Corrosion prevention material selection

Corrosion prevention—material selection, corrosion inhibition, cathodic protection, galvanic corrosion prevention, and extensive nondestructive and destructive inspection protocols. ... 

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. 

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. 

Corrosion and Materials Selection 2.2.2 Prevention of Hydrogen Attack... 

The selection of materials to be used in design dictates a basic understanding of the behavior of materials and the principles that govern such behavior. If proper design of suitable materials of construction is incorporated, the eqiiipment should deteriorate at a uniform and anticipated gradual rate, which will allow scheduled maintenance or replacement at regular inteivals. If localized forms of corrosion are characteristic of the combination of materials and environment, the materials engineer should still be able to predict the probable life of equipment, or devise an appropriate inspection schedule to preclude unexpected failures. The concepts of predictive, or at least preventive, maintenance are minimum requirements to proper materials selection. This approach to maintenance is certainly intended to minimize the possibility of unscheduled production shutdowns because of corrosion failures, with their attendant possible financial losses, hazard to personnel and equipment, and resultant environmental pollution. 

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. 

One of the most effective methods of preventing corrosion is the selection of the proper metal or alloy for a particular corrosive service. Once the conditions of service and environment have been determined that the equipment must withstand, there are several materials available commercially that can be selected to perform an effective service in a compatible environment. Some of the major problems arise from popular misconceptions for example, the use of stainless steel. Stainless steel is not stainless and is not the most corrosion-resistant material. Compatibility of material with service environment is therefore essential. For example, in a hydrogen sulfide environment, high-strength alloys (i.e., yield strength above 90,000 psi or Rc 20 to 22) should be avoided. In material selection some factors that are important to consider are material s physical and chemical properties, economics and availability. 

Material selection should be compatible with all states of the C02 stream. They should be defined to prevent corrosion and maximum material stress. In addition, eligible materials need to be qualified for the potential low temperature conditions that may occur during a pipeline depressurisation situation. 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

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. 

The selection of an alloy for a specific application is based on the cost and the corrosion resistance of the alloy in the environment of interest. It is also possible to subject the chosen alloy to a process by which the corrosion resistance of the selected material can be improved within the acceptable limits. Some of the corrosion prevention and protection strategies with respect to the aluminum-based alloys are (i) design (ii) alloy selection and joint sealants (iii) aluminum thermal spraying anodic coatings (iv) inhibitors (v) conversion and organic coatings and (vi) cathodic protection. 

Corrosion occurs in various forms and is promoted by a variety of causes, all related to process operating conditions to which equipment and support structures are subjected. It is a continuous problem that can lead to contaminated process streams. This subsequently leads to poor product quality and unscheduled equipment shutdowns, the consequence of which is reduced production, high maintenance costs, and equipment replacement costs. Minimizing corrosion is a key consideration for the designer and can be accomplished in two ways (1) proper material selection for apparatus, and (2) preventive nraintenance practices. Both of these approaches must be examined. 

Inhibitors are used in severe and harsh environments encountered in rotary cookers and hydrostatic sterilizers. The medium consists of hot water, steam, and cooling water. A single approach to this problem may not provide the solution. A combination of anodic, cathodic, and filming inhibitors was selected for corrosion prevention depending on the water composition and equipment material. 

The annual cost of corrosion consists of direct costs and indirect costs. The direct costs related to corrosion consist of (i) costs of design, manufacturing, and construction (ii) material selection (iii) additional material such as increased wall thickness for corrosion allowance (iv) materials such as coatings, inhibitors, sealants, cathodic protection to prevent corrosion (v) application including cost of labor and equipment. 

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