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Gun Blue Turns to Rust

2005

We have been powder coating engine frames. These space frames are a tubular 4130 weldment. In operation, the powder coat becomes pulverized and then falls away -leaving gaps between the mating points of the frame. Over time, the constant rattling works the fasteners loose. So... as a stop-gap measure I've masked the mating faces from powder coat and used gun-blue on them to provide some corrosion inhibiting capability. This is a pretty unsatisfactory fix since the solution seems to accelerate the rusting of the parts and must be wetted with finishing oil to preserve the rust resistance.

The gun-blue MSDS says that the solution is 5% phosphoric acid (by weight) and some trace Cupric sulphate, Nickel sulphate and selenious acid. The solution is mostly water.

So my questions are:
1. why does the metal rust so quickly if it's not oiled?
2. am I better off cad-plating or zinc-plating w/yellow chromate prior to powder coat?

Thanks very much for this great website and your responses.

Luke Preisner
Aerospace Engineer masquerading as a Mechanical

Luke Preisner
Engineer in Training - San Diego, California

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Thanks for the kind words about the site. Bare steel rusts very quickly. Gun blue, aka cold black oxiding, probably doesn't actually accelerate the rusting compared to bare steel, but probably does almost nothing to retard it. The only corrosion resistance from cold bluing is due to the oil, as you note.

Yes, plating would be far better. Don't use cadmium; it's a bioaccumulative poison that should be used only when its combination of properties is indispensable (and maybe not even then). Zinc plating with chromate would be better (although it can affect powder coating adhesion if not done right).

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005