Corrosion testing in black liquors from kraft pulp and paper mill evaporators has confirmed that carbon steel has stable passivity in weak liquors (15% solids content), unstable passivity in intermediate liquors (25% and 33% solids content), and corrodes actively in strong liquors (45% and 49% solids content). The occurrence of rapid corrosion of carbon steels in evaporators handling liquors of intermediate solids content is caused by an abrupt switch from unstable passivity to active corrosion. Sudden changes in the corrosion state may be brought about by relatively small increases in the sulfidity and temperature of the liquor or by the use of modern carbon steels having higher silicon contents than those used in the past. Galvanic coupling of active carbon steel with stainless steel increases the active corrosion rate of the carbon steel.
This paper is a companion to an earlier paper by the author on the results of inspections of kraft black liquor evaporators that was presented at the 2005 TAPPI Engineering, Environmental, and Pulping Conference held in Philadelphia. Unexpectedly rapid corrosion of carbon steel in evaporator effects handling intermediate-strength black liquors is a significant problem that has resulted in leaks or failures in vapor domes, vapor separators, tube sheets, piping, and nozzles. That carbon steel serves well as a material of construction for many years but then succumbs to rapid corrosion suggests that there has been a change from passive to active corrosion conditions. In many cases the corrosion rate of the carbon steel was evidently accelerated by galvanic contact with stainless steel components.
The TAPPI paper contains a comprehensive review of the literature on corrosion of evaporators. In two previously published studies, the author has demonstrated that the corrosion rate of carbon steel in softwood black liquors increases with:
- increased solids content in the liquor
- increased sulfidity of the liquor
- increased temperature of the liquor
- increased silicon content in carbon steels
- increased velocity of the liquor.
Preet Singh has shown in the laboratory that the nature of the wood species being pulped can have a profound effect on corrosion rates in black liquors produced under otherwise identical pulping processes. Pulping of some hardwood species has been found to yield essentially non-corrosive black liquors while pulping of many softwood species yielded black liquors highly corrosive to carbon steel.
Corrosion testing was done in samples of evaporator black liquors from kraft mills in North and South America. Materials tested included carbon steels, stainless steels, and carbon steel-stainless steel galvanic couples. All corrosion tests were carried out in autoclaves of 2-litre volume type constructed using 2205 duplex stainless steel.
The liquors were all from softwood pulping in four different geographical areas:
- NE (Northeastern North America). Softwood species pulped (predominantly spruce).
- NW (Northwestern North America). Softwood species pulped (predominantly Douglas fir).
- SE (Southeastern North America). Softwood species pulped (Southern pine).
- SA (South America). Softwood species pulped (Radiata pine).
The liquor samples were shipped in plastic bottles that were completely filled to avoid air oxidation of the liquor. The samples were stored under refrigeration. All chemical analyses were done by Econotech in Delta, BC, Canada, a company that specializes in analysis of liquors from kraft pulp mills worldwide. The liquors were analyzed for inorganic constituents hydroxide, sulfide, thiosulf