Yellow spots on alumina coating

[powerman]

hi, after plasma spray alumina on 316L plate (the plate undergo milling/machining process before), some small yellow spots appear on the alumina coatings. Is it because base plate not clean properly before blast? Thanks.

[Gordon]

Hi powerman

Do the yellow spots appear straight after spraying or develop over time?

Could be a number of contamination sources; substrate, powder,powder feeder, air jets, surrounding environment.

On 316 stainless steel I would recommend a NiCr anti-corrosion type bond/buffer layer below alumina coating if service conditions are going to involve moisture or corrosive environment. Stainless steels starved of oxygen can be prone to crevice corrosion (they can't produce/repair the normally protective oxide film), alumina coating effectively provides this environment.

[powerman]

the yellow spot develop over time. As customer required alumina coating only, so we did not spray bond coat. Somemore NiCr very expensive now. :>

[Gordon]

Hi Powerman

It could be a simple staining problem from external environment which should only effect aesthetics (appearance) or far more worrying, staining produced from substrate corrosion. If its corrosion, then staining will probably be followed by blistering of the coating. . Was the coating sealed?

[powerman]

The coating was not sealed properly. However, we will rework to see if this happens again.

[DMan]

Dear Members

Any update on this yellow spots issue?

I used SFP 105 pure alumina over Bond coat. I applied a sealant too.
The coating was really smooth and good.It appeared complete white with no spots.
But During grinding after a few passes, yellow spots started appearing.

This is the first time for me with pure alumina.

Can someone have more inputs and if anyone has already a solution to the issue
Regards
DMan

[Lemster68]

I don't have a certain answer for you, but questions about your grinding process. Maybe your wheel is too soft, smearing/mixing in with the surface. What media is your grinding wheel composed of? Coolant?

[DMan]

Hello
Trying to find out where I went wrong.
In the mean time, can you let me know if there's any cleaning agent which can remove the yellow stains...or any way I can superficially hide the same on the white alumina..
Or reprocessing is the only way ?
Warm regards
DMan

[Lemster68]

Try to grind first, then apply sealant. Maybe grinding is causing the sealant to discolor? Are you using a diamond wheel to grind?

[Lemster68]

Try to grind first, then apply sealant. Maybe grinding is causing the sealant to discolor? Are you using a diamond wheel to grind?

[DMan]

Yes. I am using a diamond wheel to grind.
Sealant is Diamant dichtol 1532 which I applied after coating .
I tried removing the spots by grinding. The more I grind..the more they seem to open up from below.