|
|
|
Latest News > Article Archives >
Back to
Article
http://www.ArcorEpoxy.com/news/archives/articles/corrosion_in_stainless_steels_199704.htm
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
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.
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
Latest News > Article Archives >
Back to
Article
http://www.ArcorEpoxy.com/news/archives/articles/corrosion_in_stainless_steels_199704.htm