Chrome Oxide Coating: Free Chrome

[rayudugirish]

Dear Gordon,
We have sprayed Chrome oxide using plasma spray system. In metallographic inspection a shiny phase is observed. Well these shiny bright phases are observed in many of the chrome oxide coating micro graphs generated by other people .

Can you please let me know how these shiny bright phase can be minimized and from where do they come. We are using 99.95% pure Chrome oxide of H C Starck.

Regards
Girish

[SimonH]

Good morning Girish,

I would suggest checking with HC Starck as to whether the Chrome Oxide that they are supplying you with is their acid-washed material or from their re-oxidising manufacturing process.

[Gordon]

Hi Girish

The metallic phases seen in chromium oxide coatings comes mainly from two sources - free chromium metal in the chromium oxide powder and copper/tungsten from plasma gun hardware. The other possible source is from entrained dust from a booth that has seen spraying of other metallic coatings and has not been cleaned sufficiently or from powder feeders which have not been clean out properly from previous use.

For most applications very small amounts of metallic inclusions will not have any detrimental effect. Laser engraved anilox rolls on the other hand can be very sensitive to this issue.

Many years ago I did some work (from the plasma spraying side) on free chromium (copper/tungsten/other metallic contamination is another matter) that was tuning up in our coatings. I came to the conclusion that little could be done with spraying parameters to improve matters very much, even injecting oxygen into the plasma only had a minor effect. Free chromium appeared to be tightly wrapped up in the chromium oxide powder particles, even acid washing of the powder only led to a minor reduction. Now I think you can get powders that are pretty much completely free of metallic contamination, though you may need to pay a little extra.

If your application really needs extremely low free metal content chromium oxide coatings, then request a high purity low free metal powder. H.C. Stark I'm sure will be very aware of this problem