Surface Engineering to Combat Corrosion and Wear

SURFACE ENGINEERING is a multidisciplinary activity intended to tailor the properties of the surfaces of engineering components so that their function and serviceability can be improved. 

The desired properties or characteristics of surface-engineered components include:

• Improved corrosion resistance through barrier or sacrificial protection
• Improved oxidation and/or sulfidation resistance
• Improved wear resistance
• Reduced frictional energy losses
• Improved mechanical properties, for example, enhanced fatigue or toughness
• Improved electronic or electrical properties
• Improved thermal insulation
• Improved aesthetic appearance

Surface Engineering to Combat Corrosion and Wear

The Economic Effects of Corrosion and Wear. The progressive deterioration, due to corrosion and wear, of metallic surfaces in use in major industrial plants ultimately leads to loss of plant efficiency and at worst a shutdown. Corrosion and wear damage to materials, both directly and indirectly, costs the United States hundreds of billions of dollars annually. For example, corrosion of metals costs the U.S. economy almost $300 billion per year at current prices. This amounts to about 4.2% of the gross national product. However, about 40% of the total cost could be avoided by proper corrosion prevention methods. Table 1 provides a breakdown of the cost of metallic corrosion in the United States. Similar studies on wear failures have shown that the wear of materials costs the U.S. economy about $20 billion per year (in 1978 dollars) compared to about $80 billion annually (see Table 1) for corrosion during the same period. Table 2 illustrates the extent of wear failures by various operations within specific industrial segments. Highway vehicles alone use annually 14,600  1012 Btu/ton of energy represented in lost weight of steel and 18.6% of this energy could be saved through effective wear-control measures.

Table 1

Table 2

Corrosive Wear

Complicating matters is the fact that the combined effects of wear and corrosion can result in total material losses that are much greater than the additive effects of each process taken alone, which indicates a synergism between the two processes. Although corrosion can often occur in the absence of mechanical wear, the opposite is rarely true. Corrosion accompanies the wear process to some extent in all environments, except in vacuum and inert atmospheres. Corrosion and wear often combine to cause aggressive damage in a number of industries, such as mining, mineral processing, chemical processing, pulp and paper production, and energy production. Corrosion and wear processes involve many mechanisms, the combined actions of which lead to the mutual reinforcement of their effectiveness. As listed in Table 3, 17 synergistic relationships among abrasion, impact, and corrosion that could significantly increase material degradation in wet and aqueous environments have been identified.

The combined effects of corrosion and wear can also lead to galvanic corrosion in some applications, such as crushing and grinding (comminution) of mineral ores. Wear debris and corrosion products that are formed during comminution affect product quality and can adversely affect subsequent beneficiation by altering the chemical and electrochemical properties of the mineral system. Electrochemical interactions between minerals and grinding media can occur, causing galvanic coupling that leads to increased corrosion wear.

Table 3 

Methods to Control Corrosion

Owing to its many favorable characteristics, steel is well suited and widely used for a broad range of engineering applications and is referenced here to demonstrate the various corrosion-control steps that can be considered. Steel has a variety of excellent mechanical properties, such as strength, toughness, ductility, and dent resistance. Steel also offers good manufacturability, including formability, weldability, and paintability. Other positive factors include its availability, ferromagnetic properties, recyclability, and cost. Because steel is susceptible to corrosion in the presence of moisture, and to oxidation at elevated temperatures, successful use of these favorable characteristics generally requires some form of protection.

Methods of corrosion protection employed to protect steel include:

• Altering the metal by alloying, that is, using a more highly alloyed and expensive stainless steel rather than a plain carbon or low-alloy steel
• Changing the environment by desiccation or the use of inhibitors
• Controlling the electrochemical potential by the application of cathodic or anodic currents, that is, cathodic and anodic protection
• Applying organic, metallic, or inorganic (glasses and ceramics) coatings

The application of corrosion-resistant coatings is one of the most widely used means of protecting steel. There are a wide variety of coatings to choose from, and proper selection is based on the component size and accessibility, the corrosive environment, the anticipated temperatures, component distortion, the coating thickness attainable (Fig. 1), and costs. Painting is probably the most widely used engineering coating used to protect steel from corrosion. There are a wide variety of coating formulations that have been developed for outdoor exposure, marine atmospheres, water immersion, chemical fumes, extreme sunlight, high humidity, and moderately high temperatures (less than about 200 °C, or 400 °F). The most widely used corrosion-resistant metallic coatings are hot-dipped zinc, zinc-aluminum, and aluminum coatings. These coatings exhibit excellent resistance to atmospheric corrosion and are widely used in the construction, automobile, utility, and appliance industries. Other important coating processes for steels include electroplating, electroless plating, thermal spraying, pack cementation aluminizing (for high-temperature oxidation resistance), and cladding (including weld cladding and roll-bonded claddings)

Figure 1 

Methods to Control Wear

There are many types of wear, but there are only four main types of wear systems (tribosystems) that produce wear and six basic wear control steps. The four basic tribosystems are:

• Relatively smooth solids sliding on other smooth solids
• Hard, sharp substances sliding on softer surfaces
• Fatigue of surfaces by repeated stressing (usually compressive)
• Fluids with or without suspended solids in motion with respect to a solid surface

The wear that occurs in these tribosystems can be addressed by coatings or by modifications to the substrate metallurgy or chemistry. The six traditional techniques applied to materials to deal with wear produced in the preceding tribosystems include:

• Separate conforming surfaces with a lubricating film.
• Make the wearing surface hard through the use of hard facing, diffusion heat treatments, hard chromium plating, or more recently developed vapor deposition techniques or high-energy processes (e.g., ion implantation).
• Make the wearing surface resistant to fracture. Many wear processes involve the fracture of material from a surface; thus toughness and fracture resistance play a significant role in wear-resistant surfaces. The use of very hard materials such as ceramics cemented carbides, and hard chromium can lead to fracture problems that nullify the benefits of the hard surface.
• Make the eroding surface resistant to corrosion. Examples include the use of cobalt-base hard facing alloys to resist liquid erosion, cavitation, and slurry erosion; aluminum bronze hard facing alloys to prevent cavitation damage on marine propellers or to repair props that have suffered cavitation damage; nickel-base hard facing alloys to resist chemical attack; and epoxy-filled rebuilding cement used to resist slurry erosion in pumps.
• Choose material couples that are resistant to interaction in sliding (metal-to-metal wear resistance). Hardfacing alloys such as cobalt-base and nickel-chromium-boron alloys have been used for many years for applications involving metal-to-metal wear. Other surface engineering options include through-hardened tool steels, diffusion (case)-hardened surfaces, selective surface-hardened alloy steels, and some platings.
• Make the wearing surface fatigue resistant. Rolling-element bearings, gears, cams, and similar power-transmission devices are often worn by a mechanism of surface fatigue. Repeated point or line contact stresses can lead to subsurface cracks that eventually grow to produce surface pits and eventual failure of the device. Prevention is possible through the use of through-hardened steels, heavy casehardened steels, flame-, induction-, electron beam-, or laser hardened steels.

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