Adhesion and corrosion issues associated with non-conductive ceramic (NCC) coatings
During the past two decades, non-conductive ceramic (NCC) coatings have been widely applied to connectors to prevent cathodic delamination. The ceramic comprising the NCC coating cannot support the electrochemical reactions (such as the reduction of dissolved oxygen to produce hydroxyl ions) that normally cause adhesive bonds to fail under cathodic delamination conditions. While the wide-scale adoption of this technology initially reduced the number and frequency of cable connector debonding failures, an upsurge in cable failures over the last five years has revealed that NCC coatings, as originally designed and implemented, are now much less effective in preventing cathodic delamination. Standard NCC-coated connectors now fail rapidly under cathodic delamination conditions rather than lasting the 10-15 years projected by accelerated life testing results obtained when NCC coatings were first being evaluated. Many recent cable connector failures have been traced to debonds between the NCC coating and the primers used to adhere polyurethane encapsulants to the connectors. While the exact mechanism via which these debonds occur has not yet been definitively established, deterioration of the NCC coating's “seal-coat” layer under the localized highly alkaline conditions that develop during cathodic delamination is believed to play a critical role in the debonding process. Corrosion-based failures of NCC coatings applied to connectors not subjected to cathodic delamination conditions have also been observed. These failures tend to be restricted to connector backshells made from highly corrosion-resistant metals/alloys (e.g., titanium, inconel etc.) and result from dissimilar metals or metal alloys being in contact with each other in seawater-thereby initiating and sustaining a galvanic corrosion cell. The end result of the process is the dissolution of the NCC coating's “metal matching layer”, which undermines the NCC layer and causes it to debond from the connector backshell. Examples of the above debonding scenarios obtained from autopsies of failed cables will be used to emphasize the need for new/improved technologies for preventing the cathodic disbondment of cable connector backshells from their overmolding encapsulants.