Effective control of Galvanic Corrosion

[JohnShiell]

Hi,

I am looking for any advice for what must be a very common engineering problem.

I am up-grading a product for the Marine Market and unfortunately, the product is an Aluminium Housing bolted together with Stainless Steel Fasteners. The Housing cannot be anodised for EMC reasons.

I am concerned that the Fasteners will react with the Aluminium Housing causing excessive corrosion. I have thought about using Plastic Insulators under the heads of the Fasteners but this gives me concerns as some of the fasteners are load bearing.

My latest thought was to use some sort of sealant that covers the Fastener after assembly. This would keep out the Seawater and significantly reduce the likelyhood of corrosion taking place.

The client will not authorise the use any Cadmium or RTV / Silicon based products.

Has anyone succesfully overcome this problem with the restraints listed?

 

[rnd2]

Without knowing exactly what is required here are some options for you to consider.
Option 1) There are electrical insulation papers that are available as tubes and washers and are load carrying. Nomex, Pyrolam, Mylar are candidate materials and I'm sure there are others. The key to these materials is they will not extrude under load. Some of these materials have successfully insulated thousands of railway fishplates.
Option 2) If the item is going to be subject to full water imersion or regular splashing then I suggest that you fix a zinc anode to the aluminium.
Option 3) Paint the entire structure with a two part epoxy and top-coat with a two part polyurethane paint.
Option 4) a combination of the first three options.

 

[MikeHalloran]

Random thoughts:

Electrical engineers are quite fond of aramid papers. I have tested them; they don't decompose rapidly when wet, but they _do_ wick salt water very nicely. That's one material to cross off your list.

Mylar has been used, under large 'fender washers', to protect the paint under door striker bolts in auto bodies for decades. You need to check the bearing stress; it would probably extrude out from under a normal sized washer.

Just covering the exterior of the fasteners will not be enough to protect them. I have become quite fond of Loctite Pipe Sealant with Teflon, for excluding corrodents from the interior of fastened joints, and for its mild anti-seize and threadlocking behaviors.

A popular alternative to silicone RTV in marine environments is 3M 5200 sealant/adhesive, which I believe is urethane- based.

Cadmium is an environmental problem for the applicator, and was never all that good at corrosion resistance anyway.

The customer's reluctance to anodize the housing is no doubt because the insulating oxide surface would interfere with the continuous ground contact required for effective EMI shielding. Of course, seawater will quickly remove all of a bare aluminum housing, so that's not really an option. You might consider a nickel- plated steel EMI shield/liner inside a plastic housing.

Take a good look an an outboard motor, preferably partly disassembled. They seem to be universally painted, post- assembly, with at least two thick coatings, of materials that are differently colored, possibly different chemically, e.g. the epoxy/polyurethane already suggested.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA