Effect of corrosion inhibitors and scale inhibitor additives on the growth of sulphate scales at metal surfaces

From an oilfield scale prevention perspective, interest has focused on understanding the relationship between heterogeneous nucleation and growth on pipeline surfaces rather than homogeneous nucleation and growth from bulk solution. Furthermore, it is recognised that homogeneous nucleation, and not surface nucleation, is conventionally examined in industry standard bulk "jar" type tests for the assessment of scale inhibitor performance. The trend towards understanding heterogeneous nucleation and growth has gained interest because surface growth is recognised as a much more serious issue than homogeneous "bulk" precipitation. Recent work examining the relationship between bulk and surface growth of barite scales has shown that low concentrations of scale inhibitor can promote surface growth by effectively inhibiting bulk (homogeneous) precipitation and that the presence of film forming corrosion inhibitors can minimise surface growth without impacting bulk precipitation.

This current work examines further the role of scale inhibitor and corrosion inhibitor species on both homogeneous and heterogeneous scale nucleation and growth at metal (Hastelloy C276) surfaces. Of particular importance is the change in composition of the scale formed on the metal surfaces in the presence of certain additives. The composition of the surface precipitate is shown to change from a 2:1 ratio of Ba2+:Sr2+ towards a 10:1 ratio of Ba2+:Sr2+. The impact of scale inhibitors when tested at concentrations below that required to completely inhibit scale therefore has a significant impact not only on the amount of surface growth as previously described, but also on its composition. The move towards higher barium containing sulphate scale creates a more chemically resistant scale growth making its subsequent removal less efficient.

This study has examined two conventional scale inhibitor additives (PPCA and DETPMP) and two conventional corrosion inhibitors (an imidazoline and a quaternary ammonium salt).