The Oxalic Acid Etch Test as
per ASTM A262 (Practice A) is used as a method to rapidly screen certain
grades of stainless steel which are essentially free of susceptibility to
intergranular attack associated with chromium carbide precipitates. This test is
used to accept material but not to reject it.
This test may be used in conjunction with other tests to
provide a rapid method for identifying those specimens that are certain to be
free of susceptibility to rapid intergranular attack in these other tests.
Such specimens have low corrosion rates in the various hot acid tests, requiring
from 4 to 240 hours of exposure. These specimens are identified by means of their
etch structures, which are classified according to the following criteria:
The Oxalic Acid Etch Test may be used to screen specimens intended for
testing in the following test:
Practice B—Ferric Sulfate–Sulfuric Acid Test,
Practice C—Nitric Acid Test,
Practice E—Copper–Copper Sulfate–16 % Sulfuric Acid
Practice F—Copper–Copper Sulfate–50 % Sulfuric Acid
Each 'Practice' specification
contains a table (To
obtain Specifications) showing which classifications of
etch structures on a given stainless steel grade are equal to acceptable, or
possibly, 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 grades of stainless steels and the hot acid tests for which the oxalic acid etch test is applicable are
listed in Table 2 of the Specification.
Extra-low–carbon grades, and
stabilized grades, such as 304L, 316L, 317L, 321, and 347, are tested after
sensitizing heat treatments at 650 to 675°C (1200 to 1250°F), 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 675°C (1250°F).