High-temperature alloys are crucial to many important technologies that underpin our civilization. All these materials rely on forming an external oxide layer (scale) for corrosion protection. Despite decades of research on oxide scale growth, many open questions remain, including the crucial role of the so-called reactive elements and water. Here, we reveal the hitherto unknown interplay between reactive elements and water during alumina scale growth, causing a metastable 'messy' nano-structured alumina layer to form. We propose that reactive-element-decorated, hydroxylated interfaces between alumina nanograins enable water to access an inner cathode in the bottom of the scale, at odds with the established scale growth scenario. As evidence, hydride-nanodomains and reactive element/hydrogen (deuterium) co-variation are observed in the alumina scale. The defect-rich alumina subsequently recrystallizes to form a protective scale. First-principles modelling is also performed to validate the RE effect. Our findings open up promising avenues in oxidation research and suggest ways to improve alloy properties.