Alkali induced high temperature corrosion of stainless steel: The influence of NaCl, KCl and CaCl 2

Abstract

The influence of KCl, NaCl and CaCl 2 on the oxidation of 304-type (Fe18Cr10Ni) stainless steel at 600 °C in 5 %O 2 + 40 %H 2O was investigated. Prior to exposure, a small amount of the preferred salt (cation equivalent: 1.35 μmol/cm 2) 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, CaCl 2 is not very corrosive. At temperature, CaCl 2 is rapidly converted to CaO. Small amounts of CaCrO 4 form where CaO is in direct contact with the scale. CaO also reacts with the scale to form Ca 2Fe 2O 5. © 2012 The Author(s).