Black spots in galvanized metal

[w9ran]

I'm looking to identify the black spots that are created when galvanized sheet metal (HVAC duct material) is heated to 600-800 degrees C (medium cherry). As the metal is heated sparks fly off the surface and white powderey residue remains, which I assume is zinc oxide as it turns yellow when hot and white after cooling. But the material of interest is the small black spots that form in a random way on the surface. These black areas exhibit the unusual electrical characteristic of negative resistance when placed in a proper circuit, but the composition of this material is unknown, and identification is the first step toward being able to produce it more consistently. The attached image shows several examples that I hope are helpful in identifying the black material. Thanks for any assistance.

 

[desertfox]

Hi w9ran

Well I am no expert but the black spots just look like corrosion on the base metal surface, iron oxide? maybe.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[mfgenggear]

W9RAN
why are you baking at 600-899 c when the melting point of zinc alloy is at 420 c, what is left cooked zinc alloy.
by the way investigate about the toxicity of zinc alloy.

 

[weldstan]

Why are you destroying the galvanize by heating in that range? Guessing iron oxide for the black spots. Is this soe sort of experiment?

 

[w9ran]

Thanks for the replies. I am aware of zinc toxicity and only do this outdoors with proper PPE. This is indeed a 'home science experiment' attempting to replicate some work done in the early 20th century by Russian scientist Oleg Vladimirovich Losev who discovered some principles of solid state physics that would be overlooked for decades. He worked with a material known as zincite, Zn0 in crystal form, and it was later discovered that heating zinc in the form of galvanized metal could produce these areas which similar semiconducting properties, but not very consistently, and of unknown composition. They may be something other than ZnO, which is white in color (yellow when heated). I plan to put the heated metal aside and look at zincite, galena (lead sulfide), and iron pyrite next.

 

[WKTaylor]

Unbelievable lack of materials and finishes knowledge.

The temperature range 600-to-800C [in air or similar] is entirely unsuited for any carbon or alloy steel... and the galvanized coatings will do exactly as described... and spot rust will 'pop-up' in bare areas.

This temperature range is best suited for BARE A286... or maybe... SStl 321A... or nickel alloys 600 or 625.

Note1. IVD aluminum coatings over the SStl may help the duct survive in a externally corrosive environment.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov