Nanoindentation study of corrosion-induced grain boundary degradation in a pipeline steel

Abstract

High-strength low-alloy steels used for oil and gas pipelines are vulnerable to intergranular stress corrosion cracking in moderately alkaline soils. The mechanism of corrosion-induced embrittlement under such conditions is not yet understood. Nanoindentation was used to detect localized degradation of mechanical properties near internal grain boundaries of X-70 steel undergoing intergranular corrosion at active dissolution potentials at pH 8.2. The measurements identified a one-micron thick mechanically-degraded layer with 25% reduced hardness near corroded grain boundaries. It is suggested that the corrosion process may introduce an active softening agent, possibly non-equilibrium lattice vacancies generated by oxidation. KeywordsIntergranular corrosion, Dissolution-induced degradation, Stress corrosion cracking, Grain boundary softening, Nanoindentation Disciplines Aerodynamics and Fluid Mechanics | Aerospace Engineering | Nanoscience and Nanotechnology | Structures and Materials CommentsThis is a manuscript of an article published as Yavas, Denizhan, Pratyush Mishra, Abdullah Alshehri, Pranav Shrotriya, Kurt R. Hebert, and Ashraf F. Bastawros. "Nanoindentation study of corrosion-induced grain boundary degradation in a pipeline steel." Electrochemistry Communications 88 (2018) This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. A C C E P T E D MA N U S C R I P T AbstractHigh-strength low-alloy steels used for oil and gas pipelines are vulnerable to intergranular stress corrosion cracking in moderately alkaline soils. The mechanism of corrosion-induced embrittlement under such conditions is not yet understood. Nanoindentation was used to detect localized degradation of mechanical properties near internal grain boundaries of X-70 steel undergoing intergranular corrosion at active dissolution potentials at pH 8.2. The measurements identified a one-micron thick mechanicallydegraded layer with 25% reduced hardness near corroded grain boundaries. It is suggested that the corrosion process may introduce an active softening agent, possibly non-equilibrium lattice vacancies generated by oxidation.

Morphology and stress evolution during the initial stages of intergranular corrosion of X70 steel

“…Nanoindentation measurements revealed an elastic modulus of 28 GPa for the product layer, much smaller than the value of about 200 GPa for dense iron oxide [22,25]. The low modulus of the film suggests high porosity, also consistent with a precipitated film.…”

“…2 and 4 (a), and an underlying compact layer between the precipitate and the steel. Evidence for such a nonporous corrosion product was obtained by nanoindentation measurements, which revealed submicron-thick regions close to the metal with elastic modulus approaching that of compact iron oxide [21,22]. Figure 6 shows an EDS line scan across a corroded grain boundary in a sample exposed for 2 hr at -0.521 V. The major species in the corrosion product layer are iron, oxygen and carbon, consistent with a product layer composed of iron hydroxy-carbonates [15].…”

“…However, very few studies exist on stress evolution accompanying corrosion along grain boundaries. Recently, we reported a nanoindentation study of corroded X70 pipeline steel which suggested that corrosion directly induces mechanical property changes at grain boundaries [21,22]. The experiments revealed 1 ߤm-thick mechanically degraded layers adjacent to corroded grain boundaries with 25% lower hardness relative to the grain interior.…”

“…above, the presence of this compact layer was revealed by nanoindentation measurements [21,22]. Since adherence of the surface film with the metal requires compatibility of the net in-plane strain at the metal-film interface, the formation of the compact layer must result in elastic or plastic strain within the film.…”

“…Therefore, despite the uncertain transport number, it is likely that the volume expansion due to oxide formation is accommodated by plastic rather than elastic deformation. The yield stress of the compact corrosion product was estimated to be 330 MPa from nanoindentation hardness measurements [22]. At the force levels in Fig.…”