Investigation of Stress Corrosion Cracking of Storage Tanks used to Store Methanol for Gas Hydrate Inhibition
Methanol is normally used in the oil and gas industries for inhibiting gas hydrate. Before use, the methanol is normally stored in tanks often made of carbon steels and stainless steels, and these tanks are prone to corrosion when exposed to methanol. Many investigations have been carried out by researchers to understand the phenomenon of stress corrosion cracking (SCC) in ethanol. Very few investigations have, however, been carried out on methanol stress corrosion cracking. Even the work that has been done, has involved performing slow strain-rate tests, so the contribution of residual stress alone has not been investigated. This work, therefore, focuses on investigating the effect of residual stress brought about by elevated temperatures, arising from welding processes, on carbon steel during construction of methanol storage tanks. The samples used during the test were made from X65-grade carbon steels, and the test was performed at ambient temperature. The samples were immersed in aerated methanol and purged methanol; and the results obtained suggest that the use of nitrogen in the purging of methanol plays an active role in localized corrosion of carbon steels, as indicated by Fourier Transform Infrared (FTIR) peaks. Cracks were found to have been initiated by pit coalescence and propagated by crack coalescence along the surface of the samples studied. SEM analysis revealed cracks through the thickness that were covered by corrosion product. EDS analysis of the corrosion product formed on samples exposed to purged methanol can be used to confirm the active role of nitrogen in the localized corrosion of the samples.