This paper reviews the theory of coating deterioration mechanisms and how these mechanisms are connected to the common test method of scribe underfilm corrosion creep. Historical corrosion creepage test data of marine coating performance in different environments is analyzed. This data is used to predict the effects of coating deterioration in marine environments as compared to rural. This analysis is used to discuss the merit of using underfilm corrosion creep of scribed panels as a coating performance indicator.
Organic coatings are the most used method of corrosion prevention and protection of metallic substrates in many industries. Owners in both the public and private sectors will invest significant resources into testing coating options to provide the best protection for new and existing products or infrastructure. Often, this testing defaults to some variation of accelerated salt spray testing or outdoor marine exposure with results being based on aspects such as visual measurements of rust through, corrosion creepage/undercutting, and blistering. In the past several decades, additional electrochemical analysis such as resistance measurements and electrochemical impedance spectroscopy (EIS) have been included in many studies, but the former visual indicators of corrosion still often take precedence. This paper will review the protective mechanisms of organic barrier coatings, analyze historical corrosion creepage data, and discuss the effectiveness of using creepage measurements as a coating performance metric for coatings in marine environments.
Organic coatings have been studied for many years. It is well understood that they are effective at preventing or delaying corrosion on steel substrates, but despite the amount of research, the mechanisms of protection are not fully understood. Several theories have been proposed, but it is largely agreed that these coatings provide a barrier to the substrate from the external environment. The most important aspect of an effective polymeric protective coating is its resistance of ionic movement to the surface of the metal from the external environment.1,2 Additionally, the coating may provide resistance inhibition preventing ion transfer from anodic and cathodic locations of the steel under the coating. Thus, the coating prevents ion transfer from both the external environment and from point-to-point at the coating and steel interface. Therefore, deterioration of protective coatings seems to be related to the formation of conductive pathways from the external environment to the metal substrate or the impact of water sorption into the coating. This has been shown through testing of immersed coated steel in which higher electrolytic resistance equated to better corrosion protection.3 Similar concepts are proposed for coatings in atmospheric exposure, with the added complication of varying cycles of moisture exposure and generally higher levels of oxygen available for diffusion to any areas of exposed substrate.
