Intergranular corrosion acceptance criteria clarification 1

[hsemus]

Nickel alloy 625 pipe is tested by using ASTM G24 method A for the loss of weight. As part of the acceptance criteria stipulated by the client required to check the weight loss @ 120hrs and also examine at X20 magnification-there should be no integranular attack presence. But the test report obtained shows that there is a 0.06mm depth of integranular attack identified at x20 magnification. So could you please advise if this X20 magnification is a realistic test on a test specimen which is solution for 120hrs with a acid solution . My understanding is that the grain boundaries will subject to corrosion attack for Nickel 625 material when performing the ASTM G24 and this corrosion cause this reduction in weight . So the corrosion seen during the X20 magnification is imminent. Please advise you view on my understanding.

 

[EdStainless]

Did anyone look at the surface before the test?
I have seen material from a mill that has been blasted and pickled that has grain boundaries ditched deeper than that.
And the spec should not say No attack, they should have a measured limit. Saying none is too subject to interpretation. And hence the reason why people use the weight loss as a limit.

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P.E. Metallurgy, Plymouth Tube

 

[hsemus]

Hi EdStainless ,

The surface was fine . As part of the test the surface is grit blasted to the required surface finish as per the standard.

I agree that there should be limit on the inter granular crack measurement . But G24 doesn't required this .

So my question is , will you expect to see corrosion in the test specimen when performing G24test . My understanding is that the rate of corrosion can only measured due to the accelerated corrosion occurred in the material due to the high concentrated solution we applied during the G24 test. If there is no inter granular attack then there is no loss of weight.

 

[EdStainless]

There will always be some weight loss in this test, even if you don't see it at the grain boundaries.
I would expect to be able to always see the grain boundaries after this test, but the issue is the depth.
So the material was grit blasted which hid the original surface. Was it aggressively pickled to remove embedded foreign material after blasting? The test solution may be removing some of the residue.
Has the material been re-heat treated? What was on the original mill certificate?
625 is a stabilized alloy. There should be secondary phases in the structure and these will etch preferentially. This isn't a single phase alloy so the tested samples will never be perfectly clean.

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

ASTM G24-13 Standard Practice for Conducting Exposures to Daylight Filtered Through Glass

Surely, it is G28.

It's a little difficult to have both a weight loss criterion and a no intergranular attack criterion simultaneously. Normally applied criteria are discussed in NACE Corrosion 2007, Paper 07202.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.