Oxalic Etch Test

Definition - What does Oxalic Etch Test mean?

The oxalic acid etch test is a rapid method of screening specimens of certain stainless steel grades which are essentially free of susceptibility to intergranular attack associated with chromium carbide participates. The test is used for acceptance, but not rejection, of material.

This test may be used in conjunction with other tests to provide a quick method for identifying specimens that are certain to be free of susceptibility to rapid intergranular attack. These specimens are identified by means of their etch structures.

Corrosionpedia explains Oxalic Etch Test

The oxalic etch test was developed as a faster way to screen specimens than the Strauss and Streicher tests. In an environment of 10% oxalic acid (H2C2O4) an anodic current of 1A/cm2 is applied for 1.5 minutes at room temperature to a polished sample. By microscopic analysis it is determined if the specimen needs to undergo further testing or if it is not sensitized. If the grain boundaries appear as ditches in the micrograph, it indicates that the sample needs further testing.

Extra-low carbon grades and stabilized grades are tested after sensitizing heat treatments at 1200 to 1250°F (650 to 675°C), which is the range of maximum carbide precipitation. These sensitizing treatments must be applied prior to submitting the specimens to the oxalic acid etch test. The most commonly used sensitizing treatment is 1 hour at 1250°F (675°C).

Each practice specification contains a table showing which classifications of etch structures on a given stainless steel grade are equal to acceptable or nonacceptable performance in that particular test. Specimens having acceptable etch structures need not be subjected to the hot acid test. Specimens having nonacceptable etch structures must be tested in the specified hot acid solution.

The etch test is applicable only to those grades listed in the individual hot acid tests and classifies the specimens either as acceptable or as suspect. The oxalic acid etch test may be used to screen specimens intended for testing in:

  • Practice B-ferric sulfate-sulfuric acid test
  • Practice C-nitric acid test
  • Practice E-copper-copper sulfate-16% sulfuric acid test
  • Practice F-copper-copper sulfate-50% sulfuric acid test

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