Mechanism of salt spray corrosion of X70 high grade pipeline steel welded joints

A neutral salt spray test was carried out for the welded joint of X70 high grade pipeline steel. The morphology and the change of chemical composition of the X70 pipeline steel welded joint before and after salt spray corrosion were observed by SEM and EDS, respectively. The phase before and after salt spray corrosion were also analyzed with X-ray Diffraction (XRD). The ingredient, and corrosion mechanism of the corrosion film on the surface of X70 pipeline steel welded joint were studied. The results show that pitting corrosion and exfoliation corrosion are the main failure modes of the salt spray corrosion. The passivation layer formed on the sample surface is destroyed by the active Cl- ions that contacts with Fe, which is the main cause of corrosive pitting. The tensile stress which is formed during the welding process becomes the source of stress. The stress corrosion cracking leads to the formation of exfoliation corrosion together with the effect of intergranular corrosion. The sample surface was covered by a certain thickness corrosion film, which prevented the sample to contact the corrosive medium and restrains the corrosion proceeding.