Green Discoloration!
10-25-2008, 06:49 PM, (This post was last modified: 10-26-2008, 08:13 PM by steve.)
#1
Green Discoloration!
Hi All,
Id like to firstly commend you on a fantastic site, full of information and advice.
I have a recent problem with a green powder residue on nickel parts after APS spraying with TBC NiCrAlY bond coat.This "powder residue" seems to turn to a green coloured oil film when left for a few hrs. Chemical analysis suggests chromium oxides but Im not sure about the source, probably chromium in bond powder. It appears to cause delamination between bond and a ZrO2 top coat. Could this be gas related (H2 -Ar)Has anyone possibly seen this before?
Best Rgds,
Steve
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10-27-2008, 04:57 PM,
#2
RE: Green Discoloration!
Hi Steve

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A green residue will probably be chromium oxide an oxidation product from spraying that powder. From how you describe the problem, it does sound somewhat excessive. I would check on your spraying parameters to make sure your not over cooking it. Powder containing excessive amount of fines will also make this situation worse. Are you spraying onto a confined area poorly ventilated - just blowing the dust around rather than it being freely removed by extraction system? Have you tried using air jets to clean as well as cool the surface?
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10-30-2008, 11:51 PM, (This post was last modified: 10-30-2008, 11:56 PM by steve.)
#3
RE: Green Discoloration!
(10-27-2008, 04:57 PM)Gordon Wrote: Hi Steve

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A green residue will probably be chromium oxide an oxidation product from spraying that powder. From how you describe the problem, it does sound somewhat excessive. I would check on your spraying parameters to make sure your not over cooking it. Powder containing excessive amount of fines will also make this situation worse. Are you spraying onto a confined area poorly ventilated - just blowing the dust around rather than it being freely removed by extraction system? Have you tried using air jets to clean as well as cool the surface?

Thanks for the advice. I'm addressing housekeeping issues at the moment. Poor ventilation is probably a factor. Could you please explain "fines" and their origin -is it a powder quality issue?
Best rgds,
Steve...
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10-31-2008, 08:20 AM,
#4
RE: Green Discoloration!
Hi,
Similar observation was also made by us. The problem is because of two reasons:
1 While coating because of attrition of the powder particles (they become fines) and they absorbed more heat and will oxidise more rapidly.
2 Because of the attrition Fines so produced needs to be removed from the job.

How to solve it?
1 Preheat the job.
2 Select Corret particle size with narrow apread. Preferably gas atomised particles.
3 Best and Immediate way is to have parallel flow air jets. This will remove the fines and lower the heat input to the system.

Try this out and hope fully the problem shall be solved.

Regards
Girish
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11-01-2008, 05:51 PM,
#5
RE: Green Discoloration!
Hi Steve

(10-30-2008, 11:51 PM)steve Wrote: Thanks for the advice. I'm addressing housekeeping issues at the moment. Poor ventilation is probably a factor. Could you please explain "fines" and their origin -is it a powder quality issue?

Most powders will contain some degree of "fines" the very fine particulates that end up not being completely removed during grading. A powder may be described as being say -105 +45 microns typical size range, unfortunately, that does not ensure it will not contain a certain level of much finer particles. If it is a powder problem, then changing supplier/manufacturer or powder batch may help.

As Girish commented "fines" can also be generated within the process. The more you can do to avoid producing and then including this unwanted material on or in the coating the better.

Certainly improving your house keeping and ventilation will bring big benefits generally, even if it does not completely solve this specific problem, it will be worth doing anyway Big Grin
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11-02-2008, 10:37 PM,
#6
RE: Green Discoloration!
Thanks Gordan and Girish for your time and advice. I will look further into the powder size spread and improving the cooling jet position.
Regards,
Stephen...
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