To understand the process governing iron corrosion in clay over centuries, the chemical and mineralogical properties of solids formed by free or anodically activated corrosion of iron in water-saturated clay at 90 degrees C over 4 months were probed using microscopic and spectroscopic techniques. Free corrosion led to the formation of an internal discontinuous thin (<3 microm thick) magnetite layer, an external layer of Fe-rich phyllosilicate, and a clay transformation layer containing Ca-doped siderite (Ca(0.2)Fe(0.8)CO(3)). The thickness of corroded iron equaled approximately 5-7 microm, consistent with previous studies. Anodic polarization resulted in unequally distributed corrosion, with some areas corrosion-free and others heavily corroded. Activated corrosion led to the formation of an inner magnetite layer, an intermediate Fe(2)CO(3)(OH)(2) (chukanovite) layer, an outer layer of Fe-rich 7 A-phyllosilicate, and a transformed matrix layer containing siderite (FeCO(3)). The corroded thickness was estimated to 85 microm, less than 30% of the value expected from the supplied anodic charge. The difference was accounted for by reoxidation at the anodically polarized surface of cathodically produced H(2)(g). Thus, free or anodically activated corroding conditions led to structurally similar interfaces, indicating that anodic polarization can be used to probe the long-term corrosion of iron in clay. Finally, corrosion products retained only half of Fe oxidized by anodic activation. Missing Fe probably migrated in the clay, where it could interact with radionuclides released by alteration of nuclear glass.