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Samples selected should be characteristic of the
material and contain a representation of the failure or corrosive attack. For
comparative purposes, a sample should also be taken from a sound and normal
section. Sampling handling is a paramount issue on which the whole remaining
analysis depends. Fracture surfaces must be protected from damage during
shipment by rigorously careful packaging. Surfaces should not be touched,
cleaned or put back together. .Surface chemistry must not be contaminated by
careless handling.
Materials specifications and service history reveal much about the nature of failure. If submitting a sample for analysis background information will need to be provided. A sample form that we find helpful is shown on the following page. Take copious notes. Do not rely on memory
Samples can be removed by acetylene torch, air-arc, saw, trepan, or drill. All cuts with an acetylene torch should be made at least six inches and cuts by air-arc at least four inches away from the area to be examined to avoid altering the microstructure or obscuring corrosive attack.
If pipe failures are involved, careful observation of the pipe conditions is important both prior to sample removal and as the cut separates the two ends of the pipe, as those may indicate stress conditions in the pipe at the time of failure. All of these characteristics should be noted and documented photographically. Be careful to include in the samples any failure-related materials such as coatings, soils in which a pipe may have been buried, corrosion deposits, waters, etc.
It is vital to prevent liquid samples from going septic. If bacterial content is a potentially important issue the samples must be taken in clean containers, refrigerated and delivered to microbiological labs for culturing within 24 hours. If bacterial content is irrelevant to the study, then two drops of household bleach per quart of sample will sterilize the contents. Note that the bleach addition will change the sodium and chlorine contents of the samples. A detailed knowledge of the final purpose for the samples has to control how they are to be handled.
