MECHANISTIC INVESTIGATION OF STRESS CORROSION CRAKING OF PIPELINE STEEL IN NEAR-NEUTRAL PH ENVIRONMENT

摘要

Stress corrosion cracking (SCC) has been identified as one of the primary reasons for failures in high-pressure buried pipelines [1-2]. Today, most researchers agree that SCC of pipeline steels in environments at near-neutral pH results from the synergistic effects of mechanical straining, local anodic dissolution, and hydrogen absorption [3-6]. However, it is not clear the primary mechanism of SCC in the various composition of the soil electrolyte at the various type and values of mechanical loading. This work explores the effect of corrosive compounds on the crack growth rate in Х-70 pipeline steel under cyclic mechanical tensions to determine the primary mechanism of near-neutral pH SCC.The crack growth rate was determined using X-70 steel specimens. A mixture of NS4 solution with a borate buffer (pH 7.0) were used as background solutions. Various compounds that affect steel dissolution and hydrogen absorption rates were added to the solutions.The cyclic crack resistance of pipe steel in NS4 buffer solution at pH 7.0 is mainly determined by the regularities of the corrosion fatigue. Adding an anodic dissolution stimulator (sulphide ions) or a hydrogen absorption promoter (iodide ions) nearly does not affect the crack growth at high amplitudes of the stress intensity factor (ΔK). The rate of crack growth depends on the presence of a corrosive medium only at small amplitudes of loading (ΔK ～ 5 Mpa⋅m~(0.5)). The crack growth is accelerated in the presence of sulphide and thiourea; these species act as metal dissolution activators, and are hindered after adding a corrosion inhibitor (Catamin AB) to a sulphide-containing medium. The cathodic polarization of pipeline steel inhibits crack growth at low ΔK values. Therefore, the local anodic dissolution of the metal is the primary crack growth mechanism in X-70 pipeline steel in NS4 buffer solution at small amplitudes of loading.