Local supersaturation and growth of protective scales during CO2 corrosion of steel

Carbon dioxide corrosion of carbon steel has been studied extensively due to its importance in oil and gas pipelines. One of the most effective ways of rendering protectiveness to the oil-field components against CO2 corrosion is to induce the formation of protective surface scale, essentially composed of iron carbonate (siderite). However, crystal growth of siderite from solution is surface reaction rate controlled and the time required for the formation of a protective siderite scale is relatively long and precipitation of siderite also requires appropriate temperature, pH and high degree of supersaturation.

The already available models on corrosion rates used for pipeline corrosion management are in good agreement in terms of non-scaling conditions. However, these models differ under conditions where a protective siderite scale forms. At temperatures above 60°C, carbon dioxide pressures more than 0.5bar and pH above 6, a very protective crystalline siderite scales found to forms which causes diminution of the corrosion rate to low values. This effect is therefore incorporated in empirical models which account for a scaling factor. Other models have also been developed in an attempt to calculate the corrosion rate from first principles. However, these models cannot reasonably incorporate the effect of formation of the scale. This is because the required kinetic factors describing the nucleation and growth of the scale were not precisely known. These models therefore fail for scale forming conditions.

The critical role of supersaturation in controlling the kinetics of formation of crystalline siderite scale is expected to have a correlation with the solution flow. In order to address this apparent sensitivity of a protective siderite scale formation on the solution flow rate, and to understand the critical dependence of the corrosion rate of steel on the formation of a protective crystalline corrosion scale in carbon dioxide saturated brine at elevated temperatures, researchers at University of Auckland in New Zealand led by Professor David E Williams, in collaboration with scientists at Qatar University led by Professor Aboubakr Abdullah studied the effect of flow on the initial formation kinetics of crystalline scales. They demonstrated the critical dependence on the local supersaturation of the kinetics of formation of a protective crystalline scale on the surface of carbon steel in the course of carbon dioxide corrosion in brine at temperatures of practical interest. Their work is published in Corrosion Science which one of the most prestigious journals in the areas of Corrosion.

In their work, the authors developed a model which is in excellent agreement with their experiments finding. In their model, the authors proved that the total current is the sum of a current due to dissolution of iron and a current due to crystalline layer growth. They showed that the dissolution current as well as the surface supersaturation are critically dependent on the thickness of an initially-formed amorphous layer. The researchers on the basis of their in-situ synchrotron X-Ray diffraction measurement inferred that the amorphous layer dissolves as a carbonato-iron complex. The surface concentration of this layer depends on the electrode potential.

The research teams constructed a simple transport reaction model where they showed that the supersaturation was determined by the precipitation rate constant of the colloidal FeCO3, the thickness of the diffusion boundary layer and by the product of the current due to iron dissolution. Through the model, the authors also showed that the crystal growth rate varies quadratically with supersaturation at pH 6.8, and linearly at pH 7.3.
The effects of electrode potential, microstructure, flow, and surface roughness were simply to change supersaturation by changing the current density per unit-projected area passing via the initially formed amorphous layer. The variation of brine concentration had no significant effect under pure CO2 enviroment.

The authors also demonstrated the sensitivity of the crystallinity of the final scale to solution flow. They showed that the siderite was the first crystalline product and chukanovite followed, with a delay time that decreased with increasing pH. The ratio of chukanovite to siderite was low at high pH and increased with decreasing pH, and possibly through a maximum. From the outcomes of the study, the authors advanced ideas relating to the importance of local microenvironments as well as local fluctuations in the mass-transport rate.