Corrosion Behavior of Pure Fe, Cr, and Mo Under Solid NH4Cl and Oxygen-Free Water Vapor Environment

The corrosion behavior of pure Fe, Cr, and Mo was studied at 125°C and 3.5 MPa solid NH4Cl + oxygen-free water vapor environment. The corrosion kinetic rates of these three metals are obtained by a weight-loss method as pure Fe > Cr > Mo. Through scanning electron microscopy, energy dispersive spectrometer, x-ray diffraction, and electron probe microanalyzer, the corrosion products, surface morphology, cross-sectional morphology, and element distribution of metals in the environment were understood. The results show that Cl− is enriched on the surface of the substrate, and participates in the corrosion reaction by the ionic cycle, which promotes the corrosion and dissolution of the metal substrate. In an oxygen-free environment, the corrosion products of metals are mainly chlorides, and a small amount of oxides are produced by hydrolysis. The corrosion rate of metal is related to the density of its surface product layer. A corrosive medium can diffuse to the surface of the substrate through the loose product layer. The denser the product layer, the more it can protect the substrate.