Single-tube Preheater Corrosion Cracking Research

Monotubular heater is an important apparatus in Bayer process of alumina extraction used in Pingguo Aluminum Company. However, since several years cracking in its weld joints occur and the production process is then interrupted by leaking of liquor. There is a pressing need to ascertain major causes of heater cracking and take protective methods to increase the use period of the heater.Corrosion cracking in weld joints of low alloy steel was studied by constant load tests, electrochemical methods and optic microscopy. Results showed that the nature of heater cracking was stress corrosion cracking (SCC) in hot caustic solutions. In high temperature and strong alkaline solution stress corrosion occured on weldment and heat effected zone of carbon steel or low alloy steel which led to formation of shallow cracks. Then acid pickling promoted crack tip dissolving and progagating along grain boundaries. Ruoding was not an effective corrosion inhibitor. Acid pickling with Ruoding rendered shallow cracks deepen and even punch. The worsening of weld joints was not due to hydrogen penetration and hydrogen embrittlement.Post-weld heat treatment was an important measure to prevent SCC of the heater. It reduced residual welding stress obviously and so resulted in the lower susceptibility to SCC, but it did not eliminate defective welding organizations. The optimum heat treatment temperature was 620 癈 and its holding time was 1 hour. After such a heat treatment, the SCC of weldment was retarded.With regard to acid pickling of monotubular heater which had been suffered from SCC, it was vital to select Changcheng 3# or Lan-826 instead of Ruoding as corrosion inhibitor. Acid pickling with these two mix-type corrosion inhibitors prevented weldment from further corrosion cracking. The optimum concentration of Changcheng 3# or Lan-826 was 0.3%, acid pickling temperature was 60癈 and its holding time was less than 5 hours.