Galvanic corrosion measurements were conducted (ASTM G-71) between drill collar and shock sleeve alloys and cyclic potentiodynamic polarization scans (ASTM G-61) on drill collar alloys for the determination of susceptibility to localized corrosion in simulated drilling fluid. The drill collar alloys selected for the program were (1) AG-17, (2) P550,
(3) P530, (4) P530HS, (5) NMS 100, (6) SMF 166, (7) 15-15LC Modified, and (8) Staballoy 734, and the shock sleeve alloys chosen were Spinodal (CDA 969 Bronze), Inconel 718, Monel K-500, and Beryllium Copper.
For the galvanic coupling between drill collar and shock sleeve alloys, drill collar materials were anodic to the shock sleeve materials in all cases examined except for Alloy 734 versus Spinodal (CDA 969 Bronze), Monel K-500, and Beryllium Copper. The shock sleeve materials with the least and the most galvanic interaction with the drill collar alloys was Spinodal (CDA 969 Bronze) and Beryllium Copper respectively.
The general corrosion resistance of the drill collar alloys varied substantially with the chemical composition. The corrosion resistance of the materials increased with the increasing pitting index (PI) value, in general. The only exception was NMS 100 which produced a higher corrosion resistance while having a lower pitting index value. One possible reason for this behavior is the lower carbon and the higher silicon levels in this material compared to the other alloys with similar pitting index values.
The surface evaluations after tests indicated a relationship between the affected area of corrosion/pitting and pitting index values for PI >16. The affected area of corrosion decreased with increasing pitting index value for the drill collar alloys with PI > 16.
Among the drill collar alloys tested, Alloy 734 was the best having the lowest corrosion rate and the lowest affected area of corrosion, and being the cathode during galvanic coupling in most cases. The next best alloys were NMS 100, followed by 15-15LC Modified.