Mechanism of grooving corrosion resistance in Cu-Ti added high frequency electric resistance welded steel pipes

The effect of the sulfides on corrosion and the observation of the microstructure of the inclusions especially, the sulfides, were investigated to elucidate the mechanism of the grooving corrosion resistance in Cu-Ti added high frequency electric resistance welded steel pipes. Main conclusions are as follows. (1) The constitution of the sulfides changes from MnS to CuxS+MnS and Ti2S by the addition of Cu and Ti and the relative small sulfides less than 0.1 μm increase. (2) MnS overall covered with CuxS precipitates in Cu-Ti added steel. CuxS in Cu-Ti added steel more precipitates than that in Cu-Ti free steel because of the difference in Cu content. (3) CuxS+MnS and Ti2S are not corroded in 3% sodium chloride solution (NaCl) from the in-situ observation. Therefore, it was supposed that the grooving corrosion of the welded portion because the constitution and the morphology of the sulfides varied by the addition of Cu and Ti. That is, it is caused by preventing MnS itself from dissolving for the environment by the formation of CuxS.