Page 202 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

180 CORROSION CAUSES

3. Chemical pulping uses various chemicals to produce long, strong, and stable
fibers and to remove the lignin that bonds the fibers together. The two main
types of chemical pulping are Kraft (sulfate) pulping and sulfite pulping. The
corrosion rates in these processes can be significant depending on the chemi-
cals and materials used. Traditionally, carbon steel has been used as a material
of construction. Because of erosion from particles in solution and corrosion
because of the increase in sulfur content, stainless steel type 304L and 316L
have been used as cladding for the carbon steel digesters. It is advisable to check
for intergranular SCC, which may occur in a heat-affected zone (HAZ) of the
weldments (36). Specific stainless steel grades used in Kraft pulping digesters
and connecting pipes are duplex stainless steels 2205 and 2304 and austenitic
stainless steel 312 (36).
The acid pulping process uses sodium bisulfite or magnesium bisulfite in a pulp
digester at a pH of 3. Type 316 stainless steel is normally used as a minimum alloy
because the sulfur dioxide can be converted into sulfuric acid. Sulfuric acid can cor-
rode the stainless steel depending on the pH, temperature, and pressure of the system.
SCC has been observed in the HAZ of weldments in the pulp digesters in the presence
of sodium hydroxide. Because of SCC susceptibility of the austenitic stainless steels,
duplex stainless steel 2205 is often used in pressure vessels and tanks.
Kraft pulping chemical recovery consists of passage of black liquor along with
the slurry passes through evaporators, recovery boilers, and causticizers to eventually
produce white liquor. Corrosion on the fireside of the recovery boiler is accelerated
by the presence of reduced sulfur species. The hydroxide mixtures present in black
liquor are extremely corrosive to the recovery boilers made of type 304 stainless steel
(37). Several phenomena in the recovery section cause different forms of corrosion
such as: (i) corrosion under ash build-up; (ii) corrosion in the thin condensation layers;
and (iii) high-temperature metal/gas interactions.
(i) Ash build up on the heat exchanger tubes can occur in recovery boilers when
the incineration of the liquid waste is incomplete. The deposited ash decreases
the efficiency of the heat exchanging process. Underdeposit corrosion such as
crevice or pitting corrosion may occur.
(ii) Condensation can occur in the ductwork between the recovery boiler and the
off-gas scrubbers when the off-gases cool down to a temperature below their
flash point before reaching the scrubber. Localized attack in the condensate
phase can be severe (>1000 mpy) and can be accelerated by alternate conden-
sation and revaporization. The concentration of corrosive species in the thin
condensed layer is the highest just before complete revaporization.
(iii) High-temperature metal/gas interactions in the recovery boiler tubes are
oxidation, carburization, and sulfidation. The rates of the processes vary with
the concentration of the burned black liquor waste and the temperature of the
recovery boiler. High-temperature gaseous attack does not require an aqueous
or molten salt electrolyte. Continued scale growth on the metal surface leads
to metal consumption and decreased wall thickness of boiler tubes and boiler
walls.