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The Effect of Environmental Factors on the Rate of Cathodic Delamination

One of the leading causes of polymer-metal debonds in the marine environment is a process called “cathodic delamination” or “cathodic debonding).” The driving force for cathodic delamination is typically either a large galvanic difference between two dissimilar metals in contact with each other, or a cathodic protection system used to prevent susceptible metals comprising ship hulls from corroding in sawater. Any hardware grounded or attached to a cathodically polarized metal substrate can be unintentionally affected by the cathodic current. The voltages used by commercial cathodic protection systems trigger a significant increase in the pH of seawater immediately adjacent to cathodically polarized surface either by causing dissolved oxygen and water to react and produce hydroxyl anions or, less commonly, by converting hydrogen cations to hydrogen gas. The resulting increase in pH at the metal surface is sufficient to initiate and propagate the debonding of most polymers, paints and primers from the cathodically polarized surface. Significant increases in the useful service lifetimes for polymer-metal bonds in seawater can be realized by slowing or preventing the cathodic delamination reactions that create the high pH microenvironments on inadvertently cathodically polarized surfaces. While significant gains can be achieved by using coatings and primers that are resistant to high pH conditions, it is also important to understand how environmental factors affect the rate of cathodic delamination. Increases in temperature, salt concentration and dissolved oxygen concentration all accelerate the rate of the cathodic delamination reaction that generates hydroxyl anions on a monel 400 alloy surface cathodically polarized by a zinc sacrificial anode. Known or suspected environmental adhesive bond stressors such as thermal shock, hydrostatic pressure cycling and high hydrostatic pressure dwell times do not have a significant effect on the rate of cathodic delamination for PR-1547 polyurethane bonded to monel 400 alloy (cathodically polarized by zinc sacrificial anodes) via PR-420 primer. Interestingly and somewhat unexpectedly, alternating week-long wet/dry cycling of similar samples reduced the rate of cathodic debonding to approximately half of what would have been expected by simply considering the amount of time the samples spent submerged in salt water. Understanding the effect of environmental factors on the rate of cathodic delamination is very important for the design of accelerated life tests used to assess the long-term reliability of many kinds of hardware in the marine environment. Accelerated life testing experimental protocols can be streamlined and costs reduced by eliminating hydrostatic pressure, wet/dry and thermal shock requirements and focusing solely on temperature increases as the accelerant for the cathodic delamination process.