On the detection and mitigation of corrosion under insulation of mild steel

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Corrosion under insulation(CUI) is a form of localised corrosion which frequently occurring in carbon/low alloy steels. The inherent difficulties on CUI detection make them hard to reveal outside and it is always too late until the problem becomes severe. Integrity loss and associated maintenance cost related to CUI show an increase every year and CUI comes up to be one of the greatest corrosion issues. Currently applied techniques on CUI detection are mainly non-destructive tests such as radiography and ultrasonic methodologies. However, those detection approaches involve high processing cost and they are unable to obtain corrosion rate data to estimate the lifespan of desired materials. In our research, an in-house built electrochemical probe targets at obtaining electrochemical data such as corrosion rate and corrosion potential as well as impedance data from corroded mild steel(MS) piping systems under insulation layer. The monitoring probes are cost-effective and easy-processing, which provide an inventive electrochemical method for CUI detection. Moreover, sacrificial zinc protection is used as cathodic protection (CP) for the mitigation of CUI occurring in MS pipes. It was identified that the efficacy of CP through insulation depends upon the ionic resistivity of the insulation and the moisture content (by volume) must be >25% for efficient ionic migration through the insulation. The maximum throwing power of CP (~75%) through the insulation was achieved when Zn was in direct physical contact with MS, with the Zn: MS area ratio being 1:10. In addition, the mechanism of CUI was studied as a function of moisture content within insulation and pH values of the solution. It was indicated that high moisture content and lower pH of the solution accelerate CUI behaviour.    

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