Corrosion between dissimilar metals

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tornitore45
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Corrosion between dissimilar metals

Post by tornitore45 » Fri Dec 01, 2017 3:37 pm

Is there a table listing compatibility of fastener and base metal?
Mauro Gaetano
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Atkinson_Railroad
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Re: Corrosion between dissimilar metals

Post by Atkinson_Railroad » Fri Dec 01, 2017 4:09 pm

Dissimilar metal mating issues is a very useful topic for discussion.

John

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10KPete
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Re: Corrosion between dissimilar metals

Post by 10KPete » Fri Dec 01, 2017 6:11 pm

Google up "galvanic table".

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Bentworker
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Re: Corrosion between dissimilar metals

Post by Bentworker » Fri Dec 01, 2017 6:21 pm

The table explains why I had to cut aluminum seatposts out of titanium bicycle frames more than once.
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tornitore45
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Re: Corrosion between dissimilar metals

Post by tornitore45 » Sat Dec 02, 2017 3:27 pm

Contrary to expectation, according to the various tables, use of stainless steel fasteners on steel construction is not a good idea.
I am adding some steel brackets and such to the frame of my RV trailer. Have to choose if I rather have the steel screws rust or the SS screws corrode the frame?
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rudd
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Re: Corrosion between dissimilar metals

Post by rudd » Sat Dec 02, 2017 4:12 pm

I work in building design. Lots of people seem to think you can join any two metals as long as you use SS fasteners. I usually ask them to perform an experiment for me and get back to me with the results.
Take a good sized bar of SS, open the hood of your car, and lay it across the battery terminals. Let me know what happens. Joining dissimilar metals in a corrosive environment is a difference of degree, not a difference of kind.

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Re: Corrosion between dissimilar metals

Post by John Hasler » Sat Dec 02, 2017 10:53 pm

tornitore45 wrote:Contrary to expectation, according to the various tables, use of stainless steel fasteners on steel construction is not a good idea.
I am adding some steel brackets and such to the frame of my RV trailer. Have to choose if I rather have the steel screws rust or the SS screws corrode the frame?
It's a bit more complicated than that. As the Wikipedia article notes, it depends somewhat on the nature of the electrolyte. Also, when the area of the less noble metal exposed to the electrolyte is large compared to the exposed area of the other metal corrosion is often negligible. A major advantage of SS fasteners is that in most environments the passivation layer isolates it from the electrolyte. My aluminum stock trailers use SS fasteners.

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WesHowe
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Re: Corrosion between dissimilar metals

Post by WesHowe » Sun Dec 03, 2017 10:24 am

rudd wrote:Take a good sized bar of SS, open the hood of your car, and lay it across the battery terminals.
Somehow, I don't think that expresses what you were trying to say. Any bar of metal laid across the two terminals of a charged car battery will generate a current of 700 or so amps for a brief period. Bad effects will happen, and not from corrosion.

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ChipMaker4130
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Re: Corrosion between dissimilar metals

Post by ChipMaker4130 » Sun Dec 03, 2017 10:33 am

I just use plated fasteners and anti-seize compound. Lasts for many years in a normal atmosphere. In the days of cadmium plating and lead-base anti-seize, it would last even longer.

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Re: Corrosion between dissimilar metals

Post by wrenchalot » Sun Dec 03, 2017 10:48 am

Years ago, I built something out of 2024-T3 aluminum and 6061-T6 ; they were in contact with each other; 20 years later, I took them apart for cleaning out, just to find lots of "exfoliation" on one of them; although they were both al., the alloys were different.
Given a choice, I would use SS screws and lots of grease upon installation, and have a look at them in one or two years; also, I would grease the steel brakets upon inst..
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Rick
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Re: Corrosion between dissimilar metals

Post by Rick » Mon Dec 04, 2017 9:43 am

Also take too consideration more than just the two metals
I use to help my Uncle at the sailing center he managed on the Chesapeake bay. He had trouble with people that rented the sailboats messing with the turnbuckles on the main stays (these were small two person dagger board boats). So the solution was to use Loctite on the turnbuckles so they couldn't be turned by hand. Turns out if you use Loctite on an all stainless turnbuckle and expose it to salt water the three parts of the turnbuckle become one. No amount of heat would help (including just about melting them) in getting them loose. So all new turnbuckles with lock nuts this time. Something in the Locktite /seawater combo was the best we could figure.
Rick

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10KPete
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Re: Corrosion between dissimilar metals

Post by 10KPete » Mon Dec 04, 2017 4:56 pm

Stainless steel must have exposure to oxygen (air) to maintain the chromium oxide layer which is what resists corrosion and keeps the surface bright. If the surface is sealed from the air the oxide layer goes away and the material is easily corroded. If there is any small crevice for moisture to get in, crevice corrosion will happen with the resulting holes in the material looking like termites have been at work.

The Loctite had crevices (cracks) from the expansion and contraction of the sstl and with salt (chloride) exposure.... game over!

See it all the time in the boat yard here.

Pete
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