Antonio Contreras-Cuevas

Electrochemical Noise Study on Corrosion of Low Carbon Pipeline Steel in Sour Solutions

Electrochemical Noise (EN) measurements for low carbon pipeline steel (X52) samples were carried out in 3.5% NaCl aqueous and NACE solutions. Both testing solutions were saturated with hydrogen sulphide (H2S) at 20 and 60 oC, under static conditions. EN data was analysed in the time domain. Parameter such as Electrochemical Noise Resistance (Rn) and Localization Index (LI) were determined from the analysis in the time domain. Also, Rn results are compared with traditional Linear Polarization Resistance (LPR) data. A superficial analysis was carried out by a scanning electron microscopy (SEM) with Energy Dispersive X-Ray analyses (EDX). A good correlation was observed among the used techniques and relevant electrochemical data related to the corrosion phenomena was obtained. According to Rn, the corrosion rate (CR) values of the steel sample immersed in 3.5% NaCl are higher than the CR of the steel samples immersed in NACE solution. The morphology of the corrosion process obtained by SEM was a localized type and this behaviour was corroborated by the results obtained by LI where these LI values are 1 or close to 1.

Electrochemical and Tension Tests Behavior of API 5L X60 Pipeline Steel in a Simulated Soil Solution

In this work electrochemical impedance spectroscopy (EIS) and slow strain rate tests (SSRT) were used for the evaluation of API 5L X60 carbon steel in contact with a simulated soil solution called NS4. EIS monitoring before and after performing the tension tests was carried out. SSRT were carried out in NS4 solution at room temperature to simulate dilute ground water that has been found associated with stress corrosion cracking (SCC) of low carbon steel pipelines. A strain rate of 1x10-6 sec-1 was used. According to the analysis of SSRT, the X60 pipeline steel is highly resistant to SCC. In order to perform the electrochemical test, two working electrodes were considered, a complete specimen, before the SSRT and a fractured specimen after the SSRT. The analyses of results show that the electrochemical response was different in each samples. The corrosion rate (CR) obtained by the two corrosion techniques revealed that the CR of the fractured specimen was higher than the CR of the complete specimen. This behavior is attributed to the fact that the fractured specimen present a high degree of tortuosity and this condition activate the corrosion process. In addition, according to the cathodic Tafel slope, the reduction reacction was influenced by a difusion process. A combine fracture type in SSRT was observed: ductil and brittle with a transgranular appearance. Some pits and internal cracks close to the fracture zone were observed. The failure process and mechanism of X60 steel in NS4 solution are controlled by dissolution and hydrogen embrittlement.

Electrochemical Characterization of the Structural Metals Immersed in Natural Seawater: In Situ Measures

This work presents the electrochemical corrosion results of the structural metals, aluminium (Al), brass and copper (Cu), immersed in coastal waters of Veracruz Port in Mexico at room temperature, atmospheric pressure and eight weeks of the exposition time. The electrochemical technique used was electrochemical noise (EN). A typical three-electrode electrochemical cell was used. Where the reference electrode was the silver/seawater (Ag/SW) and two nominally identical metallic samples were used as working electrodes (WE1 and WE2). The metallic samples of Al, brass and Cu were used as working electrode. The potential and current fluctuations were measured simultaneously between the two working electrodes (current measured) and the Ag/SW electrode (potential measured). The electrochemical noise measurements (ENM) were analysed by three different methods: Potential and current versus time (transients), Localization Index (LI) and Electrochemical Noise Resistance (Rn). The results shown a good correlation between the superficial analysis and the results obtained by the ENM. In addition, Cu presents the highest corrosion rate and, a corrosion attack was obtained by localization index; this behaviour was confirmed by superficial analysis.

Effect of Li on the corrosion behavior of Al-Cu/SiCp composites

The corrosion behavior in NaCl solutions at different pH values of Al-Cu-Li/SiCp and Al-Cu/SiCp composites fabricated by co-spraying technique was studied. The corrosion behavior of the composites was also compared with the corrosion behavior of pure aluminum. Corrosion potential measurements and potentiodynamic polarization were used to elucidate the corrosion behavior of the composites. In addition the microstructure and morphology of the composites were studied through X-ray diffraction (XRD) and scanning electron microscope (SEM). It was found that the corrosion rate of Al-Cu-Li/SiCp and Al-Cu/SiCp composites was higher than corrosion rate of pure aluminum for all NaCl solutions tested. Both composites and pure aluminum showed the typical “U” behavior of the corrosion rate as a function of pH. The addition of lithium had little effect on the corrosion behavior of the composites.

The Role of Residual Stresses in Circumferential Welding Repairs of Pipelines in SCC Susceptibility

The main objective of the present study is to measure the residual stresses in the circumferential welding joints of X52 pipeline steel with multiple welding repairs, and then relate these residual stresses to study the influence on the high pH stress corrosion cracking (SCC) susceptibility. Four conditions of shielded metal arc welding (SMAW) repairs of the girth weld were evaluated. The residual stresses were measured through X-ray diffraction (XRD) on the internal side of the pipe in longitudinal and circumferential direction. Residual stresses in the circumferential and longitudinal direction reach values of about 98 and 74% of the yielding strength (360 MPa) respectively. The effect of residuals stresses in the high pH-SCC susceptibility of X52 pipeline steel was evaluated through slow strain rate tests (SSRT) in a simulated soil solution. Relation between SCC index and residual stresses on the SCC susceptibility was analyzed. Results of SCC index taking account the ratios obtained from the mechanical properties of the welding joints evaluate (containing different levels of residual stresses) showed good SCC resistance. It was observed that increasing the magnitude of residual stresses, the SCC susceptibility increases. For all the SSRT specimens the failure occurs in the base metal and heat affected zone (HAZ) interface.

Borides Precipitation in the FeAl40 Intermetallic Compound Produced by Atomization-Deposition Process

The microstructure of FeAl40 intermetallic alloy produced by spray atomization and deposition, with boron additions and reinforced with Al2O3 particles was studied. This technique allowed the co-deposition of particulate reinforcement and the addition of boron in order to obtain a boron concentration of 0.4 at. %. The additions of alumina particles produce a grain refinement. High resolution transmission electron microscopy observations shows a precipitation of boron in the FeAl matrix in despite of the rapid solidification process. Fe3B precipitates were found which is a metastable phase formed during the rapid solidification process. In addition, FeB precipitates were observed.

Residual Stress Assessment of Multiple Welding Repairs of Girth Welds in Pipeline Used in Oil Industry

This work presents the residual stress assessment of multiple welding repairs in the same area in seamless API X52 low carbon pipeline. Four conditions of shielded metal arc welding (SMAW) repairs and one as welded specimen of the girth weld were evaluated to determine changes in the microstructure (metal base, weld bead and heat affected zone) to evaluate their effect on the residuals stresses and mechanical properties of the welded joints. One of the mainly adverse effect of residuals stresses are in the susceptibility of stress corrosion cracking (SCC) of buried pipelines. The residual stresses were measured through X-ray diffraction (XRD). Samples were evaluated on the internal side of the pipe in longitudinal and circumferential direction. Circumferential residual stresses are greater than longitudinal stresses. Microstructural characterization of the welding joints through scanning electron microscopy (SEM) was performed. Relation between microstructure, mechanical properties and residual stresses was carried out. In general, the grain size increases with the number of repairs, and consequently there is an increase in residual stresses. Significant reduction in Charpy-V impact resistance with the number of weld repairs was observed overall in the weld fusion line. The hardness and strength increase in the first repair and in subsequent repairs decrease. As increasing the average grain size, the hardness and the absorbed energy decreases. Generally, the residual stresses showed a tendency to decrease in the first repair and after showed an increase with the number of repairs. It is clear that residual stresses depend more than the position of measurement than the welding repair number, which is directly relate with the microstructure and phases presented.

Electrochemical Characterization of X60 Steel Corrosion at Different Overvoltages: A Cathodic Protection Study

This paper presents an electrochemical characterization of X60 steel corrosion using polarization curves. In order to get a surface analysis, a Scanning Electron Microscopy (SEM) was used. Analyses by Energy Dispersive Spectroscopy (EDS) were done to characterize the corrosion products films formed on surface of the steel sample at the different overvoltages studied. The electrolyte was a synthetic soil solution and all electrochemical parameter was measured at room temperature, atmospheric pressure and 24 hours of the total exposure time. It is important to point out that these parameters were measured in the protection potential (versus saturated calomel electrode, SCE) and at different overvoltages. The results of the polarization curves show that the corrosion rate increased as the overvoltage also increased and this behaviour was corroborated by the superficial analysis obtained by SEM; where the corrosion morphology was localized and the aggresivity increased with the overvoltage. In addition, in all overvoltages, the anodic reaction was limited by a mass transfer process. A localized corrosion form was found in all tests.