Metal Finishes: A Concise Overview

Types of metal finishes

Metal finishing involves treating the exterior of a metal product or component with a thin layer of augmenting material or removing material from the component’s surface. Metal finishes improve metal surfaces in a variety of ways, including:

  • Improved durability
  • Corrosion resistance
  • Chemical resistance
  • Electrical resistance
  • Abrasion resistance
  • Reducing friction
  • Making a surface conductive
  • Decorative appeal

There many different types of metal finishes that have a variety of effects on surfaces and substrates. This guide will take you through the different types of metal finishes commercial companies use to improve their products.

Electroplating

Electroplating, i.e., electrodeposition, passes an electric current through an electrolyte solution (basically a bath filled with the electrolyte or property you’re trying to imbue upon your product). Two electrodes are placed into the solution and hooked up to a battery source that connects an electrical circuit between the electrodes, electrolyte and battery. Electricity flows through the circuit, breaking up the electrolyte which causes the metal atoms to form on one of the electrodes (which is the product you’re ultimately trying to electroplate). The electrolyte can consist of whatever type of plating your product needs. For example, a zinc plating (i.e. galvanization) will help prevent corrosion, while a gold plating provides an aesthetic appeal.

Plastics can also be electroplated on to products, and are used in a variety of applications. They’re lighter, cheaper and never rust due to their inability to conduct electricity.

Rack plating and barrel plating are both popular methods electroplating.

Rack plating is most useful when a specification is complex and requires extra attention. Product components are affixed to metal racks and dipped into the solution and pulled out when complete.

Barrel plating is most useful when looking for a low-cost, high-volume option. Barrel plating is the best option when parts are small and durable (i.e. fasteners, bolts, etc.), because parts are placed in a rotating barrel filled with your chosen electrolytic solution.

Electroless plating

The foil to electroplating is of course electroless plating. This method is non-galvanic, meaning no electric current is involved in the plating process. This process is also know as autocatalytic plating or conversion plating and no external power source is used. Instead, the part is placed in a solution filled with nickel or copper, creating a catalytic reduction of solution that breaks up the material’s ions.

Electroless plating is particularly popular in industries where corrosion resistance is important. This plating method makes parts very hard and nonporous, increasing corrosion resistance. Other benefits include reduced friction, increased strength and uniform deposition. Thus, electroless plating is great for industries like oil & gas, food & beverage manufacturing,  automotive and aerospace. Any environment that’s subject to harsh and/or corrosive environments is perfect for electroless plating.

Electrocoating

Electrocoating, i.e., e-coating involves the same principles as electroplating. An electric current and solution work together to coat a components substrate. However, the solution contains electrically charged paint particulars (typically acrylics or epoxies) that create a primer over the substrate. The primer helps with adhesion when a material needs to be painted in the field or at a shop.

Cathodic epoxy e-coatings are setting the standard for corrosion resistance, although they require topcoat for UV resistance when components are exposed to sunlight.

E-coating is known for being a very low-cost-per-square-foot finishing option because of their ability to accommodate part complexity and volume. They’re also popular because of their ability to apply to almost any metal. However, other factors like equipment repair and maintenance should be considered.

Passivation

Passivation is a chemical process in which a material becomes passive, i.e., less likely to corrode. Parts are submerged in a solution of nitric or citric acid, removing corrosive particles. Passivation reduces the amount of iron that can react with the environment (causing rust) and forms a protective shield around the metal without changing the physical appearance of the part. Passivation can be implemented on a variety of materials including silicon, aluminum, ferrous materials, stainless steel and nickel.

Case hardening

Case hardening is the process of forming a hardened layer (or case) around a metal component that acts as a protective barrier. Case hardening is an excellent option for components that are subject to abrasive and high pressure environments, but aren’t subject to corrosive environments. Note that case hardened materials cannot be welded.

There are several types of case hardening options including carburizing, nitriding, cyaniding and flame hardening.

Carburizing involves adding carbon to a component by heating it to 9000 °C and exposing it to a carbon environment.

Nitriding involves heating a material to 5000 °C and exposing it to nitrogen.

Cyaniding incorporates both nitrogren and carbon into the protective casing by dipping the component into a carburizing bath that contains cyanide.

Flame hardening involves heating a material with oxyacetylene and quenching it with water.

Each process has its pros and cons, so consult an industry professional before deciding what’s right for your product.

Powder coating

Most industrial coatings come in liquid form, but powder coatings are a powder-based alternative. Powder coatings provide greater thickness than liquid coatings, and different pigments, curatives, level agents, flow modifiers and other additives can be added to the powder to provide the desired protection and aesthetic appeal. Once the coating is selected, electrostatic spray deposition (ESD) used by coating professionals to apply the coat to the metal substrate. A spray gun is typically used to apply the coating, and then the components enter a curing oven that heats the components, causing a chemical reaction that binds the coating and substrate.

Electropolishing

Electropolishing is the exact opposite of electroplating. Rather than depositing metal ions onto the component’s surface, electropolishing removes those metal ions from the substrate. The material is immersed in an electrolyte bath and an electrical current is applied. The material becomes the anode, and ions flow from it, removing defects, rust, etc.

The end result is a surface that’s polished, smoothed and removed of bumps and pocks. Even at a microscopic level, peaks and valleys will be removed. This is an excellent option for materials that need to have a clean and pristine look.

Buff polishing

Buff polishing is an alternative to electropolishing that doesn’t involve the electrochemical process. Buff polishing is similar to buffing a car–a machine equipped with a cloth wheel polishes and buffs the surface of your components. The result is a glossy and shiny finish that’s ideal for metallic components that need a high quality look.

Brushed metal

Brushed metal is similar to buff polishing in that you’re effectively removing metal ions from your components. With brush polishing, the metal is brushed in unidirectional manner that creates a specific aesthetic. You might have seen this look on different stainless steel appliances like toasters or on the hood of your car. Brushed metal is popular in small appliance and automotive industries, and it’s commonly used on stainless steel, aluminum and nickel. When you want your product to have distinct, parallel lines, brushed polish is a good option.

Metal grinding

Again, metal grinding is a metal finish that reduces a component by grinding away metal ions. Metal grinding involves a machine that uses abrasion and friction to smooth out a metal surface. Handheld power tools, grindstones, grinding machines, bench grinders and wheel grinders are all options for various applications. Metal grinding is best suited for hard materials (e.g. hardened steel) that required shallow cuts.

metal grinding

Abrasive blasting

Abrasive blasting is a cost effective solution that applies to both facility exteriors and metal components. It combines surfacing cleaning and finishing into one process. With abrasive blasting, a high-pressure stream of abrasive media (material) is blasted against a surface to remove debris, alter shape and texture and provide a smooth finish. Abrasive blasting can also act as surface preparation for coatings and plating to increase durability.

A typical blasting system is made of an air compressor that adjusts pressure and volume, moisture separator to reduce humidity, water-assisted system for dust suppression, air supply line, blast machine to store media, remote controls and a hose and nozzle.

Abrasive blasting is a serious cost- and time-saving option for manufacturers and facility owners alike. Abrasive blasting can be performed with a variety of media including sand, aluminum, plastic, glass, corn cob, steel grit, walnut shell, silicon carbide and steel shot.