Accelerated life testing of marine cables and connectors: problems, pitfalls, and suggestions for improvements
The ability to scientifically predict the performance, degradation resistance, and usable lifetime of marine cables and connectors is highly desired and valued by design engineers and program managers. The costs associated with these items failing prematurely in their end-use applications can be quite high. On the other hand, significant savings can be realized if the service life of such hardware can be reasonably extended beyond several repair/refit cycles. Historically, the most frequently encountered failure mechanism for marine cables and connectors has been delamination of the polymeric overmolding compound from metal connector backshells. Most outboard cables are connected to cathodically protected metal surfaces which induce a cathodic potential within metal connector backshells. Corrosion reactions occurring on cathodically polarized metal surfaces then cause metal-polymer adhesion failure by a variety of mechanisms. Over the years, several testing protocols have been developed either by or for the U.S. Navy to use increases in temperature to accelerate this so-called "cathodic delamination" process. The purpose of these accelerated life test (ALT) protocols is to determine the resistance to degradation of various metal-primer-polymer combinations used on cables and connectors in a marine environment. Unfortunately, many of the existing ALT protocols exhibit technical and/or scientific deficiencies that compromise their ability to provide meaningful data for the prediction of service life, and the understanding of the cause(s) of hardware failure in the marine environment. These problems include, but are not limited to: choice of failure mechanism model; differences in activation energy for materials/reactions/processes; maintenance of the proper corrosion current density and voltage; and maintenance of the proper ALT fluid medium chemistry. All of these issues are important for the successful completion of an ALT, yet ALTs are often conducted without addressing one or more of these critical issues.