Recovery boiler air-port corrosion - Part 3: Corrosion of composite tubes in molten NaOH

The corrosion mechanism of Type 304L stainless steel cladding of composite boiler tubes in molten NaOH (380 - 400 C) was studied using linear polarization measurements and conventional coupon tests. Inconel 600 had the best corrosion resistance, followed by Type 310 and Type 304L. Corrosion resistance depends on the tenacity and morphology of the protective corrosion layer formed on each alloy. The main corrosion mechanism for, Type 304L is crevice corrosion in the Presence of peroxide ions (Or2-). A study of electrochemical potential in molten NaOH shows that Type 304L has a much larger anode potential against Ni than carbon-steel 1010, Type 310, and Inconel 600. Once the Type 304L cladding on a composite tube is completely penetrated by crevice corrosion, An electrochemical corrosion cell is established. The carbon-steel substrate is cathodically protected, while the Type 304L cladding becomes a sacrificial anode and corrodes at an accelerated rate.This work is part of the lesearch on "Recovery Boiler Fireside Deposits," supported by ABB Combustion Systems, Ahlstrdm Corp., Aracruz Cellulose SA, Babcock & Wilcox Co., Champion International Corp., Diamond Power Specialty Co., E.B. Eddy Forest Products Ltd., Gotaverken Energy Systems, Jansen Combustion and Boiler Technologies Inc., James River Corp., Tampella Power Corp., Union Camp Corp., Westvaco Corp., Weyerhaeuser Co., Willamette Industries, and the Ontario Ministry of Colleges and Universities through its URIF progra