Abstract:
The goal of this work is to gain an improved understanding on the formation of crystalline corrosion product scales during carbon steel corrosion in CO2 environments using in situ synchrotron techniques, and the effect of corrosion product deposition on the corrosion rate. Particular objectives were: To elucidate the reasons for different growth habits in the formation of crystalline corrosion product layers (i.e. controlling factors in the precipitation process). To improve the understanding of the formation of highly protective layers. To study the effect of organic additives in modifying corrosion scale formation. The synchrotron X-ray diffraction (XRD) technique was used to monitor the growth of crystalline phases on carbon steel in situ while the corrosion process is accelerated using electrochemical control. The test solution was 0.5M NaCl, saturated with CO2 at 1 bar with varying test conditions: temperature, pH, applied potential and presence of solution additives (scale inhibitor, acetic acid, Cr3+ and Mg2+). A critical factor in controlling the corrosion rate was the formation of a compact scale of cubic siderite crystals. Once this scale formed, the corrosion rate was reduced significantly. In some cases, significant amount of chukanovite was formed after siderite and resulted in feathery platelet crystals. The results of this study suggest that a very high critical supersaturation was required to nucleate crystalline siderite and that crystalline phases were nucleated from a precursor amorphous and unprotective layer. The transformation kinetics of the amorphous layer into crystalline phases can have significant influence on the properties of the formed scale. Crystalline siderite nucleation and growth was sensitive to temperature, pH, stirring rate, surface roughness, steel microstructure and presence of certain cations in the solution – particularly Cr3+. Cr3+ derived from dissolution of Cr-containing low-alloy steel had a particularly strong effect. It decreased the critical supersaturation for siderite and modified the growth rate of the scale, thereby promoting the formation of an adherent and protective scale. Chukanovite phase was observed in experiments following siderite precipitation. The formation of chukanovite appeared to be strongly pH sensitive and is proposed to have an important effect on the scale morphology and consequent corrosion protection.