Electrochemical Corrosion Of Stainless Steel Under Ultrasound

304 stainless steel(304ss) is the material long-term used in Pressurized water reactor(PWR). The important coolant in process PWR is water, which is easily decomposed to hydrogen radicals, hydroxyl radicals, etc.under high temperature and high pressure. Therefore, it is of great significance to understand the impact of these species on the corrosion resistance of stainless steel. But it is very difficult to measure the influence of these ingredients in actual reactor core. As many studies found, the product such as H, OH by water splitting in nuclear industry is a great similar to that in Ultrasonic environment. So it is important to study the corrosion behavior of stainless steel under ultrasound. The effect of ultrasound on corrosion of metals is mainly to change the nature of material, increase the analysis current and improve the electroplating quality etc. The research on the effects of ultrasound with corrosion inhibitor is much fewer. This thesis has tried to combine ultrasonic with inhibitor to preliminarily discuss the mechanism of ultrasound by adding corrosion inhibitor which can inhibit the corrosion of 304 ss.The concentration of hydroxyl radicals was measured in water under ultrasound, as well as the effects of ultrasonic time, frequency and the impulse mode. The weight loss test, electrochemical methods and Field Emission Scanning Electron Microscope(FSEM) were applied to investigate the corrosion inhibition performance and mechanism under silent or ultrasound with inhibitor potassium dichromate. At the same time, X-ray photoelectron spectroscopy(XPS) was used to analyze the composition and valence state of the elements in the surface passive film of stainless steel.At first, the results showed that the amount of hydroxyl radicals linearly increased with the time, and raised with the increase of ultrasound frequency. The numbers of hydroxyl radicals produced by impulse ultrasonic were more than that by continuous ultrasound in the same time.Secondly, the weight loss test and FSEM indicated that the corrosion of 304 ss was inhibited by ultrasonic cavitation in the blank solution. Potassium dichromate was a "dangerous inhibitor" for 304 ss under quiescence, but ultrasonic could significantly improve the corrosion inhibition performance of potassium dichromate. Ultrasonic cavitation enhanced the corrosion resistance of 304 ss with the increase of temperature. The electrochemical experiment indicated that ultrasound made the pitting potential and Rct increase at the same temperature.The Mott-Schottky curve showed that the semiconducting property of the passive film changed from n type under quiescence to p type under ultrasound. The density of vacancies was less, the conductivity of passivation film was worse, so the corrosion rate was smaller. These phenomena had a greater relationship with the strong oxidizing hydroxyl free radicals generated by ultrasound. The XPS indicated that the main elements of the passive film on stainless steel surface were Fe, Cr, O and Ni and the main component was Fe, Cr, Ni oxides or their hydroxides.