自然海水中低合金铸铁点蚀的化学-电化学溶解机理

CHEMICAL-ELERTROCHEMICAL DISSOLUTION MECHANISM FOR PITTING CORROSION OF LOW ALLOYED CAST IRONS IN NATURE SEAWATER

根据自然海水中低合金铸铁试验结果,在蚀坑内受扩散和迁移控制的质量传输、化学溶解和电化学溶解的基础上,建立自然海水中铸铁点蚀的化学-电化学溶解模型,并推导出蚀坑内电压分布的方程式。此式表明,蚀坑内的电压取决于蚀坑内各类活性腐蚀因子的化学势梯度和相应粒子的电压降。试验结果表明,自然海水中铸铁表面钝化膜的诱发期的局部破坏与海生物的局部附着和生物粘膜下R（R=wCl-wOH-）值密切相关,在其蔓延生长过程中,蚀坑内外的电压降、蚀坑内活性粒子的活度和pH值密切相关。但自然海水中的卤素离子的存在和摩尔分数对铸铁点蚀影响最大。

A chemical-electrochemical dissolution model for localized pitting corrosion of low alloyed cast irons in nature seawater is set up on the base of its really exposed experiment results, chemical dissolution, electrochemical dissolution and mass transport governed by diffusion and migration of active ions inside the occluded pitting. An equation for the potential ofmetal inside the occluded pitting is deduced. The equation shows that the potential depends on the chemical potential gradient of the active ions in the occluded pitting and partial potential drop corresponding to the same ions. Experimental results show that local marine biofouling and R(R=wCI-/wOH-under the biofilm play an important role on the initiation localized pitting corrosion of low alloyed cast irons, potential drop and mobility of active ions and pH inside occluded pitting on propagation of the corrosion inside pitting. However a face must be clearly understood that halogen existing and its concentration in nature seawater badly influence on the localized pitting corrosion of cast irons.