Transgranular Stress Corrosion Cracking of High-Pressure Pipelines in Contact with Solutions of Near Neutral pH

Abstract

Although intergranular stress corrosion cracking (IGSCC) of high-pressure gas pipelines has been known for more than 20 years, a transgranular form (TGSCC) was detected more recently. Instances of TGSCC have been associated with dilute solutions with pH values in the region of 6.5 because of the presence of carbon dioxide (CO 2 ). Such pH values indicate relatively little, if any, cathodic current reaches the pipe surface, since hydroxyl ions would be generated and pH would increase to values in the region of 10 if current did reach the pipe surface. Slow strain rate testing (SSRT) of pipeline steel specimens in dilute solutions of pH in the region of 6.5 suggested dissolution and hydrogen (H) ingress into the steel are involved in the crack growth mechanism. The initiation of TGSCC in specimens subjected to cyclic loading and maximum stresses approximating those of an operating line was facilitated by pitting. The geometry of the pits allowed the localized generation of solutions of lower pH than that of the bulk solution outside the pits, thereby facilitating dissolution and H discharge.

Application of SSRT to estimate the effect of corrosive medium on the liability of X70 pipe steel to stress corrosion cracking

“…2. Comparison of the crack growth rates under static loading of pipe steel [7] with the characteristics of its cracking resistance obtained by SSRT shows that both methods give qualitatively similar results as regards the effect of soil (sub-film) electrolyte components on the liability of steel to SCC. 3.…”

“…Hence, the applicability of SSRT for studies of steel SCC under conditions of intense active dissolution is limited. Anodic polarization tests of specimens with a preformed crack show that local metal dissolution is the predominant SCC mechanism in the vicinity of the corrosion potential [7].…”

“…Taking into account that steel dissolution is the main reason of crack growth in pipe steels in weakly acidic media [7], it is of interest to consider whether anodic corrosion inhibitors can be used to hinder metal SCC.…”

“…The NS4 solution (pH 6.5 -6.8) is generally used as the test medium to study the SCC of pipe steels. The ionic composition of this solution is based on analyses of liquids found under delaminated underground pipeline coatings [7]. The SSRT method makes it possible to study the effect of separate soil electrolyte components using NS4 as the reference solution.…”

“…In those places where crevices are found on the outside pipe walls, the liquid under delaminated isolating coating (sub-film electrolyte) is a dilute aqueous solution of inorganic salts (mainly carbonates) and organic acids [6,7,8]. The sub-film electrolyte may contain dissolved hydrogen sulfide and bisulfide ions that are products of sulfatereducing bacteria life activity [6,8].…”