X70钢近中性pH环境中电化学及应力腐蚀行为与机理研究

发布时间：2018-04-24 21:06

  本文选题：X70钢 + 近中性pH值　； 参考：《北京科技大学》2015年博士论文

【摘要】：本文围绕X70钢在近中性pH值环境中的应力腐蚀行为与机理,对其裂纹扩展行为进行了研究,探讨了溶液pH值、Cl-浓度、阴极电位、交流干扰、塑性变形及氢等因素对其电化学和应力腐蚀行为的影响,并采用电偶腐蚀的研究方法对应力腐蚀裂纹尖端阳极溶解过程进行了模拟,最后对管线钢近中性pH环境中应力腐蚀裂尖非稳态电化学过程进行了分析。 研究结果表明,循环载荷作用下X70钢在近中性pH值环境中发生裂纹扩展,扩展速率为4.28×10－3mm/cycle。裂纹扩展过程中,裂尖发生了酸化及Cl-浓聚,裂尖pH值约为4.0,Cl-浓度达4mol/L。裂纹扩展速率受到阴极极化的影响：在弱阴极极化作用下,氢致韧性效应使X70钢裂纹扩展速率下降；在强阴极极化作用下,氢脆效应导致裂纹扩展速率升高。不同条件下的裂纹扩展行为均表现为应力腐蚀疲劳特征。 电化学及慢应变速率拉伸试验结果表明：溶液的酸化促进了X70钢的阴极反应过程,随着pH值的降低,其SCC敏感性先略微减小后线性增大,在pH=5.5时最低,在pH=4.0时最高。Cl-浓度的增大加速了X70钢的腐蚀,当浓度增大到100倍时,其SCC敏感性增大。在溶液酸化和Cl-浓聚共同作用下,X70钢的SCC敏感性进一步增大。在近中性及其酸化环境中时,X70钢组织中的铁素体优先溶解,晶界、碳化物、M/A岛等富碳相保留。Al-Mg-O-S-Ca及富Si夹杂发生了优先溶解,且其周围易萌生SCC裂纹。 阴极极化改变了X70钢的应力腐蚀行为,弱阴极极化作用(-775mV)下,X70钢SCC敏感性下降。随着极化电位负移,在阳极溶解和氢脆效应共同作用下,其SCC敏感性增大。阴极极化条件下,交流干扰使X70钢电位发生了明显偏移。阴极电位较正时,交流干扰使其电位负移,阳极溶解过程被促进。阴极电位较负时,交流干扰使其电位正移,阴极还原过程促进较为明显。由于交流电的“趋肤效应”,电极边缘区域腐蚀比中心区域严重。在弱阴极极化条件下,交流干扰使得X70钢的SCC敏感性增大。 塑性变形促进了X70钢的腐蚀并提高了其SCC敏感性：首先,塑性应变导致X70钢的位错密度及表面粗糙度增大,促进了X70钢的阴极反应过程；其次,塑性应变使得X70钢的显微硬度、屈服强度及氢吸附点数量的增多,促进了氢的吸附及向钢中的渗透,提高了X70钢的氢浓度。同时,进入钢中的氢在高应变条件下促进了X70钢的阳极溶解。 通过电偶腐蚀研究方法得到的X70钢裂尖的阳极溶解与塑性应变量及应变速率呈如下函数关系：i=i0+(5.13+1gε)△ε。基于以上公式,采用法拉第定律及Shoji模型计算得出的裂纹扩展速率比实测值低2个数量级,这可能是由于计算中未考虑裂尖新鲜金属表面及氢的作用。 最后对管线钢近中性pH环境中应力腐蚀裂尖非稳态电化学过程进行了分析,提出了综合裂尖新鲜金属表面、裂尖介质环境、裂尖高应变、氢富集等因素的裂尖阳极溶解理论计算公式。该公式首次在氢-应力协同作用模型基础上引入了裂尖新鲜金属表面、裂尖介质环境等作用系数,对近中性pH SCC裂纹扩展速率预测模型的建立具有重要意义。
[Abstract]:In this paper , the behavior and mechanism of stress corrosion in the near neutral pH value environment are studied . The effects of pH value , Cl - concentration , cathodic potential , AC interference , plastic deformation and hydrogen on electrochemical and stress corrosion behavior are discussed . The anodic dissolution process of stress corrosion cracking tip is simulated by the method of galvanic corrosion . Finally , the non - steady state electrochemical process of stress corrosion cracking tip in pipeline steel near neutral pH environment is analyzed .

The results show that the crack propagation rate is 4.28 脳 10 - 3 mm / cycle in the near neutral pH value environment under cyclic loading . The crack tip has acidification and Cl - concentration . The crack tip has a pH value of 4.0 and Cl - concentration of 4 mol / L . The crack growth rate is affected by the cathodic polarization : Under the action of weak cathode polarization , the crack growth rate is decreased under the effect of weak cathodic polarization .
Under the action of strong cathodic polarization , the crack growth rate is increased due to hydrogen embrittlement . The crack propagation behavior under different conditions is characterized by the stress corrosion fatigue characteristic .

The results of electrochemical and slow strain rate tensile tests show that the acidification of the solution promotes the cathodic reaction process . With the decrease of pH value , SCC susceptibility is increased slightly , and the SCC sensitivity is increased when the concentration increases to 100 times . In the near - neutral and acidic environment , the SCC sensitivity is increased . In the near - neutral and acidic environment , the ferrite preferentially dissolves , grain boundaries , carbides , M / A islands and other carbon - rich phases . The Al - Mg - O - S - Ca and Si - rich inclusions are preferentially dissolved , and SCC cracks are easily initiated around them .

Under the condition of cathodic polarization , AC interference makes the potential negative shift and the anodic dissolution process is promoted . As the cathode potential is negative , AC interference makes the potential positive shift , and the cathodic reduction process is promoted . Because of the " skin effect " of alternating current , the corrosion of the electrode edge region is more severe than the central region . Under the condition of weak cathode polarization , the AC interference increases SCC susceptibility of the steel .

The plastic deformation promotes the corrosion and SCC sensitivity . First , the plastic strain results in the dislocation density and the surface roughness increase , which promotes the cathodic reaction process .
Secondly , the plastic strain increases the microhardness , yield strength and the number of hydrogen adsorption sites , promotes the adsorption of hydrogen and the penetration of hydrogen into the steel , and improves the hydrogen concentration .

Based on the above formula , the crack propagation rate calculated by using Faraday ' s law and Shoji model is 2 orders of magnitude lower than the measured value , which may be due to the failure to consider the action of fresh metal surface and hydrogen in the calculation .

At last , the non - steady state electrochemical process of stress corrosion cracking tip in the near neutral pH environment of pipeline steel was analyzed , and the theoretical calculation formula of crack tip anodic dissolution was put forward . The formula of crack tip anodic dissolution was put forward based on hydrogen - stress co - action model . It is of great significance to establish the prediction model of crack propagation rate near neutral pH SCC .

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG172.9;TG142.1

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