Intergranular corrosion of CN7M (Alloy 20) 1

[bruv]

We have been asked to supply some castings in ASME SA351 CN7M (cast Alloy 20), with the following additional requirement:

"Intergranular corrosion test in accordance with ASTM G28A or equivalent"

We have supplied plenty of castings in this material over the years, but never before with a corrosion test requirement.

Is G28A a suitable test for this alloy? If so, what results would you expect? If not, what would you suggest as an equivalent alternative? Would one of the tests in ASTM A262 be suitable?

 

[SJones]

Try reviewing NACE Corrosion 2007, Paper 07202: An Explanation Of Corrosion Acceptance Tests And Their Applicability To Field Use (authored by Special Metals)

Steve Jones
Materials & Corrosion Engineer

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

 

[TVP]

Technically, ASTM G28 is for wrought Ni alloys, not castings. Having said that, it appears that your customer has some experience testing castings based on this method, so I would certainly investigate the details. The wrought version of alloy 20, UNS N08020, is supposed to be tested to 120 hours of exposure according to ASTM G28, so that would be the starting point absent any additional information from the customer. There is no equivalent ASTM standard for castings.

 

[bruv]

SJones: thanks for the link, I've downloaded the paper and will review it shortly.

TVP: shortly after reading SJones post I reviewed G28 more carefully and discovered N08020.

We are being asked more and more often to supply castings with G28A corrosion testing, presumably because there is no equivalent test for cast material. Our clients have, so far, set different maximum allowable corrosion rates for wrought and cast materials on their spec sheets.

 

[MikeMet]

We have used G28 for qualification of Alloy 20 for years. My only advice would be to experiment with times and temperature for heat treatment. You will find that slight differences in both can make a significant difference in G28 results.