A. Contreras

Corrosion kinetics of pipeline carbon steel weld immersed in aqueous solution containing H2S

Abstract One important challenge in the petroleum industry is reduce the problem produced by the corrosion of structures or pipeline steel caused by contact with the aggressive fluids, such as aqueous solution containing H2S or CO2. In this work, the corrosion of weldment of API X52 pipeline steel immersed in NACE solution saturated with H2S was studied. In the corrosion tests, the electrochemical techniques, polarisation curves and linear polarisation resistance were carried out in samples of API X52 pipeline steel focus in the three different zones of the weldment: heat affected zone, weld bead and base metal. In addition, a superficial analysis was made in order to identify the film of corrosion products. The temperature increment and the H2S dissolved in the NACE solution increase the corrosion rate of the three different zones. The heat affected zone was the more affected by the corrosion test and the phases mackinawite, troilite and pirrothite were identified in the film of corrosion products.

Evaluation of SCC susceptibility of supermartensitic stainless steel using slow strain rate tests

Abstract The susceptibility to chloride stress corrosion cracking (Cl-SCC) of supermartensitic stainless steel (SMSS) was evaluated at different temperatures through slow strain rate tests. In order to evaluate the Cl-SCC susceptibility the reduction in area and the time to failure ratio were considered. It is clear that all specimens tested in the chloride solution exhibited a generally high resistance to chloride SCC. However, according to the slow strain rate tests results, at lower temperature (5°C) the susceptibility to Cl-SCC increases compared to the other temperatures studied; also an increase in the corrosion rate was observed as the temperature decreased according to electrochemical impedance spectroscopy measurement. The corrosion mechanism observed on the material under dynamic stress was a combination of high activation resistivity with a diffusive process on the metallic interface given by a thin film from a protector layer of chromium oxides. This mechanism generates a high corrosion resistance and therefore good performance to chloride induced SCC.

A novel experimental arrangement for corrosion study of X60 pipeline steel weldments at turbulent flow conditions

Electrochemical noise (EN) measurements were carried out to study the effect of turbulent flow conditions on corrosion kinetic of API X60 pipeline steel weld immersed in synthetic seawater. In order to control the hydrodynamic conditions, two rotating cylinder electrodes were used. The EN data were analysed by three different statistical methods: currents transients, noise resistance and localisation index (LI). On the other hand, the spectral method was used in order to get the noise impedance. The superficial analysis using a scanning electron microscopy was carried out. According to EN analyses, the current transients indicate that the aggressiveness of the corrosion increased as the rotation speed also increased. The highest corrosion rate values were obtained at turbulent flow conditions. In the superficial analysis, a localised corrosion form was found in all corrosion processes; these results are agreed with the results obtained by LI.

Stress Corrosion Cracking Behavior of X60 Pipe Steel in Soil Environment

Stress corrosion cracking (SCC) susceptibility of API X60 pipeline steel in a soil solution by slow strain rate tests (SSRT), and surface fracture analysis was investigated. The SSRT were performed at strain rate of 25.4 × 10 -6 mm/sec in a glass autoclave containing the soil solution called NS4 with pH of 3 and 10 at room temperature and 50°C. Both anodic and cathodic polarization potentials of 200 mV referred to E corr was applied. The results of ratio reduction area (RRA), time to failure ratio (TFR) and elongation plastic ratio (EPR) indicate that X60 pipeline steel was susceptible to SCC at pH 3 and cathodic polarization of -200 mV at room temperature and 50°C. Scanning electron microscopy (SEM) observations of these specimens showed a brittle type of fracture with transgranular appearance. The SCC process and mechanism of X60 steel into NS4 solution was hydrogen based mechanism. With the different applied potentials the dominance of SCC process changes. At low pH the temperature effect on SCC susceptibility is more noticeable at 20°C. However at high pH this effects changes, being the steel more susceptible to SCC at 50°C.

Study of the Mass Transport on Corrosion of Low Carbon Steel Immersed in Sour Solution Under Turbulent Flow Conditions

A corrosion process can be influenced, in different ways, by the relative movement between the metal and the corroding environment. This relative movement can increase the heat and mass transfer of reactants towards and from the surface of the corroding metal, with a consequent increase in the corrosion rate. Also, if solid particles are present, removal of protective films, erosion and wear on the metallic surface can occur. The corrosion of the metallic structure under turbulent flow is complex, but this problem has been studied mainly in the oil industry (Garnica-Rodriguez et al., 2009; Genesca et al., 2010; Mora-Mendoza et al., 2002; Papavinasam et al., 1993; Poulson, 1993), where, the flow and some gases are very important in the behaviour of the phenomenon processes. This oil industry has processes that involve the movement of corrosive liquids in metallic structures, for example, the transport of mixtures of liquid hydrocarbons and gas with water through pipes. Therefore the influence of flow on the corrosion processes is an important issue to be considered in the design and operation of industrial equipment. This influence is complex and many variables are involved. Many observations of flowaccelerated corrosion problems have been documented (Dean, 1990; Garverick, 1994; Poulson, 1993). One aim that has been so much studied in the petroleum industry is the effect of flow and dissolved gases, such as hydrogen sulphide (H2S) and carbon dioxide (CO2). The most common type of flow conditions found in industrial processes is turbulent and according to increasing of the necessity to describe the corrosion of metals in turbulent flow conditions some laboratory hydrodynamic systems have been used with different degrees of success (Poulson, 1983, 1993, 1994). Among these hydrodynamic systems, rotating cylinder electrodes (RCE), pipe segments, concentric pipe segments, submerged impinging jets and close-circuit loops have been used and have been important in the improvement of the

Cathodic Protection Effect on the Assessment of SCC Susceptibility of X52 Pipeline Steel

Assessment of anodic and cathodic potentials on stress corrosion cracking (SCC) of API X52 pipeline steel through slow strain rate tests (SSRT) was studied. The SSRT were carried out in a NS4 solution to simulated dilute ground water that has been found to be associated with SCC of pipelines. SSRT were performed and evaluated in air and in the NS4 solution at room temperature at an extension rate of 1×10 -6 in/sec. Tests were performed at controlled electrochemical polarization potentials, both anodic and cathodic (100, 200, 400 mV) versus the open circuit corrosion potential. The results of reduction in area ratio (RAR), time to failure ratio (TFR) and plastic elongation ratio (PER) of the specimens tested in the soil solution indicate that X52 pipeline steel was susceptible to SCC at cathodic potentials. These specimens showed a brittle type of fracture with transgranular appearance. The SCC proceess and mechanism of X52 steel in the NS4 solution is mixed-controlled by both anodic dissolution and the hydrogen involvement. At positive potentials the SCC is based mainly on the anodic dissolution mechanism. When the applied potentials shifted negatively, the SCC on the steel follows mainly hydrogen embrittlement mechanism. This mechanism was confirmed through the internal cracks observed in the specimens.

Assessment of pH and temperature effects on stress corrosion cracking of 1018 low carbon steel

Abstract Stress corrosion cracking tests (SCC) were performed on 1018 low carbon steel into 0·5M NaCl solution at different pH values and temperatures of 25, 50 and 70°C using the slow strain rate testing technique. Evaluations were complemented with hydrogen permeation measurements and electrochemical impedance spectroscopy. Studies show hydrogen diffusion effects depending on temperature and pH. The maximum hydrogen diffusion was observed at 70°C (pH<1). The SCC susceptibility was measured as the percentage reduction in area and the time to failure and was maximal at pH<1 at the temperatures studied. The most likely mechanism for the cracking susceptibility of 1018 steel in the chloride solutions seems to be hydrogen assisted anodic dissolution assisted by hydrogen embrittlement. After failure, the fracture surfaces were observed by scanning electron microscopy and chemical analysis was obtained by X‐ray energy dispersive spectroscopy. The specimens tested in air exhibited a ductile type of failure, whereas, in the corrosive solution the specimens showed a brittle fracture.

Assessment of SCC Susceptibility of Supermartensitic Stainless Steel Through Slow Strain Rate Tests

The 13%Cr supermartensitic stainless steels (SMSS) are a new group used as pipelines for oil and gas transport. The optimized microstructure of these materials offers good corrosion resistance in environments containing CO2 and H2 S. In addition to reducing carbide precipitation, the low carbon content (about 0.01 wt-%) of supermartensitic steels SMSS provides good weldability with conventional arc welding techniques. The susceptibility to stress corrosion cracking (SCC) through slow strain rate tests (SSRT) of SMSS and its comparison with a Hastelloy C-22 in substitute ocean water was carried out. Tests were supplemented by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The SSRT were performed in air and in a chloride solution at 5, 15, 25 and 35°C. The susceptibility to SCC was evaluated considering the reduction in area ratio and time to failure ratio. According to these considerations it is clear that all the specimens tested in the chloride solution exhibited high resistance to chloride SCC. The specimens tested in air exhibited a ductile fracture. In the corrosive solution, the fracture was a combination of both, ductile and brittle fracture with a transgranular appearance. An evident temperature effect on the steel was observed. The major susceptibility to SCC was presented to the lower temperature (5°C). According to EIS evaluations, the corrosion rate increased with the temperature, which promoted an increase in the SCC susceptibility at a low temperature. The corrosion mechanism observed on the material under dynamic stress was a combination of high activation resistivity in combination with a diffusive process on the metallic interface given by a thin film of a protector layer of chromium oxide.Copyright