FERRIC CHLORIDE
Ferric chloride is a strong oxidizer and is recognized for inducing pitting and crevice corrosion in stainless and nickel alloys. Solutions of ferric chloride have been used to evaluate the corrosion resistance of various alloys. While the results of testing in ferric chloride can be used to compare materials, the results themselves offer only minimal guidance on the performance of a material in any particular chloride bearing environment other than ferric chloride.
The origin of the Pitting Resistance Equivalent (PRE) relationship to chloride pitting resistance dates back to work done as early as 1969 and reported by Lorenz and Medawar of Thyssen while working with austenitic stainless steels. The concept was studied extensively, and expanded to include superaustenitic and duplex alloys for pulp bleaching applications by Wensley and Reid in the early 90’s.
In general, as the PRE increases, so does the alloys resistance to chloride pitting and crevice corrosion. This relationship is particularly evidenced when tested in a severe environment such as ferric chloride.
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&D | PRE | INITIAL TEST CPT | APPROX CPT | INITIAL TEST CCT | APPROX 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.
The Test Method also provides formulas for calculating estimated starting temperatures for testing for Methods C through F. These estimated temperatures are conservative, especially for the crevice tests (see table).
CPT °C = 2.5 * % Cr + 7.6 * % Mo + 31.9 * % N – 41.0
CCT °C = 1.5 * % Cr + 1.9 * % Mo + 4.9 * % Nb + 8.6 * % W – 36.2
These equations are strikingly similar to the PREN calculation that typically uses PRE = %Cr + 3.3 * %Mo + 16 * %N. The more modern formula used by NACE International and others is %Cr + 3.3 * (%Mo + 0.5 * %W) + 16 * %N, which recognizes the benefit of tungsten.
ASTM Specification A 923, 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. A 923 actually contains three different test methods for determining acceptability. Test Method C in A 923 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 A 923 references ASTM G 48 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 should be very similar to those in the G48 Method A or E test solutions. However, because of the wide experience in the production testing of duplex alloys, testing in accordance with A923 is recommended.
