Corrosion control material selection

Material selection is the most important general procedure for corrosion prevention. Material selection, protective coatings, corrosion inhibitors, and electrochemical techniques such as CP are some of the corrosion control methods that can be used to alleviate corrosion problems. 

The various corrosion-control measures hsted previously cannot ah be used in buried exposures. From a practical point of view there is not much that can be done to modify the environment to make it less corrosive, and material selection options are often hmited by engineering requirements. Although clean, uniform, well-draining backhU materials wiU reduce corrosion activity, chemicals present in the soil will tend to leach into the backfill due to movement of water through the soil. 

Chawla, S. L. and Gupta, R. K. (1993), Materials Selection for Corrosion Control. Materials Park, Ohio, USA ASM International. 

Unfortunately, materials selection is rarely taught to designers with a focus on corrosion prevention and control. One might wonder why a practical, design-based material selection process that considers corrosion prevention and control has not already been developed. The following list identifies some of the factors that contribute to the difficulty in developing such a simplified, readily understood corrosion conscious materials selection process. 

The primaiy purpose of materials selection is to provide the optimum equipment for a process application in terms of materials of construction, design, and corrosion-control measures. Optimum here means that which comprises the best combination of cost, life, safety, and reliability. 

As many emissions involve chlorinated compounds, corrosion is a major problem in many control methods. The corrosion of columns and surface condensers can be prevented or reduced by the correct material selection. However, corrosion remains a constant threat to the interior of incinerators. Additional pollution control equipment such as scrubbers may also be required to remove acidic compounds from treated gases before discharging into the atmosphere. 

If changes have been made to the process (e.g. if incoming water quality cannot be maintained or other uncertainties arise concerning the corrosion behavior of the construction materials) it is possible to incorporate coupons or probes of the material into the plant and monitor their corrosion behavior. This approach may be used to assist in the materials selection process for a replacement plant. Small coupons (typically, 25 x 50 mm) of any material may be suspended in the process stream and removed at intervals for weight loss determination and visual inspection for localized corrosion. Electrical resistance probes comprise short strands for the appropriate material electrically isolated from the item of plant. An electrical connection from each end of the probe is fed out of the plant to a control box. The box senses the electrical resistance of the probe. The probe s resistance rises as its cross-sectional area is lost through corrosion. 

A recent NACE publication devoted entirely to the economic aspects of corrosion control contains several worked examples applicable to a number of industries. These examples serve to illustrate that anti-corrosion procedure and materials should be selected on economic grounds, and not solely on performance grounds. In presenting a proposal to management. 

All methods of corrosion control such as careful materials selection, including coating and cladding, inhibition and cathodic protection, should be regarded as an integral part of the design process. 

Since corrosion involves a reaction of a metal with its environment, control may be effected through either or both of the two reactants. Thus control could be based entirely on the selection of a particular metal or alloy in preference to all others or the rejection of metals in favour of a non-metallic material, e.g. by a glass-reinforced polymer (g.r.p.). At the other extreme control may be effected by using a less corrosion-resistant material and... 

The continuous development of new materials has resulted in changing attitudes towards materials selection for corrosion control, and the range of materials now available can be gauged from the Materials Selector Review", which becomes considerably thicker each time it is updated. Plastics are replacing metals for a variety of applications and a recent application is the use of g.r.p. in place of metals for the construction of hulls of hovercrafts the corrosive action of the high velocity spray of sea-water is such that very few metals are capable of withstanding it and the use of g.r.p. represents the... 

Henthome, M. Material Selection for Corrosion Control, Chemical Engineering, Mar. 6, 1972, p. 113. 

C. P. Dillon, Corrosion Control in the Chemical Process Industries. McGraw-Hill, New York, 1986 C. P. Dillon, Materials Selection for the Chemical Process Industries. McGraw-Hill, New York, 1992. 

The material selected for construction must be resistant against corrosion and erosion, or has to be subject to control on a regular basis, or has to be changed. 

Corrosion prevention involves inherent factors, which are within the control of the metallurgist or engineer. The three main categories considered are materials selection, design factors and life prediction analysis. 

Since corrosion is a severe problem of hydrothermolytical oxidation, corrosion control deserves close attention in the course of further development. The presented process offers the possibility to use common reactor materials e.g. high-grade steel 1.4541 or in selected applications even carbon steel St 1203. If calciumborate as corrosion inhibitor is used special alloys like hastelloy or inconel are not absolutly necessary for wet oxidation in presence of chloride-ions. 

The corrosion engineer can play a major role in system design, material selection, process or environmental control, and remediation. The focus of these efforts should not necessarily be the complete elimination or avoidance of corrosion, but rather the selection of the most cost-effective means of corrosion control and abatement. 

---