表面処理（ひょうめんしょり、surface treatment、surface finishing）は、めっきや塗装など、素材表面の性質を高めるために行われる機械工作法の一種。硬さ・耐摩耗性や潤滑性・耐食性や耐酸化性・耐熱性や断熱性・絶縁性・密着性・装飾性や美観などを向上させるのがおもな目的となる。 材料技術の一分野で、加工、熱処理、溶接、鋳造などの材料プロセス技術分野と並立する存在である。
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Underbody structural components are typically coated to provide a first line of defense against corrosion. For light truck frames, the two most common coatings are hot melt wax and electrocoat (E-coat). Paints are also used on current light truck frames. Conversion coatings enhance the adhesion of electrocoat or paint, and they are commonly used in conjunction with these two coating types. Many underbody structural components, such as front rails on passenger cars, are made from sheet steel precoated with a metallic coating, e.g., galvanized or galvanneal sheet steel. Autophoretic and powder coatings are also used on underbody structural components.
A. Application Methods
The use of spray equipment to apply coatings is proven and well defined. Spray equipment acts upon a stream of coating (solvent or water borne particles) and by various means disperses the coating into a cloud of finely divided particles. The atomized particles are then deposited on the intended surface forming a protective or decorative coating.
Dip application is a method that involves dipping a part into a coating bath, draining the part, and force drying or baking the part. Dip coatings are often used for primer and one-coat applications.
Flow coating is an automatic coating operation in which the product to be coated is conveyed through a chamber equipped with low pressure nozzles that completely flood the product with a coating. The process does not involve any atomization. It is used for large articles that would require a dipping tank of impractical size and for articles with a shape that makes spray painting impractical.
B. Types of Coatings Method
Conversion Coatings
Phosphate conversion coatings are employed to enhance paint adhesion. By enhancing paint adhesion, they indirectly enhance corrosion resistance. There are several varieties of phosphate coatings, e.g., iron, zinc or manganese.
Prior to the application of a conversion coating, the metal surfaces must be free of shop soils, oil, grease, lubricants and rust. The metal surfaces must be receptive to the formation of a uniform, adherent chemical film or coating. Surfaces may be cleaned by mechanical methods or, more commonly, by immersion or spray cleaner systems.
A phosphate coating is applied by immersing a clean metal part in a hot processing solution for 4-6 minutes, depending on the bath chemistry. The weight (thickness) of the conversion coating is dependent upon the manner in which the part is cleaned, the immersion time, the composition of the processing bath and the chemical composition of the metal itself.
Hot Melt Wax Coatings
Hot melt wax coatings are thermoplastic corrosion prevention compounds. They have a solvent or waterborne formulation. Since the 1970’s, hot melt waxes have been used extensively on underbody structural components to provide corrosion protection and enhance vehicle durability. Hot melt waxes are usually applied through a dipping process. The wax is preheated to a temperature between 125 and 195 degrees C (257 and 383 degrees F). Following an alkali cleaning and water rinsing operation, parts are immersed in the molten wax. The thickness of the wax deposited on the parts is controlled through preheat of the parts prior to dipping and the actual time of immersion in the hot melt wax. Following the immersion process, the coated parts are allowed to return to ambient temperature through a process that controls the uniformity and finish of the hot melt wax. Hot melt wax thickness is commonly specified as 75-125 micrometers (3-5 mils). Typically, hot melt wax coatings can withstand temperatures up to 143 degrees C (290 degree F) without dripping.
Electro-coat (E-coat)
Electrophoresis deposition is a process in which electrically charged particles are deposited out of a water suspension to coat a conductive part. The process is more commonly known as electro-coating or E-coating. The idea of electrically discharging polymers to coat an object was first considered in the 1930’s. Most of the basic research was conducted in Europe in the 1960’s. North American companies began electro coating in the late 1960’s, and the process has been widely used for coating metal parts ranging from simple stampings to complex auto bodies.
The process requires a coating tank in which to immerse the part, as well as temperature control, filtering and circulation equipment. Electro coating systems are known as anodic or cathodic depending upon whether the part is the anode or cathode in the electrochemical process. Cathodic systems are more common since they require less surface preparation and provide better corrosion resistance.
Electro-coating requires that the coating binder, pigment and additives be given an electrical charge. These charged materials, under the influence of an electric field, migrate through water to the part surface.
Once at the part, the charged materials give up their charge due to neutralization by electrochemically generated OH-ions (cathodic process). Upon giving up their charge, the coating materials drop out of the water suspension and coalesce as a coating on the part surfaces. Electro coat thickness typically ranges from 10 to 30 micrometers (0.4 to 1.2 mils).
Automotive parts that are electro coated usually receive zinc or iron phosphate treatment prior to deposition. This treatment enhances the application of the E-coat.
Metallic Coatings
Various types of metallic coatings can be applied to ferrous and non-ferrous substrates to inhibit corrosion and/or provide a decorative finish. The choice of a particular coating material is dependent upon the severity of the corrosive environment, whether the part is subject to wear and abrasion, and the degree of visibility of the part in service.
Four common methods for applying metallic coatings are:
- Electroplating. The coating is deposited onto the substrate metal by applying an electrical potential between the substrate metal (cathode) and a suitable anode in the presence of an electrolyte. The electrolyte usually consists of a water solution containing salt of the metal to be deposited and various other additions that contribute to the plating process.
- Mechanical plating. Finely divided metal powder is cold welded to the substrate by tumbling the part, metal powder and a suitable media such as glass beads, in an aqueous solution containing additional agents. Mechanical plating is commonly used to apply zinc or cadmium to small parts such as fasteners.
- Electro less. In this non-electric plating system, a coating metal, such as cobalt or nickel, is deposited on a substrate via a chemical reaction in the presence of a catalyst.
- Hot dipping. A coating metal is deposited on a substrate by immersing the substrate in a molten bath of the coating metal.
Many underbody structural components are manufactured from sheet steel with a metallic coating. The steel mills supply hot or cold rolled sheet in coil form with metallic coatings applied by either electroplating or hot dipping. The most commonly supplied coatings include zinc, zinc-iron, zinc-nickel, and aluminum, aluminum-zinc, tin and lead-tin.
Organic Coatings
The application of an organic coating, such as paint, is a cost effective corrosion protection method. Organic coatings act as a barrier to a corrosive solution or electrolyte. They prevent, or retard, the transfer of electrochemical charge from the corrosive solution to the metal underneath the organic coating.
An organic coating is a complex mixture of materials designed to protect the substrate and to enhance appearance. A coating is composed of binders, carriers, pigments and additives. Binders provide the major properties to the coating while the carriers (solvents and/or water) adjust the viscosity of the coating for application. Pigments impart specific properties to a coating such as corrosion resistance and color. Furthermore, when formulating a coating, the type of pigment and its volume are critical to the optimization of properties such as adhesion, permeability, resistance to blistering and gloss. Additives include thickeners, flow agents, catalysts and inhibitors.
Coating systems are often identified by the type of polymers employed. Commonly used organic coatings are:
- Alkyd and epoxy ester coatings (air dried or baked to promote cross-link oxidation),
- Two-part coatings such as urethane coatings,
- Radiation curable coatings (acrylic and epoxy polymers),
- Latex coatings such as vinyl, acrylic or styrene polymer combinations,
- Water soluble coatings (versions of alkyd, epoxy ester or polyester coatings),
- High-solids coatings and
- Powder coatings (vinyl, polyester or epoxy polymers).
Auto deposition Coatings
Auto deposition is a waterborne process that depends on chemical reactions to achieve deposition. This process has been used commercially since 1973. The composition of an auto deposition bath includes a mildly acidic latex emulsion polymer, de-ionized water and proprietary ingredients. The chemical phenomenon consists of the mildly acidic bath attacking the steel parts being immersed and causing an immediate surface reaction that releases iron ions. These ions react with the latex in solution causing a deposition on the surface of the steel parts.The newly deposited organic film is adherent yet quite porous. Thus, the chemical activators can rapidly diffuse to reach the surface of the metal, allowing continued coating formation.
The coating thickness of the auto deposition film is time and temperature related. Initially, the deposition process is quite rapid, but slows down as the film begins to build or mature. As long as the part being coated is in the bath, the process will continue; however, the rate of deposition will decline. Typically, film thicknesses are controlled from 15 to 25 micrometers (0.6 to 0.8 mils).
Auto deposition will coat any metal the liquid touches. Parts that are tubular in shape, assembled parts or parts that have intricate designs can be coated by this process. Auto deposition does not require a phosphate stage and the coating is cured at a relatively low temperature.
Powder Coatings
In the powder coating process, a dry powder is applied to a clean surface. After application, the coated object is heated, fusing the powder into a smooth, continuous film. Powders are available in a wide range of chemical types, coating properties and colors. The most widely used types include acrylic, vinyl, epoxy, nylon, polyester and urethane. Modern application techniques for applying powders fall into four basic categories: fluidized bed process, electrostatic bed process, electrostatic spray process and plasma spray process.
The electrostatic spray process is the most commonly used method of applying powders. In this process, the electrically conductive and grounded object is sprayed with charged, non-conducting powder particles. The charged particles are attracted to the substrate and cling to it. Oven heat then fuses the particles into a smooth continuous film. Coating thicknesses in the range of 25 to 125 micrometers (1 to 5 mils) are obtained. Controlling a low film thickness is difficult. A booth and collection system can be used to collect overspray for re-use
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