X70高钢级管线钢焊接接头盐雾腐蚀机理

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

利用中性盐雾试验对X70高钢级管线钢焊接接头进行室温腐蚀,通过扫描电镜和能谱仪观察了X70管线钢焊接接头盐雾腐蚀前后表面微观形貌和化学元素的变化,并采用EDS对盐雾腐蚀后焊接接头表面进行了面扫描分析.利用XRD讨论了其盐雾腐蚀前后表面物相,分析了X70管线钢焊接接头盐雾腐蚀后表面腐蚀膜的组成、作用和腐蚀机理.结果表明,盐雾腐蚀后失效主要形式是点蚀和剥落腐蚀,活性Cl-离子破坏了试样表面的钝化膜,与Fe原子接触,是发生点蚀的主要原因;焊接过程中产生的残余拉应力成为腐蚀的应力源,使材料在腐蚀介质中发生应力腐蚀开裂,在与晶间腐蚀共同作用下发生剥落腐蚀;腐蚀膜达到一定厚度后覆盖在试样表面,阻隔试样与腐蚀介质的接触,有利于抑制腐蚀的持续进行.

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.