Several candidate materials were evaluated by corrosion testing in autoclaves containing white and black liquors for batch digesters. The relationship between corrosion rate and corrosion potential was determined for ASTM SA516-Grade 70 carbon steel, UNS S30403 (Type 304L) austenitic stainless steel, UNS S31803 (“2205”) and UNS S32550 (“2605”) duplex stainless steels, and two stainless steel weld overlays, applied by the GMAW (gas metal arc welding) and SAW (submerged arc welding) processes. The tests revealed that SA516-Grade 70 carbon steel and type 304L stainless steel can experience high rates of corrosion. For the duplex stainless steels and weld overlays, corrosion resistance improved with chromium content. A chromium content of at least 25% was found to be necessary for good corrosion resistance.
Kraft digesters in the pulp and paper industry have historically been constructed from carbon steel; in particular batch digesters have been constructed with a generous corrosion allowance (at least I inch, or 25 mm). Up to about the 1970''s digesters were usually constructed from ASTM SA285-Grade C carbon steel "modified" to have a low silicon content. This modified grade is no longer readily available, so modern batch digesters have been constructed from SA516-Grade 70 carbon steel, which has a higher silicon content, and is thus susceptible to higher corrosion rates.
The batch digester operating environment is a severe one for materials. The cyclic nature of operation results in carbon steel experiencing active corrosion at the beginning of each cook, and becoming passive as the cook proceeds. Passing through the active/passive transition with each cook can result in substantial metal loss. Many carbon steel batch digesters experience corrosion rates in the order of = 50 mpy (= 1.3 mm/y). When the wall thickness approaches Code'' minimum, replacement or restoration of the vessel is necessary. Besides replacement in carbon steel, there are several alternative materials of construction for new digesters, including clad or solid construction with austenitic or duplex stainless steels. Options for restoration include application of corrosion resistant weld overlay, or coating with thermal spray; anodic protection is yet another corrosion preventive option.
The Stone-Consolidated kraft mill in Fort Frances, Ontario has several batch digesters in need of restoration and replacement. A corrosion test program was initiated to rapidly evaluate several candidate materials in laboratory autoclaves containing mixed white and black liquors from the Fort Frances mill.
The following materials were tested in the mixed white and black (3: I) digester liquors:
SA516-Grade 70 carbon steel plate (hot rolled).
UNS S31403 (Type 304L) stainless steel cladding (3 mm on SA516-Grade 70 plate).
UNS S31803 (“2205”) duplex stainless steel plate.
UNS S32550 (“2605”) super duplex stainless steel plate.
ER309LSi Stainless Steel GMAW overlay (single pass on SA516-Grade 70 plate).
E309L "Modified" (with high chromium) stainless steel SAW overlay (single pass on SA516-Grade 70 plate).
Alloy 625 Combustion Arc (CA) Thermal Spray (32 mils coating on SA516-Grade 70 plate).
