Atmospheric localised corrosion of type 304 austenitic stainless steels

T Technology > TN Mining engineering. Metallurgy

Guo, Liya ; Davenport, Alison

University:University of Birmingham

Department:School of Metallurgy and Materials

关键词: T Technology;    TN Mining engineering. Metallurgy;   

Others  :  http://etheses.bham.ac.uk//id/eprint/6458/1/Guo16PhD.pdf

来源: University of Birmingham eTheses Repository

【 摘 要 】

Atmospheric localised corrosion of stainless steel has been investigated under salt droplets containing a mixture of MgCl2 and NaCl between the deliquescence relative humidity of the two salts where there was precipitation of NaCl crystals. Dish-shaped pits and crevice-like attack could be observed.Effects of the change of relative humidity (RH) have been studied. A pit that has grown at 33% RH for 1 day will tend to repassivate when the RH is increased to 85% while pits grown at 33% RH for 3 weeks may not repassivate at 85% RH and can continue to grow when the RH is returned to 33%. A pit that has grown at 33% RH can continue to grow after 1 day at 12% RH if the RH is returned to 33%. RH fluctuations, either to low or high RH, lead to greater number of pits.Using the 1D artificial pit method, a potential sweep method was developed to investigate the repassivation process of pits in concentrated solutions, representative of atmospheric conditions. The onset of repassivation is defined as the point where there is no increase in current with an increase in potential. It has been found that repassivation takes place when the concentration of MgCl2 is lower than 3.2 M (equivalent to 64% RH) while there is no repassivation for 3.5 M and 4 M MgCl2 (equivalent to 59% RH and 50% RH respectively). A range of the repassivation potential and the critical metal ion concentration to prevent pits from repassivation was determined.