Cyclic Crack Growth Rates of X-60 Pipeline Steel in a Neutral Dilute Solution

Abstract

An experimental investigation was carried out to measure transgranular stress corrosion crack growth rates (CGR) in precracked X-60 steel specimens exposed to neutral (pH 7) dilute simulated ground water (designated NS4) while being cyclically loaded in cantilever bending. Testing was carried out over short-term (generally < 40 days) and long-term periods (~ 1 y). Loading and environmental conditions in the long-term tests were similar to those for buried natural gas pipelines, with the R ratio (minimum/maximum load) varying from 0.82 to 0.98 and frequencies ranging from 1 to 400 cycles/day. Cyclic loading conditions for specimens in the short-term tests were more severe, with R ratios ranging from 0.5 to 0.9 and frequencies from 40 cycles/day to 5,000 cycles/day. Post-mortem metallographic examination by optical (OM) and scanning electron microscopy (SEM) was used to evaluate crack growth. Average CGR from 4.50 x 10 -8 mm/cycle to 1.25 x 10 -4 mm/cycle (equivalent to 1.4 x 10 -9 mm/s to 7 x 10 -7 mm/s) were measured, depending upon applied load range. For the more severe loading conditions in the short-term tests, the observed growth was attributed to corrosion fatigue. Under the less severe loading conditions of a high R ratio and low frequency, growth was attributed to transgranular stress corrosion cracking.

Corrosion fatigue crack growth behavior of pipeline steel under underload-type variable amplitude loading schemes

“…For these lower frequencies, the effect of room temperature creep may also become significant. However, only a few investigations have been reported for the corrosion fatigue crack growth at loading frequencies lower than 10 À3 Hz [16,18].…”

Modelling environmentally assisted cracking in pipeline steels

“…As stated earlier, it is anticipated that atomic hydrogen, resulting from the cathodic reaction, diffuses into the steel and accumulates at regions of high stress triaxiality, lowering the cohesive strength ahead of a crack tip. Crack advance then occurs by tearing the weakened region, producing a quasi-cleavage fracture surface, as observed by the authors [5,15].…”

Electrochemical characteristics of pipeline-steel with planardefect epoxy-coating in simulated solutions of Ku’erle soil

“…inevitably occur on the coating surface of X70 steel pipelines or low-carbon steel underground. The category and size of the coating defects greatly influence the corrosion laws and cathodic protection effectiveness [1][2][3]. Saline soil is widely distributed in West China such as Ku'erle areas.…”