What is Metal Plating and How is It Used?

Last Updated on January 2021

Introduction

Metal Plaiting is a manufacturing process in which a thin layer of metal is used to coat a substrate. It’s beneficial particularly for products made of metal and other materials. Electroplating is the most common plaiting process, which requires an electric current. Alternatively, you can follow an electroless process – which is an autocatalytic chemical process.

Regardless of the plaiting process followed, you’ll have the following benefits:

  • Decorative appeal
  • Enhanced hardness
  • Increased solderability
  • Improved corrosion resistance
  • Enhanced paint adhesion
  • Increased magnetism
  • Material deposition
  • Altered conductivity
  • Reduced friction

Often, the plaiting process involves adding an outer coating of copper, chromium, nickel, or other metals to enhance the appearance or prevent corrosion on the parent metal.

Metal Plating History

Metal plating was discovered in 1805 by an Italian chemist known as Luigi Brugnatelli when he successfully electroplated silver medals with gold. His inventions were kept a secret by FAS (the French Academy of Sciences).

In 1940, electroplating was rediscovered by English and Russian scientists working independently. As such, the first patent for metal plating with the electric current was awarded in the same year. With that, factories in England began to mass-produce solver plated objects, such as teapots, utensils, and brushes.

Types of Metal Plating Processes

Metal plating involves one of the following processes:

Electroplating (electrolytic plating)

Electroplating is the use of electric current to drive a chemical reaction – which reduces ions on one end and adds them on the other.

It involves the immersion of the parent and plating metals in an acidic solution, through which a direct electric current is passed. Here, the parent metal serves as the cathode (or the negative electrode) in the electrolysis cell, while the plating metals serves as the anode. The electroplating metal is also contained in the solution, in an oxidized form (i.e., as an aquated cation or complex ion).

During the process, the coating metal is deposited on the cathode. At the same time, the anode (positive electrode) is dissolved to replace the removed ions/cations in the electrolytic solution. Electrolysis is governed by Faraday’s law of electrolysis and provides better control over the plating process.

The process can take minutes for thin plating or hours for thick plating.

Autocatalytic (electroless plating)

In electroless plating, a chemical reaction induces metal ion reduction. The autocatalytic process uses non-conductive substrates; and as such, the process does not require an electric current.

Here, it’s difficult to control the plating process. The plating bath has also a limited lifespan.

Autocatalytic plating is often called a conversion process, as it adds the thickness of the parent metal but does not create a direct relation between the plating and substrate metal. In fact, the process consumes part of the parent metal.

Examples include:

Immersion plating (or displacement reaction)

The process makes use of metals of varying electromotive forces. Here, a metal with a lower electromotive force is deposited in an ionized solution; and as such, displace the plating metal (which has more electromotive force).

Plating Methods

The following methods are used in metal plating:

Rack plating

In rack plating, the parent metal is hung on a rack or frame. The method is the most common thanks to its versatility and total control over rinsing and drag-out rates.

Barrel plating

As the name suggests, barrel plating takes place in a closed barrel. It’s ideal when electroplating small metal parts or when the level of processing standards/requirements is relatively low.

The process has high drag-out rates as well as high rinse water use.

Manual plating

Here, every step in the plating process is performed by a skilled technician. Manual plating is ideal for small batch plating.

Automated plating

Automated plating can be semi-automated or fully-automated. In automated plating, technicians only need to rack and un-rack the metals. In the semi-automated processes; however, the technician is also in charge of controlling hoists and rails.

Automated plaiting is used for large metals and lower production rates.

Key Metal Plating Chemicals

The key chemicals in metal plating include:

Acids and Bases

  • Hydraulic acid
  • Caustic

Coatings

  • Chromium
  • Cyanide
  • Cadmium
  • Chromium
  • Gold
  • Silver
  • Zinc
  • Brass & bronze

Solvents

  • Trichloroethylene
  • Tetrachloroethylene
  • Benzene
  • TZE

Metal Plating Process Components

Metal plating involves:

Cleaning and metal surface preparation

The metal surface must be clean (free from dirt, grime, and other contaminants) for proper bonding to occur. The cleaning process involves oxide removal, electropolishing, or alkaline cleaning.

Typically, the metal parts to be cleaned are suspended in a vat of boiling solvents. When the hot vapor reaches the cold/suspended metal, it condenses, and drip down back to the vat alongside any contaminants. As such, it leaves the suspended metal clean.

To etch the surface of the metal and enhance the adherence of the coating metal, the parent metal is sandblasted with aluminum oxide powder. Ideally, aluminum oxide roughens up the surface of the substrate, as such, the plating metal adheres better. This technique is most commonly used in the automative metal casting area.

Rinsing and drying the metal (if needed)

Once cleaned, the metal can be rinsed with water, hot water, or chemicals (if there’s a need to boost the rust resistance of the substrate).

Metal plating

As discussed earlier, metal plating can follow different processes that are electroplating, electroless plating, or immersion plating.

Case example: electroplating

To electroplate a substrate, first, hang on a copper frame. Ensure that the copper wire suspended the parent metal from the top and continues to the bottom for a complete electrical circuit.

Fill the electroplating tank/cell with a solution that’s a good conductor of electricity. Line the sides of the tank/cell with balls or bags of the coating metal.

Next, connect the support frame holding the substrate to the cathode (the negative electrode of the electrical source). Connect the coating metal to the anode (the positive terminal).

Next, a direct electric current (up to 6 volts) runs to dissolve the coating metal, which then travels through the electrolytic solution to the substrate.

Finishing and protection treatments

After plating, you can treat the resulting metal through anodizing, phosphating, or chromate conversion.

Mechanical Properties

You can plate any type of metal. However, the plating metal is determined by the desired results. For instance, cadmium or zinc plating prevents rusting of the parent metal, whereas chrome and nickel plating protects the parent material against wear. As such, it’s important to understand the engineering and mechanical properties of the coating metal as much as possible.

Nickel, zinc, and chrome are the common commercial/industrial plating metals.

Applications of Plated Metals

Plated metals have a variety  of uses including:

  • Surface protection or sacrificial coating or anodic coating that serves to protect the base metal. It’s primarily used over steel and iron.
  • Decorative coatings that serve to enhance the appearance of the base metal. Decorative coatings also protect to some extent.
  • Engineering coatings serve to impact specific properties to the surface of the base metal. For instance, the plating can be done to increase the reflectivity, conductivity, or solderability of a given metal.
  • Alloy metal plating and unusual metal plating are carried out under special conditions for specialized applications.
  • Minor metal plating for a few numbers of metals with limited applications.

Environmental Corrosion Testing

Once the metal is plated, it should be tested (usually in the environment) to gauge whether the coating is resistant to corrosion. Unfortunately, simulating the time taken for corrosion to take place is a tad challenging. Nonetheless, several tests can simulate the passage of time. Such tests include:

Fog test (or acetic acid salt spray test)

Here, a mist containing acetic acid is used to accelerate the corrosive action.

Cass (or copper-accelerated acetic acid salt spray test)

Here, a mist containing a mixture of acetic acid and copper salt is used to accelerate the corrosive action.

Corrodkote test

The plated metal is coated with kaolin, a compound that contains ferric chloride, aluminum chloride, and copper nitrate. The coated metal is then dried and put in a humidity chamber. Then, any corrosive reaction is noted.

Electrochemical corrosion test (ECT)

The plated metal is placed in an electrolyte. Then, conditions that catalyze corrosive reactions are created, and observations are made.

Sulfur dioxide test

The plated metal is placed in a chamber containing sulfur dioxide, after which observations are made of whether or not corrosion takes place.

Humidity tests

Humidity tests involve placing the plated metal in a humidity chamber or moisture/dump conditions. However, these tests are often unreliable.

In Conclusion

Metal plating is a manufacturing process in which a thin layer of metal is used to coat another. It’s carried out for various purposes, including protecting the parent metal, boosting its appearance, or altering its engineering properties.

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