Failure analysis of an oil pipe wall perforated by pitting corrosion

Abstract

Failures of underground oil pipes caused by pitting perforation are investigated in this paper. The chemical composition of the failed oil pipe was analyzed by the direct reading spectrometer, which is in accordance with China national standard GB/T699‐1999 for 20 steel. The failed parts were investigated by means of visual observation, optical microscope (OM) and scanning electron microscope (SEM). Energy‐dispersive X‐ray spectroscopy (EDS) of the inner pits were conducted. The results indicate that the pit on the failed oil pipe is composed of three smaller circular pits, on which a fatigue striation‐like pattern suggests a repetitionary pitting corrosion process, while pitting corrosion in the vertical direction is more serious. The failed position was located in the underground ditch where carbon dioxide (CO2) deposited and underwent wetting and drying cycles with an alternative of rainy and sunny days. Combined with the environment of the oil pipe, the simulation experiment, and corresponding mechanism analysis was conducted to evolve the mechanism of pitting corrosion, initiated by the synergistic effect of CO2 and chloride ions (Cl−), and impelled the production of FeCO3 corrosion agents providing an isolated space in a short time, meanwhile chloride ions proceed to propagate continuously into the inner pit, which could explain pitting corrosion is more serious in the vertical direction.

Comparisons of corrosion behaviour for X65 and low Cr steels in high pressure CO2-saturated brine

“…There is a wealth of research reaching [1,7,8,10,14,15,16,17,18,19] focusing on the influence of 1-5% Cr addition to steels to improve strength, hardenability and CO2 corrosion resistance, with the majority of observations concluding that Cr addition does improve corrosion performance [7,10] . In addition, a number of successful field trials have been reported [14,20] with the use of low Cr alloyed steels.…”

Transformation between localized and general corrosion behavior of die‐cast AZ91D magnesium alloy in 3.5 wt% NaCl aqueous solutions

“…To keep electric neutrality, chloride ions must migrate from the bulk solution to the pit walls. Meanwhile, the produced Mg 2+ diffused away, and the sites where Cl – reached will become a new anodic site because of a certain amount of H + in surrounding solutions, which further accelerated the dissolution of the metal matrix . As corrosion proceeded, these micropitting holes clustered together to form a larger localized corrosion morphology, as shown in Figure a.…”

Dual functions of chloride ions on corrosion behavior of mild steel in CO2 saturated aqueous solutions

“…The function mechanism of Cl − ions in CO 2 corrosion process proposed is illustrated in Figure . In CO 2 saturated solutions, CO 2 reacts with H 2 O and produces a great number of CO 3 2‐ , HCO 3 ‐ , and H + ions and increases acidic degree of the solutions, which reacts with steel as follows:…”