Characterization of the Crack Propagation in the API X-52 and API X-65 Steels into Cathodic Protection
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Characterization of the Crack Propagation in the API X-52 and API X-65 Steels into Cathodic Protection A. Aguilar1, R. Esparza1, M.A. Gil2, L.F. Cuahutitla2, E. Rubio-Rosas2, V. Rodríguez-Lugo2 and R. Perez1 1
Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, P.O. Box 48-3, Cuernavaca, Mor., 62251, MEXICO. 2 Centro Universitario de Vinculación, BUAP, Prolongación de la 24 Sur y Av. San Claudio Col. San Manuel Ciudad Universitaria, Puebla 72570, MEXICO. ABSTRACT Cathodic protection has been applied for many years as the best method to prevent the corrosion in systems which transported hydrocarbon pipelines. However, it has found the presence of stress corrosion cracking (SCC) in steel pipelines with high concentrations of carbonates and bicarbonates with pH final (9 to 11). The resistance to the stress corrosion cracking of the API X-52 and API X-65 steels was evaluated on compact modified wedge opening specimens (WOL). The specimens were loaded of 95% of the yield strength. The resistance of crack propagation and the corrosion rate were evaluated with different applied potentials (-850 and -650 mV), this with respect to a saturated copper/copper sulfate reference electrode. The used electrolytes were simulated soils (carbonate-bicarbonate solution). Evidence of crack propagation of the API X-52 and API X-65 steels were carried out by scanning electron microscopy. The obtained result showed susceptibility to SCC on specimens with cathodic protection. The cathodic protection applied (-850 mV vs Cu/CuSO4 electrode) decreases considerably the corrosion rate on the evaluated steels. In this work the loaded stress showed to be a very important variable on the susceptibility to SCC. INTRODUCTION The study of soil as a corrosive environment is necessary due to the large number of buried pipelines and tanks, their deterioration can represent a real economical and environmental problem though the years. Many parameters can affect the corrosivity of a soil, but the methods more frequently used to measure the corrosivity are only representative [1]. The protection of buried iron pipelines against corrosion is a complex process. The corrosion of metals is of an electrochemical nature, differences of potentials must be compensated as pipes cross terrains with different conductivities. A non-corrosive coating is used to prevent damage, and additional protection is applied by means of cathodic protection (CP) in order to control galvanic current in such a way as to avoid anodic current flow from the pipe to the soil. CP can be accomplished by two widely used methods: sacrificial anodes and impressed-current systems. A rectifier or galvanic anode is used to create a complete circuit between the anode and the pipe, throwing current through the ground and creating chemical reactions on pipe surface to prevent corrosion. The voltage differences in sacrificial anode systems are limited to approximately 1 V, while impressed-current systems can use larger voltage difference, producing more efficient protection. Neve
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