Effect of Microstructure of Low-Alloy Steel on Corrosion Propagation in a Simulated CO₂ Environment

Abstract

The paper focuses on the analysis of initiation and propagation of CO2 corrosion in several samples of low-alloy steel with different microstructures using scanning electrochemical microscopy (SECM) and other microscopy techniques. It is found that the corrosion rate and the mode of corrosion are highly sensitive to the microstructure. The overall current density is much higher and more uniformly distributed for the tempered martensite structure than for samples having either a ferritic-pearlitic microstructure or a microstructure combining ferritic, bainitic and martensitic-austenitic regions. As a result, the sample with the tempered martensite structure undergoes uniform corrosion, while the other two samples undergo selective corrosion. The SECM maps show that regions of polygonal ferrite generate larger anodic currents than the pearlitic structure in the early stages of corrosion. The residual cementite lamellae provide greater cathodic surface areas after the initial dissolution of ferritic lamellae within pearlite, promoting galvanic corrosion and subsequently enhanced dissolution of ferritic lamellae. According to SECM data, the dissolution of iron in polygonal ferritic grains is 2.4 times faster than that of ferritic lamellae in pearlitic regions.

Effect of Redox Mediators on Corrosion Behavior and Scanning Electrochemical Microscopy Response

“…Established analytical approximations can be used to relate the tip-to-substrate distance and the measured current to the RM’s kinetic rate constant at the substrate’s surface, where efforts have been made to ensure the accuracy of the fitting procedure . SECM in feedback mode has been used to estimate rate constants at distinctive features, detect surface heterogeneities, , and reveal passive film formation. , …”

“…Established analytical approxima- tions can be used to relate the tip-to-substrate distance and the measured current to the RM's kinetic rate constant at the substrate's surface, 26 where efforts have been made to ensure the accuracy of the fitting procedure. 27 SECM in feedback mode has been used to estimate rate constants at distinctive features, 28 detect surface heterogeneities, 29,30 and reveal passive film formation. 31,32 Despite their popularity in corrosion science, utilizing the analytical approximations for feedback mode has some fundamental assumptions that may be compromised when studying an actively corroding substrate.…”

Ex‐Situ Synchrotron X‐Ray Diffraction Study of Co2 Corrosion‐Induced Surface Scales Developed in Low‐Alloy Steel with Different Initial Microstructure

“…However, various CO 2 corrosion models exists within the oil and gas industry and across them, there is no consensus on exactly how the protective scale is formed or how the scale structure quantitatively controls the corrosion rate [15,16]. In CO 2 environments, the microstructural features of steel (presence of phases and their morphology, i.e., their amount, size, shape, and distribution) are found to have a significant impact on corrosion rate as well as the scaling behavior [17,18]. The role of the microstructural phases in the formation of protective scales, in particular, the iron carbonate scale, and its influence on corrosion rate has been extensively studied in the literature.…”

“…There is currently no consensus on the mechanism by which the steel microstructure resulting from the applied heat treatment could influence the growth and stability of iron carbonate layers. However, the majority of studies suggest that plain normalized steel with a ferritic-pearlitic (FP) microstructure provides considerably superior CO 2 -corrosion resistance compared to quenched and tempered steel with a martensitic microstructure [17][18][19][20][21][22][23][24][25].…”

“…The diffracted intensities were normalized using the structural factor and multiplicity for each reflection 3. The light optical micrographs of the investigated steels with 2 different initial microstructures can be found in the current authors' previous works[17,18] showing the tempered martensitic and ferritic-pearlitic microstructure for QT and FC samples, respectively.…”