發(fā)布時間：2018-04-24 21:06

  本文選題：X70鋼 + 近中性pH值��； 參考：《北京科技大學》2015年博士論文

【摘要】：本文圍繞X70鋼在近中性pH值環(huán)境中的應力腐蝕行為與機理,對其裂紋擴展行為進行了研究,探討了溶液pH值、Cl-濃度、陰極電位、交流干擾、塑性變形及氫等因素對其電化學和應力腐蝕行為的影響,并采用電偶腐蝕的研究方法對應力腐蝕裂紋尖端陽極溶解過程進行了模擬,最后對管線鋼近中性pH環(huán)境中應力腐蝕裂尖非穩(wěn)態(tài)電化學過程進行了分析。 研究結(jié)果表明,循環(huán)載荷作用下X70鋼在近中性pH值環(huán)境中發(fā)生裂紋擴展,擴展速率為4.28×10－3mm/cycle。裂紋擴展過程中,裂尖發(fā)生了酸化及Cl-濃聚,裂尖pH值約為4.0,Cl-濃度達4mol/L。裂紋擴展速率受到陰極極化的影響：在弱陰極極化作用下,氫致韌性效應使X70鋼裂紋擴展速率下降；在強陰極極化作用下,氫脆效應導致裂紋擴展速率升高。不同條件下的裂紋擴展行為均表現(xiàn)為應力腐蝕疲勞特征。 電化學及慢應變速率拉伸試驗結(jié)果表明：溶液的酸化促進了X70鋼的陰極反應過程,隨著pH值的降低,其SCC敏感性先略微減小后線性增大,在pH=5.5時最低,在pH=4.0時最高。Cl-濃度的增大加速了X70鋼的腐蝕,當濃度增大到100倍時,其SCC敏感性增大。在溶液酸化和Cl-濃聚共同作用下,X70鋼的SCC敏感性進一步增大。在近中性及其酸化環(huán)境中時,X70鋼組織中的鐵素體優(yōu)先溶解,晶界、碳化物、M/A島等富碳相保留。Al-Mg-O-S-Ca及富Si夾雜發(fā)生了優(yōu)先溶解,且其周圍易萌生SCC裂紋。 陰極極化改變了X70鋼的應力腐蝕行為,弱陰極極化作用(-775mV)下,X70鋼SCC敏感性下降。隨著極化電位負移,在陽極溶解和氫脆效應共同作用下,其SCC敏感性增大。陰極極化條件下,交流干擾使X70鋼電位發(fā)生了明顯偏移。陰極電位較正時,交流干擾使其電位負移,陽極溶解過程被促進。陰極電位較負時,交流干擾使其電位正移,陰極還原過程促進較為明顯。由于交流電的“趨膚效應”,電極邊緣區(qū)域腐蝕比中心區(qū)域嚴重。在弱陰極極化條件下,交流干擾使得X70鋼的SCC敏感性增大。 塑性變形促進了X70鋼的腐蝕并提高了其SCC敏感性：首先,塑性應變導致X70鋼的位錯密度及表面粗糙度增大,促進了X70鋼的陰極反應過程；其次,塑性應變使得X70鋼的顯微硬度、屈服強度及氫吸附點數(shù)量的增多,促進了氫的吸附及向鋼中的滲透,提高了X70鋼的氫濃度。同時,進入鋼中的氫在高應變條件下促進了X70鋼的陽極溶解。 通過電偶腐蝕研究方法得到的X70鋼裂尖的陽極溶解與塑性應變量及應變速率呈如下函數(shù)關系：i=i0+(5.13+1gε)△ε�；谝陨瞎�,采用法拉第定律及Shoji模型計算得出的裂紋擴展速率比實測值低2個數(shù)量級,這可能是由于計算中未考慮裂尖新鮮金屬表面及氫的作用。 最后對管線鋼近中性pH環(huán)境中應力腐蝕裂尖非穩(wěn)態(tài)電化學過程進行了分析,提出了綜合裂尖新鮮金屬表面、裂尖介質(zhì)環(huán)境、裂尖高應變、氫富集等因素的裂尖陽極溶解理論計算公式。該公式首次在氫-應力協(xié)同作用模型基礎上引入了裂尖新鮮金屬表面、裂尖介質(zhì)環(huán)境等作用系數(shù),對近中性pH SCC裂紋擴展速率預測模型的建立具有重要意義。
[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 .

【學位授予單位】：北京科技大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG172.9;TG142.1

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本文編號：1798250