Study On The Electrochemical Noise Characteristics And The Mechanism Of Crevice Corrosion

Crevice corrosion is a dangerous localised corrosion which widely existes and can cause great damage to the metal materials. Generally speaking, almost all the metals are able to suffer crevice corrosion, and nearly all the aggressive media can trigger crevice corrosion of metallic materials. Even though much attention has been paid on the research of crevice corrosion mechanism quite earlier, now it is still necessary for the further investigation to understand its machnism more clearly. Electrochemical noise (EN), as a non-destructive testing technology, can reflect the real status of corrosion without imposing any disturbances, and is appropriate for studying the mechanism of crevice corrosion in laboratory and monitoring it in situ in fields. Consequently, the investigation of the crevice corrosion behavior for different steels in different aggressive media using EN measurement together with other electrochemical methods has important theoretical and practical significance for the further understanding of its mechanism and for the developing of its in situ monitoring technique.In this dissertation, three typical corrosion systems were chosen for the crevice corrosion study, i.e. carbon steel (Q235) in NaHCO3 (0.5M)-NaCl (0.1M) solution, stainless steel (13Cr) in 3.5% NaCl solution and carbon steel (X52) in CO2-saturated HAc (600mg/L)-NaCl (0.1M) solution. The solutions are alkaline (pH=8.3), near neutral (pH=6.7) and acidic (pH=3.4), including the passive system and active dissolution system. EN, potentiodynamic scanning, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were employed to study the crevice corrosion behaviour and its mechanism. The main results are as follows:The crevice corrosion behavior of Q235 carbon steel in the NaHC03+NaCl solution system could be divided into three stages:an incubation stage, a transformation stage and a stable development stage. Increasing the ratio of the outside creavice area to the inside creavice area (r), the lifetime of the incubation stage increased. When r=1,10, the outer electrode was in an active dissolution state after the crevive corrosion occured, which caused the obvious negative movement of the coupled potential and the large decrease of the potential difference between the outer and inner electrodes. Therefore, the crevice corrosion rate was relative low. When r=160, the outer electrode surface was in a passive state after the crevive corrosion occured, the potential difference between the outer and inner electrodes was large, which caused the serious crevice corrosion.The crevice corrosion behavior of 13Cr stainless steel in 3.5% NaCl solution also could be divided into three stages:an incubation stage, a rapid development stage and a stable development stage. The value of r and crevice width (a) influenced the crevice corrosion occurrence and development significantly. As r increased, the incubation stage lasted longer, but crevice corrosion developed with a higher rate a increased, the occlude effect weakened and the crevice solution volume increased so that the acidification process needs more time to end the incubation stage and the corrosion was slighter than that at smaller a value. The corrosion inside the crevice began from the crevice bottom and then spread to the whole surface. In case of a low pH value and low potential, proton could be reduced on the uncorroded area of electrode to form hydrogen bubbles inside the crevice. The corrosion product accumulated at the crevice opening after coupling for a long time.The Principal Component Analysis/Hierarchical Agglomerative Cluster Analysis (PCA/HACA) could be used to analyse the EN data of the crevice corrosion in the above passive systems so as to identify the crevice corrosion stages precisely and automatically. The results agreed with those obtained from the analysis of the EN data in time domain.The crevice corrosion behavior of X52 carbon steel in CO2-saturated 0.1M NaCl solution containing HAc did not show obvious stages. The chloride enriched and the pH value increased inside the crevice. Moreover, the increase of CCl- and pH in the crevice solution are more obvious with the decrease of a. When the electrode in crevice solution was uncoupled with the electrode in bulk solution, its corrosion rate decreased obviously but its anodic process increased slightly with the exposure time. With the increase of CCl-and pH in the crevice solution, the depressing of the cathodic process is larger than the promotion of anodic process, which results in a negative shift of corrosion potential and low corrosion rate. After the electrode in crevice solution was coupled with the electrode in bulk solution, with the corrosion proceeds, the change of species (alkalization and the enrichment of Cl- ion) in the crevice due to the difficulty of mass transfer resulted in a negative shift of the electrode potential in the crevice solution and a positive shift of the electrode potential in the bulk solution. Therefore, a galvanic corrosion effect is established between the electrode in crevice solution (anode) and the electrode in bulk solution (cathode). In this case, the corrosion rate of the electrode in the crevice solution is enhanced while the corrosion rate of the electrode in the bulk solution is retarded.