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topic 27167

Galvanic corrosion issues: aluminum and stainless steel

A discussion started in 2004 but continuing through 2018


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 [last name deleted for privacy by Editor]
Operations Manager - Douglasville, Georgia


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

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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 Stransky
- Putnam, Connecticut


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


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 [last name deleted for privacy by Editor]
- Jacksonville, Florida


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 [last name deleted for privacy by Editor]
high performance architectural metals - Cranberry Twp., Pennsylvania


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 [last name deleted for privacy by Editor]
connector development - Wenatchee, Washington


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?

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


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


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.

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


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.

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

3M Marine Sealant

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.

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

March 1, 2009

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



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.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, 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, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, 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.
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



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.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, 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.

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, 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.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, 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

August 2, 2013

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

James Watts
- Navarre, Florida

August 7, 2013

thumbs up signThanks 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

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

Are stainless cables causing aluminum support posts to corrode?

July 24, 2014

Q. We are running into issues with stainless steel cable railing systems in close proximity to salt water environments. We use 1/8" stainless steel cables. The support posts are aluminum. We have the support post drilled out and then powder coated. Each hole has a plastic grommet to prevent the stainless steel cable from touching the aluminum posts.


Would using a Zinc Oxide primer help? How else can we prevent this from happening? Or should we not sell these types of systems and direct clients towards a glass system?


Hugh Hammond
- Bothell, Washington USA

July 2014

A. Hi Hugh. If there is no electrically-conductive metal-to-metal connection from the stainless cables to the support posts, it can't be galvanic corrosion; a saltwater connection doesn't count, as you need both the metallic path and the ionic path. And although I am not a corrosion expert, it doesn't look like galvanic corrosion to me either.

I think the aluminum is not properly treated. Try a sample with chromate conversion coating, followed by an e-coat primer, followed by the powder coating and I think you'll quickly prove it via no corrosion for a very long time.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

January 16, 2015

A. Hugh,

Perhaps check on a dry day for any conductivity between the aluminum and cables with an ohm-meter? To see if there really is no connection.

The corrosion does seem to be near the holes. If not galvanic (i wouldn't know), makes me wonder if (1) the aluminum was treated before the hole was drilled, and thus corrosion radiates out from there or (2) cables moving (wind) are abrading coating over time the at the drill holes (remove a grommet and inspect?) or (3) is the aluminum hollow and if so what's the interior protection? Cut a piece in half and see if the interior is extremely corroded and then it migrates from inside to out near drill hole?

Also what Ted says sounds right.

Dwayne Smith
- New York, New York USA

March 21, 2015

Q. My question is slightly different. I have 304 stainless 1/2" tension cable with 316 stainless fittings on wooden posts about 100 feet from the ocean. I noticed brown staining on the cable. Should I attach a piece of zinc or aluminium or steel to the assembly to prevent this?
Thank you

Tom Kornylak
- fort pierce, florida, usa

A. Hi Tom. A sacrificial anode is unlikely to do any good. But search this site for "passivation of stainless steel with citric acid". That should do it at least temporarily, but 304 may not be good enough to remain free of staining. Personally, I'd try the citric acid, then leave it. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

April 21, 2015

Q. When a steel alloy bonds with an aluminium alloy through a galvanic reaction, what is the material bonding the surfaces?
I have seen the aluminium seat posts bond into the steel seat tube of bicycles in northern climates where salt is used to melt road ice/snow.
Can low voltage currents reverse the effect? I have also considered the use of aqueous sodium hydroxide (lye), your thoughts on this problem.

Sean Elwood
- Toronto, Ontario, Canada

May 2015

A. Hi Sean. I think the problem is simply that aluminum has voluminous corrosion products which swell the dimensions; this same freezing together happens on aluminum to aluminum joints, brass to aluminum joints, copper plating to aluminum joints, etc. Aluminum just isn't a great material for that kind of joint. Paint, joint compounds, WD40 or almost anything will somewhat improve the situation by lessening the corrosion. Chromate conversion coating of the aluminum should help.

Once they're locked together, I doubt that electrolytic action or lye can help much.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

May 12, 2015

A. I have been in the aluminum civil and marine industry for over 25 years.
Per codes always isolate aluminum from other metals or porous material (for example concrete, wood). UHMW was found the best and cheapest way.

Never use 6000 series under water.

You can use SST 304, 316 or HDG bolts for bolted joints. As mentioned above, there is a direct relation between volumes of dissimilar metals. I used HDG and SST bolts for over 100 pedestrian bridges and its working quite well.

Alway isolate SST fasteners under water or in splash zones.

More recently I am using Xylan 1424 coating (Teflon) on SST fasteners when required.

Alex de la Chevrotiere, IWE, P.E.
- Montreal, Quebec CANADA

July 28, 2015

Q. Could anyone please tell me what would be better for holding a PVC yard sign to an aluminum U channel stake, aluminum or stainless steel pop rivets, near the ocean?

David Graham
- Florence, Oregon, USA

A. Hi David. I'd bet on the stainless rivets.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

October 27, 2016

Q. Would a chrome steel ball bearing press-fit onto an aluminum rod present a galvanic problem? Neither material would be exposed to lubricants, and both would be in a "clean room."

N Kope
- Newark, Delaware, USA

November 4, 2016

Q. What will be problems occur when stainless steel is coated with inorganic zinc silicate primer?

eydin mohammed
Aramco - Saudi arabia

November 2016

A. Hi Eydin. I haven't heard of it, but I don't think it would cause any problems.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

November 16, 2016

A. If you can get adequate adhesion inorganic zinc coatings will give excellent performance.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
plating systems & technologies banner ad

November 14, 2017

Q. My company is working on an outdoor application to be installed in a rural environment (faraway form the seaside). It consists of a large stainless steel (AISI 304) trapezoidal screw that goes inserted in a diecast aluminium (A380) fixture. One part is threaded to the other (and remains fixed thru the life of the application).

We are concerned about the effects of galvanic corrosion on this to parts, specially because the aluminium part is relatively smaller than the stainless screw.
Aesthetics is not an issue, but cost and ensuring structural resistance of the parts are.

As said, the part application will be installed outdoors, unprotected from rain. The desired life is 25 years.

We do not have a lot of experience with aluminium finishes, but making some online research we have found that anodizing could help reduce/avoid the galvanic problem. But have also read that anodizing is not easy with die cast aluminum.

The finishing we are thinking to use is: Mil-A-8625, Type II, (0.8 mil min), Class I. Sealed.

Do you think this finish will work? Do you know any better approach to solve this problem?


Raul Obis
product designer - Reus, Tarragona, Spain

November 2017

A. Hi Raul. I think this is a poor idea for a few reasons. Not only can it be difficult to anodize aluminum die castings, but it can be very hard to anodize threads -- some people feel it should not even be attempted. But if the thread area is well sealed with some kind of thread sealant, so there is no non-anodized aluminum exposed to moisture or the atmosphere, it might be okay.

A photo of the part might be helpful because it's difficult to suggest what should be done instead when we don't understand the implications of any such suggestions. And if the owner removes the screw and reinserts it without careful resealing, or the sealant deteriorates from the sun over the years, it sounds like you would get severe galvanic corrosion in the thread area.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

November 15, 2017

Q. Thank you for your answer Ted.
As I understand, you are suggesting to anodize the full part (without paying special attention to thread anodizing) and protect isolate the thread with thread sealant.

I include some pictures of our 3D model and of some prototypes we manufactured, so readers can get a better understanding of our application.

27167-4a   27167-4b  

Our manufacturer suggest to use a Chromate passivation Cr3 (Clear) as a finish. Do you think that may work properly if combined with the thread sealant?

Raul Obis
- Reus, Tarragona, Spain

November 2017

A. Hi again Raul. Thanks for the pictures.

If you are still talking about anodizing the outside of the aluminum item, but also adding chromating of the thread area, I imagine that chromates could be of at least some small value. But if you are now talking about chromating the exterior in place of anodizing it, I personally do not think that will be a satisfactory answer.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Galvanic corrosion potential with aluminum tank, stainless nipple and brass fill assembly

January 2, 2018 December 19, 2017

Q. I had a custom fiberglass boat built with an aluminum gas tank. The builder installed a 6" stainless steel nipple into the aluminum tank and a brass filler cap assembly atop the stainless nipple. The boat is used primarily in a salt water environment. After reading your forum, I am concerned! Your thoughts?

Mike Wilson
New boat owner/Inshore fishing guide - Silverhill, Alabama USA

January 2018

A. Hi Mike. I don't know exactly what "installed" means in terms of direct metal to metal connections. I've seen numerous anodized aluminum railings installed on hotel and condominium balconies with stainless steel screws, and a lot of aluminum fins directly on copper heating coils without memorable issues, but there's also a lot of difference between the conductivity of rainwater and saltwater. A direct metal to metal contact in a saltwater environment does sound to me like it could lead to pitting and corrosion issues. But again, if the metals aren't actually contacting each other, you cannot have galvanic corrosion.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

January 3, 2018

thumbs up sign Thank you for your response. The "installed" gas tank assembly is direct metal to metal connections. The brass fill cap and flange is threaded to a 6" long x 2" diameter male stainless steel nipple which is threaded into the aluminum gas tank. Although somewhat protected in a fiberglass enclosure, with only the brass fill cap and flange directly exposed to the elements, it is used exclusively in a saltwater marine environment and the enclosure is vented. Thank you again.

Mike Wilson [returning]
- Silverhill, Alabama USA

Magnesium Anode Rod

February 14, 2018

Q. I have a "salt water" pool (1/12th ocean salinity).

My second ($1000) pump is about to fail due to what I think is galvanic corrosion.
I'm wondering if there is any type of "sacrificial anode" I can attach that would prevent the issue?

The pump unit is fiber reinforced plastic.
The electric motor has a bell shaped aluminum casting that is bolted to the plastic pump with stainless steel (through) bolts and nuts. I don't think the bolts directly touch the pool water, but undoubtedly get some stray water on them.

The corrosion is such that the aluminum flange the bolt goes through entirely gets eaten away. It starts under the paint (no apparent primer or anodizing under paint). The paint flakes off and reveals just white dust left behind where the aluminum used to be...

Is there a sacrificial anode I could use?

Are there "sleeved washers" I could get to put under the bolt heads?

Rick Thorne
- San Diego, California, USA

February 2018

Hi Rick. Due to the low salt content, zinc sacrificial anodes might not work, but magnesium ones ought to. I think they're available from boat stores, or you could get one from a water heater. But, unless tightly connected to the aluminum, and wet, it won't work.

Yes, you can insulate thru-bolts. It requires a micarta or fiberglass washer under the bolt head or nut, between the stainless and aluminum, and a micarta or fiberglass tube which just fits inside the washers and is just a hair shorter than the tightened joint. It's a pretty common arrangement. Aluminum hardware, if you can find it, or nylon hardware if it's strong enough is another solution. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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