White Powder Corrosion on Bronze Casts

[cwheeler]

I am working with Bronze casts purchased from Asia and have been receiving them with a white powder material on them. Our vendor states that this is due to the high humidity present during their monsoon season. A chemical analysis revealed it to be a form of lead oxide. Has anyone had any experience with this?

 

[Metaljon]

Go to an area on the casting and scrape the surface to see whether the surface is coated with a resin or paint and the colouration of the underlying material. It is very commom practice with cheap "bronze" castings to be manfactured from aluminium. The aluminium is then painted with a paint or resin to give the colouration of bronze. The white residue is probably aluminium corrosion product aluminium hydroxide. The lead you have detected maybe from the paint.

 

[cwheeler]

Interesting. I was thinking along those lines as well, but would have thought aluminium would have shown up on the SEM/EDS or the X-Ray Diffraction tests.

Since the corrosion is not fully spread over the body, it sounds like I need to do an anlysis of the 'bronze' as well to ensure no aluminium.

 

[btrueblood]

Huh? Wouldn't you be able to tell from the weight difference alone? Who'd think aluminum could pass for bronze?

Lead is a "normal" constituent of bronzes that are intended to undergo any machining, and yes, a high humidity with condensation and/or rain soaking the parts will preferentially oxidize the lead (then any zinc) and leave the residue behind once it dries. We've never worried about it (other than to warn assemblers and other users that the powder is toxic and to wash their hands before going to lunch/smoke breaks/etc.)

 

[cwheeler]

Btrueblood - Thanks for your reply. The weight difference may tell us something, but only if a large amount of aluminum was substituted.

Regarding your note about oxidizing the lead, wouldn't this lead to a different physical manifestation than white powder? I would think this process would lead to a green or redish color on the corrosion.

Another detail that I didn't meantion in my original post is the fact that we have seen several parts where the corrosion pattern has two odd features. In several cases, the patters shows up as drip, much like you would see if you dropped paint on top of a bell and it would run down the sides. In some other cases, we've seen defined lines in the pattern where the corrosion ends in a sharp edge. These two patters occur about 25% of the time. The remainder is what you would expect from 'normal' corrosion.

 

[btrueblood]

Small amounts of Al in bronzes give it lots of strength and wear resistance (Al/Ni bronze e.g.). Generally, the aluminum will form a very tightly adhered oxide layer on the surface of the bronze, lending some passivity. You'd need an awful lot of Al for it to form a flaky white layer.

Lead oxide is white (thus its use in "lead paint"). Lead in cast bronze doesn't alloy with the parent copper, but exists as a seperate phase (this helps form chips during machining, also the lead lubricates the cutting tool). Because it isn't alloyed, the lead near the surface of a cast part oxidizes out pretty quickly, and can coat the surface of a wetted then dried part.

Zinc oxide is white too (think diaper rash ointment, or at the beach on a lifeguard's nose). Zinc, in the presence of chlorinated water or hot water, will leach out of some brasses and oxidize if no tin or arsenic is present. You said you have a bronze, which typically includes tin, so this is probably not the problem.

The green color on oxidized bronze/brass/copper comes from copper corrosion salts (chlorides, sulfates, etc., which can happen when the corrosion occurs in a slightly acid environment with those contaminants present). Red copper oxide is a pretty tightly adhered layer on the copper.

 

[Metaljon]

You need to test the composition of the substrate. If the corrosion is white then it is unlikely to be bronze. Normal corrosion of bronze generates brown, green or sometime black patina. A white residue is abnormal. The EDS shows lead which suggests some abnormal lead content in the substrate or coating. The description of the corrosion process is very odd and suggests some coating has been applied to give it the appearance of bronze.

 

[btrueblood]

Why would lead content be abnormal in bronze? Leaded bronzes are as common as dirt, for reasons posted earlier.

Any coating would be easy to find -- smack one with a hammer, or cut it with a hacksaw, and crack it open.

Aluminum is not misicble with copper beyond a few percent, so either you have Aluminum Bronze (with a few percent Al), or Aluminum alloy in a 3000-series (with a few percent Cu). Either way, the issue is not that the white powder is Aluminum oxide, but (apparently from your posting) lead oxide, so quit worrying about aluminum unless the weight is a factor of 3 too light.

Back to basics: cwheeler, what specific alloy are your castings supposed to be? You should have a CDA number for the alloy composition. What is the result of chemical analysis for the alloy in the castings you received, and how does the chemistry deviate from what you requested?