A Guide to Chemical Finishes for CNC Machined Parts

Previously published on fastradius.com on November 16, 2022

All post-processing increases part costs and production timelines, but the right surface finish has the potential to bring your design vision to life. Metal finishing for CNC machined parts typically encompasses a variety of mechanical processes, such as tumbling, brushing, and bead blasting, but metal parts may also be treated with chemical finishes such as passivation and zinc plating.

Amongst many useful results, chemical finishing can remove blemishes from a part, alter its conductivity levels, extend its lifespan, and even increase its resistance to wear and corrosion. Chemical finishes have an array of industrial applications: in the aerospace industry, for example, companies use chemical finishes to increase parts’ durability, improve thermal stability, and slow oxidation. In the consumer goods industry, chemical finishes can be found in the production of everything from enclosures and casings to sporting equipment.

While there are plenty of chemical finishes available, they aren’t necessarily interchangeable between materials. In fact, every chemical finish is typically compatible with specific materials and offers its own advantages and disadvantages. In this guide, we’ll explore several common chemical finishing processes so that you can decide which will work best for your CNC manufacturing project.

Choosing Your Chemical Finish

When choosing a chemical finish for your part, you’ll need to think about both compatible materials and end use. This means considering an array of contextual factors, including: 

• The environment your part will be used in
• Whether it requires conductive or insulating properties
• How much weight it will need to bear
• How much wear it will need to withstand
• Tolerance requirements
• Color and transparency requirements
• Surface finish standards
• Any other relevant or desired properties.

To help you evaluate your options, here are some common chemical finishes and their compatible materials:

Let’s take a closer look at these chemical processes, how they work, and how they might benefit your project. 

Anodizing

A popular aluminum finishing option, anodizing thickens the natural oxide layer on part surfaces, creating an anodic oxide film that confers increased protection and improved aesthetics. In the case of aluminum, to form the anodized protective layer, you’ll need to bathe your part in an acid electrolyte bath and then apply a cathode (a negatively charged electrode) to cause the solution to release hydrogen. At the same time, the aluminum part (the positively charged anode) will release oxygen, forming a protective oxide layer on its surface. After a part has been anodized, its surface will have microscopic pores which must be sealed with a chemical solution to prevent corrosion and any build up of contaminants. 

Anodized parts are durable and resistant to corrosion and abrasion, which can reduce maintenance costs down the line. The anodized layer is electrically non-conductive and is fully integrated with the aluminum substrate, so it won’t chip or flake away like plating and paint often do. In fact, in addition to sealing, the porous anodized layer can be painted or dyed, and since anodized finishes are non-toxic and chemically stable, they’re also more environmentally friendly. Anodizing isn’t just a finish for aluminum: the process is also possible for titanium and other non-ferrous metal parts. 

There are three different types of anodization:

• Type I (chromic acid anodizing) results in the thinnest oxide layer, which means it won’t change your part’s dimensions. Type I anodized elements will appear grayer in color and won’t absorb other colors well.
• Type II (boric-sulfuric acid anodizing) has better paint adhesion and is slightly thicker than Type I. With Type II anodizing, you can easily create anodized parts that are blue, red, gold, black, or green.
• Type III (hard sulfuric acid anodizing) is the most common form of anodizing. It has the clearest finish, which means it can be used with more colors. It’s worth noting that Type III anodizing results in a slightly thicker finish than Type II anodizing.

The increased durability, abrasion resistance, and corrosion resistance of anodized parts, and the high level of dimensional control that the process offers, makes anodizing particularly popular in aerospace and construction. Beyond those industries, anodized metal components are found in a wide variety of applications including curtain walls, escalators, laptops, and more.

Despite its broad applications, there are drawbacks to anodization:

• Anodizing metal will change the dimensions of your part, so you’ll need to consider the oxide layer when determining dimensional tolerances or use chemical or physical masks to ensure specific areas of your part remain untreated.
• It can be challenging to achieve a true color match if your anodized components aren’t treated in the same batch. Color fading may also occur.
• Anodizing a metal part will increase its electrical and thermal resistance. In some cases, this might be the intention, but in others, you may need to use a mask to ensure your part retains its full conductivity in certain sections.
• Anodizing will increase your part’s surface hardness.

Passivation

This popular metal finishing process prevents corrosion in stainless steel parts, helping them retain their cleanliness, performance, and appearance. Not only will passivated parts be far more resistant to rust, and thus better suited to use outdoors, they’ll also be less likely to pit, last longer, be more aesthetically pleasing, and more functional. Accordingly, passivation is used across a variety of industries, from the medical industry where sterilization and longevity are key, to the aerospace industry where businesses seek high steel quality and tight dimensional tolerances.

Passivation involves the application of nitric or citric acid to a part. While nitric acid has traditionally been the typical choice for passivation, citric acid has recently increased in popularity because it can produce shorter cycle times, and is safer and more environmentally friendly. During the passivation process, parts are submerged in an acid-based bath to remove any iron and rust from their surfaces without disturbing the chromium. The application of acid to stainless steel removes any free iron or iron compounds from its surface, leaving behind a layer composed of chromium (and sometimes nickel). After exposure to the air, these materials react with oxygen to form a protective oxide layer. 

It’s important to bear in mind that passivation can extend part production time. Before a part can be passivated, it must be cleaned to remove any greases, dirt, or other contaminants, and then rinsed and soaked (or sprayed). While submersion is the most common passivation method because it offers uniform coverage and can be completed quickly, an acidic spray may be used as an alternative. 

Black Oxide Coating

A finish for ferrous metals like steel, stainless steel, and copper, the black oxide coating process involves immersing parts in an oxide bath to form a layer of magnetite (Fe₃O₄), which offers mild corrosion resistance.

There are three types of black oxide coating:

• Hot black oxide: The hot black oxide coating process involves dipping a part into a hot bath of sodium hydroxide, nitrites, and nitrates in order to turn its surface into magnetite. After bathing, parts will need to be submerged in alkaline cleaner, water, and caustic soda, and then coated with oil or wax to achieve the desired aesthetic.
• Mid-temperature black oxide: Mid-temperature black oxide coating is very similar to hot black oxide coating. The main difference is that coated parts will blacken at a lower temperature (90 – 120 °C). Since this is below the boiling point of the sodium and nitrate solution, there’s less need to worry about caustic fumes.
• Cold black oxide: While hot and mid-temperature black oxide coating involves oxide conversion, cold black oxide relies on deposited copper selenium to alter a part. Cold black oxide is easier to apply but rubs off more quickly and provides less abrasion resistance.

Parts that have received black oxide coating will have greater corrosion and rust resistance, be less reflective, and will have much longer life cycles. The oil or wax coating will add water resistance and may also make your parts easier to clean by preventing harmful substances from reaching the metal interior. Black oxide coating will also add thickness, making it ideal for drills, screwdrivers, and other tools that require sharp edges that won’t dull over time.

Chem Film

Chem film, also known as chromate conversion coating, or by its brand name Alodine®, is a thin coating typically used on aluminum (although it can be applied to other metals) to prevent corrosion and improve adherence of adhesives and paints. Chem film finishes often have proprietary formulas, but chromium is the main component in every variety. A chem film finish can be applied via spraying, dipping, or brushing, and, depending on product and formula, may be yellow, tan, gold, or clear in color.

While other finishes reduce thermal and electrical conductivity, chem film finishing allows aluminum to maintain its conductive properties. Chem film is also relatively cheap and, as noted, provides a good base for painting and priming (for additional time-saving benefits). Since it’s prone to scratches, abrasion, and other superficial damage, however, chem film isn’t ideal for projects in which aesthetic appearance is a top priority.

Electropolishing

Electropolishing is an electrochemical finishing process commonly used to remove a thin layer of material from steel, stainless steel, and similar alloys. During the electropolishing process, a part is submerged in a chemical bath and an electric current is applied to dissolve its surface layer. Various parameters affect the part’s finish, including the chemical composition of the electrolyte solution, its temperature, and the part’s exposure time.

Electropolishing generally removes between 0.0002 and 0.0003 inches from an object’s surface, leaving smooth, shiny, and clean material behind. Other benefits of electropolishing include improved corrosion resistance, increased part longevity, improved fatigue strength, a lower coefficient of friction, reduced surface roughness, and the elimination of surface defects such as burrs and micro-cracks.

Electropolishing is compatible with steel, stainless steel, copper, titanium, aluminum, brass, bronze, beryllium, and more. It’s worth noting that electropolishing is faster and cheaper than manual polishing, though it still takes time and won’t remove 100% of rough surface defects. 

Electroplating

Electroplating is effectively the reverse of electropolishing. Instead of removing a layer of metal to achieve a finished surface, electroplating deposits an additional outer layer, increasing a part’s thickness. Compatible with cadmium, chrome, copper, gold, nickel, silver, and tin, electroplating creates smooth parts that experience less wear and tear over time thanks to their additional protection from corrosion, tarnishing, shock, and heat. Electroplating can increase adhesion between the base material and its additional outer coating, and, depending on the type of metal used, can make your part magnetic or conductive.

In contrast to other CNC machining finishes, electroplating isn’t particularly eco-friendly since it creates hazardous waste that can seriously pollute the environment if disposed of improperly. Electroplating is also relatively costly, as a result of the metals and chemicals (and other necessary materials and equipment) that it requires, and can be time-consuming, especially if a part requires multiple layers.

Chrome Plating

Chrome plating, or chromium plating, is a type of electroplating that involves adding a thin layer of chromium to a metal part to increase its surface hardness or resistance to corrosion. The addition of a chrome layer can make cleaning a part easier and improve its aesthetics, and nearly all metal parts can be chrome plated, including aluminum, stainless steel, and titanium.

The chrome plating process generally involves the degreasing, manual cleaning, and pretreatment of a part before it is placed in a chrome plating vat. The part must then stay in the vat long enough for the chrome layer to reach a desired thickness. Since the process consumes electricity, and involves multiple steps, chrome plating is a relatively expensive finishing process.

Polytetrafluoroethylene (Teflon™) Coating

Polytetrafluoroethylene (PTFE) coating, commonly known as Teflon™, is available in powder and liquid forms, and is used across the industrial landscape. Some PTFE applications only require one coat, but others need both a primer and a topcoat to ensure maximum protection. The finish can be applied to a range of metals including steel, aluminum, and magnesium.

PTFE-coated parts have non-stick surfaces, a low coefficient of friction, and are highly resistant to abrasions. Since PTFE coating has low porosity and surface energy, coated parts will be resistant to water, oil, and chemicals. PTFE can also withstand temperatures up to 500°F, can be easily cleaned, and offers great electrical insulation and chemical resistance.

Due to its chemical resistance and non-stick properties, PTFE is often used to coat fuel pipes and to insulate circuit boards in computers, microwaves, smartphones, and air conditioners. It is also commonly used to coat medical tools and equipment, and cookware. Although it is popular across industries, the PTFE coating process is relatively expensive and isn’t as long-lasting as other chemical finishing options.

Electroless Nickel Plating

Electroless nickel plating refers to the addition of a protective layer of nickel-alloy to metal parts. In contrast to the electroplating process, which involves an electric current, electroless nickel plating involves the use of a nickel bath and a chemical reducing agent like sodium hypophosphite to deposit a layer of nickel-alloy (often nickel-phosphorus) onto parts. The nickel-alloy deposits uniformly, even on complex parts with holes and slots. 

Parts finished with nickel plating have increased resistance to corrosion from oxygen, carbon dioxide, salt water, and hydrogen sulfide. Nickel-plated parts also have good hardness and wear resistance and, with additional heat treatment, can become even harder. Electroless nickel plating is compatible with a variety of metals, including aluminum, steel, and stainless steel. 

Electroless nickel playing has its challenges. Common problems include the build up of contaminants in nickel baths, rising phosphorus content, and subsequent reductions in plating rates. Additionally, the wrong temperature or pH level can create coating quality issues like pitting, dullness, and roughness. Electroless nickel plating isn’t suitable for rough, uneven, or poorly machined surfaces, and parts will need to be cleaned of soaps, oils, and dirt before the plating process can begin.

Different types of electroless nickel plating coatings are categorized by the percentage of phosphorus in the alloy by weight. Different levels of phosphorus content also offer different levels of corrosion resistance and hardness:

• Low phosphorus nickel (2 – 4% phosphorus): Low phosphorus electroless nickel has an as-plated hardness between 58 and 62 Rc, and is highly resistant to wear. It has a high melting point and good corrosion resistance when exposed to alkaline conditions. Low phosphorus electroless nickel deposits are compressively stressed and are usually more expensive than medium and high phosphorus nickel.
• Medium phosphorus nickel (5 – 9% phosphorus): Medium phosphorus nickel plating offers a middle ground between low and high phosphorus nickel. It is resistant to corrosion in alkaline and acidic environments and has a fast deposition rate (18 to 25 µm per hour). The as-plated hardness of medium phosphorus nickel can be anywhere between 45 and 57 Rc, and the plating can be heat treated to reach 65 to 70 Rc.
• High phosphorus nickel (>10% phosphorus): Since high phosphorus deposits of electroless nickel plating are amorphous, parts won’t end up with phase boundaries or grain, increasing their corrosion resistance and making them ideal for use outdoors or in extreme environments. High phosphorus electroless nickel plating also offers ductility, high thickness, and stain resistance, and will make it easier to polish or solder your final product.

Zinc Plating

Zinc plating, or zinc chromate, is a popular chemical finish that protects steel parts from moisture and corrosion. Zinc-plated products have increased longevity, improved aesthetic appeal, and a more uniform appearance. Zinc plating can also alter a part’s color to silver-blue, yellow, black, or green. Another significant benefit of zinc plating is its potential to protect a part’s surface for years: even if the coating becomes scratched, the zinc will react to the atmosphere and quickly oxidize. Since zinc is chemically susceptible to acids and alkalis, however, zinc plating may not be sufficient for parts destined for wet or extremely humid environments.

There are a few different types of zinc plating. Electro-galvanization requires an electrical current to coat the part in a thin layer of zinc, whereas hot-dip galvanization requires parts to be submerged in a hot zinc bath. Electro-galvanization is the cheaper process, but hot-dip galvanization is better for parts that will be used in aggressive environments or that will experience a lot of wear.

Following the zinc plating process, parts can undergo a secondary procedure for increased protection and improved performance. The ASTM B633 standard, the most widely used standard for zinc plating, includes four types of zinc plating:

• Type I: Type I has no supplementary treatment.
• Type II: Type II involves a colored chromate treatment.
• Type III: Type III uses a colorless chromate treatment.
• Type IV: Type IV uses a phosphate conversion treatment.

Achieving Quality Finishes With SyBridge

Chemical finishing offers numerous ways to achieve the surface quality and performance levels that you need for your part, but not every finishing process will be suitable for every material and end-use. To determine which chemical finish is right for your part, you’ll need to have a strong understanding of critical factors, such as how much corrosion, friction, and wear resistance your final part needs, the environment in which it will be used, and its required conductive or insulative properties. 

Given the importance of those considerations, it’s worth finding a manufacturing partner to help you select a suitable finish, and ensure that it offers the best quality and cost efficiency possible. At SyBridge, our expert team of designers and engineers can offer insight not just into the chemical finishing process, but material selection, tooling, and suitable CNC technologies. If you want to know more about the finishing options available for your next CNC machining project, get in touch with us today. If you’re ready to get started, create your account, upload your designs to get an instant quote, and start making new parts and products in just a few simple steps.