| Dew point corrosion(DPC)is defined as the condensation of acid vapors including SO2、SO3、NOx、HCl、H2S、H2O etc below the saturation temperature.A novel DPC simulation set-up is established so as to realize the in situ electrochemical test and electrochemical impedance spectroscopy(EIS)and electrochemical noise(EN)are carried out by two electrode cell system and three electrode cell system,respectively.In situ and ex situ research methods of DPC are compared and the research results on the hydrochloric acid DPC behaviour of Q235 steel and 304 stainless steel show that these two DPC test methods are essentially different.Effect of HCl concentration on the hydrochloric acid DPC behaviour of Q235 steel at 50℃,70℃ and 90℃ is investigated using the DPC simulation set-up and the EN data is analyzed by Hilbert-Huang transform.The results indicate that corrosion rates of Q235 steel is increased with the increasing HCl concentration in the electrolyte layer in hydrochloric acid DPC environment and effect of HC1 on the corrosion of Q235 steel consists of two simultaneous processes:accelerating galvanic corrosion between phase and phase in the steel and promoting uniform corrosion of the steel.As HCl concentration in the electrolyte layer is low,galvanic corrosion effect is dominant in the corrosion process and the corrosion of Q235 steel show localized corrosion characteristic.With the increase of HCl concentration in the electrolyte layer,effect of uniform corrosion is enhanced and the corrosion of Q235 steel is characterized by uniform corrosion.The EN data analysis results after Hilbert-Huang transform is in good consistent with the corrosion morphologies.As a novel time-frequency analysis method for EN data,it provides a new way to obtain more information concerning the corrosion process.Influence of microstructure on the hydrochloric acid DPC behaviour of Q235 steel is discussed.Ferrite and pearlite scatter randomly along rolling direction of the Q235 steel and pearlite shows a net-like feature with ferrite distributing between pearlite nets along normal direction of Q235 steel.Weight loss of Q235 steel along rolling direction is higher than that along normal direction,which depends on Faraday process and non-Faraday process.From the point of view of Faraday process,volume of cathodic phase in the rolling direction of Q235 steel is higher,but cathodic density is lower than that in the normal direction,which leads to nearly the same weight loss resulting from Faraday process in two directions.From the point of view of non-Faraday process,pearlite is easily broken off in the rolling direction and not in the normal direction,which results in more weight loss due to non-Faraday process in the rolling direction.Pitting resistance of 304 and 316L austenite stainless steel in the hydrochloric acid DPC environment is evaluated by EIS and EN and the results show that pitting resistance of 316L stainless is better than 304 stainless steel.The shot noise analysis results and corrosion morphologies observations by SEM indicate that pitting generation rate and growth rate of 316L stainless steel are both lower than that of 304 stainless steel.Chemical compositions of passive film are analyzed by XPS and the results demonstrate that the states of Fe and Cr in the passive film of these two stainless steels have no difference.Reasons for the pitting performance of the two stainless steels can be summarized as two aspects:1.chromium enrichment and lower Fe elemental concentration in the passive film of 316L stainless steel;2.there exists Mo which can improve the pitting resistance in the passive film of 316L stainless steel. |
