IMPROVED PRE-PASSIVATION OF MILD STEEL HEAT EXCHANGER TUBES
Kane, A.R., Battams, M.J., Connor, J.N., Xavier, A., del Aguila, D.
Unpassivated mild steel exposed to high temperature Bayer liquor can suffer catastrophic corrosion damage in relatively short periods. The formation of a durable passivation layer is therefore a key process in ensuring long heat exchanger tube life.
Based on accumulated observations and field data, a pre-passivation process was adopted at QAL in which mild steel heater tubes are exposed to low flows of low temperature spent liquor for a period of up to 12 hours.
In conjunction with a chemical supplier, a new acid inhibitor was developed which in laboratory testing showed a significantly reduced corrosion rate when acid cleaned metal coupons were subsequently exposed to hot synthetic Bayer spent liquor.
A series of plant trials have been undertaken to validate the laboratory results under plant operating conditions to determine whether a reduction in pre-passivation times could be implemented.