Anodizing vs Electroplating: A Complete Engineering Comparison

An anodized and an electroplated component appear similar. It requires high expertise to differentiate between the two, especially by observation. The main difference between anodizing and electroplating is that anodizing adds a protective layer to a component to enhance strength, corrosion resistance, and friction resistance. The two techniques can also be used as decorative enhancements in the beauty and cosmetics industry. This guide examines the technical differences between anodizing and electroplating to help readers clearly understand the two methods.

How does Anodizing Work?

Anodizing is an electrochemical oxidation process typically applied to aluminum components. It is used to condition them to meet specific roles as required by the user.

During anodizing, two metals are submerged in a chemical solution called an electrolyte. The common electrolytes used in anodizing are sulfuric and chromic acid solutions. One metal is connected to the positive terminal of the power supply, making it an anode. The second metal is connected to the negative terminal, making it the cathode.

The power supply is switched on to provide an electric current. As the electric current passes through the bath, it dissociates water molecules, releasing oxygen ions. The oxygen ions then attach to the surface of the anodic metal, initiating a chemical reaction with it. The reaction leads to a metal oxide. Unlike painting or electroplating, the metal oxide layer integrates with the underlying substrate, so it never peels or chips off. In the case of aluminum, the aluminum oxide provides structural strength, making it applicable in highly loaded environments.

There are three types of anodizing, depending on the required wear resistance, corrosion resistance, and appearance. These types are summarized in the table below;

How does Electroplating Work?

In electroplating, an electric current dissolves metal ions in one of the metal rods and deposits them onto the target metal. The metal added is known as the deposition metal, and the underlying material or work piece is known as the substrate material.

The main components of an electroplating setup include the cathode (the object to be plated), the anode (a metal supplying ions for plating), and the electrolyte solution containing dissolved ions. DC power supply provides the electrical energy required for deposition. Electroplating also includes a process control system for controlling process parameters.

The commonly used metal in electroplating is nickel. Suppose steel has to be electroplated with nickel; the nickel is treated as the anode while the steel is the cathode. At the anode, nickel metal loses electrons to become an ion.

Ni → Ni²⁺ + 2e⁻

At the cathode, nickel ions around the steel gain electrons to form nickel metal.

Ni²⁺ + 2e⁻ →Ni

As a rule of thumb, the work piece is usually treated as the cathode. The deposited metal categorizes electroplating. For example, nickel plating deposits nickel on the work piece while zinc plating deposits zinc on the work piece.

Which Materials can be Anodized or Electroplated?

Material selection is one of the clearest differences between anodizing and electroplating. Anodizing is strongly associated with aluminum and its alloys, magnesium, and titanium. For conventional anodizing, engineers usually select aluminum. However, different aluminum alloys anodize differently, affecting coating formation, appearance, and uniformity. For instance, copper and zinc exhibit different behavior when combined with other alloying constituents such as silicon and magnesium.

Materials selection for electroplating, on the other hand, is broader. Some commonly used substrates include carbon steel, copper, stainless steel, aluminum, brass, zinc alloys, and nickel alloys. The common deposited metals are nickel, zinc, copper, chromium, silver, and gold.

How do Anodizing and Electroplating compare for Coating Thickness?

There is no universal coating thickness set for anodizing or electroplating processes. The right thickness depends on several factors, such as the substrate, dimensional tolerance, the part’s application, and the corrosion environment.

Anodizing thickness

For thin decorative finishes, the anodizing thickness is as low as a few micrometers. This thickness can reach 100 µm for hard anodizing. The Aluminum Anodizers Council classifies architectural aluminum into two classes. Class I provides an approximate minimum thickness of 18 µm, while Class II provides an approximate minimum thickness of 10 µm.

Electroplating Thickness

The coatings in electroplating vary by application, ranging from thin decorative deposits to thicker engineering coatings used for structural strength. For instance, decorative chromium is a few micrometers thick. At the same time, nickel is several to tens of micrometers thick. Hard chromium can be tens or hundreds of micrometers thick, depending on the specific application.

Which Process creates the Harder Surface?

The hardness achieved by anodizing and electroplating depends on the methods and materials used. Choosing aluminum material, for instance, leads to micro hardness values of up to 400 HV, depending on the alloy used. The hard anodizing process creates a ceramic-like aluminum oxide layer that enables the use of aluminum in corrosive environments.

Surface hardness is even higher for engineering hard chromium electroplating. The hardness level can range from 750 HV to 1050HV.

Surface hardness cannot be used to determine which process is superior to the other. There are other mechanisms to consider, such as corrosion mechanism, coating adhesion, and substrate hardness.

Which Provides Better Corrosion Resistance?

There is no better method than the other in achieving corrosion resistance. The aluminum oxide layer from anodizing makes aluminum resist environmental attack. However, after anodizing, the layer is sealed to reduce permeability because of its porous anodic structure. With proper hard anodizing and sealing, aluminum finds extensive application even in highly corrosive environments.

The effectiveness of electroplating in achieving corrosion resistance depends on the coating architecture and the nature of the deposited metal. Engineers widely use zinc in electroplating because of its sacrificial corrosion protection. ASTM B633 provides guidelines on electrodepositing zinc on steel and iron. It provides that, for effective corrosion protection, the part must undergo supplementary treatments such as passivation.

Neither anodizing nor electroplating can achieve more corrosion resistance than the other without better treatment. It is important to seal or perform post-process treatment to avoid cracks, pores, and discontinuities and to enhance corrosion resistance.

When should Engineers Choose Anodizing instead of Electroplating?

There are some areas where anodizing is better suited than electroplating. First, anodizing is highly preferred in areas where aluminum is used as structural components. In such applications, engineers usually prefer to retain the aluminum appearance. The method is even suitable when engineers seek to maintain black, clear, or other colors. For black color retention, a specialized dyeing-based black anodizing process is used. Second, anodizing is used where the coating should be permanently or strongly integrated with the substrate. This helps to prevent peeling of the coating, thus maintaining corrosion resistance. Third, engineers prefer anodizing when they don’t require a thick metallic repair coating.

Examples of the application areas include electronic housings, automotive trim, camera bodies, optical equipment, and heat sinks. Anodizing is categorized into decorative and engineering applications. The guidelines for decorative and protective anodizing of aluminum are provided in ISO 7599. On the other hand, ISO 10074 provides guidelines for hard anodizing in engineering applications.

When should Engineers choose Electroplating Instead?

Sometimes engineers design a system or component to have a surface that exhibits a specific metal property. This can effectively be achieved by electroplating the substrate with the desired metal. Some of the properties engineers seek in different coating metals include electrical conductivity, low contact resistance, high surface hardness, and wear resistance. Sometimes manufacturers intend to achieve solderability, low friction, metallic appearance, and chemical resistance.

Typical applications include zinc-plated steel fasteners for corrosion resistance, copper-plated components for electrical contact, and gold-plated metal chains for beauty.

الخاتمة

Anodizing and electroplating are different in several ways. The main difference is that anodizing enhances the naturally occurring oxidation process, whereas electroplating applies one metal to another metal surface. While anodizing provides enhanced corrosion resistance and decorative options, it typically cannot achieve the same level of wear resistance as certain surface treatments, such as hard chrome plating or physical vapor deposition (PVD) coatings. Additionally, anodizing doesn’t work on all metals. The final decision must be made based on all product materials, application conditions, performance requirements, and budget, taking the overall context into account.