Electrochemical Formation Of Highly Pitting Resistant Passive Films On Stainless Steel Surface

Pitting is one of a localized corrosion of stainless steel in corrosive media. Generally in the case of halide anions environment it is more prone to pitting, chlorides and bromides have the strongest corrosion to metal. The corrosion resistance of stainless steel is due to the thin oxide film on surface, called a passivation film, stainless steel start pitting from the passivation film. The different nature of the passive film, such as chemical composition, electrical properties and thickness determines the pitting corrosion resistance of stainless steel. Generally agreed that the passive film is formed due to the presence of chromium oxide, with the increase of chromium oxide, pitting resistance of stainless steel becomes better. Although stainless steel has good corrosion resistance to general corrosion, it is very sensitive to localized corrosion, pitting is one of the most serious localized corrosion of stainless steel, which makes the device easy to perforate, there is a big risk in actual production.In order to improve the pitting resistance of stainless steel, a lot of work have done to find methods to improved the passivation film of stainless steel. These methods focus on removing surface impurities, changing the chemical property of the passivation film and the element distribution in the film or to increase the Cr/Fe ratio and so on.Cyclic voltammetry (CV) is a commonly used simple electrochemical techniques.316,304stainless steel are modified by cyclic voltammetry in the phosphate buffered solution. Electrochemical polarization curves, electrochemical impedance spectroscopy, salt spray test are uesed to evaluate pitting corrosion resistance of stainless steel. Also discussed the effect of scan number, scan rate, chloride concentration, temperature on the results. Studies have shown that after surface treatment the passive film of the sample showing better pitting resistance compared to the unmodified sample. SEM pictures show: after corroded in a constant temperature salt fog box for30days, the corrosion degree of unmodified sample surface is significantly serious than the samples after electrochemical modification. Inorganic non-metallic coating is a common method of surface modification, Put304stainless steel coated with chromium phosphate into box-type resistance furnace sintering at a certain temperature and calcination temperature conditions, after repeated experiments, analysis the chromium phosphate coating performance on surface. XRD, SEM results shows:after repeated sintering experiments, get a thicker coating of dense amorphous chromium phosphate on the surface. Electrochemical polarization curves show that the sample surface is coated with a chromium phosphate compared to sample without treatment, exhibits better corrosion resistance.