DESCRIPTION
General attack is
typically caused by uniform general corrosion. Uniform corrosion can be
described as follows: Corrosion reaction that takes place uniformly over
the surface of the material, thereby causing a general thinning of the
component and an eventual failure of the material.
Prevention or
Remedial Action
- selection of a more
corrosion resistant alloy (i.e. higher alloy content or more inert alloy)
- utilize coatings
to act as a barrier between metal and environment.
- modify the environment
or add chemical inhibitors to reduce corrosion rate.
- apply cathodic protection.
- replace with corrosion
resistant non-metallic material.
Standard Test
Methods
- ASTM G-31 - laboratory
immersion corrosion testing of metals.
- ASTM G-4 - corrosion
coupon tests in plant equipment.
- ASTM G-54 - practice
for simple static oxidation testing.
- ASTM G-59 - practice
for conducting potentiodynamic polarization resistance measurements.
- NACE TM0169 - laboratory
corrosion testing of metals for the process industries.
- NACE TM0274 - dynamic
corrosion testing of metals in high temperature water.
- ASTM B-117 - salt
fog testing.
- ASTM G-85 - modified
salt spray (fog) testing.
- ASTM D-2776 - test
for corrosivity of water in the absence of heat transfer, by electrical
methods.
- ASTM D-2688 - test
for corrosivity of water in the absence of heat transfer, by weight loss
methods.
- ASTM G-91 - test
method of monitoring atmospheric SO2 using the sulfation plate
technique.
Evaluation
of General Corrosion
The predominant standard
utilized for general corrosion assessment is ASTM G31. This standard gives
guidelines for conducted simple immersion corrosion tests. Important considerations
when conducting such tests in either the laboratory, field or plant setting
are:
- Adequate solution
volume for the surface area of corroding specimens in test.
- Electric isolation
of the specimens from other specimens and any dissimilar metals in the
system.
- Exposure of specimens
to more than one phase, if applicable, since corrosion rates can change
substantially in the different phases especially as water and impurity
contents vary.
- Other test conditions
such as flow rate, temperature, and aeration can produce variable results
and locally high corrosion rates.
Methods of specimen
surface preparation and post-test cleaning should be controlled as defined
in the test standards.
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