Microstructure evolution and electrochemical corrosion behaviour of API X70 linepipe steel in different environments

摘要

Pipeline steels are extensively used in various applications like oil and gas transmission, offshore oil drilling and pressure vessels due to its higher toughness and strength values. To provide higher atmospheric corrosion protection and to improve mechanical properties than normal steels, wide variety of pipeline steels are manufactured using different heat treatment procedure. In present work, a heat treatment procedure was used to alter the microstructure of base metal and to study its effect on the electrochemical corrosion behaviour of X70 pipe line steel. Heat treatment of X70 steel was performed at two austenitizing temperatures (e.g. 1000 degrees C and 850 degrees C) followed by quenching and subsequent tempering at 300 degrees C, 450 degrees C and 600 degrees C. Linear Sweep voltammetry technique of electrochemical corrosion was used to study the corrosion behaviour of heat treated specimens in sea water pH-8.2, 5%NaCl + 10(-2) mol/l sodium thiosulphate pH-3 and 5%NaCl + 10(-3) mol/l sodium thiosulphate pH-5 solution. Microstructure of different heat treated samples shows the presence of different phases developed during heat treatment. Polygonal ferrite and fine grained ferrite microstructures (at 600 degrees C) developed during heat treatment inhibits the corrosion rate as compared to the tempered martensitic (300 degrees C) microstructure.