Introduction to finish materials

Finishing is performed to achieve the desired characteristic of the surface finish of a manufactured part. The most common surface finishes address aesthetic characteristics, but they also aim to alter adhesive, soldering, resistance, hardness, electrical conductivity, or even friction control properties.

A broad spectrum of Finishing materials can be applied to the various finishing processes, including Anodizing, Dip Coating, Electroplating, Electropolishing, Electroless Plating, Powder Coating, and Spraying.

Generic finish materials

Anodizing aims to increase the thickness of the natural oxide layers on the surface of metal parts. Electrolytic passivation is the process used in anodizing, which aims to achieve the desired set of properties, including corrosion and wear resistance, enhanced adhesion for paint and glue, and aesthetic effects.

Anodizing is particularly beneficial to achieve a significantly thinner coat than paint or powder. It is also durable, hard, abrasion-resistant, long-lasting, affordable, environmentally friendly, and an excellent electrical insulator. Likewise, anodizing eliminates peeling and chipping while offering excellent corrosion protection and color fade resistance.

The main materials used with Anodizing include: chromic acid, sulfuric acid, and hard anodize plating.

The process of anodization based on Chromic acid is also known as Type I anodization. This is also the oldest anodizing process to use chromic acid. Chromic acid produces thin, 0.5 μm to 18 μm, opaque films that are soft, ductile, and to a degree self-healing. They are harder to dye and may be applied as a pretreatment before painting.

The process of anodization based on Sulfuric acid is the most widely used solution known as Type II anodization. Coatings of a moderate thickness of 18 μm to 25 μm are typically achieved with Sulfuric acid.

The process of anodization based on increasing thickness is known as Type III anodization. Hard anodize plating is typically applied to heavy wear industrial parts intended for use in aggressive or highly corrosive applications. It can be made between 25 μm and 150 μm thick. Anodizing thickness increases wear resistance, corrosion resistance, the ability to retain lubricants and PTFE coatings, and electrical and thermal insulation.

Dip coating is immersion, or dipping, whereby a substrate is coated with a powder by immersing it in a solution of coating material at a constant speed. This industrial process is suitable for manufacturing high-volume products such as coated fabrics or prophylactics and specialized coatings in the biomedical field. Numerous chemical and nanomaterial engineering research processes are used in academic research to study the use of Dip Coating to create thin-film coatings. It is possible to achieve a uniform, high-quality film even on bulky, complex shapes with the Dip Coating technique.

The main materials used with Dip Coating include rubber-like paint and silicone coating.

Rubber-like paint is water-based paint that has a latex binder.

Silicone coating films can be applied to silica-based substrates such as glass to form a covalently bonded hydrophobic coating. Many fabrics can be coated or impregnated with silicone to form a strong, waterproof composite such as silnylon.

Electroplating: is adopted by a wide variety of industries, this is a finishing process that coats metal objects with a thin layer of a different metal to achieve the desired property that the original workpiece lacks. The process is called electrodeposition. Electroplating aims to modify the surface properties of an object, such as abrasion, wear or corrosion resistance, lubricity, or aesthetic qualities. Additionally, electroplating is sometimes performed to increase undersized parts' thickness or form objects with electroforming.

The main materials used with electroplating include cadmium plating, chrome plating, copper plating, gold plating, hard chrome plating, nickel plating, rhodium plating, silver plating, tin plating, tin-lead plating, zinc plating, and zinc-iron plating.

Cadmium electroplating delivers a robust and versatile metallic coating. Cadmium is a soft white metal that corrodes before the substrate material when plated onto steel, cast iron, malleable iron, copper, and powdered metal. Cadmium plating also offers an exceptional bonding surface for adhesives, making it ideal for use in aircraft manufacturing and salt-water environments.

Chrome electroplating distributes a thin layer of chromium onto a metal object for decorative purposes or to achieve corrosion resistance, ease of cleaning, or increased surface hardness properties.

Copper plating uses electrolysis to deposit a thin, solid, metallic layer of copper onto the surface of an object.

Gold plating distributes a thin layer of gold onto the surface of another metal, most often copper or silver (to make silver-gilt), by chemical or electrochemical plating.

Hard Chrome plating, also known as industrial chrome or engineered chrome, is performed to reduce friction and to improve durability through abrasion tolerance and wear resistance in general. Additionally, it is committed to minimizing the galling or seizing of parts, expanding chemical inertness to a broader set of conditions (especially oxidation resistance), and bulk material for worn parts to restore their original dimensions.

Nickel plating distributes a thin layer of nickel onto a metal object for decorative purposes, achieve corrosion resistance, wear resistance, or bulk up worn or undersized parts for salvage purposes.

Rhodium plating is occasionally used on white gold, silver, or copper and its alloys. First, a barrier layer of nickel is usually deposited to prevent contamination of the rhodium bath, which slightly dissolves the aforementioned metals because of the sulfuric acid present in the bath composition.

Silver plating distributes a thin layer of silver onto an object as a cost-effective alternative to using solid silver. Popular silver plating examples include musical instruments, cutlery, various vessels, and candlesticks.

Tin plating is principally adopted to protect both ferrous and nonferrous surfaces. It is particularly useful in the food processing industry because of its non-toxic, ductile, and corrosion-resistant properties.

Tin-lead plating is primarily adopted for corrosion protection and as a base for soldering. Tin-lead does not oxidize quickly in air and is soft and very ductile. Its color can range from matte gray to very bright gray.

Zinc plating distributes a thin layer of zinc onto metal objects to prevent oxidation by forming a barrier. This barrier acts as a sacrificial anode if it becomes damaged.

Zinc-Iron plating provides improved corrosion protection versus zinc alone. Likewise, it has even deposit distribution and superior lubricity, ductility, and hardness.

Electropolishing is often referred to as the opposite process of electroplating, whereas the material is removed from a metallic workpiece instead of added. Also referred to as electrochemical, anodic, or electrolytic polishing, the process is employed to polish, passivate, and deburr metal workpieces. Electropolishing applies a scientific method where the material is removed using electrical currents, oxidization, and electrolytes.

Electropolishing is particularly beneficial with stainless steel since it removes iron from the surface, which enhances the chromium/nickel content to achieve a superior passivation state. It also provides a clean, smooth surface that facilitates sterilization and an aesthetically pleasing appearance. Additionally, it can reduce the size of parts when needed.

The main materials used with electropolishing are usually concentrated mixtures of sulphuric acid and phosphoric acid, mainly adopted for their high viscosity. However, there are also reported perchlorates with acetic anhydride and methanolic solutions of sulphuric acid. These mixtures are also known as electropolishing solutions.

Sulphuric acid is a mineral acid with the molecular formula H2SO4. It is a colorless, odorless gluey liquid that is soluble in water. Its acidic solid nature makes it highly corrosive. It is also hygroscopic, meaning it readily absorbs water vapor from the air. Even at moderate concentrations, sulphuric acid is very dangerous when directly contacting the skin.

Acetic anhydride, or ethanoic anhydride, is the chemical compound (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a colorless liquid that smells strongly of acetic acid, formed by its reaction with moisture in the air.

Phosphoric acid (also known as orthophosphoric acid or phosphoric(V) acid) is a mineral (inorganic) and weak acid having the chemical formula H3PO4. Orthophosphoric acid refers to phosphoric acid, the IUPAC name for this compound. The prefix ortho- is used to distinguish the acid from related phosphoric acids, called polyphosphoric acids. Orthophosphoric acid is a non-toxic acid that is a solid at room temperature and pressure when pure. In addition to being a chemical reagent, phosphoric acid has a wide variety of uses, including as a rust converter, food additive, dental and orthopedic etchant, electrolyte, soldering flux, dispersing agent, industrial etchant, fertilizer feedstock, and a component of home cleaning products.

Electroless plating is a purely chemical process that aims to toughen a component, enhance its appearance, and improve friction and corrosion resistance. It is also known as autocatalytic plating, and, as its name suggests, no electricity is used in this plating process. Electroless plating is particularly beneficial for industries that want a simplified, cost-effective coating part with metal. Parts made of intricate, complex forms or susceptible to heavy corrosive factors are prime candidates for electroless plating.

The main materials used with Electroless Plating include electroless copper plating, nickel PTFE plating, and phosphorus electroless nickel.

Electroless copper plating chemically deposits a coat of copper onto the desired segment of a part or segment of a part.

Electroless Nickel-PTFE (phosphorous PolyTetraFluoroEthylene) plating coats metal surfaces via an autocatalytic chemical reaction to achieve a repeatable coating of uniform thickness. This process makes it possible to coat parts with sharp edges, deep recesses, seams, threads, and complex geometries.

There are three phosphorus electroless nickel plating types: low, medium, and high. Low-phosphorus treatment is applied for hard-ness deposits up to 60 Rockwell C. Medium-Phosphorus Electroless Nickel (MPEN) refers to the nickel-phosphorus alloy deposited by the electroless process. The resulting alloy consists of medium levels of phosphorus. Medium levels vary as a percentage of weight between 4–10%. High Phosphorus Electroless Nickel (HPEN) is ideal for industry standards requiring protection from highly corrosive acidic environments such as oil drilling and coal mining, thanks to its high corrosion resistance.

Powder coating coats workpieces with a free-flowing, dry powder to create a harder, tougher finish than conventional paint. Most commonly applied to metals, it is used to coat household appliances, aluminum extrusions, drum hardware, and automotive and bicycle parts. Usually, thermoplastic or thermoset polymers are applied using an electrostatic process, followed by a curing process of heat. Curing time with powder coating is remarkably faster than liquid coating. Also, its powdered state eliminates the disadvantages of running and sagging.

The main materials used with powder coating include powder primer and powder coating.

Powder primers add extra corrosion resistance and a better surface for an enhanced aesthetic appearance. Different primers include Eposeal, Epoxy Polyester NZP, Epozinc 1000, Epozinc ZL, and many others.

Powder coating materials can be broken down into two main categories: thermosets and thermoplastics. Thermosetting primers incorporate a cross-linker that reacts with other chemical groups upon being baked, improving the final performance properties. Thermoplastic primers are not subject to additional actions during the baking process, which completes the final coating.

The most common polymers include polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy), and acrylics.

Spraying is a technique that involves spraying either paint, ink, varnish, or other materials to achieve the desired finish properties of a workpiece. Usually coupled with compressed air, spraying employs airbrushes or spray guns that can be hand-held or automated, respectively. Airbrushes are mostly utilized for workpieces requiring greater detail, while spray guns are better for covering large surfaces with an even liquid coat.

The main materials used with spraying include:

The conductive coating allows an electric current to run over the surface of a non-conductive material.

Liquid primers provide a smooth layer upon which subsequent coat materials will improve adherence.

Liquid topcoat provides a superior quality finishing coat on top of a primed surface and offers excellent corrosion and chemical resistance.

Rubber-like paint is water-based paint that has a latex binder.

Silicone coating films can be applied to silica-based substrates such as glass to form a covalently bonded hydrophobic coating. Many fabrics can be coated or impregnated with silicone to form a strong, waterproof composite such as silnylon.


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