Ferric Chloride Testing Of Ni Alloys

[SJones]

NORSOK M-605 requires an ASTM G48 method A test, 50 deg C 24 hours, for ASTM A494 castings in N06625 but not for pipe, tube, forgings, fittings etc. The acceptance criteria are the same as for austenitic alloys: No pitting at x20 magnification and max weight loss of 4g/m2.

Q1. Why only test castings?

Q2. Is a G48 A test for N06625 and N08825 of any value as a production quality control test for all product forms used in piping systems?

Q3. If they are of any use, is it valid to adopt the same criteria as for austenitic and duplex alloys?



Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Goahead]

While I have no answer for the questions that bother you, I found some information that may have some interest.

The ferric chloride test described in ASTM G 48 is performed on specimens with flat-parallel surfaces for attachment of crevice formers. It is used for Crevice Corrosion Susceptibility determination by immersion.

ASTM Specification G 78 provides guidance for conducting crevice corrosion tests for stainless steels and nickel-base alloys in sea-water and other chloride-containing environments. Results can be affected by several factors.

http://www.welding-advisers.com/

 

[EdStainless]

Practice A and C are pitting tests. They do not require any specific product form for testing, no ground surface, no crevice blocks.

My best guess is that the 50C requirement for 625 castings is not applied to other product forms because it is far below what you would expect from wrought products.
Since the FeCl solution isn't stable above 50C, and even the acidified solution used in practice C and D isn't usable above 85C, I am not sure that you could do pitting tests on good 625. Though 85C wouldn't be a bad acceptance criteria for forged/wrought products.

I very much prefer practice C. The acidified solution is a little more aggressive, much more stable, and more repeatable.

If you want to use it as a test I would use it to qualify sources/processes. You need to select a temperature that is reasonable for the alloy that you are testing. Either test to find hte actual CPT, or select a temp and make sure that everything passes.

In duplex alloys there are other issues. If you conduct a test per ASTM A923 you are testing for secondary phases. You must use freshly ground samples, no acid cleaning allowed. This is based on the intermetallics disolving faster. You can also do G48s on duplexs. Similar test solutions, different results and reasons.

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Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[SJones]

Thanks Ed. You just confirm my view that it is pointless really testing 625 and 825 products to the same standard as super suplex and high alloy austenitic materials. Much research was undertaken to arrive at a standard test method for duplex materials in the oil & gas industry, including test temperature selection, ref TWI et al. The same has not been done for nickel alloys and, so, what can really be the value of the tests? Something for the NORSOK people in Norway to contemplate.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[EdStainless]

There is a lot of published data on all of the high alloy materials. You can find out what is typical or reasonable that way.
Corrosion testing is a poor acceptance test since the sample size is so small. It is great for qualification of a process.
Just make sure that you don't set unreasonable test ctireria.
G48Practice C is a fine test for almost all high alloy stainlesses, nickle alloys and duplexes. As long as the material should have a pitting temp above 25C then it is just a matter of setting the temp.
Testing everything to one standard is pointless. You only really need to test the alloy with the lowest corrosion resistance in that case. Test the 825 or the 2205 (I would have to look them up) and don't bother with the 625 and 2507 since they are better.
Except for the 625 castings, are they really any more pitting resistant than the 825? Depends on chemistry and how they were annealed. Testing will tell you.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[SJones]

Ed,

You have indicated the crux of the matter: setting reasonable testing procedures and acceptance criteria. Many thanks for your efforts in replying but we are not bothered about all the other alloys you mention. From my point of view, the testing and acceptance criteria for duplex alloys are nailed down and, until the same level of research and agreement has been achieved for Ni alloys in the oil & gas industry, I for one shall not be calling for ferric chloride testing of 825 and 625 whether it be for qualification or quality control.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[EdStainless]

Steve, Until they are nailed down, I would demand testing. Especially for castings. I have seen 625 castings that were so bad that they wouldn't pass 30C in a Practice C, not quite what I was paying for.

There is far more data available on the Ni alloys. THe info on duplex is more organized because there are so few players.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[SJones]

Ed,

I would say that it is a case of picking suppliers correctly. NORSOK does have a system for supplier evaluation and material qualification outlined in standard M-650; however, a review of that document shows (again) a bias toward duplex and 6Mo alloys although, ironically based on your noted experience, the only Ni alloy product form addressed is castings. There is an attempt to maintain a listing of qualified suppliers at

http://www.teknologisk.no/norsok-m650/

but you will note that it is not exactly highly populated at the moment.

In terms of developing standard tests, I would guess that the focus has been on duplex alloys since they offer a "lower cost haven" above the lower performance austenitics and before having to resort to the Ni alloys that have the the big $$$ capex that everyone wants to avoid if at all possible.

In the absence of a standardised test for Ni alloys that would suit the oil & gas industry, the only other recourse is simulated service testing even if it does mean dealing with 20 or 30 bars of H2S in our case!!

As an aside, had the poor casting you experienced also been solution annealed and would solution annealing be an improvement factor for poor casting control?


Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/