Chemical Treatment of Machining Parts

All post-processing adds to part cost and production time, but the right surface preparation has the potential to turn your design vision into reality. Metal finishing for machining parts typically includes a variety of mechanical processes such as tumbling, brushing, and sandblasting, but metal parts can also be treated with chemical treatments such as passivation and galvanizing.

Chemical finishing has many useful effects, such as removing imperfections from a part, changing its conductivity, extending its life, and even improving its wear and corrosion resistance.

Chemical finishing has a variety of industrial applications: In the aerospace industry, for example, companies use chemical finishing to increase part durability, improve thermal stability, and slow oxidation. In the consumer goods industry, chemical finishing can be found in the production of everything from housings and cases to sporting equipment.

While there are many chemical treatments available, they are not necessarily interchangeable between materials. In fact, each chemical treatment is generally compatible with specific materials and has its own advantages and disadvantages.

In this guide, we’ll explore several common chemical finishing processes so you can decide which one is best for your CNC manufacturing project.

1.Choosing Your Machining Parts Chemical Finish

When choosing a chemical coating for your machining parts, you need to consider compatible materials and end-use applications. This means considering a range of contextual factors, including:

1. The environment in which your machining parts will be used
2. Whether it needs to be conductive or insulating
3. How much weight it needs to bear
4. How much wear it needs to withstand
5. Tolerance requirements
6. Color and transparency requirements
7. Surface finish standards
8. 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 can benefit your project.

2.Anodizing

Anodizing is a popular aluminum surface treatment that thickens the natural oxide layer on the surface of the part to form an anodic film that provides better protection and aesthetics.

For aluminum, to form the anodized protective layer, you soak the part in an acidic electrolyte and then use the cathode (negatively charged electrode) to cause the solution to release hydrogen gas. At the same time, the aluminum machining parts (positively charged anode) will release oxygen to form a protective oxide layer on its surface.

After the part is anodized, micropores appear on its surface, which must be sealed with a chemical solution to prevent corrosion and the accumulation of any contaminants.

Anodized machining parts are durable, resistant to corrosion and wear, and can reduce subsequent maintenance costs. The anodized layer is non-conductive and fully bonded to the aluminum substrate, so it will not chip or peel like electroplating and painting.

In fact, in addition to sealing, the porous anodized layer can be painted or dyed, and because the anodized coating is non-toxic and chemically stable, it is also more environmentally friendly. Anodizing is not just a surface treatment for aluminum, titanium and other non-ferrous metal parts can also use this process.

1) Three different types of anodizing:

1. Type I (chromic acid anodizing) produces the thinnest oxide layer, which means it does not change the size of the part. Type I anodized components will be grayer in color and do not absorb other colors well.
2. Type II (boric acid sulfuric acid anodizing) has better paint adhesion and is slightly thicker than Type I. With Type II anodizing, you can easily create blue, red, gold, black or green anodized parts.
3. Type III (hard sulfuric acid anodizing) is the most common form of anodizing. It has the clearest surface, which means it can be used in more colors. It is worth noting that Type III anodizing produces a slightly thicker surface than Type II anodizing.

The increased durability, wear and corrosion resistance of anodized machining parts, as well as the high degree of dimensional control that the process offers, make anodizing particularly popular in the aerospace and construction industries. In addition to these industries, anodized metal machining parts are widely used in applications as diverse as curtain walls, escalators, laptops, etc.

2) Disadvantages of Anodizing:

1. Anodizing metal will change the dimensions of the machining parts, so you will need to account for the oxide layer when determining dimensional tolerances, or use chemical or physical masks to ensure that specific areas of the part are untreated.
2. It will be difficult to achieve a true color match if your anodized parts are not processed in the same batch. Fading may also occur.
3. Anodizing a metal part will increase its electrical and thermal resistance. In some cases, this may be intentional, but in other cases, you may need to use masks to ensure that your part retains its full conductivity in certain sections.
4. Anodizing increases the surface hardness of a part.

3.Passivation

This popular metal finishing process protects stainless steel machined parts from corrosion, helping them maintain their cleanliness, performance, and appearance. Not only are passivated parts more resistant to rust and therefore better suited for outdoor use, they are also less prone to pitting, last longer, and are more aesthetically pleasing and functional.

As a result, the passivation process is used in a variety of industries, from the medical industry, where sterilization and longevity are key, to the aerospace industry, where companies seek high-quality steel and tight dimensional tolerances.

Passivation involves applying nitric or citric acid to the part. While nitric acid has traditionally been the typical choice for passivation, citric acid has recently become more popular because it can reduce cycle times and is safer and more environmentally friendly.

During the passivation process, the machined parts are submerged in an acid bath to remove any iron and rust from their surface without disturbing the chromium. Applying the acid to the stainless steel removes any free iron or iron compounds from its surface, leaving behind a layer composed of chromium and sometimes nickel. Upon exposure to air, these materials react with oxygen to form a protective oxide layer.

It is important to remember that passivation can extend the production time of machining parts. Before passivating a part, it must be cleaned to remove any grease, dirt or other contaminants, then rinsed and soaked (or sprayed). While soaking is the most common passivation method because it provides even coverage and can be completed quickly, acid sprays can also be used as an alternative.

4.Black Oxide Coating

The black oxide coating process is a surface treatment for ferrous metals such as steel, stainless steel and copper, where the part is immersed in an oxide bath to form a layer of magnetite (Fe3O4), which provides mild corrosion resistance.

1) Three types of black oxide coatings:

1. Thermal black oxide: The thermal black oxide coating process involves immersing the part in a hot bath of sodium hydroxide, nitrites and nitrates to turn its surface into magnetite. After soaking, the part needs to be immersed in an alkaline cleaner, water and caustic soda, and then coated with oil or wax to achieve the desired aesthetic.
2. Medium temperature black oxide: Medium temperature black oxide coating is very similar to thermal black oxide coating. The main difference is that the coated parts will turn black at a lower temperature (90 – 120°C). Since this temperature is below the boiling point of the sodium and nitrate solutions, there is no concern about corrosive fumes.
3. Cold Black Oxide: While hot and medium temperature black oxide coatings involve oxide conversion, cold black oxide relies on deposited copper selenium to transform the part. Cold black oxide is easier to apply, but will fall off more quickly and has less wear resistance.

Machining parts coated with a black oxide coating will have greater resistance to corrosion and rust, be less reflective, and last longer. An oil or wax coating will add water resistance and also prevent harmful substances from getting inside the metal, making your parts easier to clean.

Black oxide coatings also increase thickness, making them ideal for drills, screwdrivers, and other tools that need a sharp edge that won’t dull over time.

5.Chemical Film

Chemical Film, also known as chromate conversion coating, or by the brand name Alodine®, is a thin coating typically applied to aluminum (although it can be applied to other metals) to prevent corrosion and improve adhesion of adhesives and paints.

Chemical film coatings typically have proprietary formulas, but chromium is the primary ingredient in each coating. Chemical film coatings can be applied by spraying, dipping, or brushing, and may be yellow, tan, gold, or clear, depending on the product and formulation.

While other surface treatments reduce thermal and electrical conductivity, chemical film treatments allow aluminum to maintain its conductivity. Chemical film is also relatively inexpensive and, as mentioned earlier, provides a good base for painting and priming (saving even more time).

However, since chemical film is susceptible to scratches, abrasions, and other surface damage, it is not ideal for projects where aesthetics are a top priority.

6.Electropolishing

Electropolishing is an electrochemical finishing process that is typically used to remove thin layers of material on steel, stainless steel, and similar alloys.

During electropolishing, parts are immersed in a chemical bath and an electric current is applied to dissolve their surface layers. Various parameters affect the surface finish of machining parts, including the chemical composition of the electrolyte solution, the temperature, and the exposure time of the machining parts.

Electropolishing typically removes 0.0002 to 0.0003 inches of residue from the surface of an object, leaving a smooth, shiny, and clean material. Other benefits of electropolishing include improved corrosion resistance, longer component life, improved fatigue strength, lower coefficient of friction, reduced surface roughness, and the elimination of surface defects such as burrs and microcracks.

Electropolishing is suitable for steel, stainless steel, copper, titanium, aluminum, brass, bronze, beryllium, and more. It is worth noting that electropolishing is faster and cheaper than hand polishing, although it still takes time and cannot remove 100% of rough surface defects.

7.Electroplating

Electroplating is actually the reverse process of electropolishing. Instead of removing a layer of metal to get a finished surface, electroplating deposits an additional outer layer, which increases the thickness of the component. Electroplating is compatible with cadmium, chromium, copper, gold, nickel, silver, and tin, and produces smooth parts.

Due to its additional protection against corrosion, rust, shock, and heat, it will show less wear over time. Electroplating can increase the adhesion between the substrate and its additional outer coating, and depending on the type of metal used, it can make your part magnetic or conductive.

Compared to other CNC machining surface finishes, electroplating is not particularly environmentally friendly, as it produces hazardous waste that can seriously pollute the environment if not properly disposed of.

Electroplating is also relatively expensive, as it requires metal and chemicals (among other necessary materials and equipment), and can be time-consuming, especially when multiple layers are required on a machining part.

8.Chrome Plating

Chrome plating is an electroplating process that adds a thin layer of chrome to metal machining parts to increase their surface hardness or corrosion resistance. Adding a layer of chrome makes machining parts easier to clean and improves their aesthetics, and almost all metal machining parts can be chrome plated, including aluminum, stainless steel, and titanium.

The chrome plating process typically involves degreasing, manually cleaning, and pre-treating the machining parts before placing them in a chrome plating tank. The machining parts must then remain in the tank long enough for the chrome layer to reach the desired thickness. Because the process is power-intensive and involves multiple steps, chrome plating is a relatively expensive finishing process.

9.PTFE Coating

Polytetrafluoroethylene (PTFE) coatings, available in both powder and liquid forms, are widely used in industry. Some PTFE applications require only one coat, but others require a primer and topcoat to ensure maximum protection. The coating can be applied to a variety of metals, including steel, aluminum, and magnesium.

PTFE coated machining parts feature a nonstick surface, a low coefficient of friction, and high wear resistance. Because PTFE coatings have low porosity and surface energy, coated parts are resistant to water, oil, and chemicals. PTFE can also withstand temperatures up to 500°F, is easy to clean, and provides excellent electrical insulation and chemical resistance.

Due to its chemical resistance and nonstick properties, PTFE is commonly used to coat fuel lines and insulate circuit boards in computers, microwave ovens, smartphones, and air conditioners. It is also commonly used to coat medical tools and equipment, as well as cookware. Although the PTFE coating process is popular across industries, it is relatively costly and is not as long-lasting as other chemical treatments.

10.Chemical Nickel Plating

Chemical nickel plating involves adding a protective layer of nickel alloy to metal machining parts. Unlike electroplating processes that require an electric current, chemical nickel plating requires the use of a nickel bath and a chemical reducing agent such as sodium hypophosphite to deposit a layer of nickel alloy (usually nickel-phosphorus) on machining parts. The nickel alloy is deposited evenly, even on complex machining parts with holes and slots.

Nickel-plated machining parts have greater resistance to corrosion from oxygen, carbon dioxide, salt water, and hydrogen sulfide. Nickel-plated parts also have good hardness and wear resistance, and their hardness can be even higher after additional heat treatment.

Chemical nickel plating is compatible with a wide range of metals, including aluminum, steel, and stainless steel.

Chemical nickel plating has its challenges. Common problems include the accumulation of contaminants in the nickel bath, rising phosphorus levels, and subsequent reductions in plating speed.

In addition, the wrong temperature or pH can cause coating quality issues such as pitting, dullness, and roughness. Chemical nickel plating is not suitable for rough, uneven, or poorly machined surfaces, and soap, oil, and dirt need to be removed from machining parts before starting the electroplating process.

1) Different types of electroless nickel coatings are classified by the weight percentage of phosphorus in the alloy

Different levels of phosphorus content also provide different levels of corrosion resistance and hardness:

1. Low phosphorus nickel (2 – 4% phosphorus): Low phosphorus electroless nickel deposits have a hardness between 58 and 62 Rc and excellent wear resistance. It has a high melting point and good corrosion resistance in alkaline conditions. Low phosphorus electroless nickel deposits have compressive stress and are generally more expensive than medium and high phosphorus nickels.
2. Medium phosphorus nickel (5 – 9% phosphorus): Medium phosphorus nickel deposits are between low phosphorus nickel and high phosphorus nickel. It is corrosion resistant in alkaline and acidic environments and has a fast deposition rate (18 to 25 µm per hour). Medium phosphorus nickel deposits have a hardness between 45 and 57 Rc and the deposits can be heat treated to 65 to 70 Rc.
3. High-phosphorus nickel (Phosphorus content > 10%): Because high-phosphorus deposits of electroless nickel are amorphous, machining parts do not show phase boundaries or grains, which improves their corrosion resistance and makes them ideal for use outdoors or in extreme environments. High-phosphorus electroless nickel also provides ductility, high thickness, and stain resistance, and makes the final product easier to polish or weld.

11.Galvanizing

Galvanizing, or zinc chromate, is a common chemical treatment that protects steel machining parts from moisture and corrosion. Galvanized products last longer, are more beautiful, and have a more uniform appearance.

Galvanizing can also change the color of machining parts to silver-blue, yellow, black, or green. Another important advantage of galvanizing is that it protects the surface of the part for many years: even if the coating is scratched, the zinc reacts with the atmosphere and oxidizes quickly.

However, because zinc is chemically susceptible to acids and bases, galvanizing may not be sufficient for machining parts in wet or extremely humid environments.

There are several different types of galvanizing. Electrogalvanizing requires an electric current to coat machining parts with a thin layer of zinc, while hot-dip galvanizing requires immersing machining parts in a hot zinc bath. Electrogalvanizing is a less expensive process, but hot-dip galvanizing is better suited for machining parts that will be used in harsh environments or subject to heavy wear.

After the galvanizing process, machining parts can undergo a secondary treatment to enhance protection and improve performance. The ASTM B633 standard is the most widely used standard for galvanizing and includes four types of galvanizing:

1. Type I: Type I has no supplemental treatment.
2. Type II: Type II involves a colored chromate treatment.
3. Type III: Type III uses a colorless chromate treatment.
4. Type IV: Type IV uses a phosphate conversion treatment.

12.Achieving a High-Quality Finish

Chemical finishing offers a variety of methods to achieve the desired surface quality and performance levels for machining parts, but not every finishing process is suitable for every material and end use.

To determine which chemical finish is right for your machining parts, you need to have a solid understanding of key factors, such as how much corrosion, friction and wear resistance the final machining part requires, the environment in which the machining parts will be used, and the conductive or insulating properties required.

Given the importance of these considerations, it’s worth finding a manufacturing partner to help you select the right surface finish and ensure it provides the best possible quality and cost-effectiveness.