The service life expectancy of a product is defined by
level of degradation that will be designated as failure. This would ideally be
found in a product specification or warranty, a document which summarizes
product quality requirements, desired outcomes, and expectations. A product
specification may include requirements that the product meet certain accepted
standards such as those defined by ASTM, NACE, or other self-regulatory bodies.
A well–written specification indicates such characteristics as load-bearing
capacity and life expectancy. (reference:
(reference: Dr. Zee)
Many specifications recognize that perfect materials do not exist. Major construction codes may do the same thing. They make allowance for the presence of defects or corrosion loss by establishing limits on defect type, size, location, and distribution. Imperfections such as surface laps, tears and casting and forging defects are recognized in ASME and AFS materials specifications as acceptable within certain limits. However, in the real world it has been found that specifications or standards have commonly not been put in place prior to putting a product in service. Having such specifications would provide a valuable reference guide should a product fail in service, helping the analyst determine whether the failure was reasonably to be expected.
For example, recently a widely used type of aluminum scaffolding collapsed in service. The owner wanted to know if he had a cause of action against the manufacturer. The product had been in service for over five years, three of which had been under a prior owner. A full failure analysis found no defect in the product and found it fully within the specifications. Apparently the product simply wore out and finally failed. It may have suffered overload under the prior ownership, but that was unknowable. The manufacturer was protected by the load-limit specifications and by the finding of no manufacturing or material defects.
In another example, a small area of discoloration in the paint of an expensive new car may be considered unacceptable because of the desired high quality of appearance. By contrast, pinholes of various sizes through epoxy coatings on gas lines may not be considered unacceptable, because corrosion of the pipeline in service will be prevented by cathodic protection. Corrosion resistance is the issue of greatest importance for the pipeline, not appearance, thus the criteria for failure are quite different, even though coating quality is an important issue in both cases.
The service life expectancy must always be tailored to the product application. For example, most coating specifications are designed for products to be used for above- ground corrosion protection. Few of these specifications are particularly relevant to underground coating applications where cathodic protection may be in place.