Changes in pulping methods have generally increased the cooking temperatures along with the sulfidity and alkalinity of pulp mill liquors. These changes have led to accelerated corrosion rates in carbon steel equipment used in pulp mills. There is need to define environmental limits of candidate alloys, typically used in old as well as new pulp mill equipment. Depending upon the type of pulping liquor and environmental conditions, different mitigation methods can be employed to safely continue to use the old pulp mill equipment. Corrosion resistant metallic coatings have been used on carbon steel equipment surface by thermal spray or weld overlay methods. At the same time, corrosion resistant alloys are being used for new pulp mill equipment. Duplex stainless steels like 2205 and 2304 have been selected for new digesters, flash tanks, and other pulp mill equipment. Present study presents laboratory data on corrosion and stress corrosion cracking susceptibility of selected duplex stainless steels and compares it to the carbon steel and austenitic stainless steels in different pulp mill environments.
Duplex stainless steels were found to be resistant to general as well as localized corrosion under majority of pulp mill environments. Although duplex stainless steels offer superior stress corrosion cracking (SCC) resistance in sulfide containing caustic solutions, but may become susceptible to SCC at high temperatures in certain sulfide containing caustic environments.
Changes in pulping methods have been introduced in pulp mills to increase fiber yield and product quality. Typically, these changes lead to increased temperature, sulfidity, and alkalinity in pulping liquors. As a result, carbon steel equipment like digesters, flash tanks and other pulp mill equipment have seen an accelerated corrosion rate (1-7). Depending upon the type of pulping liquor and environmental conditions, different mitigation methods are being employed to continue to use the old equipment safely. Corrosion resistant metallic coatings like duplex stainless steels have been employed by thermal spray or weld overlay methods to protect carbon steel equipment (8, 9). Some mills are using anodic protection to protect carbon steel equipment (10). Corrosion resistant alloys are being selected for new pulp mill equipment. Duplex stainless steels like 2205 and 2304 have been selected for new digesters and other pulp mill equipment and have generally performed well (5-7, 11, 12, 16-19). Wensley (3) reported results from autoclave tests where selected materials were exposed to the black liquor from a digester, operating with low residual alkali. Increasing temperature was found to accelerate the corrosion rate of C-steels in flash tank liquors. At 145oC the active corrosion rate of carbon steel was less than 2 mm/y (~80 mpy). At 175 oC the active corrosion rate of carbon steel exceeded 10 mm/y (~400 mpy). Austenitic and duplex stainless were reported to be
highly resistant to corrosion for 145 oC to 175 oC temperature range.