X70异种钢焊接接头的电偶腐蚀行为

Galvanic corrosion of X70 dissimilar weld joints

利用SEM和光学显微镜对X70异种钢焊接接头进行显微组织观察,并在模拟土壤溶液中测量各区极化曲线以及焊缝和X70钢以不同面积比偶合时的电偶腐蚀电流.结果表明,该接头熔合区很窄,无类马氏体存在,焊缝为细小树枝晶,Nb和Mo元素严重偏析于枝晶间,在实验溶液中该区呈钝化状态,具有优异的耐蚀性.热影响区粗晶区主要为粗大贝氏铁素体和分布不均匀的长条状M-A组元,而X70母材为细小贝氏体组织,二者腐蚀行为无明显差别,阳极极化电位超过150mV时前者的极化率略高于后者.在焊缝与X70的电偶腐蚀中,阴极反应受氧扩散控制,随阴/阳极面积比增大,X70腐蚀电流密度急剧增加,具有近似"汇集原理"的腐蚀特点,偶合时焊缝阴极极化电位很高,受到完全保护.

The microstructures of X70 dissimilar weld joints were studied by optical microscope and SEM. Galvanic corrosion at different area ratios was investigated in simulated soil solution. The results indicated that the fusion zone was so narrow that no martensite-like structure was found. Fine dendrites were obtained in the weld zone, in which Nb and Mo seriously segregated in interdendritic. The corrosion resistance of the weld zone was excellent because of its passive film. There were coarse bainitic ferrite and nonuniformly distributed strip M-A constituent in the coarse-grained heat-affected zone （CGHAZ）, and fine bainite existed in X70 base material. Their corrosion behaviors were not obviously different, but as the anode polarized potential exceeded to 150 mV the polarizability of CGHAZ was relatively high. The cathodic reaction rate was dependent on oxygen diffusion when the weld zone was coupled with X70 pipeline steel. The corrosion current density of X70 pipeline steel increased rapidly with increasing area ratio. The corrosion characteristic was similar to ＂Catchment Principle＂. Because of high cathodic potential the weld zone did not suffer corrosion.