Base metal diameter decrease after Phosphate coating

[tzallas]

We are producing a 4140/4145 120 KSI part with a section along the shaft that has been HVOF Coated (WC-17Co)
The final piece is at about 5 in OD

Phosphate vendor wanted to test a sample of a partially coated piece to see effects of the phosphate coating on the HVOF section, in case there were any unforeseen results.

Results came back. HVOF section is perfectly fine and was inert during the dipping process, as expected.

However, the base metal has "shrunk" slightly, or to be more accurate, diameter has decreased (material removed)

Test piece Original OD: 48.89 mm
Test piece Post Phosphating OD: 48.67
220 μm difference.

Coating weight Specified at 500 mg/ft^2
Zinc/Manganese phosphate

There is a perceptible transition step, from steel to HVOF portion, which can be blended in, but more importantly, my question is, is this common during phosphating?
I have never come across this "shrinkage" before in all the years we have been specifying Phosphate coating and apparently it is common, as per phosphate vendor at least.
I understand that Phosphating is a surface conversion, but was not aware of the base metal removal during it.

Is this true? or have they just not accounted for variations in the test piece, or been sloppy with their calipers/micrometers, different measuring location etc

Puzzled and surprised about this, any insight would be appreciated

Thank you

 

[vipereyes]

I have found this to be true. I have measured numerous steel springs before and after our zinc phosphate process and they always weight slightly less afterwards.

 

[tzallas]

I have found this to be true. I have measured numerous steel springs before and after our zinc phosphate process and they always weight slightly less afterwards.

Interesting....seems like I just assumed all this time and never relayed back.
Good to know for tight tolerance situations

Thank you