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Study on the Cathodic Protection Process and Susceptibility of Hydrogen Embrittlement of X70 Steel in Simulated Deep Ocean Environment

Author: ZhangLin
Tutor: DuMin
School: Ocean University of China
Course: Marine Chemistry
Keywords: X70 steel Calcareous fouling layer Hydrogen embrittlement Deep sea Cathodic Protection
CLC: P75
Type: Master's thesis
Year: 2011
Downloads: 118
Quote: 1
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Abstract

With the offshore oil and gas development, the use of the rapid development of cathodic protection, offshore structures, and deep-sea facilities has become a hot issue of Metal Corrosion and Protection of research in the field of. Appropriate and effective cathodic protection potential not only widespread corrosion inhibition of steel, but also reduce the fatigue of the steel to improve the safe use of steel structures coefficient. In addition, cathodic protection calcareous layers can also improve the performance of steel corrosion fatigue, prevent fatigue crack growth, and enhance the effect of cathodic protection. But if the cathodic protection potential is too negative, cause the cathode hydrogen evolution, has led to the risk of hydrogen embrittlement fracture of high-strength steel. Therefore, in the design of cathodic protection, how to choose the right cathode control potentiometer is essential. According to the calcareous layers forming characteristics of hydrogen embrittlement of factors and actual sea environmental conditions, this paper to simulate typical of the marine environment by changing the temperature, dissolved oxygen, by potentiodynamic polarization, potentiostatic polarization method, slow strain rate tensile Law and Devanathan-Stachurski double electrolytic cell test, a comparative study of cathodic protection behavior of X70 steel in different simulation environments and its susceptibility to hydrogen embrittlement. By potentiodynamic polarization and potentiostatic polarization experiments analyzed the cathodic polarization regularity of X70 steel in different simulation environments. The potentiodynamic polarization Experimental results show that relative to the shallow water environment, X70 steel in simulated deep-sea environment (4 ℃, 3.0mg / L) and analog thermocline environment (8 ° C, 1.5mg / L) in the hydrogen evolution potential shifts, and The most positive potential of the hydrogen evolution in the simulated the thermocline environmental X70 steel. -1000mV (SCE, the same below) in the shallow sea and deep-sea environment, the potential loss to the hydrogen evolution reaction significantly. Thermocline environment, the potential of-950mV hydrogen evolution reaction. The potentiostatic polarization experiment results show that the analog the Deep and analog thermocline environment of X70 steel cathodic polarization current density is small, the rate of decrease of the polarization current density is very slow. Linear polarization in different experimental environments, with the potential negative shift, polarization resistance there is a great value. Shallow marine environments, the potential of-900mV polarization resistance value, the polarization resistance in analog the Deep and analog thermocline environment-1000mV. The surface composition analysis showed that the main component of the calcareous scale layer are CaCO ₃ and Mg (OH) _{the 2}. When the polarization potential is small, the main component of the deposited layer CaCO ₃, with the increase of the polarization potential of Mg (OH) ₂ content increased. Shallow water environment, the scale layer of calcareous easy to form-900mV and-950mV is suitable polarization potential formation of good calcareous dirt layer. Analog the deep and analog thermocline environment, calcareous fouling layer is difficult to form the deposited layer in order to form a better potential to reach-1000mV, compared to shallow water environment poor, and a relatively thin, dense. Devanathan-Stachurski sided electrolytic cell testing techniques and slow strain rate tensile method different simulation environments cathodic polarization potential of the hydrogen embrittlement susceptibility of X70 steel investigated. The hydrogen permeation experimental results show that with the negative shift of the cathodic polarization potential, the diffusion coefficient of hydrogen in the steel decreases, the concentration of hydrogen atoms in the surface of the material gathered increases, the order of the size of the diffusion coefficient of hydrogen in the material under cathodic polarization gt; shallow water conditions simulated thermocline conditions the gt; simulated deep-sea conditions. The slow strain rate tensile test results showed that, regardless of the maximum tensile strength, yield strength, strength index and susceptibility to hydrogen embrittlement, fracture elongation, fracture energy plastic indicators with the the polarization potential negative shift gradually decreases, hydrogen embrittlement coefficient gradually increase, increased sensitivity to hydrogen embrittlement. In shallow water, the polarization potential of-950mV, hydrogen embrittlement coefficient close to the danger zone, showing the morphology of the quasi-dissociation fracture characteristics. Polarization potential of-1050mV, hydrogen embrittlement coefficient into the brittle fracture zones and fracture energy of less than 80%, is a brittle fracture. Simulated deep sea environment, the polarization potential of-950mV, hydrogen embrittlement coefficient in a safe area, the fracture still microporous aggregation mainly ductile fracture. Polarization potential of-1050mV, hydrogen embrittlement coefficient entering the danger zone, showing the morphology of the quasi dissociation fracture characteristics. Analog thermocline conditions under X70 steel hydrogen embrittlement sensitivity between shallow marine environment and simulation of the deep-sea environment. Finally, various experimental results of X70 steel in seawater cathodic protection the control potentiometer concluded that the potential in the shallow water environment and simulated thermocline environment should be controlled in-900mV ~-950mV-950mV ~-1000mV. Simulation of deep-sea environment protection potential control-1000mV, the better the protection.

Study on the Cathodic Protection Process and Susceptibility of Hydrogen Embrittlement of X70 Steel in Simulated Deep Ocean Environment

Abstract

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