Sweet corrosion scale: Structure and energetics of siderite facets

•DFT calculations of sweet corrosion scale (FeCO3) reveal its atomic structure and classify surfaces protective against or vulnerable to corrosion.•First principles thermodynamics predict a range of FeCO3 crystal habits depending on environmental changes, providing a direction for protective corrosion scale engineering.•Crystallographic orientation of predicted FeCO3 facets are confirmed through a combination of EBSD and confocal microscopy.