HEAT TREATMENT OF LOW CARBON STEEL (LCS) TUBES TO IMPROVE CORROSION RESISTANCE

Heat exchangers in the Bayer process are routinely cleaned with dilute sulphuric acid to dissolve Desilication Product (DSP) scale attached to the internal surface of the tubes. The scale deposit reduces the heat transfer coefficient of these units consequently reducing the energy efficiency of heat recovery.

During the cleaning process, the tubes become exposed to the dilute acid cleaning solution causing corrosion that reduces the life of the tubes. The corrosion attack is minimised by adding corrosion inhibitors to the acid solution but the damage still remains significant for the life expectancy of the tube.

Another variable with significant impact on corrosion resistance under these conditions is the type of microstructure produced in the metal of the heat exchanger tube during manufacture. Optimum microstructures can double tube life based on laboratory results. This type of microstructure can be obtained by selective heat treatment at the steel mill.

The paper shows the results of different heat treatments on the microstructure of standard LCS heat exchanger tubes. The improvement in corrosion performance is verified using a laboratory simulation acid circulation rig (ACR) with state-of-the-art electrochemical monitoring.