How And Why Does Scale Stick—Can the Surface Be Engineered To Decrease Scale Formation and Adhesion?

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Abstract In many production fluids encountered in the oil and gas industry, the formation of mineral scales such as BaSO4 and CaCO3 causes major problems with respect to flow assurance. It has recently become apparent that in order to fully understand a scaling system both an appreciation of the bulk precipitation and surface deposition characteristics must be obtained [1]. In surface deposition the important steps are (i). nucleation of crystals, (ii) the growth of these crystals at the surface site and (iii) finally the adhesion of crystals to create a scale layer. In some instances where scaling causes operational problems (e.g in heat exchangers in desalination units) application of ‘non-stick’ materials are considered as a promising method to alleviate scale deposition. To date, there have been few instances where surface modification or surface coatings have been considered seriously for scale control in the oil and gas industry. In this paper results are reported from experiments to investigate calcium carbonate scale formation and adhesion at solid surfaces. Two aspects relevant to adhesion are considered: 1. the influence of the substrate on the extent of surface scale formation; 2. the critical shear stress required for removal of scale. Four substrates were investigated in the study, UNS S31603 (stainless steel), electrochemically pretreated UNS S31603 and two coatings: DLC (diamond like carbon) and PTFE (polytetrafluoroethylene). Deposition experiments were conducted from supersaturated solutions of calcium carbonate at 50°C. In order to investigate the critical shear stress for removing scale on different substrates, a submerged impinging jet (SIJ). It has been shown that the amount of scale formed at the surface is dependent on surface energy and on physical surface roughness. The surface energy, and in particular the polar component, is an important factor in adhesion – as are the morphology and crystal arrangement in the surface scale deposit.