Study On The Cathodic Protection Process And Susceptibility Of Hydrogen Embrittlement Of X70 Steel In Simulated Deep Ocean Environment

With the rapid development of exploitation and utilization about marine oil and natural gases,the cathodic protection technological research of offshore structures and deep-sea facilities has become a hot issue in corrosion and anticorrosion areas.Proper cathodic protection potential not only can control general corrosion of steel,but also can reduce fatigue value and improve the safe use coefficient of steel structure.In addition, calcareous layer generated in the process of cathodic protection also can prevent the spreading of cracks, increase resistance against fatigue corrosion and enhance the cathodic protection. However, over-protection during cathodic protection would increase the generation and absorption of cathodic hydrogen at the metal surface, this could result in hydrogen embrittlement of the high-strength steel.Therefore, in the cathodic protection design, how to choose appropriate effective cathodic protection potential is very important.According to the characteristics of calcareous deposits formation, influencing factors of hydrogen embrittlement and real-sea environmental conditions, real marine environment was simulated through temperature and dissolved oxygen in this thesis. The cathodic polarization behavior and susceptibility to hydrogen embrittlement of X70 steel in different experiments environmental conditions were studied comparatively by using potentiodynamic polarization method, potentiostatic polarization method, SSRT and Devanathan-Stachurski bipolar cell.Through potentiodynamic polarization curves and potentiostatic polarization method, the cathodic polarization characteristics of X70 steel in different experiments environmental conditions were analyzed. The results of potentiodynamic polarization showed that the hydrogen evolution potential of X70 steel in simulated deep sea(4℃,3.0mg/L) and in simulated maximum gradient layer environment(8℃,1.5mg/L) move toward the positive direction compared with the shallow water and that in simulated maximum gradient layer environment is the most positive. When the polarization potentials are negative to -1000mVSCE in shallow water and in deep water, hydrogen evolution reaction is very conspicuous. In simulated maximum gradient layer environment, hydrogen evolution reaction is prominent at -950mV.Potentiostatic cathodic polarization results showed that the cathodic polarization current densities of X70 steel are small and polarization current densities decline slowly in simulated deep sea and in simulated maximum gradient layer environment.The electrochemical linear polarization measurements showed that the polarization resistance of the specimen has a maximum value with the decrease of polarization potential in different experimental environments. In the shallow water, the polarization resistance is maximum when applying potential is -900mV; when it is -1000mV, the maximum of polarization resistance appears in simulated deep sea and in simulated maximum gradient layer environment.Surface composition analysis showed that the main components of calcareous deposits are CaCO₃ and Mg(OH)₂. At smaller cathodic potential, the main components of calcareous deposits are CaCO₃; as the potential becomes more negative, the content of Mg(OH)₂ increases. In the shallow water, the formation of deposits is very easy. A good dense deposit can be obtained at -900mV and-950mV. In simulated deep sea and in simulated maximum gradient layer environment, the deposit is difficult to format.When applying potential is -1000mV, deposits can be formed but compactness is not good compared with the shallow water.The susceptibility of X70 steel to hydrogen embrittlement was investigated in different experiments environmental conditions by means of Devanathan-Stachurski bipolar cell and slow strain rate test. Hydrogen permeation results showed the hydrogen diffusivity in steels decrease and hydrogen concentration on the surface increase with a shift to the negative cathodic potential direction. The order of magnitude of hydrogen diffusion coefficient in X70 steel is: the shallow water>simulated maximum gradient layer environment>simulated deep sea. SSRT results indicated there is no correlation between maximum tensile strength, yield strength, stress at failure, and hydrogen embrittlement. But the elongation, time-to-fracture, and fracture energy decrease and hydrogen embrittlement coefficient increase with a shift to the negative cathodic potential direction, hydrogen embrittlement sensitivity boosts up. When applying potential is-950mV in the shallow water, hydrogen embrittlement coefficient gets close to the dangerous zone of HE, fracture surface exhibits quasi-cleavage fracture; at -1050mV, hydrogen embrittlement coefficient has reached into the fracture zone of HE and fracture energy ratio is less than 80%, the fracture is brittle. In simulated deep sea, at -950 mV, hydrogen embrittlement coefficient is located in safety zone of HE, fracture surface is characteristic of ductile dimple fracture pattern; at -1050mV, hydrogen embrittlement coefficient has reached into the dangerous zone of HE, the fracture surface exhibits considerable quasi-cleavage fracture. In simulated maximum gradient layer environment, the susceptibility to hydrogen embrittlement of X70 steel is between in shallow water and in deep water.Finally, combining the experimental results of X70 steel, the conclusion was obtained. The appropriate cathodic protection potential for the material is between -900mV and -950mV in the shallow water and the appropriate cathodic protection potential is between -950mV and -1000mV in simulated maximum gradient layer environment. In simulated deep sea, the optimum protection potential is -1000mV.