Study On Corrosion Mechanism Of 20CrMo Sucker Rod

The service environment of sucker rod became more and more deterioration with increasing mining age in our country. The sucker rod break accidents occurred unceasingly mainly because of corrosion fatigue. The study on sucker rod failure was almost focused on the fatigue, and seldom on corrosion because of the harsh corrosion fatigue test conditions. Based on this situation, the corrosion rate, corrosion morphologies, corrosion production and corrosion mechanism of 20CrMo sucker rod were studied in this paper on the base of fatigue test results. Firstly, a set of clamps were designed to connect the fatigue test machine and sucker rod. According to the fatigue test results, the best fatigue resistance sucker rod was chosen as corrosion samples. The electrochemical test and static immersing test were done in 3.5% NaCl solution at various temperatures and pH. The corrosion potential curve and Tafel curves were got from electrochemistry test at various conditions. The results indicated that the corrosion potential was reduced with temperature increasing or pH reducing in open corrosion solution system. In CO₂ corrosion solution system, the corrosion current and corrosion rate were increased with temperature increasing, and had those maximum at 70℃, and then they would decrease with temperature increasing. The corrosion potential was reduced, the corrosion current and the corrosion rate increased with temperature increasing or pH decreasing in O₂ corrosion solution system. Corrosion rates were got from static corrosion weightlessness test. The corrosion morphologies and products were also studied in this paper. The results showed that the character of corrosion was mainly pitting corrosion, and products were FeCO₃ and FeO(OH) in CO₂ solutions. In O₂ solutions, the corrosion were mainly helcoid corrosion, and the products were Fe₃O₄ and FeO(OH). To comparison, the corrosion behaviors of sucker rod plated tungsten alloy coatings were also carried out in this study. It didn't suffer corrosion in the above two corrosion solutions, which indicated that the tungsten alloy coating could enhance the behavior of corrosion resistance of 20CrMo sucker rod.