Ferric Chloride Corrosion Testing

ASTM has issued G48, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution.

G48 contains six different test methods for evaluating the pitting and crevice corrosion resistance of stainless and nickel alloys. These are:

  • Method A - 6 % FeCl3 pitting test
  • Method B - 6 % FeCl3 crevice corrosion test
  • Method C - 6 % FeCl3, 1% HCl critical pitting test, 72 hour test period
  • Method D - 6 % FeCl3, 1% HCl critical crevice corrosion test, 72 hour test period
  • Method E - 6 % FeCl3, 1% HCl critical pitting test, 24 hour test period
  • Method F - 6 % FeCl3, 1% HCl critical crevice corrosion test, 24 hour test period

Methods A and B are designed to be individual tests performed at a given temperature (usually 22° C or 50° C) for a specific amount of time, most commonly 72 hours. However, these parameters can be modified to suit individual test needs. Note that these are not acceptance tests as written in G48 as there are no acceptance criteria. They can be used as acceptance tests if criteria (e.g maximum pitting depth, maximum weight loss, etc.) are provided along with specific instructions on the test temperature and time period to be utilized.

G48 C & DPREInitial Test CPTApprox. CPTInitial Test CCTApprox. CCT
316L 24.3 17.8 15 -8.1 0
2205 34.9 42.9 30 2.8 20
AL-6XN 43.2 61.2 78 4.5 43
2507 37.5 49.2 70 5.8 40
ZERON 100 38.8 49.8 75 11.0 40
625 49.2 76.2 >85 29.7 50
C276 70.9 112.9 >85 42.5 80

Methods C through F are designed as incremental tests and require multiple specimens. These tests identify the minimum (critical) temperature needed to initiate pitting or crevice corrosion. There are no acceptance criteria for any alloys. These tests offer a means to evaluate and compare the expected performance of various alloys. Because of the extensive testing required and time consuming nature, these tests are generally not used for production testing, even when acceptance criteria are provided.

ASTM Specification A923, Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels, has become the standard acceptance test for Alloy 2205 (UNS S32205) plate products. A923 actually contains three different test methods for determining acceptability. Test Method C in A923 is a ferric chloride pitting test. Alloy 2205 material tested in a 6 wt. % solution of ferric chloride (pH controlled) at room temperature for 24 hours with a maximum corrosion rate of 10 mg/sq. decimeter/day (MDD) is considered acceptable per this standard.

Method C of A923 references ASTM G48 and the test method in A923 is very similar to Methods A and E in G48. The pH is adjusted to 1.3 so it is slightly less acidic than the G48 Method E practice, but the results in A923 Method C will be very similar to those in the G48 Method A or E test solutions. However, since the tests are slightly different, material tested in accordance with one method cannot be certified to the other without re-testing. Because of the wide experience in the production testing of duplex alloys, testing in accordance with A923 is recommended.