Mr Ed
All good questions but I have few answers!
This is the test result as reported on the MRC.
As the test is ASTM G48 - A, I assume that the test-piece was 50mm x 25mm x unknown t and prepared according to G48.
Temperature was 30ºC, duration 24 hours.
Per ASTM A923, the maximum acceptable corrosion rate (as weight loss) for S31803 & S32205 in the 6% ferric chloride test (ASTM G48, Method A) at 25 °C is 10 mdd. In customary units, this is
0.1 g/m2/day or 0.01 mg/cm2/day
The slightly higher test temperature (30 °C) wouldn't double the corrosion rate. Hence, the material is unacceptable.
Hold on Ken, those are two different tests, designed to find two different things. They just happen to both use FeCl3.
The A923 must be preformed on a freshly ground surface with no pickle or passivation. You are strictly looking for intermetallics.
The G48 is a pitting test, so not pits means that you pass. G48 A is a 72 hour test though, so it makes me wonder, did they really do a Method E.
The other not so slight difference is that G48 C, D, E, F and A923 all use an aicdified solution. G48 A and B use straight FeCl3.
Converting the units is also a pain. Whoever thought that 10 mg/dm2/day was cute need to be kicked. I guess that converts to 1g/m2/day or 0.1mg/cm2/day. This makes your 0.2g/m2 not bad at all.
Ask to see their procedure outlining the test method. Ask to see the coupons. My hunch is that they are not sure what they did.
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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net
Ed is correct on the units; A923 gives (dm2) which I misinterpreted as 0.1 m2;
correct is (0.1 m)2 = 0.01 m2.
Ferric chloride solutions are inherently acidic: pH 2 @ 0.1 Molar and pH 1 for 40 wt% FeCl3 (per MSDS's). In ASTM G48 Method A, the 6 wt% (~0.39 Molar) solution is naturally acidic. In ASTM A923 Method A, either HCl or NaOH is added as needed to adjust the pH to 1.3. So, A923 simply standardizes the initial pH.
In ASTM G48 Methods C, D, (E, & F)* a specified addition of HCl is made, giving a more acidic pH ~ 0.6.
*My copy of ASTM G48-99 only goes up to Method D.
The surface preparation is very importance; a freshly polished sample having a thinner passive layer will show greater weight loss. Both G48 & A923 mention wet polishing with 120 grit paper (unless need to evaluate mill finish or as-fabricated welds).
Ed is correct in asking about the testing details.
Whilst Ed and kenvlach get their standards and area conversions in a muddle, the answer to your question is: yes, it is acceptable. As to being high, the usually adopted criterion is (used to be when NORSOK advocated it) 4.0 grammes per square metre.
I just want to make a point, you can't use one test to qualify both the intermetallic condition and the pitting resistance of a duplex stainless.
For the A923 test you must use a freshly ground surface, for G48 it is preferable to use the actual production finish that will see service.
The reason for adding HCl to the solution is to have more uniform test conditions and better solution stability at higher temps.
While your test results look OK. You need clarification as to what they tested. There is no such thing as a 24 hour G48-A.
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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net
