Abstract: A scanning micro-electrode like a needle was used to measure the potencial distribution on the speciment surface of 921 steel and it's welding joint in the marine water and study the behaviour of the active process of localized corrosion. The three dimensional potencial distribution maps that the results were made by test can show the potencial distribution on the specimen surface and the begining and developing of the corroded active points. They can also indicate the positions of anode and cathode area and give the changed regulation with time. They were very direct and accurate. The results of polarized conditions gave the very govd date for protection.