The role of hydrogen in the process of stress corrosion cracking of pipeline steels in dilute carbonate-bicarbonate solution

摘要

Stress corrosion cracking (SCC) in buried high-pressure pipelines occurs on the outside surface where ground water is in direct contact with pipelines through disruptions of the protective coat-ing. Instances of SCC have been related to an electrolyte composed of alkaline carbonate-bicarbo-nate [1-3]. SCC of pipeline steels in high concen-tration of carbonate-bicarbonate solution (pH of about 10) has been well studied [4-7]. However, recent research [8] has shown that the chemistry of the solution beneath coating disbondments on pipelines is concerned with much lower carbonate-bicarbonate concentrations, and hence lower pH (about 7). SCC is transgranular in low-pH solution and is intergranular in high-pH solution [9-11]. Only a few studies have focused on the SCC of the pipeline steels in dilute carbonate-bicarbonate solu-tions in the presence of chloride ions. Since the low pH of solution facilitates hydrogen accumulation in stressed region in steel, hydrogen plays an important role in SCC [12]. It is important to clarify whether hydrogen could diffuse into the materials and its distribution status around the crack tip, if the dissolved hydrogen has influence on SCC of the pipeline in NS-4 solution. The research described in this paper was carried out with the following objectives: firstly to determine hydrogen reduction and enrichment near the crack tip, and secondly to investigate effects of hydrogen on the anodic SCC of the pipeline steels in dilute carbonate—bicarbonate solution.