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topic 705

Where can I find information on screw finishes?


(1996)

We join aluminum die castings with threaded fasteners. We try to use commonly available carbon steel screws. We have used zinc plated fasteners with great success, but the call outs all call for "...with yellow dichromate". How and/or where can I learn about screw finishes, dichromate, etc., so that I can make informed decisions in our search to reduce cost, insure effective corrosion protection, and save the environment?

John Scarborough
- Schlumberger


(1997)

John,
Carbon steel against aluminum will rust fairly fast.

Most if not all zinc plated screws are chromated after plating. Most use a clear chromate because it has more eye appeal. Yellow has significantly better salt spray corrosion resistance than clear. Olive drab is even better but is not generaly available as it is ugly.

Cadmium is less prone to cause corrosion of the aluminum than zinc is, and it can receive a similar chromate.

Since you are using carbon screws, I assume that stainless is not required.

I know Schlumberger as oil field related. You must be in another division and indoors to be able to use carbon screws with any success.

James Watts
- Navarre, Florida


(1997)

John,

I would like to suggest an interesting source of informations on screw finishes, the series of British standards BS 7371-1 [link by ed. to spec at TechStreet] to BS 7371-11 [link by ed. to spec at TechStreet], all standards dealing only with screw finishes.

You can get them to a reasonnable cost from the BSI in London UK and possibly at a reduced cost from the MFA (Metal Finishing Association) also in UK. I don't remember if the MFA offers this service to members only or to all. Address of the MFA is available through the homepage of www.finishing.com !

One word of introduction to these standards : BS 7371-1 [link by ed. to spec at TechStreet] is a general introduction to all screw finishes and to Torque/Tension. In the other parts you will find details about all coatings and finishes, with practical guidelines for the choice of the coating thickness and of the passivate in case of zinc electroplating.

Regarding contact with Aluminium, pure zinc will consume itself rapidly, I would suggest a coating made of zinc and aluminium flakes in a chrome binding matrix, providing controlled cathodic protection : see BS 7371-11 [link by ed. to spec at TechStreet].

Regards

Frederic Gheno
France



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