Corrosion occurs
at grain boundaries in austenitic stainless steels because grain
boundary segregation or precipitation produces local galvanic
cells. In austenitic stainless steels and nickel alloys,
chromium carbides can precipitate at grain boundaries. The
formation of the carbides removes the chromium from the
austenite adjacent to the boundaries. The low-chromium austenite
at the surface of grains is anodic to carbides and corrodes.
The following tests are carried as per the ASTM A 262
specification:
Oxalic Acid
Test, ASTM A 262, Practice A
The oxalic acid etch test is a rapid method of screening those
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.
Ferric Sulfate - Sulfuric Acid, ASTM A262 - Practice B
This test is based on weight loss determinations and provides a
quantitative measure of the relative performance of the material
evaluated. The procedure includes subjecting a specimen to a 24
to 120 hour boil in ferric sulfate - 50% sulfuric acid. This
procedure measures the susceptibility of stainless steels to
intergranular attack associated with the precipitation of
chromium carbides at grain boundaries.
Nitric Acid, ASTM A262, Practice C, (Huey Test)
The specimens are boiled for five periods, each of 48 hours, in
a 65 per cent solution of nitric acid. The corrosion rate during
each boiling period is calculated from the decrease in the
weight of the specimens. Properly interpreted, the results can
reveal whether or not the steel has been heat-treated in the
correct manner. The customer must specify the maximum
permissible corrosion rate and, in applicable cases, data on
sensitizing heat treatment.
This procedure may also be used to check the effectiveness of
stabilizing elements and of reductions in carbon content in
reducing susceptibility to inter-granular attack in
chromium-nickel stainless steels
Copper - Copper Sulfate - 16% sulfuric acid, ASTM A262 -
Practice E (Straus Test)
This procedure is conducted to determine the susceptibility of
austenitic stainless steel to inter-granular attack associated
with the precipitation of chromium-rich carbides. Once the
specimen has been subjected to the solution boil, it is bent
through 180° and over a diameter equal to the thickness of the
specimen being bent. This test is based on a visual examination
of the bent specimen.
Copper - Copper Sulfate - 50% sulfuric acid, ASTM A262 -
Practice F
This test is based on weight loss determination which provides a
quantitative measure of the relative performance of the material
evaluated. It measures the susceptibility of “as received”
stainless steels.
Salt Spray (Neutral / Fog), ASTM B117
This is the most commonly used salt spray for testing of
inorganic and organic coatings, especially where such tests are
used for material or product specifications. Salt Spray testing
is a tool for evaluation the uniformity of thickness and degree
of porosity of metallic and nonmetallic protective coatings. A
number of samples can be tested at once depending upon their
size CASS Test, tarnishing test are also done.
Pitting Corrosion Test, ASTM Gr 48
This Procedure is used to assist in the selection of test
methods that can be used in the identification and examination
of pits and in the evaluation of pitting corrosion to determine
the extent of its effect. The importance of this evaluation is
to be able to determine the extent of pitting, either in a
service application where it is necessary to predict the
remaining life in a metal structure, or in laboratory test
programs that are used to select the most pitting-resistant
materials for service. ASTM Gr 48 Method A and ASTM A923 Method
C are typical pitting corrosion tests performed.
Hydrogen-Induced Cracking (HIC) Test, NACE TMO284
This test method evaluates the resistance of pipeline and
pressure vessel plate steels to Hydrogen Inducted Cracking
caused by hydrogen absorption from aqueous sulfide corrosion. An
unstressed test specimen is exposed to a solution at ambient
temperature and pressure and after a specified time, the test
specimen is removed and evaluated.
Sulfide Stress Corrosion Cracking (SSCC), NACE TM 0177
The polythionic acid (sulfurous acid and hydrogen sulfide)
environment provides a way of evaluating the resistance of
stainless steels and related alloys to inter-granular stress
corrosion cracking. This practice can be applied to wrought
products, castings, weld metal of stainless steels or other
materials to be used in environments containing sulfur or
sulfides
Chloride Stress Corrosion Test as per ASTM Gr 36 Specification
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Corrosion test as per ASTM Gr 35 specification |
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Ammonia Vapour Test |
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Corrosion test as per ASTM A 761 Specification |
Crevice corrosion
Crevice corrosion is a corrosion occurring in spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices.
Ammonia Vapour Test
This test method is an accelerated test to determine if a copper alloy product will be susceptible to stress-corrosion cracking when exposed to a particular atmospheric condition during service with the appropriate risk level.
