"In-situ" Measurement of Corrosion Rate of Zinc-5% Alluminum Alloy Plated Steel Sheet by Using AE Method

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In laboratory, the corrosion rate of metals in aqueous solutions is measured by using the methods of corrosion weight loss measurement, polarization resistance measurement and a polarization curve measurement. However, these methods cannot be used as the "in-situ" mesurement. of the corrosion rate. Since corrosion rate changes greatly with various environmental conditions, development of the new method for "in-situ" mesurement is desired. We have investigated the hydrogen damage in steels caused by cathodic polarization using the acoustic emission (AE) method. During the cathodic polarization, AE generated in a corrosion reaction was detected in addition to AE resulting from hydrogen damage. Consideration of the corrosion environment where the AE was detected, the AE was estimated to be due to the evolution reaction of hydrogen gas.On the other hand, galvanizing of steel sheets has been carried out for the purpose of improvement of the corrosion resistance, and the zinc-plated steel sh … More eets have been used as building materials, and outer panel of an electric product and an automobile. In order to protract the usable period and use in more severe environment, the more excellent plating process in corrosion resistance has been developed, and zinc-alloy plating of zinc-aluminum, zinc-nickel, etc. has been performed. The zinc-5%aluminum alloy plated steel sheet is higher in corrosion resistance. It is reported that the corrosion resistance is improved by alloying of aluminum to zinc, and the improvement is owing to formation of corrosion protective films of Zn_6Al_2(OH)_<16>CO_3・4H_2O and ZnCl_2・4Zn(OH)_2H_2O as corrosion products. However, a reason of the improvement is not clear for details. For development of the zinc alloy plating with higher corrosion resistance, it is important to clarify the reason of the improvement in corrosion resistance caused by alloying of aluminum.In the present work, the following were investigated (1)the cause of AE detected for the corroding metal specimen in aqueous solutions, especially our estimation that the AE is caused by the evolution of hydrogen gas, (2)application of the AE method to "in-situ" measurement of corrosion rate, (3)development of the equipment measuring corrosion rate under the "in-situ" condition by using the AE method, and (4)the corrosion process of zinc-5%aluminum alloy plated steel sheet in aqueous solutions. Less