Investigating the Corrosion Characteristics of Carbon Steel and the Efficacy of Phenolic-Epoxy Coatings in Enclosed Environments

Abstract

Organic coatings are widely used for the corrosion protection of metals in marine environments because they provide good protection against corrosion at a relatively low cost. In marine environments, active corrosion in enclosed systems, such as ballast tanks or corrosion under insulation (CUI), cannot be effectively halted by organic coatings. The maximum operating temperature for organic coating systems is often defined by dry heat exposure, but this is not highly relevant to enclosed environments underwater immersion where the maximum operating temperature of the coating is dramatically reduced. In this study, the ability of phenolic-epoxy coatings to prevent corrosion in enclosed environments was investigated. In enclosed environments, metals are often exposed to aggressive conditions, such as high temperature (isothermal or cyclic conditions), corrosive salts such as seawater and various pH levels. To evaluate the performance of phenolic-epoxy coatings under enclosed conditions, the performance of freshly coated carbon-steel panels was compared with that of coated specimens after thermal treatment at 120 C. Case study I focused on simulating the effect of carbon steel in closed environments (i.e., CUI conditions or ballast tanks). To evaluate the corrosion behaviour and mechanism when the samples were immersed in 3.5 wt. %NaCl solution vs. exposed to condensation conditions. Electrochemical methods were applied to monitor corrosion. In case study II, a phenolic-epoxy coating was exposed to different heating conditions (isothermal or cyclic temperatures) based on high dry temperatures. The highest performance of the coated panels was observed after exposure to thermal cycling (22–120 C) for 40 d, whereas cracking of the coating was observed after treatment at 150 °C for 3 d. In case study III, the corrosion performance of panels coated with commercial phenolic epoxy was analysed after exposure to thermal cycling (22–120 C for 40 d) and compared to that of freshly coated specimens. Accelerated corrosion tests were conducted in (3.5 and 5.0 wt. % NaCl solutions) in an enclosed system at 80 C for 60 d. The effect of chloride ions on the coatings was investigated by visual observation (degree of blistering and degree of degradation around an artificial scribe in the coating) and electrochemical measurements. More significant degradation of the freshly coated panels was observed after exposure to 3.5 wt. % NaCl than 5.0 wt.% NaCl. The exposure of the organic coatings to heat provided improved resistance against severe marine conditions. Case study IV compared the performance of phenolic-epoxy-coated panels before and after thermal exposure (thermal cycling; 22–120 C for 40 d). The effect of the solution pH (4 or 8) on the coated specimens was evaluated in simulated seawater (3.5 wt. % NaCl solutions) at 80 C for 60 d. These tests aimed to mimic the acidic/alkaline marine environments as different enclosed environments with different pH ranges. The results of electrochemical measurements and the degree of blistering and delamination around scribed regions of the coated panels showed that the alkaline 3.5 wt. % NaCl solution caused greater coating degradation than a similar acidic solution. In contrast, thermal treatment of the coated panels improved corrosion protection against severe marine conditions at both pH levels. Overall, this study identified the conditions under which phenolic-epoxy coatings provide maximum corrosion protection to steel in enclosed environments. In conclusion, the case studies demonstrate that heat treatment of the coated panels improves their coating performance in enclosed environments.