Stress corrosion cracking (SCC) on the external surface of pipelines contributes to the major failure of pipelines. The great majority of SCC is intergranular and occurs in a carbonate-bicarbonate environment. Metallurgical factors affecting SCC are still vague and therefore have been studied. Uniform microstructures, not mixed structures, are favorable for suppressing SCC. Low-C steels produced in a process such as thermomechanical-controlled processing are less susceptible to SCC. The presence of locally soft microstructures decreases resistance to SCC (mixed structure and decarburized structure). However, SCC resistance is high on hard layers, like grit-blasted surfaces.
[Show abstract][Hide abstract] ABSTRACT: This paper reviews the current understanding of the mechanisms of stress corrosion cracking of pipeline steels. The similarities, the differences and the influencing factors are considered for the “high pH” stress corrosion cracking caused by a concentrated bicarbonate-carbonate solution, and for the “low pH” stress corrosion cracking due to a diluter solution. For high pH stress corrosion cracking, it is well accepted that the mechanism involves anodic dissolution for crack initiation and propagation. In contrast, it has been suggested that the low pH stress corrosion cracking is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen into the pipeline steel. But the precise influence of hydrogen on the mechanism needs to be further studied.
[Show abstract][Hide abstract] ABSTRACT: Electrochemical impedance spectroscopy, corrosion potential measurements, and surface analysis by scanning electron microscopy/energy-dispersive x-ray spectroscopy (SEM/EDX) and Raman spectroscopy were used to investigate the localized dissolution of millscale-covered pipeline steel surfaces. The porous millscale originally present on the pipe surface exerts an influence on the corrosion of the pipeline and may contribute to the eventual onset of stress corrosion cracking (SCC). Three regions in the corrosion potential-time plot were observed after exposure to an aqueous environment, corresponding to the initial attempts at breakdown of the millscale, coupling of the dissolution of the underlying steel to reductive dissolution of the millscale, and active corrosion of the steel at the base of pores in the film supported by water reduction either on the metal or on the millscale surface. The corrosion rate increases as the dissolved carbon dioxide (CO2) concentration increases. Changes in the solution resistance, polarization resistance, and pore resistance are related to the corrosion kinetics and growth of the pores. The porous structure of the millscale increases the possibility of local separation of anodic and cathodic sites, which would promote localized corrosion at the base of pores. These stress-raising pits eventually could act as precursor sites for the initiation of SCC.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a fundamental study of the influence of carbon steel microstructure on the corrosion rate. Subsequently, the corrosion performance of various grades of carbon steels were evaluated in stirred autoclaves under elevated carbon dioxide and temperature conditions. Corrosion and penetration rates were determined via mass loss and optical microscopy, respectively. It was found that the corrosion rate of carbon steel line pipe is influenced by microstructure. More specifically, a relationship between localized corrosion susceptibility and the presence of pearlite bands in the steel microstructure was found. However, no correlation was evident between minor elemental concentrations (i.e., Ni, Cr, Mo) and corrosion resistance. It has been proposed that the corrosion stability of the various microstructures may arise from variations in the distribution of carbon bearing phases within the steel. In the banded ferrite/pearlite structure, the carbon-bearing phase (pearlite) is distributed in layers whereas in the other structures the carbon-bearing phases are much more evenly distributed. This study reports on the corrosion resistance of carbon steels in relation to their chemical and physical properties.
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