Abstract
Preferential attack behavior of welded parts of carbon steel pipes used for piping of industrial water was investigated by several kinds of corrosion tests and electrochemical measurements in chloride solutions. The attack under consideration was classified into two types: the grooving corrosion that occurs along the welded seam of electric resistance welded (ERW) steel pipes and selective corrosion at the weld metal of girth welded joints of steel pipes. The attack occurred preferentially at the portion whose corrosion potential is least noble than any other parts of the weld of carbon steel pipes. The potential of the welded seam was 20 to 60mV lower than that of the base metal of the ERW steel pipe. Difference in the polarization behavior between them was large in an ordinary carbon steel pipe, while the difference is small in a grooving corrosion resistant 0.2% Cu-0.08% Ni steel one. The evaluation of the susceptibility of welded steel pipes to selective corrosion can be done by the accelerated test, in which the specimen was polarized potentiostatically at -0.5V vs. SCE for 48hrs in 3% NaCl solution at 25°C. The test result was in good agreement with those of a loop test in synthetic sea water and a long term piping test in a chloride containing solution. The evaluation of the corrosion resistance at the girth welded parts as well as grooving corrosion resistance was carried out by the accelerated corrosion test. The use of a 0.5% Cu-0.3% Ni steel as a welding material was effective to improve the localized corrosion resistance of the welded joint of carbon steel pipes.
