Corrosion resistance of white cast irons in caustic solutions

High chromium white cast irons have proven to be a useful family of materials for wear resistant applications in high temperature Bayer liquor, as they combine high abrasion resistance with reasonable corrosion resistance. Plant experience has shown that certain alloy grades exhibit poor performance in high temperature applications at the end of the digestion circuit and it has been suggested that this is related to the corrosion resistance of these materials. In this paper, the corrosion behaviour of three grades of high chromium white cast iron was studied in pure caustic solutions at temperatures from 170ºC to 280ºC, with exposure times of up to 57 days. It was found that at lower temperature, the corrosion proceeded by a mechanism of degradation of the carbide-matrix interface with significant corrosion of the carbide component of the alloy. At higher temperatures, preferential corrosion of the matrix occurred in lower alloy grades, leaving carbide particles exposed to mechanical damage whilst in higher alloy grades, corrosion occurred by corrosion penetration along the carbide-matrix boundary. Corrosion resistance also correlated with the microstructure of the alloys and a number of the alloy constituents, including chromium content. This work will assist in our understanding of the mechanism of degradation of white irons in Bayer liquor and in the selection of white cast iron grades for application in high temperature digestion circuits.