Investigation Of Stress-induced Hydrogen Diffusion And Hydrogen-induced Cracking In Marine Steels

With the development of marine resources,more and more high-strength steels and stainless steels have been used in marine engineering construction.However,the service environment of these marine steels is harsh:high hydrostatic pressure(HP),cathodic protection as well as acidic solution can cause hydrogen embrittlement or corrosion that will lead to unexpected failure.Furthermore,the service conditions of these steels are normally accompanied by applied external stress,which could cause the redistribution of hydrogen and eventually change the cracking features.Extensive researches have been conducted to reveal the stress-induced hydrogen diffusion around notches and crack-tips in the past decades,however,the materials in services are often smooth to avoid stress concentration,in which,unfortunately,the impact of external-stress induced hydrogen redistribution to cracking features was rarely reported.Based on this,the slow strain rate tensile(SSRT)test method combined with finite element calculation were conducted to study the stress-induced hydrogen diffusion and fracture behavior of Q960E martensitic steel.Moreover,materials used in the marine environment should meet both strength and corrosion resistance requirements.Hence,2205 duplex stainless steel(2205 DSS)that meet this criterion is widely used in marine environment.Cathodic protection is often required for materials in deep-sea environments,however,the hydrogen embrittlement phenomenon,hydrogen-induced cracking mechanism under cathodic protection haven't been well addressed.Additionally,hardly any investigations have been conducted on the evolution behavior of the surface passivation film under deep-sea conditions of 2205 DSS.Hereby,hydrogen embrittlement and fracture mechanisms of 2205 DSS,together with the corrosion and stress corrosion cracking behavior of 2205 DSS under deep-sea conditions were systemically studied in this work.The following results were obtained:(1)SSRT tests combined with finite element simulations were used to determine the true critical hydrogen concentration.Results indicated that 1.8 ppm was the critical hydrogen concentration that could cause hydrogen-induced brittle cracking in Q960E steel.The size of brittle zone increases with the decreasing strain rate.Even if the initial hydrogen content inside the material is lower than this critical value,hydrogen could still accumulate through stress-induced diffusion and eventually reached the critical concentration at a slow strain rate,and causing brittle cracking.On the other hand,when the hydrogen content in the material is constant,the size of the brittle zone(L)and the strain rate(?)satisfy an exponential function relationship:L=3.02e-13167?.When the strain rate is constant and the initial hydrogen concentration inside the material varies,the area of the brittle zone(S)and the initial hydrogen concentration(C0)exhibited a logarithmic function relationship,for example,at strain rate of 5×10-6/s,S=3.931nC0+1.26.(2)Sprindle-shaped products induced by immersing in simulated seawater under high HP or by electrochemically hydrogen charging in the ferrite phase of 2205 duplex stainless steel were observerd.It was found that the sprindle-shaped products seriously deteriorated the mechanical properties of 2205 DSS.The microcracks mainly initiate at the interface between hydrogen charging product and the ferrite matrix.However,the elongation recovers to that of the hydrogen-free sample after removing the hydrogen charging product at high temperature.Ssecondary ion mass spectrometry(SIMS)result indicates that H content in the hydrogen charging product is higher than the normal ferrite area,and the X-ray diffraction shows the characteristic peak of iron hydride at 40.07°.Moreover,the differential scanning calorimeter(DSC)test demonstrates that the phase decomposition temperature of the product is 268?,which coincides with the phenomenon that it dissolves at the high temperature caused by the focused electron beam during transmission electron microscopy(TEM)analysis.All experimental results indicate that the hydrogen charging product is either the hydride of FeH or(Fe,Ni)H.(3)The dynamic hydrogen charging during SSRT test combined with electrochemical method were employed to study the influence of hydrogen on the mechanical properties of 2205 DSS.It was observed that hydrogen mainly affected the elongation of 2205 DSS,but had little effect on the strength loss.When the hydrogen content of 2205 DSS is lower than 8 ppm,hydrogen embrittlement sensitivity increased obviously,otherwise,the increment was small.This is attributed to the fact that hydrogen would preferentially reach the saturation state and generate hydrides in ferrite phase,where the microcracks are preferentially to initiated.Studies have proved that the hydride-induced brittleness is the main reason of hydrogen-induced cracking of 2205 DSS.(4)Electrochemical method was employed to study the effect of HP and pH on the corrosion behavior of 2205 DSS in 3.5 wt.%NaCl solution,and constant load test was used to examine the effect of HP and pH on the stress corrosion of 2205 DSS.Results indicate that both high HP and low pH promote pitting of 2205 DSS,and the compositional change in the oxide film at high HP is responsible for the decrease in the corrosion resistance of 2205 DSS.Moreover,when the HP is increased from 0.1 to 10 MPa,the optimal cathodic protection potential is positively shifted from-0.9 to-0.7 VAg/AgCl,which is attributed to the enhancement of hydrogen adsorption and entry by HP.In acidic environment,when HP elevated,the stress corrosion sensitivity of the material will also increase significantly.Thus,the stress corrosion cracking mechanism of 2205 DSS under deep-sea environment should be caused by the combination of anode dissolution and hydrogen-induced cracking.