SELECTION OF MATERIALS FOR CORROSIVE ENVIRONMENT 9.1.1 MATERIALS EVALUATION AND SELECTION Material selection is critical to engineering design. Corrosion may be minimized by employing an appropriate design. The selection of appropriate materials in a given environment is a key factor for corrosion control strategy. The material selected has to meet the criteria for mechanical strength, corrosion and erosion resistance for specific service conditions. For instance, high performance corrosion resistant alloys are specified for valves and piping systems in aggressive environments encountered in refineries and chemical processing

9 . 2 FACTORS AFFECTING THE PERFORMANCE OF MATERIALS The following is a list of factors affecting the performance of materials: (1) Expected performance and functions of the product. (2) Physical characteristics. (3) Strength and mechanical characteristics. (4) Corrosion and wear characteristics. (5) Fabrication parameters. (6) Recycling possibilities.

CORROSION AND WEAR CHARACTERISTICS Improper selection of materials without consideration of their corrosion behavior in aggressive service environment can lead to premature failure of components and plant shutdown. To avoid failures caused by corrosion: (a) Materials selected should be compatible with the environment. They must possess sufficient resistance to corrosion for the designed life. (b) Appropriate preventive maintenance practice must be adopted to maintain the integrity of the equipment/component. The selection of materials must be based on an extensive knowledge of the service environment. It is to be realized that the behavior of a material may dramatically change when exposed to a corrosive environment. The behavior of materials largely depends upon the following: (a) Corrosive medium parameter. (b) Design parameters. (c) Materials parameters.

Corrosive Medium Parameters Corrosive medium parameters include (Fig.1): (a) Composition of the corrosive medium. For instance, if a metal is exposed in seawater, a quantitative analysis of all constituents must be made to determine the composition. (b) Physical and external factors affecting the medium, such as pH, conductivity, temperature and velocity affect the magnitude of corrosion induced by the medium. (c) Presence of dissolved gases, such as oxygen, carbon dioxide, hydrogen sulfide in fluids promotes the corrosivity of the medium.

(d) Presence of organic matter and bacteria promote the corrosivity of the environment. For instance, sulfide reducing bacteria, desulfovibrio and Clostridium, induce corrosion by producing hydrogen sulfide, a serious corrodant for steel. Algae, yeasts and molds also contribute to corrosion. (e) Physical state of the corrosive medium has a pronounced effect on the corrosivity of the medium. For instance, a dry soil would be less aggressive to corrosion than a wet soil in which salts are dissolved.

9.2.4.2 Design Parameters Design parameters which affect the rate of corrosion include the following: (1) Stresses acting on the materials in service. (2) Relative velocity of the medium and obstacles to flow. (3) Bimetallic contacts. (4) Crevices. (5) Riveted joints. (6) Spacing for maintenance. (7) Drainage and directional orientation of loop. (8) Joints to avoid entrapment. (9) Sharp corners. (10) Non-homogeneous surface.

9.2.4.3 Materials Parameters The following are the material parameters which may affect corrosion resistance: (1) Impurity segregation on grain boundaries leads to weakening of grain boundaries and accelerates corrosion attack. (2) Microstructural constituents. A heterogeneous microstructure forms anodic and cathodic sites which promotes corrosion. (3) Surface treatment, such as galvanizing, phosphating and painting increase the resistance of materials to corrosion. (4) Alloying elements and film formation. Alloying elements in steels, such as chromium, nickel or molybdenum contribute to the production of a protective oxide layer which makes steel passive.

9.2.5 FABRICATION PARAMETERS Following are the fabrication parameters required for analyzing material selection: (1) Weldability. Welding procedures, such as electric arc welding, friction welding, spot welding, need to be carefully selected to minimize the effect of corrosion. (2) Machinability. Machining operations, such as drilling, milling, shearing, turning, may lead to enhancement of corrosion if they are not properly controlled. (3) Surface modification procedures, such as cladding, galvanizing and metallizing (metallic coatings) increase the resistance of the materials to corrosion.

9.3 THE MATERIAL SELECTION PROCESS With the phenomenal growth in the development of new materials and availability of over 40 000 metal alloys and a larger number of nonmetallic materials, the process of selection of materials has become very complex and challenging. The selection process should start at the drawing board stage where all important factors affecting the products, such as the choice of engineering design and processing methods should be considered. 9.3.1 FACTORS IN MATERIALS SELECTION 9.3.1.1 Physical and Mechanical Factors Amongst the physical factors the size, shape and the weight of material are important factors.

The mechanical properties commonly used for design are the following: (1) Density (p) (2) Modulus of elasticity (£) (3) Strength (a) (4) Ductility (5) Fracture toughness (6) Fatigue (7) Corrosion fatigue (8) Creep (9) Impact (10) Hardness. 9 . 4 MATERIALS CLASSIFICATION The world of materials basically comprises families of polymers, metals, ceramics, glasses, natural materials and composites, which can be synthesized by combination of the above materials. Table 9.1 shows the subdivisions of the material kingdom in families, classes, subclasses and

FACTORS A F F E C T I NG THE CORROSIVITY OF SEAWATER A. Movement Moving seawater is less corrosive than stagnant seawater. Under stagnant conditions, oxygen concentration cells are setup on the surface of materials and corrosion is accelerated. Salt water must be rinsed without delay from stainless steel equipment before plant shutdown, otherwise heavy damage by pitting may be caused to the equipment necessitating replacement. B. Corrosive agents There is a large variation in the total dissolved solid contents (TDS) from one ocean to another. The total dissolved solid content in the Baltic Sea is represented to be on an average of 8000 ppm as against the TDS content of 44000-60000 ppm in Arabian Gulf water. The degree of corrosivity of seawater varies with the TDS content. The chloride ion is the most aggressive (corrosive) constituent in seawater, as it has a tendency to penetrate the passive films on the metal surface and destroy them. A typical seawater composition is shown in Table 9.4.

9.5.3 INDUSTRIAL E N V I R O N M E N T The industries contributing to the industrial environment include chemical, petrochemical, fertilizer, pulp and paper, and all other process industries. The following are the major corrosive fluids encountered in a wide spectrum of industry: (a) Acids (Inorganic). Acids, such as hydrochloric, hydrofluoric, sulfuric and sulfurous, are encountered in highly corrosive environments in refineries. Acids (Organic). Acids, such as formic, acetic, propionic, dicarboxylic and napthenic, are organic acids present in crude oil. (b) Strong alkalies. (c) Saltwater. (d) Dissolved gases, such as hydrogen sulfide (H2S), carbon dioxide (CO2) and oxygen (02).

(e) Pollutants, such as particulate matter, sulfur oxides, carbon monoxides, nitrogen dioxides, ozone and lead. (f) Soil pollution, such as bacteria, oil spills, natural gas contaminants sewage contaminants and pesticide degradation products. The success of service performance of materials would depend on their ability to offer sufficient resistance to corrosion in industrial environments. 9.5.4 O I L F I E L D ENVIRONMENT The corrosion problems caused by environment can be classified in to two categories: (a) downhole environment and (b) surface environment. Whereas the surface environment is mainly the corrosion related to the absorption and atmospheric contaminants, the downhole environment is highly aggressive as it may contain brine, carbon dioxide, acids and hydrogen sulfide, all of which are highly corrosive. The downhole environment is also subjected to elevated temperatures. Corrosion protection of drilling equipment is a challenging job in the oil industry.