Evolution of dissolution process at the interface of carbon steel corroding in a CO2 environment studied by EIS,”

a b s t r a c t The evolution of interfacial phenomena during CO 2 corrosion of C1018 carbon steel was characterized by EIS (Electrochemical Spectroscopy Impedance) and LPR (Linear Polarization Resistance). Turbulent conditions were simulated by a channel flow cell with deoxygenated 3 wt.% NaCl solution at 80°C and pH 6 during 158 h. EIS helped in the characterization of the dynamic mechanism during the formation of the unprotective porous Fe 3 C layer, and subsequent precipitation of the protective FeCO 3 layer inside the cementite. The experimental response of the active states at the interface was characterized by electrical passive elements with constant phase parameter analogs showing good agreement with the experimental results

The Influence of pH on localized corrosion behavior of X65 Carbon Steel in CO2-Saturated Brines

Pitting and localized corrosion of carbon steel is considered to be a complex process influenced by a wide range of parameters such as temperature, bulk solution pH and chloride ion concentration. Solution pH is known to influence corrosion product characteristics and morphology in CO2 and H2S-containing corrosion systems. However, from the perspective of pitting corrosion in CO2-saturated environments, the extent to which bulk pH of solutions and the presence of corrosion products influence localized attack is still not clearly understood. This paper presents an investigation into the role of pH on the characteristics of corrosion product and pitting corrosion behavior of X65 carbon steel in CO2-saturated brine. Pitting corrosion studies were conducted over 168 hours at 50°C in 3.5 wt.% NaCl solutions at different bulk pH (buffered to pH values of 6.6 and 7.5 in some cases) in order to understand and correlate the role of pH on corrosion product morphology, chemistry, initiation and propagation of pits within each distinct environment. Corrosion product composition and morphology are identified through a combination of electrochemical and surface analysis techniques, which include Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The extent of corrosion damage of the carbon steel is evaluated through the implementation of surface interferometry to study discrete pit geometry; namely, the size, depth and aspect ratio. Results indicate that the process of pit initiation and propagation of carbon steel in CO2 corrosion environment is different depending upon bulk solution pH. At low pH (pH values starting at 3.8), pitting initiates faster and propagates steadily along with significant uniform corrosion due to the formation of ‘amorphous’ form of FeCO3. At higher pH, uniform corrosion is significant, while pitting initiates with increasing protection from crystalline FeCO3. At a pH value of 7.5, pitting corrosion initiation occurs after and/or during pseudo-passivation is achieved due to the formation of a ‘protective and pseudo-passivating’ FeCO3 film

Corrosion Behavior of Metal Active Gas Welded Joints of a High-Strength Steel for Automotive Application

© 2017, ASM International. In this work, the corrosion behavior of metal active gas-welded joints of a high-strength steel with tensile yield strength of 900 MPa was investigated. The welded joints were obtained using two different heat inputs. The corrosion behavior has been studied in a 3.5 wt.% NaCl aqueous solution using electrochemical impedance spectroscopy and potentiodynamic polarization tests. Optical microscopy images, scanning electron microscopy and transmission electron microscopy with energy-dispersive x-ray revealed different microstructural features in the heat-affected zone (HAZ) and the weld metal (WM). Before and after the corrosion process, the sample was evaluated by confocal laser scanning microscopy to measure the depth difference between HAZ and WM. The results showed that the heat input did not play an important role on corrosion behavior of HSLA steel. The anodic and cathodic areas of the welded joints could be associated with depth differences. The HAZ was found to be the anodic area, while the WM was cathodic with respect to the HAZ. The corrosion behavior was related to the amount and orientation nature of carbides in the HAZ. The microstructure of the HAZ consisted of martensite and bainite, whereas acicular ferrite was observed in the weld metal