how to stop galvanic corrosion- S.S. to alum.

rfuerst911sc · 08-04-2013, 01:35 PM

I'm restoring a 1968 truck camper that has an aluminum skin, I believe the alloy is 5005 anodized. I will replace quite a few rusted steel screws that are in contact with the aluminum and I want to use stainless steel. What is the best way to stop or retard galvanic corrosion between the two metals ? Some of these screws are going into wood and others are going into aluminum beams that make up the structure. I'm thinking maybe nylon washers under the screw heads ? Is there some " goop " to put on the threads before installing ?

Jim Bremner · 08-04-2013, 01:36 PM

grease

nota · 08-04-2013, 01:52 PM

why not use aluminum screws
sure they are not super strong
but how much load is on them

or rivets

GWN7 · 08-04-2013, 01:55 PM

If the screws are rusted they are not stainless. Test them with a magnet. Stainless is not magnetic. Stainless is one metal that doesn't react very much aluminum unless there are other chemicals present. Buy good quality stainless replacement screws and spray them with plastic coat or even varnish for a protection layer.

Zeke · 08-04-2013, 02:25 PM

Quote:

Originally Posted by GWN7

If the screws are rusted they are not stainless. Test them with a magnet. Stainless is not magnetic. Stainless is one metal that doesn't react very much aluminum unless there are other chemicals present. Buy good quality stainless replacement screws and spray them with plastic coat or even varnish for a protection layer.

He didn't say they were.

I have read over the years about SS. Apparently it is not all that strong when the alloy most rust proof is used. Therefore, not very good for fasteners. I don't know what the marine crowd does, but they wouldn't have SS and AL tied together that I know of.

Varnish won't last long in the elements. Richard lives where it rains a lot and gets hot. Tough environment for any coating. Would clear anodizing work better?

I think that's why a lot of AL skins are riveted with AL rivets. So, I vote for AL screws. Maybe SS if they aren't subject to a lot of stress.

Quote:

Originally Posted by Jim Bremner

grease

Dielectric?

74-911 · 08-04-2013, 02:46 PM

Quote:

Originally Posted by Zeke

I don't know what the marine crowd does, but they wouldn't have SS and AL tied together that I know of.

SS to AL fittings are actually very common in the sailing world.... AL masts with many SS bolts/ screws attaching things to it and galvanic corrosion is definitely a problem, particularly in salt water environments.

There are several ways to minimize the corrosion, i.e. Coat all the threads liberally with Teflon pipe dope is what I have done (several other things such as a lanolin based grease also helps).

Red88Carrera · 08-04-2013, 02:47 PM

On my 5th wheel, any fastener that comes in contact with aluminum, is aluminum. Although it has a Filon exterior, all aluminum window frames are held in with aluminum screws.

rfuerst911sc · 08-04-2013, 02:48 PM

I'm going to assume the OEM used steel screws for strength so that's why I want to replace with stainless. So is this really as simple as adding dielectric grease to each one before tightening ?

URY914 · URY914's Garage

There are different grades of SS hardware. Ace Hardware and Home Depot are the low end of the scale. Go to a real marine hardware dealer. You should have a West Marine somewhere up there.

mreid · 08-04-2013, 03:46 PM

Anti-seize:

red-beard · red-beard's Garage

Different forms of stainless react differently. We use 316 with 6061 Aluminum with a type of nylon washer.

Nostril Cheese · 08-04-2013, 07:15 PM

anti-seize paste or teflon.

Zeke · 08-04-2013, 07:18 PM

Quote:

Originally Posted by red-beard

Different forms of stainless react differently. We use 316 with 6061 Aluminum with a type of nylon washer.

I can never figure out what it is that you do for a living.

GWN7 · 08-04-2013, 07:23 PM

also look for supplies (screws & other fasteners) at a aircraft supply place.

sammyg2 · 08-04-2013, 07:57 PM

Anyone who says SS doesn't rust is not informed on the subject. There are many, many different stainless alloys. Most 400 series stainless alloys will rust.

Use a high quality never-sieze compound on the screws. Not copper coat.

SteamWolf · SteamWolf's Garage

316 stainless won't corrode. 304 will show signs of rusting. Both will react and cause the alloy to corrode.

It needs to be insulated, either with nylon sleeved washers, or by chemical means such as lanolin grease or non melting grease such as "Aqua-Lube" (The stuff the Europeans use in boat trailer wheel bearings, not the alternative to KY Jelly lol).

John Rogers · 08-04-2013, 08:20 PM

If you could get the skin completely off, you could use some of 3M's super double sided tape like the stuff that is used to hold ambulance side walls in place. They can't use screws because of the germ hiding places screws provide.

WolfeMacleod · 08-05-2013, 12:02 AM

Quote:

Originally Posted by GWN7

If the screws are rusted they are not stainless. Test them with a magnet. Stainless is not magnetic.

There are several grades of Stainless which are ferrous. Some austenitic grades, like 304, become magnetically attracted after cold working.
Several grades, like 440B, can be made permanently magnetic.

DanielDudley · 08-05-2013, 01:44 AM

I don't think you need to do anything except swap the screws out. How long did the plain steel screws last ?

Don't over think this.

red-beard · red-beard's Garage

Quote:

Originally Posted by Zeke

I can never figure out what it is that you do for a living.

My wife's side of the business is metals. We're working with a company in Mexico building heliports for off-shore platforms. They are made with 6061 aluminum. We also supply all build the lighting and solar energy systems for these heliports.

My side of the business is trying to take the solar side to other industrial applications.