Caustic Corrosion in Black Liquor Evaporators, 1996 Engineering Conference Proceedings
Today, the dry solid content of black liquors before combustion has been increased from 70 wt.% of d.s. to almost 80 wt.% of d.s., sometimes even above that. In many cases this means that the evaporation temperatures of kraft black liquors increase, too. The corrosion of ten different grades of stainless steel and nickel compounds was measured at the temperatures of 150°C and 170°C for periods of 14 and 40 days. The high testing temperatures were reached by installing the test coupons and the black liquor in 4 cylinders that were rotated in hot oil during the test. For some of the tests, black liquors from two different mills were modified by adding NaOH or H2 S04 . The black liquors used in the tests were chemically analyzed before and after the corrosion tests. It can be concluded that duplex stainless steels are more resistant to caustic grain boundary corrosion than normal grades of stainless steel. At high levels of NaOH, depending on the test temperature, grain boundary corrosion of stainless steels became evident. So, if the rate of corrosion should not exceed the level of 0.1 mm/y, a temperature increase from 150°C to 170°C means that the critical concentration of NaOH should be decreased by almost 50 %.