Oxalic acid etch tests

Figure 38 Comparison of data from SL-EPR, acid ferric sulfate, and oxalic acid etch test for seven separate heats of Type 304 and 304L stainless steel. Note that for low levels of sensitization, the SL-EPR can quantitatively distinguish among degrees of sensitization. At higher levels, the coupon exposure tests are more discriminating. (From Ref. 32.)...

The term stainless alloys includes the austenitic and ferritic stainless steels with 12 % or more chromium and certain austenitic nickel-rich chromium-bearing alloys. From this section through the discussion on the oxalic acid etch test, the text is an update and condensation of material in Ref 9. AU figm es are from this publication except where other references are shown. The latter are reprinted with permission, copyright NACE International. All rights reserved by NACE. 

The potential of the specimen in the oxalic acid etch test while being anodically etched at lA/cm is 1.7 V vs calomel scale. In austenitic stainless steels only hromium carbides are detected and sigma phase when present as a second phase also in stabilized ferritic stainless steels. 

Specimens with step and dual structures do not drop grains in boiling acid tests, and therefore have acceptable or passing corrosion rates in weight-loss tests. Materials represented by such specimens can therefore be accepted for plant use on the basis of the 1.5 min oxalic acid etch test. Only those materials that have a ditch structure must be submitted for testing in the hot acid tests to determine whether or not their corrosion rates fall below or above the acceptance rate for the given alloy. 

The oxalic acid etch test has been incorporated as Practice A in ASTM A 262 for screening a large number of wrought and cast austenitic stainless steels. It has also been included in ASTM A 763 for screening of stabilized ferritic stainless steels, Practice W (Table 6). 

For comparisons, data from the EPR, ferric sulfate, and oxalic acid etch tests have been plotted on log-log scales in Fig. 15, along with information on pitting in the EPR structures. It has been observed that P values on nonsensitized (step structure) AISI 304 or 304L material exceeding 0.10 coulomb/cm are evidence of significant amounts of pitting in the EPR etch structures. 

The line at 2.0 coulomb/cm has been drawn because Clarke et al. [20] selected it as the upper limit for acceptance testing of incoming stainless steel to ensure a "lack of discernible intergranular carbide precipitation. This is in agreement with our findings as shown in Fig. 15, i.e., the limit of 2.0 coulomb/cm essentially covers the range of step structures in the oxalic acid etch test. However, any P value in excess of 0.10 coulomb/cm denotes the presence of some random pitting in the microstructure. 

In terms of the data in Fig. 15, the requirement as mentioned above for freedom of sensitization corresponds to a requirement for a step structure in the oxalic acid etch test. Even though the oxalic acid etch is not a quantitative test, the step microstructure can be readily identified and has been used for laboratory acceptance testing in industry for more than 40 years to identify and screen acceptable material which needs no further testing in one of the boiling tests that provide quantitative data. The much simpler oxalic acid etch test also has been used as a nondestructive test on equipment in chemical plants. 

DL data have been plotted in Fig. 17 as a function of corrosion rates in the ASTM standard ferric sulfate test and the classifications obtained in the oxalic acid etch test. They show that the current ratio is very sensitive for detecting the absence of sensitization and for differentiating mild degrees of sensitization for which the oxalic acid test shows step or dual structures. Current ratios are in the range of 0.0001-0.001 for step structures and between 0.001-0.05 for dual structures. Corrosion rates in the ferric sulfate test do not differentiate between these small levels of sensitization, However, for severely sensitized materials with ditch structures, current ratios become less effective in making distinctions between medium and severe levels of sensitization, while the corrosion rates vary over a wide range. These specimens have DL ratios in a wide band extending from 0.05-0.3. 

FIG. 17—Correlation of the DL test with the ferric sulfate and oxalic acid etch tests on AISI 304 stainless steels [22]. ( NACE International. All Rights Reserved by NACE reprinted with permission.)... 

The advantages [22] of the DL over the SL EPR test are its relative independence not only of the surface finish, but eJso of the presence of nonmetallic inclusions that cause pitting in the SL test. It is more reproducible than the SL test and less sensitive to variations in scan rate and solution composition. It provides a quantitative, nondestructive method for detecting relatively mild degrees of sensitization which match the metallographic classifications obtained in the oxalic acid etch test. 

Information on "acceptable and nonacceptable corrosion test results has been given for the two EPR tests in the section above on these tests. Acceptable etch structures (step, and in some cases, dual) are also identified for the oxalic acid etch test. The absence of fissures in bends after certain copper sulfate tests is considered an indication of acceptable results. However, in the case of weight-loss tests (nitric acid, ferric sulfate, and certain copper sulfate tests) the standard ASTM test methods merely show how corrosion rates are calculated without identification of rates which are evidence of the onset of susceptibihty to intergranular attack. 

Since intergranular corrosion is limited to corrosion at or immediately adjacent to grain boundaries and is detectable at the microstructural level, simple corrosion rate calculations using weight loss measurements provide no information. Standard ASTM tests, such as the oxalic acid etch test... 

ASTM A 262 Practice A is a rapid procedure in which specimens are electrolytically etched and examined metal-lographically for attack at the grain boundaries. It has been widely used as a quality control screening test to approve acceptable material. Specimens that fail the oxalic acid test are then evaluated by one of the other tests in ASTM A 262 and are accepted or rejected based on these results. Although the oxalic acid etch test is done quickly, it can be tedious, because examination of all fields is required. 

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