Design considerations for accelerated life tests conducted on cable hardware subject to cathodic delamination
The accelerated life test (ALT) is a tool used by cable manufacturers and cable end-users to determine how long a given piece of hardware will last once deployed into the marine environment. To conduct an ALT, the failure mechanism for the hardware in question must be known, and the kinetics of that process must be determined via experimentation. Typically, Arrhenius kinetics are assumed and the activation energy for the dominant degradation process must be measured. Cathodic delamination is a major failure mechanism for undersea cable connectors. Although the mechanism via which cathodic delamination weakens/destroys metal-polymer bonds is well known, good quality kinetic data for the process (from which Arrhenius activation energies can be derived) is generally lacking. Our research into the cathodic debonding process has revealed that there are three major cases that must be examined separately to obtain the kinetic parameters needed to make the time-temperature conversions for an ALT. Case (1) concerns thin polymer films (e.g., paints) applied directly over, and completely covering, a cathodically-polarized metal surface. Here, the water and oxygen needed for the cathodic reduction of dissolved oxygen reaction on the metal surface can diffuse directly and rapidly through the polymer layer. Two different methods for calculating the diffusivity of water through the polymer layer will be discussed. Once water diffusivity at three different temperatures has been measured, the natural logarithm of the diffusivity values can be plotted versus the reciprocal absolute temperature to obtain the slope of a line from which the Arrhenius activation energy for water diffusion through the polymer can be calculated. Cases (2) and (3) involve macroscopically thick layers of polymer with exposed polymer/metal bond lines. If metal-polymer bonds susceptible to attack by cathodic delamination are present, debonding will proceed inwards from the exposed edges. Measurements of the rate at which the width of the debonded region grows at three different temperatures can be used to calculate an Arrhenius activation energy. Case (3) is more difficult because it concerns materials/coatings that are resistant to cathodic delamination; no obvious debonding occurs during the early part of the ALT. For this case, a reaction acceleration factor (RAF) must be experimentally determined by measuring the corrosion current density (icorr) for the reduction of water reaction using the two metals involved (usually a sacrificial anode and the metal surface of interest). The icorr value for the sacrificial anode/metal substrate pair is measured using a potentiostat at the anticipated service temperature and also at the intended ALT temperature. The ratio of these two values is the RAF and can be used directly to determine time-temperature conversion factor for the ALT.