超声滚压20CrMnTi纳米化表面对局部腐蚀萌生行为的影响

Influence of Surface Nanocrystallization of 20CrMnTi on Behavior of Localized Corrosion by Ultrasonic Surface Rolling Processing

目的研究不同静压力条件下,形变诱导纳米化表层对20CrMnTi局部腐蚀萌生行为的影响。方法采用超声滚压技术(USRP,Ultrasonic surface rolling processing)强化20CrMnTi表层,并运用激光共聚焦显微镜、EBSD与电化学方法进行表征,分别评价形变诱导纳米化表层的腐蚀行为以及局部腐蚀行为对表面应力集中状态以及微裂纹萌生行为的影响。结果在弱碱性腐蚀条件下,20CrMnTi表层经超声滚压处理后,试样的点蚀击穿电位出现显著负移。同时,随着滚压压力升高,20CrMnTi表层钝化出现了显著的滞后,致钝电流与维钝电流同步增长,并伴有钝化区间缩小。借助激光共聚焦显微镜高精度三维重构,观测到在模拟弱腐蚀环境下,20CrMnTi超声滚压表面会显现出沿滚压方向凸出的波纹状局部腐蚀纹理,随滚压压力增长,波纹界面位置局部腐蚀损伤呈现一定的加深与拓宽,具有较高的裂纹萌生风险。通过EBSD表征发现,在超高频冲击作用下,20CrMnTi表层晶粒会转化为晶粒取向随机分布的细化表层,但由于波纹两侧晶粒存在取向差异,在滚压头高频多点的冲击作用下,不同取向晶粒的细化机制存在差别,导致波纹两侧晶粒构成电位差异明显的局部腐蚀微电池,呈现较高溶解选择性。结论在超声滚压过程中,20CrMnTi表层晶粒在局部微区范围内会出现明显的细化程度差异,在电化学作用下,细化晶粒间的微区电位差异会导致表面出现选择性溶解,并在晶粒折叠界面处产生波纹状局部腐蚀坑,且伴随超声滚压压力增大,波纹状腐蚀坑会出现明显的拓宽与加深,具有较高的疲劳裂纹萌生风险。

The work aims to study the effects of deformation-induced nano-surface on the local corrosion initiation behavior of 20CrMnTi under different static pressure conditions. The surface layer of 20CrMnTi was strengthened by ultrasonic rolling technique (USRP) and characterized by laser confocal microscopy, EBSD and electrochemical methods. The effects of corrosion and local corrosion of deformation-induced nano-surface on stress localization and microcracks were evaluated. Under the condition of weak alkaline corrosion, after the 20CrMnTi surface layer was ultrasonically rolled, the pitting breakdown potential of the sample showed a significant negative shift. At the same time, with the increase of rolling pressure, the surface passivation of 20CrMnTi showed significant hysteresis, and the passivation current and the blunt current increased synchronously, accompanied by the reduction of the passivation interval. Moreover, with the high-precision three-dimensional reconstruction of the laser confocal microscope, it was observed that under the simulated weak corrosion environment, the 20CrMnTi ultrasonic rolling surface showed a corrugated local corrosion texture protruding along the rolling direction and with the rolling pressure increasing, the local corrosion damage at corrugated interface showed a certain deepening and broadening, which led to a high risk of crack initiation. Through EBSD characterization, it was found that under the action of ultra-high frequency impact, the surface layer of 20CrMnTi was transformed into a refinement surface layer with randomly distributed grain orientation, but due to the orientation difference of crystal grains on both sides of the corrugation, under the impact of the high-frequency multi-point of the rolling head, there were differences in the refinement mechanism of grains with different orientations, so the local corrosion micro-cells with obvious difference in crystal grain composition on both sides of the corrugation showed higher solubility selectivity. Under the ultrasonic rolling process, the surface grain of 20CrMnTi shows obvious difference in the degree of refinement in the local micro-area. Under electrochemical action, the difference in micro-area potential between the refinement grains leads to selective dissolution on the surface. Corrugated local corrosion pits are generated at the grain folding interface, and as the ultrasonic rolling pressure increases, the corrugated corrosion pits are significantly broadened and deepened, and the risk of fatigue crack initiation is high.