Page 323 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

PULP AND PAPER INDUSTRIAL SECTOR 301

resulting corrosion can be controlled. Another useful practice consists of scheduled
cleaning and proper equipment maintenance to prevent the buildup of pulp and
decrease the amount of stagnant solutions.

4.29.2 Process Design and Corrosion Inhibitors

Corrosion rates in mill equipment also depend on the chemical composition within
each section of the process. The chemical composition varies from mill to mill
depending on the processes involved, the closure of the mill’s systems, the desired
paper grade, the speed of the process, and the amount of pulp or paper being
produced.
In general, the corrosivity of an environment will increase with an increase in
temperature, low pH, dissolved solids content, and chloride and sulfur compound
concentrations. Thus by monitoring the process and maintaining the proper temper-
ature, pH, dissolved solids content, chloride, and sulfur compound concentration the
extent of corrosion can be controlled.
In some processes, addition of corrosion inhibitors can control the corrosion. Mea-
surements of the process chemistry should be made to ensure that the correct dosage
of inhibitor is maintained.
Corrosion control in the pulp and paper industry consists of verification of average
corrosion rates, using weight loss coupons, regular inspection, and preventive main-
tenance. Preventive strategies are considered during design and construction phases
when new equipment is considered or existing equipment is refurbished or repaired.
Corrosion prevention techniques minimize corrosion initiation altogether, while cor-
rosion control techniques minimize the propagation of ongoing corrosion.

4.29.3 Weight Loss Coupons
Weight loss in coupons exposed to the corrosive medium gives the average corrosion
rate in the atmosphere of a pulp or paper plant. This technique is cheap and easy
to use as it involves the determination of weight of metal coupons before and after
exposure for a known length of time. From the loss in weight of the coupon, the
average corrosion rate can be calculated. An advantage of this technique is that several
coupons of different materials can be exposed to the environment simultaneously.
By performing weight loss tests, technical personnel can estimate the corrosive-
ness of an environment over a period of time. If the corrosion rate is determined to
be high, preventive measures can be taken to prolong the remaining life of the equip-
ment. The preventive measures can include the addition of a corrosion inhibitor, a
change in the process, an application of anticorrosion coatings, or the addition of a
CP system.

4.29.4 Inspection and Preventive Maintenance
An additional and simplest and obvious method to monitor the corrosion rate is
through visual inspection of all parts of the equipment that are exposed to corrosive