When precision machine shops and metal finishing shops work together, they must speak the same language about coating thickness and color matching. 

But before even reaching that point, there are basic metal finishing terms and common MIL-Specifications that machine shops need to know. Using the proper terminology helps ensure seamless communication with your metal finisher and minimizes unnecessary delays and confusion in getting the job done correctly. In addition, most projects have many moving parts, and an opportunity to reduce friction can make a big difference. 

You probably won’t need this glossary if you’ve been communicating with metal finishers for years and are already familiar with these metal finishing terms. However, we recommend filing it away for the next time you hire a new employee or need to cross-train someone else in your shop to handle the logistics of metal finishing. 

Metal Finishing 101: Glossary of Terms 

Acid etch

Acid etching is a process metal finishers use to achieve a non-reflective, matte surface. This proprietary chemistry is preferable to standard etching because of its ability to evenly level the high peaks to the low peaks on the microscopic texture of the metal.  Many customer’s prefer the surface is blasted prior to acid etching, which adds additional handling and costs.

Anodizing 

Anodizing aluminum is an electrochemical process that produces a hard, transparent oxide layer (0.00030-0.0007” thick) on the surface of a metal part, while making it non-conductive. This oxide layer is extraordinarily porous and looks like a honeycomb when viewed under a microscope. 

When color is desired on the surface, dye or ink is deposited into the pore structure. The last process to coloring is to stabilize the surface; the pores are hydrated or sealed, making the surface user-friendly. This sealing process improves corrosion resistance, locks in the color, and stabilizes the surface. 

Bead blasting

Bead blasting involves propelling small abrasive materials, such as aluminum oxide or glass beads, against the surface of a part to clean, polish, or roughen it. Run at a very low psi, the aluminum oxide will cover minor scratches, while the glass bead creates a soft, matte finish. Many OEMs know the Ra number that is required to match their desired outcome.

Chromate conversion coating

Chromate conversion coating is a type of conversion coating used to passivate aluminum. It is primarily used as a corrosion inhibitor, primer, decorative finish, or to retain electrical conductivity. The process is named after the chromate found in chromic acid. 

Customers choose chromate conversion coating for its strong corrosion protection and conductive surface and because it’s an ideal primer for painting, adds no weight, and maintains pre-process dimensions. 

Clear RoHS Chemical Conversion 

In Europe, the RoHS (Restriction of Hazardous Substances) Directive restricts the use of certain hazardous substances, such as chromic acid (hexavalent chromium), in electrical and electronic equipment. To adhere to this restriction, metal finishers use an alternative mixture containing trivalent chromium, a more environmentally friendly metal.  

Conversion coating

A conversion coating is a chemical or electrochemical treatment that metal finishers apply to a metal surface to enhance corrosion protection, surface hardness, and the adherence of other coatings. It may also be used as a decorative primer. 

Corrosion inhibitor 

A corrosion inhibitor—also known as an anti-corrosive—is a protective substance metal finishers apply to a metal surface to prevent corrosion.  

Electrical conductivity 

Electrical conductivity is the measure of how much electrical current a material can carry. If electricity flows easily through a material, the material is considered highly conductive. An anodized coating is non-conductive. 

Hardcoat anodizing

Hardcoat anodizing differs from conventional anodizing in that the temperature in the tank is lowered to increase the current density and produce a thicker, more compact oxide layer on the surface of the metal. 

Hardcoat anodizing penetrates the metal as much as it builds up on the surface. The term “thickness” includes penetration and buildup. A typical hard anodized application will increase in dimension by 0.002”—.001” penetration of the surface and .001” buildup—and produce a hardness of 60 to 65 Rockwell on the C scale. 

This process is ideal for parts that need extra protection under extreme conditions. Unlike conventional anodize, hardcoat anodize is often left unsealed to bond more effectively with paint and preserve its high hardness. 

Laser engraving

Laser engraving involves vaporizing a metal surface to engrave deep and permanent marks. It’s an inexpensive way to mark and identify parts. 

Passivation 

Passivation enhances the formation of a protective passive layer on stainless steel, maximizing the corrosion resistance inherent in the metal. 

Chemical passivation is a two-step process: 

1. The metal finisher uses acid to dissolve any free iron or iron compounds from the surface; otherwise, this iron will create a localized site where corrosion can continue. The surface itself is not affected by this process. 
2. The metal finisher uses an oxidizer to force the conversion of chromium metal on the surface to the oxide form. This process creates a uniform oxide protective layer.  

Teflon™ impregnation

Teflon™ impregnation is applied to an unsealed or sealed surface. It locks onto the surface of the oxide layer created during anodizing and is extremely difficult to remove, providing a frictionless surface. 

The information provided here is part of Global Metal Finishing’s ongoing efforts to educate customers on metal finishing services. We’ll continue to publish and share valuable information to ensure that you get the most out of your engagement with us. 

If you’re looking for a reliable partner for your metal finishing needs, request a quote, and we’ll respond within one business day.