高温高压氯离子环境对螺杆钻具硬铬镀层腐蚀行为的影响

Effect of High Temperature and High Pressure Chloride Ion Environment on Corrosion Behavior of Hard Chromium Plating on Screw Drill

目的研究螺杆钻具表面硬铬镀层在高温高压氯离子环境下的腐蚀行为。方法利用高温高压反应釜模拟螺杆钻具的井下环境,对螺杆钻具表面硬铬镀层进行腐蚀和检测,通过正交实验法研究温度、压力和氯离子浓度对硬铬镀层腐蚀状态的影响。采用动电位扫描、腐蚀失重法,研究硬铬镀层的电化学参数及腐蚀特征。通过扫描电镜(SEM)和能谱(EDS),分析镀层腐蚀后的微观形貌和表面成分的变化。结果影响硬铬镀层腐蚀速率的最大因素是氯离子浓度,其次是温度,再次是压力。年腐蚀深度最严重为0.8925 mm/a,最大的腐蚀速率为1.9026 g/(m^2·h)。腐蚀速率随着氯离子浓度的升高而降低,随着温度和压力的升高而升高。结论推断在温度180℃、压力15 MPa和Na Cl质量浓度30 g/L的条件附近,硬铬镀层腐蚀最严重。氯离子是影响镀铬层腐蚀速率的主要因素,因为氯离子优先在敏感位置(钝化膜的薄弱点处,如裂纹)引发孔蚀核。在闭塞腐蚀电池催化作用下,镀层表面不断形成孔蚀,裂纹被加宽,最终导致少部分区域镀层剥落,影响镀层整体保护性能。

The work aims to study corrosion behavior of hard chromium plating on surface of screw drill in high tempera- ture and high pressure chloride ion environment. The hard chromium plating on surface of screw drill was corroded and tested by simulating downhole environment of screw drill with high temperature and high pressure reactor. Effects of temperature, pressure and chloride ion concentration on corrosion state of hard chromium plating were studied in the method of orthogonal experiment. Electrochemical parameters and corrosion characteristics of hard chromium plating were studied in the methods of dynamic potential scanning and weight loss corrosion. Changes in microstructure and surface composition of the corroded plat- ing were analyzed by using scanning electron microscopy and EDS. The most important factor affecting corrosion rate of the hard chromium plating was the chloride ion concentration, followed by the temperature and pressure. The most serious corrosion depth of the was 0.8925 mm/a, and the maximum corrosion rate was 1.9026 g/（m2.h）. The corrosion rate decreased with increase of chloride concentration, and increased with the increase of temperature and pressure. It is inferred that the most severe corro- sion of the hard chromium plating occurs nearly provided with the temperature of 180 ℃, pressure of 15 MPa and NaCl mass concentration of 30 g/L. Chloride ion is the main factor affecting corrosion rate of the chromium plating because the chloride ion preferentially initiated pitting nuclei in the sensitive position （the weak point of passive film, such as crack）. Under the effect of occluded corrosion cell catalysis, pitting corrosion continuously occurs on the plating, cracks are widened, and ultimately lead- ing to peeling of local plating, thus affecting overall protective properties of the plating.