ASTM G48A - surface finish

[bruv]

I have identical G48 Method A tests carried out by two different test labs. Test criteria were 24hr duration at 50C, no pitting allowed, and a maximum weight loss of 4g/sq.m. Test pieces were pickled and passivated before testing. The material in question is a nickel based casting alloy, in the solution annealed condition.

Test lab A reported a final weight loss of 4.2g/sq.m, and found pitting. Test lab B reported zero weight loss and no pitting. Test specimens were approx 50mm x 25mm x 10mm.

The only (apparent) difference between the two specimens is the surface finish. Lab A reports that the test piece was finished with 180 grit paper, lab B with 1200 grit paper. I know that G48A calls out 120 grit finish, but this appears to be only a recommendation, not a requirement.

In your opinion, would a finer surface finish give a better result (either weight loss or pitting)? If not, why would the labs use a non-recommended surface finish? I have asked, and I'm awaiting the reply. one of the two labs has offered to run some more tests at different surface finishes, and I'll report back when I get the results.

If the different surface finish should make no difference, why would I get such different results for identical test pieces from the same heat of material?

 

[EdStainless]

The surface finish depends on what you are trying to determine. If you are testing or verifying a process, solution anneal and pickle, then you want the test done on the as produced surface.
We do this all of the time on as produced material to assure that the entire process is giving us the desired results.

If you want to know what the alloy is capable of then providing a smoother finish will lead to higher values.

I suggest that you move to practice C. The temps will be a little lower and the results more stable. Even then getting two labs to agree within 5C is unrealistic.
Or you could move to using cyclic polarization electrochemistry, then you would have quantified results.

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[bruv]

Unfortunately I can't move to another test, as G48A is a customer requirement (Norsok related).

We have had CPT/G48C done on other alloys a long time ago (15yrs+). If I remember correctly, it is considerably more expensive than G48A, which is probably why it's not normally requested. I have no idea what the price difference would be now.

Cyclic polarization electrochemistry - I've not heard of it but I shall investigate. Is there an ASTM standard for this?

Thanks.

 

[CoryPad]

Any chance the parts were ground after pickling and passivation and the passivated surface was removed?

 

[EdStainless]

There is no cost difference for practice A and C. If you had a full CPT determination done that required a number of tests. That is what drove the cost.

I believe that G150 is the electrochemistry spec.

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