Alkali Induced High Temperature Corrosion of Stainless Steel: The Influence of NaCl, KCl and CaCl2

The influence of KCl, NaCl and CaCl₂ on the oxidation of 304-type (Fe18Cr10Ni) stainless steel at 600?°C in 5?%O₂?+?40?%H₂O was investigated. Prior to exposure, a small amount of the preferred salt (cation equivalent: 1.35?μmol/cm²) was deposited on the samples. Exposure time was 1–168?h. The oxidized samples were analyzed by SEM/EDX, XRD, FIB and IC. The presence of KCl and NaCl strongly accelerates high temperature corrosion of 304L. Corrosion attack is initiated by the formation of alkali chromate through the reaction of alkali with the protective oxide. Chromate formation is a sink for chromium in the oxide and leads to a loss of its protective properties. Subsequently a rapidly growing scale forms, consisting of an outer hematite layer with chromate particles on top and an inner spinel oxide layer. In contrast to NaCl and KCl, CaCl₂ is not very corrosive. At temperature, CaCl₂ is rapidly converted to CaO. Small amounts of CaCrO₄ form where CaO is in direct contact with the scale. CaO also reacts with the scale to form Ca₂Fe₂O₅.