Formation mechanism of the outer layer of duplex scales on stainless steels in oxygenated supercritical water

•Formation mechanism of the outer layer of duplex scales in oxygenated supercritical water.•Additive phosphates serve as the marker of formation process of scales.•Who is predominant mechanism: precipitation or solid-state growth?•A critical water density for distinguishing two mechanisms was proposed and verified. The dependence on water density of the formation and growth of the outer scale layer was revealed by using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to analyze the effects of additive phosphates on morphology and composition of oxide scales formed on 316 stainless steel in supercritical water (SCW) with an initial dissolved oxygen of 100 mg·L−1 at 400 °C and 600 °C. In 400 °C SCW with a high density (exceeding 100 kg·m−3) the outer layer thickens via a precipitation process, hence the promotion of pre-existent phosphates in cationic precipitation results in the presence of smaller product particles and phosphorus on the scale surface. In 600 °C SCW with a low density, the formation of the outer scale layer follows a solid-state growth mechanism.