galvanic corrosion as a protection against un-screwing fastener

[cervantes]

Hi all,

I am using black oxide steel inside aluminium part, because after galvanic corrosion between threads appeared in high humidity area, galvanic corrosion will additionaly prevent against unscrewing (instead of using Loctite on threads).

Based on my experiences it works fine.

I need to change black oxide steel to other type, because rusty head is not acceptable as a visible part.

Will there be reaction between e.g. zinc screw and aluminium part on threads? Or in this case should I start to use a glue?

thanks

 

[TVP]

Yes, there is a galvanic potential between the steel screw and the aluminum parts. However, there are literally billions of joints in the world that use a steel screw into an aluminum part. Black oxide has almost no corrosion resistance, but there are a number of options to consider, based on the exact details of your application. Here are some ideas:

1. Zinc electroplated screw and washer. Zinc plating will be ~ 8-12 micrometers thick and should have a chromate conversion coating. This provides modest corrosion resistance for the steel, but not much galvanic corrosion resistance. This is certainly much better than black oxide. Zinc-iron or zinc-nickel electroplating have improved corrosion resistance and are frequently black after application of the chromate conversion coating. Addition of a sealer or topcoat will further improve the performance, especially galvanic corrosion.

2. Zinc flake coating for screw and washer. The topcoat can even be black if you want to maintain a black color. There are a number of products available from Magni, Doerken, NOF Metal Coatings, and Atotech.

3. Phosphate and oil coatings have less corrosion resistance than zinc plating, but are less expensive. The oil is dry to the touch, so it is not "wet". The appearance is dark gray. Not good for preventing galvanic corrosion, but somewhat better than black oxide.

 

[EdStainless]

In most cases where people are 'using' the galvanic corrosion to lock fasteners in place they are willing to tolerate rust stains.
If you go to a better coating (zinc or a resin) then you should use a thread lock adhesive.

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Plymouth Tube

 

[cervantes]

TVP:
Thanks for tips. But for all these 3 I should use glue to lock screw because corrosion will be reduced?

 

[moltenmetal]

I'd vote for the zinc coating and the use of Loctite or the like. This will give more repeatable and reliable locking while also reducing the corrosion. Murphy's Law says that when you're counting on rust, it won't rust- or won't rust the way you'd like.