海洋服役典型双相不锈钢焊接区复相组织点蚀规律与机制研究

Pitting corrosion has been identified as the most important local corrosion mechanism during the service life due to the microstructure evolution of complex phases in the welding zone of duplex stainless steels. Thus, the revelation of corresponding corrosion rules and mechanisms plays a significant scientific and practical role in establishing welding industry parameters and predicting materials reliability. This project will focus on systematical study of the pitting corrosion behaviors of welding zones and typical complex phases in five different corrosion-resistant level duplex stainless steels in artificial seawater..(1) On the basis of microstructure evolution characterization of welding zone, the continuous wide-voltage and narrow-pulse technique to control the pitting size of 100 μm or submicron will be built. Meantime, the corrosion rules and mechanisms of pitting corrosion of different regional scale will be presented. In addition, the effects of precipitation phases (Cr2N,σ,χ,γ2...) and Cr-depletion zone on pitting corrosion behaviors will be studied;.(2) The influence of welding heat input and cooling rate on the microstructure evolution and pitting corrosion resistance will be examined. The differences between single-pass welding and multi-pass welding will be compared and analyzed. These results will provide supporting data and theory evidences to the optimization of industry parameters;.(3) The theoretical model will be built to explain the pitting corrosion mechanisms of duplex phases and complex phases. The acceleration and inhibition effects of different precipitation phases on pitting corrosion will be explored;.(4) Based on these findings, by using the field calibration data supporting of National Corrosion Platform website and comparing the calibration data and critical pitting temperature (CPT) values of welding specimens through interpolation method, the quantitative decrease values of CPT and indirect prediction of in-ocean corrosion behaviors will be achieved.

双相不锈钢焊接区由于复相组织演变使得点蚀成为其海洋服役中的重要腐蚀形式。对应规律与机制的揭示对于焊接工艺的制定与材料可靠性预测及保障具有重要学术与实际意义。本项目针对五种耐蚀级别的钢种焊接区及其典型复相组织在人工海水中点蚀行为进行系统研究：（1）在对焊接区微观组织演变表征基础上，建立连续电压宽、窄脉冲联合控制的100微米级与亚微米级点蚀发生技术，揭示点蚀在不同尺度区域的发生发展规律与析出相（Cr2N,σ,χ,γ2等）、贫铬区的作用；（2）给出焊接热输入、冷速的影响规律，以及单道与多道焊接的差异，为工艺优化提供依据；（3）建立理论模型揭示双相与复相组织点蚀机制，探索不同相区对点蚀的加速与抑制作用；（4）在此基础上，利用国家腐蚀平台网站的钢种实海定标样品现场数据，通过定标样与焊接样CPT内插比较方法，给出焊接导致点蚀性能下降幅度的定量数值，实现有现场依据的实海腐蚀间接预测。

随着海洋经济与海洋国防相关需求的日益增长，桥梁建设、船舶制造、油气管道等诸多领域面临着设备选材、可靠性评价和寿命预测等问题。双相不锈钢作为最具发展前景的海洋服役不锈钢种类，局部腐蚀（点蚀）是其主要的失效形式之一；特别是焊接部位偏离固溶态和稳定化处理态，即晶界偏移、二次析出相（sigma相、M23C6、Cr2N、γ2、χ）、微观组织演变、耐蚀元素扩散再分布等均对点蚀行为产生重要影响。本课题针对上述问题，采用模拟焊接热处理方式，运用临界点蚀温度（CPT）、电化学脉冲技术（PPT）、极化曲线和交流阻抗谱（EIS）等，系统研究2304、2205、2507、2707和3207五种双相不锈钢在人工模拟海水中的点蚀机制。研究揭示了高温固溶和中温敏化及相关影响因素对双相不锈钢点蚀萌生、发展、演变和生长的动力学特性；结合国家腐蚀平台网站的钢种实海定标样品现场数据，通过CPT内插比较方法，实现有现场依据的实海腐蚀间接预测。结果表明：五种双相不锈钢的最佳固溶温度随耐点蚀当量（PREN）值增大而增大，至超级双相不锈钢2707时达到1100℃；中温敏化处理阶段，CPT随冷速的减小而增大，多焊道有助于增强材料的耐点蚀性能且最佳的热输入值为2.5 kJ/mol。CPT和PPT技术均能在点蚀萌生阶段，在100微米级与亚微米尺度上观察点蚀与双相不锈钢微观组织间的内在联系，且在最佳的PPT测试周期中较低电位（0V vs. SCE）的施加时间为3s，较高电位（0.5V vs. SCE）的施加时间为1s。通过实海样品现场定标数据（316L、317L、904L、254SMO），结合实验室温度-电位（Z）曲线实现海洋实际环境下双相不锈钢间接腐蚀寿命预测。