The Role of Phosphate-Conversion Coatings in the Makeup and Sealing Ability of Casing Connections

Phosphate-conversion coatings are widely used on (premium) casing connections for protection against corrosion. These coatings provide galling protection in conjunction with lubricant. The friction and wear that occur during makeup and subsequent load cycling strongly influence the sealing performance of the metal/metal seal. Therefore, phosphate-conversion coatings play an important role in the sealing performance of metal/metal seals. An extensive test program was set up to investigate the role of phosphate coatings during makeup and in the subsequent sealing of the metal/metal seal. With pin-on-disk, anvil-on-strip, and ring-on-ring tests, the interactions between the substrate, lubricant, and phosphate coating were investigated. A comparison was made between uncoated and coated specimens using base greases and formulated greases: API-modified lubricant and two commercially available yellow dopes. The results indicate a strong influence of the phosphate coating leading to damage-free makeup, low wear, and less dependence on the lubricant for optimal sealing ability. 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 countersurface. 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.