Page 203 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

AGRICULTURAL PRODUCTION 181

Recausticizing is the process used to transform the inorganics recovered from the
recovery boiler into white liquor so that the chemicals may be recycled.
Sulfite pulping chemical recovery consists of greater amounts of sulfite, hydro-
gen sulfide, and hydrochloric acid than those used in the Kraft process. Because of
these corrosive species, the internal portions of the recovery boilers and the evapo-
rators are generally constructed of reinforced plastics, type 316L stainless steel, type
317L stainless steel, or nickel-based alloys. To prevent pitting and crevice corrosion,
scale build-up should be prevented, wet-dry zones should be avoided, and chloride
concentration should be kept to a minimum.
The bleaching is done in three to five stages in which the pH of the pulp is alter-
nated between acid and alkaline conditions. During the acid cycle, the chemical reac-
tions between the bleach and lignin bonds turn the pulp lighter in color. During the
alkaline cycle, the reaction products from the acid stage are removed (38).
Chlorine dioxide is similar to chlorine with respect to its corrosivity (36). Suitable
materials for bleach washers using chlorine and chlorine dioxide are super austenitic
6–7% molybdenum stainless steels such as 25–45 Mo or 25–6 Mo.
In recent years, less chlorine and more sodium hydroxide have been used for
bleaching. At present, chemical pulps and the deinking of secondary fibers are the pri-
mary users of chlorine bleaching techniques. The European paper and pulp industry
uses ozone, oxygen, and peroxide in place of chlorine.
Many pulp mills have a paper mill adjacent to them, thus making a transfer of the
product easy. The equipment used to store and transport the pulp can undergo crevice
and pitting corrosion.
The suction rolls used to remove water from the paper during the drying process
experience general corrosion, fatigue failures, pitting, and MIC because of exposure
to stock and white water, deposits of paper fiber present in crevices and bacterial
growth. The inside diameter of the drilled holes within the suction rolls experience
fatigue failure because of the presence of high stress concentrations. The sodium
thiosulfate present in stock and white water systems can result in severe pitting (39).
The drilled holes in the suction roles, along with crevices and deposit sites,
∘
experience the growth of microbes as a result of high temperature (40–50 C),
dissolved organic materials, and dissolved inorganic salts. Microbiological attack
occurs beneath deposits on the microbiological slime and increases with the degree
of closure of the paper mill. Corrosion problems within the paper machines are
chloride pitting and crevice corrosion, thiosulfate pitting, and microbiological attack.

3.27 AGRICULTURAL PRODUCTION

The major input items and farm production expenditures in 1997 are shown in
Table 3.14
Corrosion problems occur in the plumbing systems of agricultural sprayers.
Urea ammonium nitrate (UAN) is used as a fertilizer. Anhydrous ammonia, used in
farm fertilizers, is very corrosive. Nitrate formed on oxidation of ammonia can be
corrosive.