Laboratory corrosion studies were carried out in green, white, and black liquors from a pulp and paper mill in the Northwestern US. The corrosion potential behavior and corrosion rates of carbon steel, stainless steel, and cast irons were determined at the service temperatures of the equipment from which the liquors were sampled, including storage tanks, evaporators, and concentrators. Both low- and high-silicon content carbon steels were found to be spontaneously active and had unacceptably high corrosion rates in the green liquor, white liquor, and strong black liquor. Carbon steels were spontaneously passive in the weak black liquor and also tended to passivate in the intermediate black liquor. Nickel-alloyed cast irons used for valves in the evaporators had corrosion behavior similar to or poorer than the carbon steels. Stainless steels such as type 304L austenitic stainless steel and type 2205 duplex stainless steel were found to have excellent resistance to corrosion in all the alkaline pulping liquors.
Alkaline pulping liquors are mixtures of inorganic compounds including hydroxide, sulfide, polysulfide, thiosulfate, sulfate, carbonate, and chloride (that comprise green and white liquors). In addition, there are dissolved organic compounds in black liquors. For many years most of the equipment used to handle alkaline pulping liquors in pulp and paper mills was constructed using carbon steel. It was understood as early as 1949 that the element silicon in steel was detrimental to corrosion resistance1 and most equipment was built using specially modified low silicon steels, particularly A285-Grade C carbon steel. By 1970, however, this knowledge was lost and the modified version of this steel became increasingly difficult to obtain. A516-Grade 70 carbon steel (UNS K02700) saw increasing use from the 1970's onward due to its higher strength and cleaner microstructure. Unfortunately, the silicon content in A516-Grade 70 as well as modern A285-Grade C steel (UNS K02801) can be over ten times higher than it was in "modified" A285-Grade C steel. The result has been more rapid corrosion of the new equipment than was experienced in the past.
There have been several studies of corrosion of carbon steel in white liquors2-7. Hydroxide, sulfide, and thiosulfate have been identified as the corrosive agents in white liquor; polysulfide may be either detrimental or beneficial depending on its concentration. Higher sulfidity was found to cause increased corrosion rates in white, green, and black liquors8. Studies of corrosion of carbon steels in continuous digester liquors have revealed that increasing temperature and lowering the hydroxide content both contribute to accelerated corrosion9,10. There has been relatively little investigation of corrosion phenomena in black liquors despite the industry awareness of corrosion problems in equipment handling black liquors going as far back as the 1970's11. While some early researchers were aware of the significant role in the corrosion process played by organic wood extractives such as catechols12, the systematic investigation of the effects of different wood species on corrosion did not begin until very recently13,14.
Stainless steels are highly resistant to corrosion in most alkaline pulping liquors. One notable exception is batch digesters where the corrosion of stainless steel cladding and weld overlays can be very rapid10. There is also some field experience with corrosion and stress corrosion cracking (SCC) of types 304L (UNS S30403) and 316L (UNS S31603) stainless steels in more concentrated black liquors15.
MATERIALS
Table 1 lists the chemical composition of the materials that were tested in the alkal
