Stripping of Phosphate Coatings

[swall]

Anyone know of a method for stripping phosphate coatings (on steel) that does not use chromic acid as the stripping agent?

 

[CoryPad]

ASM Handbook has an electrolytic acid method:

1) Water rinse at 80 °C
2) Immerse for up to 2 min in 55 to 70% sulfuric acid at 20 °C, using a current density of 1000 A/m²
3) Flowing water rinse for 15 to 30 s at room temperature
4) Repeat step 3 in a second tank
5) Dip in 20% hydrochloric acid for 15 s at room temperature
6) Flowing water rinse for 15 to 30 s at room temperature

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TVP]

Steve,

Phosphate coatings can be removed with both acids and bases. Do you need a method for evaluating phosphate coating weight or do you just want to remove the coating? If it is the latter, standard products like Chemetall's Gardoclean® S 5171 are used for removing phosphate coatings from fasteners and formed parts prior to heat treating.

 

[swall]

Yes, looking for a method for phosphate coating weight.

 

[swall]

And now for my real question--we got dinged on our most recent TS audit for not having a way to tie our titration results on our phosphate baths to the actual performance of the coating system (ZnPhos+epoxy powder coat). I am leaning towards using an adhesion test, but would consider phosphate coating weight if the stripping process does not involve overly hazardous chemicals.

 

[TVP]

Interesting problem. Personally I would rather use coating weight and some type of imaging (optical microscope, electron microscope, etc.) in order to know what the coating "looks" like rather than rely on another subjective test. Did you already contact PPG/Chemetall/insert name of swall's favorite phosphate vendor regarding their stripping method?

 

[swall]

I posed the dilemma to our chemical supplier and he suggested coating weight, but we haven't talked about specific methods. I think I could live with the electrolytic sulfuric acid option, as the people on the floor are competent in handling acids and bases. At this point, I am sitting on it--have passed my tech recommendations on to our quality people and they will distill it down into qualityspeak and pass it on to the auditors.

 

[ck1999]

swall,

You should be testing salt spray ASTM B117 with ASTM D1654 for results. Your auditor should be ok if you are testing panels on a monthly basis. Your chemical supplier and/or paint supplier should be willing to conduct these test for you. Also TCP is correct as far as an electron microspcope to determine if you are achieving the correct crystal structure and density.


20% NaOH and 5% Sodium Gluconate for about 20 minutes at 160F will strip iron phosphate coatings (with minimal chelation of the iron from the substrate) not sure about zinc phosphate. You will need an analytical balance to weigh to 0.1mg

ck1999

 

[swall]

I did get a non chromic acid stripper procedure from our supplier, although at the moment I can't locate it. But if the issue comes up again, I'm ready. And no, the chemical supplier is not willing to do unlimited lab analysis for free. Apparently, a few small customers expected this into perpetuity and that ruined things for all customers. You can get some freebies--basically it seems to be on a frequency related to how often the tech rep pays you a visit. But if you want monthly SEM analyis, it will be on a fee basis.But,the auditors were satisfied with the corrective actions that were submitted. Case closed.

 

[ck1999]

That may be true for the sem but they should do salt spray analysis monthly or minimum of quartely

ck1999

 

[swall]

I should have mentioned--we have in-house salt spray capabilities.