Reactions Occurring during the Sulfation of Sodium Chloride Deposited on Alumina Substrates

Abstract

The reaction between solid and air containing 1% has been studied between 500° and 700°C. The reaction product, , forms not only on the surface of the but also on surrounding areas of the substrate due to the volatility of the at these temperatures. At the higher temperatures, the vapor pressure of is so high that the majority of the reaction product is distributed on the substrate. Above 625°C, the reaction product is a liquid solution of and that exists only so long as is supplied from the original crystal source. Eventually, the liquid solidifies by constitutional solidification is the is converted to . While it exists, the liquid solution is shown to be highly corrosive to and, on a scale of growing on alloy HOS 875, particularly attacks the grain boundaries of the scale at preferred sites where chromium and iron oxides and sulfides rapidly develop. This is proposed as one mechanism by which deposition contributes to the initiation of low temperature hot corrosion.