The Corrosion Mechanism Of Oil And Gas Carbon Steel Pipelines Under Deposits

During the exploitation of oil and gas in the sour oil and gas fields, there would be alarge number of solid particles in the oil and gas pipelines. When the fluid velocity isinsufficient, these solid particles would deposit on the pipelines surface, and then causeserious localized pitting corrosion (such as pitting, crevice corrosion, corrosion attack),namely under deposit corrosion. Corrosion under deposit is different from the corrosion inhomogeneous medium since the corrosive medium under deposit may be different from themedium in the area without deposit (such as pH value, Cl-concentration). Moreover,galvanic corrosion between the under deposit and without deposit will be formed, whichresults in serious corrosion under deposit.In this work, the corrosion behavior of X65carbon steel under different deposits, thelocal corrosion behavior under deposit, the coupling effect between under deposit andwithout deposit, the inhibition effect by adding thiourea based alkyl imidazoline quaternaryammonium salt corrosion inhibitor(CI1) and pre-film oily imidazoline corrosion inhibitor(CI2) in CO2saturated formation water was studied by weight loss measurements,electrochemical measurements, wire beam electrode (WBE) and microscopic analysis(SEM/EDS, XRD). The results show that covering with sand, clay, ferrous carbonate canreduce the corrosion of X65carbon steel slightly. These sediments can to some extentprevent the aggressive ions reach the steel surface, The formation of protective film on thesteel surface also reduces the corrosion of steel. However, covering ferrous sulfide,elemental sulfur, mixture can accelerate the corrosion of steel obviously, especially coverwith elemental sulfur which can sharply increase the corrosion rate. The corrosion rate ofthe steel covered with mixture also increased significantly since elemental sulfur play animportant role. The coverage of elemental sulfur can result in a catastrophic corrosion ofsteel. The coupled potential and galvanic current between the deposit-covered electrode andbare electrode (without deposit covered) was also measured by a galvanic corrosion testsystem. It is found that the bare electrode acts as anode while the deposit-covered electrodeacts as cathode at60°C in the initial stage. However, the polarities of these two electrodesreverse at late stage, which is attributed to the formation of FeCO3on the bare electrodesurface and then the prominently positive shift of the potential of bare electrode. However, the potential of bare electrode is always more negative than that of deposit-coveredelectrode, i.e., bare electrode always acts as anode while deposit-covered electrode alwaysacts as cathode at25°C. Microscopic analysis (SEM/EDS, XRD) demonstrate that aftercoupling for some time, a dense ferrous carbonate layer is formed on the bare electrodesurface. Current and potential maps of WBE can effectively reflect the coupling effect ofwithout deposit and under deposit and localized corrosion behavior of steel under depositsdue to the change of local environment. In the initial stage, the potential of the WBE underdeposit electrodes were positive than those without deposit. However, after coupling aperiod of time, the potential of the WBE without deposit shift positively so that thepotentials between without deposit electrodes and under deposit electrodes reversed,namely the WBE under deposit as anode when two electrode coupling. Some electrodesunder mixture sediments became severe corrosion during the corrosion process. Corrosioninhibitor cannot inhibit the corrosion under deposit when adding thiourea based alkylimidazoline quaternary ammonium salt inhibitor. Moreover, the higher inhibitorconcentration, the more serious corrosion under deposit. The corrosion of the WBE underdeposit can be to some extent inhibited by pre-filming oil-soluble imidazoline corrosioninhibitor (CI2) at initial stage. However, the electrodes under mixture sediments stillbecame severe corrosion at later corrosion process.