Effect of Tempered Martensite and Ferrite/Bainite on Corrosion Behavior of Low Alloy Steel Used for Flexible Pipe Expose
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Effect of Tempered Martensite and Ferrite/Bainite on Corrosion Behavior of Low Alloy Steel Used for Flexible Pipe Exposed to High-Temperature Brine Environment Dazheng Zhang, Xiuhua Gao, Guanqiao Su, Zhenguang Liu, Ningning Yang, Linxiu Du, and R.D.K. Misra (Submitted March 14, 2018; in revised form July 10, 2018) Immersion experiments and electrochemical measurements were carried out to study the effect of tempered martensite and ferrite/bainite on corrosion behavior of low alloy steel used for flexible pipe exposed to hightemperature brine environment in terms of microstructure morphology, corrosion rate, corrosion products characteristics, potentiodynamic polarization curves and electrochemical impedance spectroscopy. The experimental results indicated that the proportion of high-angle grain boundaries of tempered martensite steel was greater than ferrite/bainite steel. The corrosion rate of ferrite/bainite steel was lower than tempered martensite steel. With increasing immersion time, the structure of corrosion products formed on the surface of ferrite/bainite steel was more compact and denser than tempered martensite steel. In addition, the current density of ferrite/bainite steel was lower than tempered martensite steel. The charge transfer resistance and total impedance of ferrite/bainite steel were larger than tempered martensite steel. Thus, the corrosion resistance of ferrite/bainite steel exposed to high-temperature brine environment was excellent compared to the tempered martensite steel. Keywords
corrosion behavior, ferrite bainite, flexible pipe, hightemperature brine environment, tempered martensite
1. Introduction With increased exploitation of offshore oil resources, more and more flexible pipes are used for offshore oil and gas transportation. Compared with the traditional pipeline steel, flexible pipe is promising because of its excellent corrosion resistance, low cost and easy installation (Ref 1-4). Flexible pipe is composed of multi-layered structure, such as carcass layer, pressure armor layer, tensile armor layer and outer sheath. Armor layer is an important component and is made of highstrength low alloy (HSLA) steel (Ref 5-7). During service, the temperature of armor layer varies typically from 20 to 130 °C (Ref 8, 9), which depends on the operating conditions in the bore and on the thermal properties of the structure. At the same time, seawater penetrates into armor layer, and contacts steel, resulting in intractable corrosion problem caused by seawater. Therefore, it is necessary to study the corrosion behavior of armor layer steel used for flexible pipes exposed to hightemperature brine environment. Corrosion behavior of HSLA steel in seawater or corrosion solution containing chloride ion has attracted significant Dazheng Zhang, Xiuhua Gao, Guanqiao Su, Ningning Yang, and Linxiu Du, State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, Peoples Republic of China; Zhenguang Liu, School of Material Science and Engineerin
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