Verdegris and rust

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Quetzalsailor
Master of the Arcane
Posts: 1100
Joined: Fri Aug 26, 2005 9:53 am
Boat Name: Quetzal
Boat Type: LeComte North East 38
Location: Philadelphia, PA

Verdegris and rust

Post by Quetzalsailor »

A protective oxide is one which does not fall off the base metal to reveal more base metal.

I don't think either the verdegris or rust coating is protective of the cuprous or ferrous metal beneath. Both are oxides of the base metal. Happily, the cuprous metals normally corrode very slowly. Unhappily, the rust does not stop except on very peculiar steel alloys like Corten. Corten is familiar as the brown bare steel seen on bridges, guard rails and some few buildings. But, even the Corten needs lots of air and water to form its 'rust' coating; that's why you now see Corten components with their connective ends galvanized or painted (the laps at joints don't get enough water and air to form the coating).

Stainess steels and aluminum alloys form an oxide coating which is protective so long as metals distant from them in nobility are not touching them, and that the oxide is allowed to form. That's the problem with 'crevice corrosion' in stainless that's not quite well enough sealed from the elements.

Stainless components which rust are indicative of cheaper alloys and I think, long term, are problematical for marine use. That's why we're told to replace lifelines that exhibit rust. Rust on stainless fittings need to be monitored or replaced. I've noticed that often the rust on stainless fittings is associated with welding; either the weld metal is not as good an alloy or the heating has caused/allowed metals in the alloy to migrate, allowing some of those metals to rust.

I've never understood what the passivating goos do for some stainlesses, but they're supposed to work. Wichard promotes their goo.
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