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Corrosion in
Stainless Steels
A range of mechanisms
may cause corrosion and deterioration of stainless steels.
by Steven J. Suess, Technimet
Corp; New Berlin Wis
excerpted from Advanced
Materials & Processes, April 1997
Published by
ASM International, Materials Park, Ohio
Types of Corrosion
General or overall corrosion: This
type of corrosion is defined as attack in a uniform fashion over the
entire exposed area of a surface across a wide range of temperatures.
Pitting corrosion: When localized areas of the
surface passive layer are damaged, they are no longer able to protect
the underlying metal against attack. Pits can then form, which can
lead to high localized corrosin rates with little or not general
corrosion to the areas surrounding the pits.
Galvanic corrosion: If two dissimilar metals are
electrically connected and exposed to an electrolyte, the more
electrochemically active metal corrodes by galvanic action at an
increased rate.
Microbiologically induced corrosion (MIC): When a
metal is exposed to untreated aqueous substances containing living
microbial organisms, these organisms have a tendency to form colonies
on the metal surface, especially in the areas containing gross surface
irregularities such as welded joints leading to corrosive conditions.
Stress-corrosion cracking (SCC): when a stressed
metal is exposed to certain types of environments it may become
suscepitible to stress-corrosion cracking. This phenomenon involves
stresses that may be signifi8cantly below the yield strength of the
material. Stresses may; be applied or residual and are generally
tensile in nature.
Analysis of Corrosion
Failures
Examination: Prior to sectioning
any part for analysis, it is advisable to thoroughly =examine the part
visually and with the aid of stereo microscope.
Chemical Analysis: In conducting a failure
investigation on a corroded component, it is advisable to verify the
chemical composition of the base metal against its specification
requirements. After the metal grade has been established, it is
possible to conduct research to determine the material's compatibility
with various environments from published data.
Metallographic examination: To determine the extent
of thinning or pitting in a component, metallographic cross sections
may be extracted and viewed for ao0verall wall thickness, pit depth,
and pit profile. Optionally, the metallographic section may be etched
in order to reveal the microstructure.
Scanning electron microscopy (SEM): A scanning
electron microscope allows a corroded surface to be studies at high
magnifications. This is useful for evaluating a corroded surface for
intergranular features, pits, or general oxidation. If corrosion
products are still present on the surface, they can be analyzed for
elemental makeup.
Immersion testing: This technique is simple, but very
useful for establishing overall corrosion and pitting rates of a metal
immersed in a solution.
Electrochemical corrosion testing: Many
electrochemical testing techniques are capable of establishing the
corrosion rate and pitting characteristics of metals in electrically
conductive solutions.
by Steven J. Suess, Technimet Corp; New
Berlin Wis
excerpted from Advanced
Materials & Processes, April 1997
Published by
ASM International, Materials Park, Ohio
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