Sucker Rod Materials – Metallurgical Quality Evaluation and its Influence on General Corrosion Resistance

Sucker rod pumps are widely used in artificial lift technology of Oil & Gas production. Corrosion, fatigue and corrosion-fatigue are common failure modes that limit life of the sucker rods that drive these pumps. Of these, corrosion fatigue is probably the most common. The initiation site for corrosion fatigue is typically a corrosion pit. The factors that contribute to the formation of corrosion pits were investigated. Four different sucker rod materials were evaluated with respect to (1) chemistry, (2) non-metallic inclusion content and (3) microstructural investigation including banding severity. It was concluded that microstructural banding has a negative influence on the general corrosion rates. The higher values of quantitively measured banding lead to increased general corrosion rates. The electrochemical potential differences between the light and dark bands is proposed as the cause of locally accelerated corrosion resulting in pits. The dark bands in the less corrosion resistant samples were lower bainite and these were presented as being more anodic.

Introduction

Sucker rods pumps are downhole artificial lift equipment that are widely used in the Oil and Gas industry as the part of onshore wells production. This artificial lift technology maintains or increases well production. With sucker rods pumps, the risk is related to aggressive production environmental conditions that may contain carbon dioxide (CO₂) and hydrogen sulfide (H₂S) with other factors such as temperature, chlorides, and water cuts that cause materials corrosion and degradation. The cyclic up-and-down rod stroke movement lead to tension-tension and tension-compression stress conditions that provoke fatigue. The corrosion and fatigue synergy make corrosion-fatigue mode a predominant failure mechanism of sucker rods.¹

There are several known manufacturing approaches that have the potential to increase rods operational life by their influence on fatigue or corrosion resistance properties individually and synergistically. These approaches include (1) material selection, (2) surface treatments, (3) coatings, and (4) design.