Analysis of pitting corrosion of hot-rolled X70 micro-alloyed steel plates in storage condition

The paper analyses pitting corrosion form and control of X70 plate. The appearance of the pit under oxide film is shown as a small pit mouth at the interface and a big volume in the matrix. Discussion of the examination results and the survey of literature have led to the conclusion, that the reason of pitting corrosion of X70 microalloyed steel plate is the influence of chlorine ions in the cooling water and the concentrating of the aggressive species in rolling process promote the pitting corrosion further. The oxide film had little protective function to the substrate. The laminar structure and micro-cracks among the oxide film may provide the thoroughfare to reach the matrix for the aggressive species, such as chlorine ions. In the results, it is shown that the PO ₄ ^3- has inhibition effect on the pitting corrosion in the cooling water, but it needs more research in the application.