.

steven masur
- Montauk, New York

February 23, 2011appended

Q. I am constructing some window awnings and before I commence the project I wish to establish some facts. Are Aluminium and stainless steel 304 screws compatible metals in an external environment?

Pete Mattsson
House renovation hobbyist - Brisbane Australia

September 9, 2011

Q. Reading on Aluminium used in conjunction with Stainless steel. I'm a technician in the telecomm industry and come in contact with many different manufacturers' equipment meant for use outside, the casings manufactured from Aluminium or aluminium castings with all the bolts and nuts always stainless steel. I'm always wondering how is it that the industry doesn't know about dissimilar metals especially aluminium and stainless forming aluminium oxide at a very fast rate. We throw away so many pieces of working electronics due to broken casings. The aluminium flakes so badly it resembles a popular breakfast cereal and the stainless steel bolts just totally seizes inside the aluminium. If the guys installing equipment remembers they put some Copperslip paste/grease on the bolts' thread, that does seem to help, although I don't know for what duration of time before it will still form aluminium oxide. Then a question: if you sit with these bolts seized in the aluminium, any suggestions on how to dissolve the oxide and get the two separated?

Andre Botha
Telecommunication - Cape Town South Africa

June 12, 2012

Q. My question is, for protection of metal to metal contact between two different metals, say "steel & Aluminum", which one is better
A. Neoprene barrier
B. Polysurlyn barrier

Khalid Saifullah
- Jubail, Saudi Arabia


June 13, 2012

A. Hi Khalid.

To prevent galvanic corrosion the only thing necessary is to electrically isolate the two metals. For that, neoprene or polysurlyn, or almost any type or thickness of plastic will do fine.

However, the purpose of Polysurlyn (as I understand it) goes well beyond simple electrical isolation: it is intended to be a waterproof, weatherproof coating to prevent the elements from reaching the metal at all.

Regards,

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


July 12, 2012

Q. I have read through this entire thread and I'm pretty sure this hasn't been brought up yet, but what if the aluminum that is in contact with the stainless steel happens to be conversion coated?
I am designing a manifold that is ~1/4" thick and is going to be made out of 6061-T6 Aluminum that will be conversion coated per MIL-DTL-5541, Type II, Class 1A. This manifold will have 6 nozzles mounted to it using stainless steel machine screws. The nozzles are ~1" in diameter and will be made out of 304 Stainless Steel and will be Passivated per AMS2700.
Questions:
1) Will the conversion coating on the aluminum manifold provide a sufficient amount of insulation to prevent galvanic corrosion of the parts?
2) Why is galvanic corrosion not an issue when installing stainless steel helical thread inserts into aluminum plates? Is it due to the stainless steel part being much smaller than the aluminum part as mentioned above with stainless steel rivets in an aluminum plate vs. aluminum rivets in a stainless steel plate?

Mike Miller
- Columbus, Ohio, USA



August 9, 2012

Q. I currently have this issue of the cast aluminum house corroding or oxidizing real bad. Could this be caused by the cast aluminum being in contact with the stainless steel mounting brackets? This car is also near the seashore and also has 12v being used for the light.

corroded cast aluminum house 1 corroded cast aluminum house 2

thoughts?

Scott

Scott Doenges
- Michigan, USA


August 13, 2012

A. Hi, Scott.

That looks like exceptional corrosion, which usually has an exceptional cause, rather than simply mounting on stainless brackets. I can't tell from the pic whether this is inside or outside of the car. How old is it? What was the original finish -- anodized, raw, painted? I would suspect something to do with that 12 volt source rather than a mounting bracket.

Regards,

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


August 14, 2012

Q. These lights are mounted outside the vehicle. The finish is a black powder coating after being cleaned, which I'm still trying to figure out what that actually entails.

Scott Doenges
- Hidsonville, Michigan, USA


August 17, 2012

A. I think the components should be chromate conversion coated and electrocoat primed before the powder coating. Complete coverage with powder coating looks unlikely to me considering the shape of the component.

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


November 3, 2012

Impressive problem solving!
Q. I have an outdoor public art commission in Colorado where I am going to use 3/16" 5051 aluminum sheet up high on a wall for a ceramic mural backing (I am adhering tile to masonry and metal with polyurethane adhesive). I welded mild steel support brackets to hang the aluminum from. I then coated them with acid etch primer and automotive enamel. Is that enough to prevent corrosion between the aluminum and steel? Should aluminum also be coated?

Also, according to this thread, it looks like stainless steel or galvanized bolts to attach the aluminum sheet to the steel brackets is a no-no, unless they are isolated from the aluminum, correct? I really appreciate any wisdom/insights you could offer an artist!

Mario Echevarria
Public Artist - Longmont, Colorado, USA


March 29, 2013

Q. OK for those that aren't an engineer, I want to build a boat lift from 6061 Alum, what is the best type of fasteners to use? just give me a simple answer.

Terry White
- Phx, Arizona, USA


April 1, 2013

A. 316SS. Best, but not very good, because the fasteners won't corrode but the aluminum may.

Regards,

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


July 30, 2013

Q. I am working on a project for an oil company offshore. Needs to be all aluminum construction; the only other material I can use is stainless 316. I have s.s. wheels, pulleys, wire rope, aluminum structure, s.s. fasteners.
I am purchasing all fasteners as 316 s.s. and the vendor is having them sent out and Teflon coated. All surfaces in contact alum/ss I am using a gasket. Maybe a neoprene as recommended above, I am still looking for a better gasket with high compressive stress, and non-absorbent.

Randy Davis
Design Drafting - Selfemployed - Broken Arrow, Oklahoma
  ^- Privately contact this inquirer -^


August 2, 2013

A. UHMWPE or Polyurethane or teflon should work better. Will cost a bit more.

James Watts
- Navarre, Florida

misc. plating stuff
For Sale cheap



August 7, 2013

thumbsup2Thanks very much James for your comments. It looks like I will be using G10 high pressure laminate. It has very high commpressive allowable and is fairly non-absorbant. I have used it on radio frequency shielded enclosures (anechoic chambers) at the base plates for isolation from ground (10000 olms to ground) from the slightly absorbant concrete. It may be an overkill but this is a bolted connection.

Randy Davis
design drafter self-employed - Tulsa, Oklahoma



August 26, 2013

Q. Hi everybody, I am currently turning an old Airstream travel trailer/caravan into a food trailer/concession trailer/catering van. These old Airstreams have aluminium frames and aluminium outer skins.
I am wanting to fix a very shiny diamond shaped, quilted stainless steel sheet as the entire internal wall skin to the aluminium frame, which will give a fantastic reflective and shiny retro-diner look. However, I am worried about Galvanic corrosion.
Do I have anything to be worried about? What could happen?
Best regards -

Pete Vanda
Small business owner - Melbourne, Victoria, Australia
  ^- Privately contact this inquirer -^


October 3, 2013

Q. I didn't read a response to Mike Miller's question...

I'm very curious because we use stainless helicals in aluminum parts in just about everything we build. Also, a passivated SS part touching a Alodine aluminum part is not uncommon either. All of our units have to survive a salt spray test.

B. Barbour
- Maryland, USA
  ^- Privately contact this inquirer -^

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