Thermal Spray Rework Limits

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“To err is human and to forgive divine” may be a well known saying, however, this does not sit very well with thermal spray quality control personnel. Not that they do not believe in the age old adage, but that thermal spray quality control personnel want everyone to strive for perfection. They cannot stand thermal spray operators putting on the wrong thickness of coatings or using the incorrect thermal spray powder, coating on the wrong area, incorrect thermal spray masking and so on. As stringent as the thermal spray quality control department may be, there may be a few instances where an error has occurred and the thermal spray coated part needs to be stripped and recoated. There have been some cases where the error has occurred on the same part more than once either for the same or different reason, sending it to the strip and recoat journey all over again. While this does cost quite a bit of money and for that matter, any re-work costs money, there are some limitations and some things to watch out for prior to simply taking the strip and recoat route ESPECIALLY when chemical stripping is used.

It is essential that upon chemical stripping, a laboratory sample travel along side the part that is made of the same base material and has been subjected to the same chemical stripping conditions as the actual hardware being stripped. This includes the same chemical solution, time and temperature in the tank and electrical parameters if electrolytic stripping is utilized. Upon successful stripping, this sample must be evaluated for grain boundary attack and if there is any evidence of grain boundary attack or alpha case formation in titanium alloys, then the hardware is suspect and the customer must be notified for disposition prior to thermal spray. This is particularly important for aerospace hardware. Thus, the strip and recoat option is not without risks and is not a safety net for all thermal spray errors and some alloys may end up with significant metallurgical deterioration depending upon the chemical stripping technique used. Remember nature may not bail out your thermal spray errors like AIG was bailed out