Study On The Stress Corrosion Cracking Of X90 Pipeline Steel In Soil Simulated Solutions

In this work,the high strength X90 pipeline steel was investigated in near-neutral pH soil simulation solutions（NS4） by means of electrochemical experiments,slow strain rate test（SSRT）and corrosion fatigue test（CF）,and the electrochemical behavior,stress corrosion cracking（SCC）susceptibility,the hardening behavior and the behavior of crack dehisce in near-neutral environments were analyzed.At the same time,the mechanism of stress corrosion cracking under the test conditions were discussed,and stress corrosion crack growth rates were quantitatively analyzed.The main conclusions are as follows:The electrochemical results showed that:X90 pipeline steel showed obvious anode active dissolution characteristics at the static state,the E_op=-714 mV,E_corr=-743.6mV;however,dynamically,the E_op is increased firstly and then decreased with the fluctuating load N value increasing,but the anodic dissolution velocity is accelerated while the cathodic is inhibited.In 0≤N<500 times,the overall reaction is dominated by the process of cathodic reaction while it is dominated by the anodic dissolution reaction when N≥500 times,the overall reaction is a multi-steps reaction with the ion transfer charge oriented process.The SSRT results suggested that: the SSRT curves belong to typical Round-house curves under the test conditions;the strain hardening exponent n value is different at different strain range,generally,n value of base metal is higher than that of the weld joint and near-neutral pH value NS4 solution is able to cause the n value decreases.Both of pipeline steel base metal and weld joint possess obvious SCC sensitivity in near-neutral NS4 solution,and the latter is higher than the former,the main characters are loss of plasticity and secondary cracks;the sample surface pits can induce and promote micro-crack initiation and propagation;In air,the fracture mechanism belongs to dimple-microporous ductile fracture while it is the ductile-brittle mixed fracture mechanism in near-neutral NS4 solutions.Corrosion fatigue test results indicated that: the base metal fracture surface exhibits brittle fracture characteristics in the air and near-neutral environments,and many secondary cracks appear on the surface,which are transgranular and intergranular mix-cracking form,but the transgranular form is dominant,crack initiation is mixed mechanism of anodic dissolution and hydrogen embrittlement;pipeline steel SCC crack propagation possesses higher sensitivity to near-neutral solution environments.In contrast,it is less contribution that anodic dissolution possesses the crack growth rate in the whole crack process,at the same time,in near-neutral solution environments,SCC crack growth rate of the steel in the near-threshold can be described approximately by equation da/d N=4.14×10^-9（?K-6.48）^1.43 and in the stable crack growth area,by da/d N=6.80×10^-10（?K-8.63）².