The rate, extent,
and type of corrosive attack that can be tolerated in a part vary widely,
depending on the specific application, when investigating a corrosion failure,
an analyst must:
- determine the failure
mode;
- determine the failure
cause;
- estimate the extent
of damage and the likelihood of additional failures;
- design and implement
an appropriate corrective action; and
- follow up to ensure
that the corrective action is first implemented and then is sufficient
to prevent another failure
It is important to
bear in mind the difference between failure mode and failure cause. The
mode will usually be one of more of the eight basic forms of corrosion:
- general corrosion
- pitting corrosion
- crevice corrosion
- galvanic corrosion
- stress-corrosion
cracking
- erosion-corrosion
- intergranular corrosion
- dezincification
The failure cause
is the root reason for the actual occurrence of the failure. For example,
failure mode might be stress-corrosion cracking (SCC), however, failure
cause might be chloride ions that were introduced into the system or residual
stress in the component, which in turn made it susceptible to SCC.
The principal stages
of the investigation and analysis of corrosion failures are:
- collection of background
information and samples;
- preliminary examination;
- detailed examination;
and
- analysis and verification.
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