Ab initio calculations have been employed to elucidate the habit and surface reactivity of iron carbonate crystals, which are a major component of corrosion scales formed in sweet solutions. The habit is environment dependent, varying from rhombohedral to micro-facetted cylinders with trigonal pyramidal caps as a function of iron activity. Consistent with modelling, the cap facets are shown to be (104) surfaces through a combination of EBSD and confocal microscopy. Furthermore, it is concluded that reactivity is facet dependent, with the (104) surface being relatively inert. These observations have the potential to initiate new approaches to corrosion control and prediction.

Controlling corrosion with naturally occurring corrosion scales is potentially a more environmentally sustainable alternative to current approaches, including dosing of organic corrosion inhibitors. We report operando grazing incidence X-ray diffractograms correlated with electrochemical measurements to elucidate the growth and corrosion protection properties of a corrosion scale composed of FeCO3 crystallites, which is encountered in various key energy industry applications. Data, acquired as a function of time from high-purity iron immersed in CO2-saturated deionized H2O at pH 6.8 and T = 80 °C, show that the FeCO3 scale not only prevents corrosion of the covered substrate but also acts as a significant interfacial diffusion barrier for corrosion reagents and/or products once sufficient coverage is achieved. Most notably, from a corrosion engineering perspective, however, it is determined that corrosion occurring in gaps between scale crystallites remains appreciable; this important insight is gained through the analysis of electrochemical impedance spectra to estimate the variation in electrochemically active surface area as scale coverage increases. These results indicate that naturally occurring FeCO3 scales are not a tenable solution for corrosion protection, as even in their intact state they are highly likely to be, at best, semiprotective.

Despite intensive study over many years, the chemistry and physics of the atomic level mechanisms that govern corrosion are not fully understood. In particular, the occurrence and severity of highly localized metal degradation cannot currently be predicted and often cannot be rationalized in failure analysis. We report a first-principles model of the nature of protective iron carbonate films coupled with a detailed chemical and physical characterization of such a film in a carefully controlled environment. The fundamental building blocks of the protective film, siderite (FeCO3) crystallites, are found to be very sensitive to the growth environment. In iron-rich conditions, cylindrical crystallites form that are highly likely to be more susceptible to chemical attack and dissolution than the rhombohedral crystallites formed in iron-poor conditions. This suggests that local degradation of metal surfaces is influenced by structures that form during early growth and provides new avenues for the prevention, detection, and mitigation of carbon steel corrosion.

Electrochemical measurements and substrate analysis have been employed to study the corrosion of iron in sweet solution (pH = 6.8, T = 80 degrees C) over a period of 288 h. Correlated with decreasing corrosion rate, diffraction, microscopy, and spectroscopy data reveal the evolution of adhered sweet corrosion scale. Initially, it is comprised of two phases, siderite and chukanovite, with the latter affording little substrate protection. Subsequently, as the scale becomes highly protective, siderite is the sole component. Notably, siderite crystals are concluded to display a somewhat unexpected habit, which may be a trigger for local breakdown of protective sweet scales.

An electrochemical cell for in situ grazing incidence x-ray diffraction is described, along with its implementation for characterization of corrosion in a model sweet oilfield environment. In order to mimic reservoir fluids, the cell has been designed to facilitate measurements from substrates immersed in solutions with low concentrations of dissolved oxygen. In addition, the temperature of the solution can be varied from room temperature up to ~ 80°C. Diffraction patterns acquired with synchrotron radiation from X65 pipeline steel immersed in CO2-saturated aqueous NaCl solution (pH = 6.8, T = 80°C) are presented. These data demonstrate the formation and temporal evolution of a multicomponent corrosion scale, which includes siderite (FeCO3) and chukanovite (Fe2(OH)2CO3).