Failure mechanism of polyurethane coated steel subjected to erosion-corrosion

Puget, Yannick (1999) Failure mechanism of polyurethane coated steel subjected to erosion-corrosion. University of Southampton, Doctoral Thesis.

Abstract

Numerous applications in the marine industry require pipes to carry seawater. Due to the aggressive nature of the water and to the particles carried by the flow, the internal surfaces of these pipes are subjected to erosion-corrosion. In order to increase the pipe lifetime, the internal pipe surfaces can be protected by barrier coatings. Hence, in a pre-screening program, the performance of a range of coatings on steel was evaluated, including ceramic, polymeric and metallic. The tests were made using a slurry jet impingement apparatus (30° impact angle, jet velocity 10-20 m/s, sub-angular silica sand, 235 μm diameter). Flexible polyurethane (PU) coatings offered the best combination of cost and erosion resistance.

Consequently, the mechanisms of degradation of PU coatings on steel subjected to slurry erosion-corrosion were identified, using a purpose-built apparatus. The samples were subjected to a slurry jet made of 3.5-13% wt. NaCl solution, containing 0-4% sand particles (90° impact angle, jet velocity 3-4 m/s). The corrosion reactions present on the substrate were examined using the electrochemical noise technique "in-situ". The mechanism of material removal by the sand particles was micro-cutting and ploughing at low impact angle, and fatigue crack propagation and intersection at normal impact. The coatings with no defect remained fully protective for at least 10 months when exposed to a purely corrosive solution, and until total stripping by the sand particles when exposed to a corrosive slurry. In contrast, for the coatings with defects, corrosion reactions were rapidly initiated on the substrate after solution transport via the defects to the coating/metal interface. This was indicated by the increase of the current spontaneously generated by the substrate above the lower level of current detection by the instrumentation. At the base of the defects, the corroding metal area increased as the corrosion reactions etched the metal, whilst the coating gradually delaminated from the substrate. During the exposure to a corrosive slurry, initially embedded defects were opened by the erosion, leading to the initiations of additional corrosion reactions on fresh metal areas. However, no synergism was detected between the erosion and the corrosion with these test conditions.

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