The role of phosphate conversion coatings in make-up of casing connections

Phosphate conversion coatings are widely used on (premium) casing connections for protection against corrosion. Next to that, in conjunction with the lubricant these coatings provide galling protection. The friction and wear that occurs during make-up and subsequent load cycling determines the sealing performance of the metal-to-metal seal. Therefore, phosphate conversion coatings play an important role in the sealing performance of metal-to-metal seals. An extensive test program was set-up to investigate the role of phosphate coatings during make-up. With pin-on-disc, anvil-on-strip and ring-on-ring tests the interactions between substrate, lubricant and phosphate coating was investigated. A comparison was made between uncoated and coated specimens using base greases, and formulated greases: API modified and two commercially available yellow dopes. The results indicate a strong influence of the phosphate coating leading to damage free make-up, low wear, and less dependence on the lubricant. This is attributed to the formation of a hard and smooth dissimilar surface, the ability to adsorb the lubricant and the generation of a transfer layer on the uncoated counter surface. It is concluded that taking the interaction with phosphates into account could enable lubricants to be tailored for sealing performance and thus can ease the transition to environmentally friendly rated lubricants