Study On Sacrificial Anode Protection Performance Of Hot-dip Zn And Zn-Al Alloys In Seawater

In the seawater, Zn-Al alloy coatings protect steel substrate. On the initial stage, Zn-Al alloy coatings and corrosion products make the steel subject insulated from seawater. With the corrosion going on, the flaws in the coatings get the steel substrate exposed to seawater. At the same time, the steel substrate is protected by the coating as an anode. Presently, many scholars have studied the uniform corrosion of the coatings. However, the cathodic protection of the coating has only been studied qualitatively. Therefore, the dissertation evaluated the cathodic protection qualificationally according to the sacrificial anode electrochemical test standard, which would provide scientific data to the new corrosion resistant coatings in seawater.Nine types of alloys were made and according to the standard of GB 17848-1999, their potentials were tested, their anode depletion rate ,coating consume thickness and current efficiencies were calculated and their corrosion products' falling off and corrosion uniformity were observed at 20℃,35℃ and 50℃. It was shown that Zn is the best in the electrochemical protection performance, Zn-55Al-1.6Si is the worst, and the others are between them. It was also indicated that with the temperature rising, the electrochemical protection performance for all specimens become worse. Therefore, it was proved that though on the initial stage, the more Al, the lower corroding rate, on the late stage, the high content of Al is disadvantage to protect the steel substrate. In order to study the reason of the low current efficiency, the sections of Zn-30Al-0.2Mg-0.2Si, Zn-30Al-0.2RE-0.2Si and Zn-55Al-1.6Si were observed by the scanning electron microscope (SEM) after the experiments at 50℃. It was figured that the grain-boundry corrosion is more serious with the more Al. The influence of Al to sacrificial anode property was analyzed and explained theoretically.Mathematical grey-box interrelation analysis method was used to study the correlation between five selected alloy elements and open circuit potential, working potential, anode depletion rate, coating consume thickness, current efficiency and corrosion uniformity. It was showed that Zn is the most important factor to affect alloys' cathodic protection performance, while Al and Si are inferior to Zn. It was indicated that Mg is the key element to affect corrosion uniformity of alloys. Therefore, It was educed that Zn is the best element for improving the alloy's electrochemical protection performance, Al is the worst, and more Mg will decrease the property.