Analysis of the corrosion scales formed on API 5L X70 and X80 steel pipe in the presence of CO2

Longitudinal welded joints samples of X70 and X80 API grade steel pipes were exposed to a deaerated 1% NaCl solution containing CO2, at temperatures in the range of 25 ºC to 80 ºC for 48 hours. Analogous samples were exposed to the adjacent vapour phase. The layer of corrosion products which formed on the base metal and welded regions of the samples was investigated using mass loss determinations, scanning electron microscopy and X-ray diffraction. The corrosion products formed in tests at 60 ºC and were composed of FeCO3. The FeCO3 which had initially formed at 80 ºC was observed to have subsequently decomposed, resulting in a layer of Fe2O3.