Metal surfaces are generally protected by a oxide layer that forms on them and guards against further attack from corrosive elements. This protective layer can be destroyed through chemical or electrochemical dissolution.

In chemical dissolution, the protective layer is removed by a chemical process using either an acidic or an alkaline solution depending on the pH value in the local region. In electrochemical dissolution, depending on the electro­chemical potential, the metal can undergo either transpassive or active dissolu­tion. All forms of electrochemical corrosion require the presence of aggressive ionic species (as reactants, products, or both), which in turn requires the exis­tence of an aqueous environment capable of stabilizing them.

Stainless and nickel-chromium alloys experience high corrosion rates at supercritical pressure but subcritical temperatures because of transpassive dis­solution (Friedrich et al., 1999), where the nickel or iron cannot form a stable insoluble oxide that protects the alloy. Under supercritical conditions, the acids are not dissociated and ionic corrosion products cannot be dissolved by the solution because of the solvent’s low polarity. Consequently, corrosion drops down to low values.

Electrochemical corrosion requires the presence of ionic species like halides, nickel-based alloys, and compounds. These show high corrosion rates, which decrease at higher temperatures. High-pressure water in an SCW reactor pro­vides favorable conditions for this, but once the water enters the supercritical domain the solubility and concentration of ionic species in it decrease, although the reaction rate continues to be higher because of higher temperatures. The total corrosion reduces because of decreased concentration of the reacting species. Thus, corrosion in a plant increases with temperature, reaching a peak just below the critical temperature, and then reduces when the temperature is supercritical. The corrosion rate increases downstream, where the temperature drops into the subcritical region.

At a relatively low supercritical pressure (e. g., 25 MPa), the salt NaCl is not soluble. Thus, in an SCW a reaction that produces NaCl, the salt can pre­cipitate on the reactor wall. Sometimes water and brine trapped between the salt deposit and the metal can create a local condition substantially different from conditions in the rest of the reactor in terms of corrosion. This is known as underdeposit corrosion.

In general, a reaction environment that is characterized by high density, high temperature, and high ion concentration (e. g., acidic) is most conducive to cor­rosion in an SCW reactor. Rather than the severity of corrosion in terms of whether the flow is supercritical or subcritical, the density of the water should be the major concern.