Corrosion between different metals

Powder coating. Will slow the process.

 

There are special paints you can use as well. GOOGLE!

 

Aluminum and stainless will not corrode together. At least not in this lifetime.

 

Ahhh, dissimilar metals, I think over time they will still corrode. I believe anti-seize products are your friend here.

 

Amazingly on surface one would say dissimilar metals corrode period. But anyone who has worked on say any Ferrari with steel studs in an aluminum block and say an NSX with same steel in their aluminum block will notice an immediate difference. Aluminum is not aluminum. So if the chassis aluminum is as good as the block aluminum I would say don't worry. If the chassis is like an NSX aluminum worry! I always thought the challenge cage design lacking and it is especially poor where it connects to the chassis. So much more can and should be done especially when steel cages are only bolted to an aluminum chassis. I would never race a factory challenge cage.

 

You must be making funeral plans.

One of the most common sources of cursing and hand-wringing in the sailing world is when someone tries to remove something mounted to an aluminum mast; a winch or cleat for example. In nearly every case, the mounting hardware is SS and the mast is aluminum. The non patented process involved is something sailors call "time weld". After a few years the SS mounting bolt and aluminum mast become one and you can guess the rest of the story; broken bolts, drilling out the bolt, use of an Easy-Out, stripped threads, etc. etc. Galvanic corrosion between aluminum and SS is a VERY serious concern.

Of course we Ferrari types try to keep our cars from direct contact with salt water, but I would NOT want to have a non-insulated SS component against an aluminum part. The aluminum acts as an anode to the stainless steel. The Anodic Index shows that stainless steel and aluminum are just about as far apart ( this is bad ) as you can get for ordinary materials, unless you consider gold an ordinary structural metal (some Ferrari owners might).

 

Galvanic corrosion is essentially a battery, key thing required is an electrolyte, ie water. The presence of salt and acids accelerate the corrosion process.

Aluminium oxidises rapidly to alumina on the surface which protects the aluminium from further oxidation. Scratching the surface exposes aluminium which can then oxidise again. Alumina is very hard, while aluminium is soft, so where there is constant rubbing, the alumina wears the aluminium down.

Steel rusts rapidly and needs a protective coating of zinc, paint etc, but coating strength is the real issue. Any movement between a painted steel and aluminium joint in the presence of water will create a corrosion and wear issue. This can take a long time or a short time depending on the conditions.

I would put an insulating layer of rubber, Teflon etc between the two, make sure that the bolts are torqued properly to prevent movement of the joint and you should be ok.

 

Being an avid sailor and having owned several sailing craft over the years I can fully support the comments above about trying to remove fittings from a mast. Given that there are different environments and with a auto the situation is much different. I am also a fan of old British cars especially the Austin Healeys. They are made with body parts of both steel and aluminium. The upper shroud front and rear are aluminum and while the side fenders are steel separated by a nice SS bead that works especially well in isolating the parts and preventing corrosion. BB made a good point about the different forms of aluminium, there are differences there that can make a big difference. Just a thought.

 

When building the Maranello, didn't Ferrari have to use some type of spacer or material to separate the steel frame from the aluminum body to prevent galvanic corrosion? I recall reading that years ago when the Maranello first came out. Frank

 

just a tid bit on aluminum- the oxide formation protects cuz it is one of the most DENSE oxides of metals, preventing oxygen from penetrating and causing more oxidation. Thats why steel/iron rusts easily, cuz iron oxide is no way near as dense as aluminum oxide. Rust is so flaky/lose, as we have noticed perhaps. It rusts forever, deeply, until no more iron left to oxidise.

On the boat thing- gotta remember also contributing is the extremely salty environment. unlike ferarris parked in garage/driven away from salt water, etc. (yous dont park the ferrari on the beach, now, do you??)

 

I design commercial airplane repairs for a living. Corrosion is a fact of life. There is no avoiding it.

That being said, there is a lot you can do to slow it way, way down.

For airplane parts with dissimilar metals... ( and that is most of them with all the alloys that are out there... ) the standard is two coats of zinc chromate epoxy primer on both parts with a poly-sulfide sealant between both parts. The primer protects the parts from atmospheric contaminants and the sealant barrier prevents the electrolytic contact.

 

Good tip, NW. Now, if one wanted to get a hold of a consumer quantity of the above, where does one get them? NAPA paint store? other?

 

Results for zinc chromate - Search Result | SkyGeek.com

 

Try Dupont paint products. Their Nason line of paints offers an etch primer with the qualities stated earlier. Use a Dupont paint store in your area. Most body shop supply house offer this product.

http://pc.dupont.com/dpc/en/US/html/visitor/common/pdfs/b/product/nsn/Nason/491-17.pdf

 

thanks. good to know, people.

 

be sure to include the fasteners ( bolts, nuts, washers ) in the isolation process in addition to the parts being joined

also the same also applies when joining parts to carbon fiber,