Preferential weld corrosion of X65 pipeline steel in flowing brines containing carbon dioxide

Abstract

The aim of this research was to investigate the cause of the severe localised corrosion that sometimes occurs at welds in carbon steel pipelines carrying hydrocarbons and inhibited brines saturated with carbon dioxide. A rotating cylinder electrode apparatus was designed so that electrodes machined from the weld metal, heat affected zone and parent material of welded X65 pipeline steel could be galvanically coupled and tested in high shear stress conditions. The galvanic currents flowing between the weld regions were recorded using zero resistance ammeters, and their self-corrosion rates were found by polarisation resistance measurements. The total corrosion rate of each weld region was obtained from the sum of the self-corrosion and galvanic contributions. In uninhibited conditions, the weld metal and heat affected zone were both cathodic to the parent material, and localised corrosion was prevented. However, when an oilfield corrosion inhibitor was present, a current reversal took place, which resulted in accelerated weld corrosion. Electrochemical impedance spectroscopy showed that the inhibitor film had lower electrical resistance and was less protective on the weld metal than on the parent material. At the highest shear stress, a second current reversal could occur when the inhibitor was removed from all regions of the weld, and there was a return to the original galvanic behaviour. It was concluded that preferential weld corrosion was caused by unstable conditions in which the inhibitor film was selectively disrupted on the weld metal but remained effective on the other weld regions.

Analysis of the corrosion scales formed on API 5L X70 and X80 steel pipe in the presence of CO2

“…The presence of FeCO 3 on the surface of the steel after exposure to the test media was confirmed by the SEM analysis as shown in Figure 8 (in this example, the film observed was that formed on the surface of one of the immersed steel specimens). Alawadhi and Robinson 6 , reported that the stability of the protective iron carbonate layers formed increased with test temperatures above 60 °C. By contrast, the mass loss results of the present study indicate that the formation of a FeCO 3 layer can in fact increase localized corrosion when the layers formed are discontinuous and porous.…”

“…Additionally, when welds are present, the microstructure of the weld metal is typically acicular ferrite, which becomes cathodic relative to the base metal when exposed to the corrosive solution. The microstructures which result from welding under these conditions can thus lead to an acceleration of the process and result in preferential weld corrosion (PWC) phenomena as well as the localized increase in hardness, grain size and the presence of other aligned phases 6 . As mentioned above, the adhesion of the FeCO 3 film and the degree to which it can behave in a protective manner are related to the presence of Fe 3 C in the steel since, under certain circumstances, the cementite phase can anchor the film on the substrate.…”

“…This carbonic acid lowers the pH of the solution and increases the corrosion rate of steels exposed to this medium. Despite the many studies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] undertaken to investigate CO 2 corrosion (also known as sweet corrosion), questions are still raised regarding the mechanism responsible for its occurrence. Many parameters are involved in the process and even small differences in chemical composition between one steel and another can strongly influence scale formation during CO 2 corrosion 12 .…”

“…Due to the material loss rates resulting from internal corrosion, it becomes necessary to thoroughly characterize the behaviour (for example: CO 2 corrosion behaviour at low partial pressure, under supercritical condition) of high strength steels which are used for oil and gas pipelines such as API 5L X65, X70 and X80 [1][2][3][4][5][6][7][8] . Well fluids and other products are transported in the oil and gas industry using pipelines.…”

Metallurgical Characterization of API X65 Steel Joint Welded by MIG Welding Process with Axial Magnetic Field

“…Many high-grade pipelines have been developed for offshore and onshore applications [1][2][3][4][5][6][7][8] , through adding microalloying elements, thermomechanically controlled process (TMCP) and other processes 9,10 , inducing a higher capacity for hydrocarbon transportation and significant cost savings by reducing the pipe wall thickness X65 grade steel is commonly used for manufacturing pipe for transporting hydrocarbons [11][12][13][14][15][16] . Recently, the authors demonstrated that X65 grade steel may reach similar tensile strength to that presented by the X80 grade steel via a non-conventional heat treatment 17 .…”