Coating Processes

There are a number of methods, ranging from simple to sophisticated, that are used to apply coatings, and there are advantages and disadvantages to each method. The quality of the application of a coating is critical because any defect or significant porosity in the coating can result in severe localized corrosion. Selection of a coating application method is usually based on the type of coating (i.e. metallic, ceramic, organic), the type of substrate to be coated, the amount of surface area that will be coated, and whether there are any environmental regulations or restrictions. Application methods for metallic coatings include cladding, electrodeposition (electroplating), flame spraying, vapor deposition, and hot dipping. Application methods for ceramic coatings include diffusion, spraying, and chemical conversion. Application methods for organic coatings include brushing, rolling, and spraying. Materials and application methods of metallic, inorganic, and organic coatings will be described below.

Hot-Dipping

Hot dipping designates the coating application process of immersing a metal substrate in a molten metal bath, which is usually aluminum, zinc, tin, or lead. Since the applied coating consists of a molten metal, the melting temperature of the metal coating should be relatively low. Hot dipping can be either a continuous or batch process. Hot-dip galvanizing is the most common metal coating method; it involves the application of a thin layer of zinc to carbon steel. The zinc layer provides cathodic protection of the steel thereby protecting the steel from corrosion. Figure 42 shows the service life of hot-dip galvanized steel in different environments.

Electrodeposition

Electrodeposition, also called electroplating, is a process where a thin metal layer is deposited on a metal substrate in order to enhance the surface properties, including its corrosion resistance. The metal substrate is placed in an electrolytic solution containing dissolved metal ions, which will ultimately become the coating. An electrical current is passed through the solution, between two electrodes, causing the ions to deposit on the cathode (metal substrate) resulting in a metallic coating.

Characteristics of the coating are dependent on control of the processing parameters including temperature, current density, residence time and composition of the solution. The physical and mechanical properties of these coatings can be altered by varying the processing parameters. They can be made to be thick or thin, hard or soft, or have a layered composition.

A variety of metals are available for use as electrodeposited coatings and include aluminum, chromium, iron, cobalt, nickel, copper, zinc, rhodium, palladium, silver, cadmium, indium, tin, rhenium, platinum, gold, lead, brass, bronze and a number of other alloys. As with all coating application methods, electrodeposition has its advantages and disadvantages.

Electroless Plating

Electroless nickel plating is similar to the electrodeposition process except that it does not require an external electrical current to be applied. It is a chemical reduction process where nickel ions are driven to the surface of the substrate metal by a reducing agent which is also present in the host solution. If processing conditions are properly maintained and the composition of the aqueous solution is uniform, the deposition of the nickel should be uniform over the entire surface of the substrate, even if it has a complex geometry.

Cladding

Metal claddings typically provide corrosion protection by acting as a barrier and a sacrificial coating. The cladding method involves a thin metal layer that is installed on the metal substrate by pressing, rolling or extrusion. This produces a metal layer with essentially zero porosity. An advantage is that this allows a thin piece of expensive, corrosion resistant material to be used on an inexpensive thicker piece of metal that is susceptible to corrosion instead of using the corrosion resistant material as the entire piece.

Thermal Spraying

Thermal spraying is a coating process in which a material feed is melted by a flame and sprayed by compressed gas onto a substrate; when the molten droplets/particles hit the substrate they flatten and adhere to the surface to form a coating. The process involves the build-up of these flattened particles which melt to form a cohesive coating that adheres to the substrate and covers the entire surface, while filling irregularities on the surface. Bonding between the coating and substrate usually results from mechanical interlock or diffusion and alloying. Therefore, surface preparation of the substrate is an important aspect in the quality of the coating. Often, it is required for the surface to be roughened in order to promote good mechanical adhesion between the coating and substrate. Thermal spraying can be performed using flame spraying, electric arc, or plasma arc.

Physical Vapor Deposition

There are several coating application methods which are subsets of the physical vapor deposition category. These include sputtering, evaporation, and ion plating. PVD processes involve plasma bombardment to deposit the metal over the entire area of the substrate.

Sputtering

Sputtering is the process where a target material is bombarded by gas ions causing atoms to be ejected and consequently deposited onto the substrate. Some advantages and disadvantages of this process are given in Table 54.

Evaporation

Evaporation is a relatively simple process that involves the vaporization of a metal, which is subsequently deposited on a substrate. The adhesion of coatings deposited by this method is only marginal and uniformity is difficult to achieve. Therefore, the evaporation method is not typically used for corrosion prevention applications.

Ion Plating

Ion plating is a process in which ions are driven from a plasma by an electrical bias on the substrate where they are deposited. Alternatively, the coating can be applied using an ion beam deposition technique, where plasma ions bombard the substrate to create nucleation sites for a neutral ion species. The neutral species can then deposit onto the nucleation sites, resulting in the formation of a coating.

Laser Surface Alloying

Laser surface alloying involves feeding the metal to be deposited into a laser beam. The laser beam melts the metal and deposits it on the surface of the substrate, where heat is transferred and a strong metallurgical bond is formed.

CVD

Chemical vapor deposition processes involve coating a substrate by chemical means, namely by reacting a precursor gas on the metal substrate. The gas is mixed in a chamber causing it to become reactive and is then sent to another chamber to be deposited onto the substrate. The gas mixture reacts at the surface of the substrate, which is heated in order to drive the endothermic reaction, to ultimately form the coating. It is important in this process to maintain a non-contaminated system. Table 55 lists some of the advantages and disadvantages corresponding to the various coating application methods.

Brushing

Brushing is perhaps the most intuitive coating application process, and is used to apply organic based coatings. It is a manual application method, and there are numerous types of brushes that can be used. It is very important to select the appropriate type of brush with the proper bristles in order to produce a high quality coating. The brush size, shape, and bristle type are all important considerations when selecting a brush for a specific coating application. This is because poor brush selection can lead to uneven or discontinuous coating application, runs, drips, or other unfavorable coating characteristics. A standard wall brush is often used for applying coatings to structural steel or similar surfaces. Oval-shaped brushes are used for other structural and marine applications, and are also used to apply coatings near rivets, boltheads, piping, railings and other difficult to reach areas.

Brushes are made with either synthetic, typically nylon, or natural fibers for bristles. The advantage to using a brush with synthetic bristles is that it has a very good resistance to abrasion and are good to use on rough surfaces such. Brushes with synthetic bristles are also less expensive than those employing natural fibers. One of the primary disadvantages to synthetic bristles is that they may be susceptible to strong solvents such as ketones. Natural bristles are more expensive and sensitive to water, but they have a good resistance to strong solvents and are capable of a much finer, uniform coating application.

An advantage to the application method of brushing is the ability to perform what’s called striping. Striping is used to apply the coating around irregular areas that cannot be easily or properly coated through a spraying or other coating technique. Areas that typically require striping include edges, rivets, fasteners, corners, boltheads, and welds. It is a recommended procedure because it can provide the proper coating thickness around these irregular areas, which would not be able to be achieved otherwise. Striping is not used, however, for coatings that have a solute that must remain in suspension, such as zinc-rich coatings. The brushing application method can also achieve complete coating penetration in particularly porous surface areas on a substrate.

A disadvantage of the brushing application method is that it is time consuming as opposed to the spraying methods. Also when applied over a large surface area it is very difficult to maintain a uniform coating thickness through brushing, and therefore it is not a practical method for components or systems with large areas. Furthermore, after the coating dries the surface may have brush marks or slight grooves left over from the bristles. This usually is only a detriment to the appearance rather than the functionality. Another disadvantage to using the brushing method is that it is a difficult technique to use for coatings containing a high solid content and also for fast drying coatings.

Brushing is most commonly used for applying oil-based or water-based coatings to surfaces with small or irregular areas. There are proper techniques in applying the coating that give the best results in the end product. Either an experienced professional or a well-trained technician should be used to apply coatings on critical assets or components.

Rolling

Rolling is another manual coating application process, and it requires a roller assembly consisting of a core roller and a cover to absorb and apply the coating material. The assembly can vary in diameter as well as length, and there are also various cover materials. Common cover materials are polyester, nylon, mohair, and lambskin. Of course, the cover material is usually selected to suit the type of surface to be coated.

There are three types of roller cores: pipe rollers, fence rollers, and pressure rollers. Pipe rollers are used just as the name suggests: for coating surfaces such as pipes. The surfaces usually are contoured and need the roller to flex and cover the surface. Fence rollers use roller covers that have an extra long fiber length, which enables them to simultaneously coat both sides of a surface such as fence wire. Pressure rollers are more sophisticated and have a feed line that moves the coating material to the inside of the roller core from a pressurized tank. The core is a porous material which allows the coating to pass through to the surface of the cover, and thus pressure rollers can provide continuous application of the coating.

Rolling is a good application method for coating large, flat surfaces. A disadvantage is that it is much more difficult to achieve coating penetration into porous or cracked surfaces using the rolling coating application method, and is therefore not recommended for rough or irregular surfaces. Rolling does provide a fine quality finished surface on smooth surfaces. Rolling is a faster process than brushing, but is slower than other coating methods such as spraying.

The roller coating application method is typically used to apply oil-based and water-based coatings, and can also be used to apply epoxy and urethane coatings. This method is not recommended for applying coatings containing a high solids content, zinc rich coatings, or high performance coatings and linings. As with the brushing application method there are proper techniques that result in uniform and quality application of the coating on the substrate.

Spraying

There are several variations of the spray coating application method, including high volume-low pressure spraying, airless spray, air-assisted airless spray, plural component spray, and electrostatic spray. Conventional spraying simply uses compressed air to atomize coating particles and propel them toward the substrate. Though simple, the efficiency with which the coating successfully reaches the intended surface is low: ~ 25-30%. Conventional spraying is used to apply coatings such as latex paints, lacquers, stains, sealers, zinc-rich mixtures, alkyds, and epoxies.

An advantage of the spray application technique is that it requires significantly less time than brushing and rolling, and therefore it can be used to coat large surface areas. It also results in a smooth, uniformly coated surface compared to brushing and rolling, and does not leave brush or speckle marks or a textured appearance. Spraying equipment can also be used to clean off the surface prior to applying the coating. Spraying can produce a high quality, smooth surface.

A low efficiency for the amount of coating that is deposited on the substrate is one disadvantage to the spray application method. Spraying can be a slower process than other coating methods. It also is sometimes difficult to coat hard to reach areas, such as edges corners, and irregular surfaces with spraying. Since the equipment required for spraying is more expensive than that used for other coating methods, it must be cleaned after each use and properly maintained to ensure durability of the equipment.

High volume low pressure spraying is a spraying technique that uses approximately the same amount of compressed air as conventional spraying but requires less pressure to atomize the coating material. This results in a lower velocity air/coating stream and consequently improves the transfer efficiency from ~30% to up to 70%. This effectively reduces the coating costs by preserving more coating material. The negative side of high volume low pressure spraying is that the application time needed to coat an equivalent surface area compared to conventional spraying is increased. Furthermore, this spraying technique may not be suitable for applying more viscous coatings due to the low pressure requirement.

Airless spraying is another spraying technique that uses a fluid pump to pressurize and propel the coating material onto the substrate. Advantages to using this technique include good surface penetration (e.g. cracks, porous surfaces), better irregular surface coverage (e.g. corners, edges), quick film buildup, rapid area coverage, and higher viscosity coating materials. The coating material transfer efficiency is usually between 30 and 50%. One of the disadvantages to airless spraying is that it is difficult to adjust and change the equipment configurations (e.g. nozzles, orifices) while in the field. It also does not atomize the coating material as well as the conventional spraying method. Poor application techniques using this particular method can result in coating deficiencies such as solvent entrapment, voids, runs, sags, pinholes, and wrinkles.

A variation of the airless spraying method is the air-assisted airless spray, which incorporates the advantages of the airless spray method and the conventional spray method. For instance, it combines the fine atomization abilities of the conventional spray with the improved production and surface penetration characteristics of the airless spray. This method allows the coating material to be joined with a compressed air jet after it has been atomized in the absence of air. This results in a further atomization of the coating material before it reaches the substrate. This combined method is useful for applying fillers, glazes, lacquers and polyurethanes.

Plural component spraying is a complex application method that mixes coating components immediately before the coating is propelled to the substrate. This method is used for high-solids coatings and for coatings with a short cure time, such as epoxies. This method can be performed by any of the spraying methods mentioned above. This method is used to apply polyesters, polyurethanes, vinyl esters, and epoxies.

Electrostatic spraying is also a coating application process that utilizes the various atomization methods mentioned above (i.e. conventional, airless, air assisted airless). It utilizes an electrostatic, high voltage supply to direct the atomized particles to the substrate by electrostatic attraction. This technique is used to coat irregularly shaped substrates such as cables, piping, and fencing. The advantages of this coating method are that it improves the coating material transfer efficiency, has a good rate of application, and has good atomization properties. A disadvantage to this method is that it has a tendency for non-uniform deposition of the coating near irregular shaped objects on a surface. Furthermore, it requires special formulation of the coating material.

Proper application techniques are critical when using spraying techniques in order to achieve a high quality, uniform coating on the substrate. Therefore, it is very important that the applicator have either the necessary experience or training in order to produce acceptable coating results.