Studies of corrosion and stress corrosion cracking behaviour of x80 pipeline steel
Creators: name: Xue, Huibin
Publisher: name: University of Calgary
Year: 2011

description: Bibliography: p. 174-186
description: Many pages are in colour.
description: Corrosion and stress corrosion cracking (SCC) have been identified as essential threats to the integrity of pipelines. A research program was developed to investigate the fundamentals of corrosion and SCC of the high-strength X80 pipeline steel under conditions relevant to pipelines operation. The microstructure of X80 steel consists of polygonal ferrite and bainitic ferrite matrix, with martensite/austenite constituents distributing along grain boundaries. The inclusions existing in the steel include those enriched with Si, A b O3, Si-ferric carbide and Al-Mg-Ca-O mixture, respectively. The majority of inclusions are Si-enriched. Upon hydrogen-charging, cracks can initiate in the steel in the absence of external stress, which are primarily associated with the Si- and Al2O3-enriched inclusions. The diffusivity of hydrogen in X 80 steel at room temperature is 2.0x 10-11 m2/s, and the estimated hydrogen trapping density in the steel is as high as 3.33 x l027 /m3. Moreover, the effects of welding on corrosion and SCC of X80 steel were investigated. There is the smallest hydrogen permeation rate and hydrogen diffusive coefficient, but the highest hydrogen trapping density at heat-affected zone (HAZ), while the base steel has the highest hydrogen diffusivity and a low hydrogen concentration as well as the lowest hydrogen trapping density. These results are attributed to the typical microstructures of the individual zones of welded X80 steel. In near-neutral pH environn1ent, SCC of X80 steel is attributed to a synergistic effect of hydrogen and stress on the local dissolution of steel at the crack tip. The hydrogen and stress enhanced anodic dissolution of steel was characterized by an in-house photo-electrochemical measurement system, where the photocurrent density was measured on the steel. It is determined that, of the total photocurrent density measured at the crack tip of the charged steel, the photo-induced current density, hydrogen-enhanced dissolution current density and photo-oxidative current density of hydrogen atoms contribute approximately 65.8%, 12.8% and 21 .4%, respectively. A higher photocurrent density is measured at crack tip than the region ahead of the crack for both the charged and uncharged electrodes, demonstrating the high electrochemical activity at the crack tip, and the accumulation of hydrogen atoms locally. The passivity and pitting corrosion behaviour of the steel in a high pH carbonatebicarbonate solution were investigated. It is found that a stable passivity can be established in the absence and presence of chloride ions in the solution. Hydrogencharging does not alter the transpassive potential, but increases the passive current density. When chloride ions are introduced, pitting corrosion will initiate. Photo illumination can enhance the activity of the steel electrode, resulting in an increase of photo-induced anodic current density. Furthermore, the pitting corrosion of the steel under sand bed in COrcontaining systems was characterized and "visualized" in-situ by a localized electrochemical impedance spectroscopy technique. Moreover, the pitting growth rate was measured by monitoring the local current density through a scanning vibrating electrode. A galvanic effect model was proposed to illustrate the initiation and growth of corrosion pits. It also confirmed that the increases of the chloride concentration and flow velocity of the solution favored the growth of corrosion pits.