Page 322 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

300 CORROSION CONTROL AND PREVENTION

4.28.4 Remedial Action and Diagnostic Analysis
Corrosion monitoring must be coupled with diagnostic work and remedial action.
In some cases of corrosion, remedial action may be obvious or easily deduced. In
other cases, diagnostic work precedes a decision or remedial action. The options for
remedying corrosion problems in a process plant are: (i) install a CP system; (ii)
install an anodic protection system; (iii) change equipment design; (iv) improve feed
stock purity; (v) alter process variables; (vi) change the alloy/material; (vii) institute
inhibitor additions; (viii) institute planned maintenance.
When the remedial action is not known, diagnostic action should precede
remedial action. The root cause of the corrosion failure must be investigated. For
example, NDTs such as radiography, magnetic particle testing, liquid penetrant,
acoustic emission, and ultrasonic testing may be used to detect cracks in the
metal/alloy sample. Options for remedial measures are: (i) complete replacement of
the sample/equipment, using a corrosion-resistant alloy, clad material, or application
of anodic protection, use of protective coatings or corrosion inhibitors.

4.29 PULP AND PAPER INDUSTRIAL SECTOR

Corrosion control methods used in this industrial sector consist of: (i) equipment
design; (ii) process design; (iii) the use of corrosion inhibitors.

4.29.1 Equipment Design
Pulp and paper industrial equipment design consists of proper material selection
in conjunction with the process chemicals and prevention of stagnant fluids in the
process equipment. In the absence of corrosion, low-alloy carbon steel would be
the material of choice if corrosion were not a problem. However, for many pro-
cesses, stainless steel and even nickel-base and titanium alloys are required for better
performance in corrosive environments. At present, current US paper mills are con-
structed of about one-third carbon steel and two-thirds stainless steels. There are
several grades in the group of stainless steels. The relative cost of the stainless steels is
dependent on the concentration of the major alloying elements (Cr, Ni, Mo), the vol-
ume produced, and the form in which it is supplied such as tube, pipe, plate, or block.
It is useful to note that stainless steels are 10–20% more expensive than low-alloy
carbon steels.
Stagnant and slow-flowing process fluids in pulping equipment can occur in
crevices and water traps. Fluid stagnation can lead to an increase in concentration of
chemicals and the localization of a severe environment in which pitting and crevice
corrosion may occur. By designing drain holes and easy access to the equipment,
stagnant solution can be removed. Slow-flowing liquids containing a solid fraction of
pulp may deposit a layer of pulp at the bottom of pipe and reservoirs. Underdeposit
corrosion modes such as crevice corrosion and pitting may occur. The equipment if
designed with sufficiently fast and/or turbulent flow, deposit formation and hence the