The Zinc Plating Process

The greatest enemy of all types of metal is rusting and corrosion. The corrosive effect of rust is very destructive. Reports from various institutes show that the cost of losses in the United States alone on rusting is about $276 billion dollars each year and a global figure of about $1 trillion dollars. The industries affected by this devastating effect of rusting includes Production and Manufacturing, Transportation, Government, Utilities and Infrastructure. A process known as zinc plating is frequently used to protect metals such as iron and steel against the relentless forces of corrosion. Zinc plating involves the electrodeposition of a thin coating of zinc metal onto the surface of another metal object, known as a substrate. The zinc coating creates a physical barrier that prevents rust from reaching the underlying metal surface. Zinc is chosen because of its innate ability to fight corrosion. In fact, zinc is often referred to as the corrosion-prevention workhorse.

While the use of zinc compounds began more than 2,500 yeras ago, the production of metallic zinc first occurred in India around the 15th century. Today, nearly one-third of all zinc metal is used for galvanization, which is the coating of a metal surface to protect against rusts. Zinc plating is the most commonly used method of galvanization.

Plating With Zinc

Zinc is a relatively recent process, and it did not gain widespread use until approximately 1980. Prior to that time, cadmium was the preferred metal for electroplating in applications where maximum protection against corrosion was required. However, cadmium is a highly toxic substance that can have a cumulative poisoning effect over an extended period of time.

While Zinc cannot quite match cadmium in terms of its anti-corrosion capabilities, it's viewed as a more bio-friendly alternative. These days, the use of cadmium plating is generally restricted to applications where plating with zinc won't provide sufficient corrosion protection or additional levels of protection are required. However, the use of cadmium in plating operations must comply with current environmental regulations.

Step-by-Step Zinc Plating Process:

Zinc plating is a relatively complex process that requires a high level of experience and expertise. Specialized equipment and machinery are also required, including cleaning stations, pickle stations, plating stations, rectifiers and anodes. Here's an overview of how a typical zinc plating process works:
  • Choosing the appropriate zinc plating procedure - The chosen method could involve rack plating, where parts are affixed to metal racks which are placed on a machine and transferred through the process. Rack parts remain stationary on the Rack ensuring no damage occurs to the part. Barrel plating is normally used for springs, fasteners and smaller parts. The parts are placed inside a barrel and rotated, which provides a more uniform finish.
  • Preparing and cleaning the surface - It's essential to thoroughly clean the surface of the substrate prior to plating. Any debris or contaminants remaining on the surface will prevent proper adhesion of the zinc coating. An alkaline detergent solution is typically used for surface cleaning and degreasing, which is followed by the application of an acid treatment to remove surface rust and mill scale. This latter procedure is referred to as pickling.
  • Preparing the plating solution - Zinc plating requires the immersion of the substrate into a specially formulated electrolyte solution, which is referred to as the plating bath. The bath consists of the zinc metal ionic solution and various chemicals that facilitate plating processes. They also help produce the desired chemical and physical properties of the finished product. Specific types of zinc electrolyte solutions include acid zinc and alkaline zinc.
  • Acid Zinc - This is a widely used plating technology known for its high efficiency, fast deposition and superior covering power.
  • Alkaline Zinc - This offers less plating efficiency than acid zinc and a slower electrodeposition rate, but it provides better thickness distribution and ductility.
  • Introduce the electrical current - Electroplating is also known as electrodeposition because an electrical current is used to deposit metal ions onto the surface of the substrate. In the case of zinc plating, the substrate serves as the cathode. A DC current originating at the anode is introduced into the bath and flows to the substrate. The zinc ions are then deposited onto the surface. The current flows from the cathode back to the anode to complete the circuit.
  • Post-treatment procedure -Upon completion of the electrodeposition process, the parts are ready for post-treatment. This normally involves rinsing the parts in water to remove any remaining contaminants and plating bath remnants. In cases of heavy contamination, the parts may need to be rinsed several times. The final step is to thoroughly dry the zinc-plated parts. In situations where additional corrosion protection is required, the application of passivates and sealers can be included in the post-treatment process.

Factors Impacting Zinc Plating Results:

There are a variety of factors that can influence the outcome of a zinc plating project, most of which can be effectively managed and controlled by and experienced metal finishing provider. Some factors include:
  • Temperature -The temperature of the plating bath will also have a direct impact on the zinc plating outcome. Higher bath temperatures tend to reduce hydrogen diffusion on the cathode and increase the consumption of brightners and other additives. There's also a close relationship between temperature and current density. When both are increased, the result will be a brighter zinc deposit. When the temperature increases but current density remains unchanged, the formation of larger metallic crystals will occur.
  • Current Density -The density of the DC current flowing from the anode to the cathode can have a significant impact on the thickness of the zinc coating. The higher the current density, the greater the thickness of the coating.
  • Concentration of Zinc Deposit - The level of zinc concentration in the plating solution will affect the brightness and texture of the plated product. Higher concentrations will produce a rougher surface, while lower concentrations will result in a brighter finish with fine crystals. Other controllable factors that affect the result when plating with zinc include:
  • Concentration of hydrogen ions
  • Substrate surface condition
  • Position of the anode and cathode
  • Additives such as surfactants and brightners
  • Duration of plating time
  • Concentration of pollutants and contaminants in the plating bath
  • Efficiency of the rinsing operation
  • Degree of filtration of the zinc plating bath

How Does Zinc Coating Prevent Corrosion?

One of the most important benefits of zinc plating is that it will significantly increase the corrosion resistance of the underlying substrate. But how does this actually occur? In addition to forming a physical barrier, the zinc serves as a sacrificial coating. This means the zinc coating will corrode instead of the metal substrate that it protects. Additionally, zinc reacts with the oxygen in the atmosphere to form zinc oxide. The zinc oxide subsequently reacts with water to form zinc hydroxide, which bonds with carbon dioxide to produce a thin layer of zinc carbonate. The zinc carbonate adheres to the zinc on the coated metal to provide even more corrosion protection.

When To Avoid Zinc Plating?

While there are plenty of benefits to zinc plating, the process is not ideal for every situation. Specifically, zinc plating should be avoided with critical steel parts that are exposed to temperatures of 500° F or higher. It should also not be used in marine or seawater applications or in tropical environments. These environments can contribute to the formation of bulky corrosion. Zinc plating should not be used on products or equipment kept in confined areas that are subjected to heavy moisture condensation. It is also not recommended to apply zinc plating on equipment with moving parts that come in contact with one another.

Zinc Plating Longevity:

There are a variety of factors that contribute to how long a zinc coating will provide reliable protection against corrosion. The quality of the coating is of vital importance, which is why you should carefully evaluate the company you choose to provide your zinc plating services. Thicker coatings and the use of the right post-treatment products can also increase the protective capabilities. The surrounding environment also plays a critical role in the formation of corrosion.

Zinc Plating Applications:

Plating with zinc has many industrial applications. Zinc can provide a corrosion-resistant coating on smaller metal parts such as screws, fasteners, nuts and bolts. In general, most hardware parts are coated with zinc. Zinc plating has also gained widespread use in the automotive industry as a means of protecting parts such as power steering components, brake calipers and brake pipes.

Additionally, zinc plating is used in the production of tanks, armored personnel carriers and other heavy military vehicles. Zinc plating can also serve as a protective undercoating prior to painting, as it can promote greater paint adhesion.

Measuring The Effectiveness Of Zinc Electroplating:

A good way to assess the effectiveness of a zinc coating, or a substrates ability to resist corrosion in general is through the application of a procedure known as salt spray testing. Frequently used by the automotive industry, salt spray testing consists of spraying the parts with a 5% salt solution dissolved in water in a closed cabinet at a constant temperature determined by many variables. The tester records how long it takes for the appearance of "white" rust, a powdery substance that often forms on the zinc-coated steel, as well as the more destructive "red" rust. This indicates whether or not the zinc plating process needs to be adjusted to comply with the desired manufacturing tolerances.
Matthews Plating is your source for cost-effective zinc plating solutions that can be customized to the needs of your industrial or manufacturing operation. We offer zinc plating that can provide a protective coating for metal stampings, clamps, nuts, bolts, screws, metal brackets and other essential parts. You'll also have peace of mind knowing your zinc plating services are provided by a company with more than 50 years of metal finishing innovation. Contact us today to learn more about the many benefits of zinc plating, or send us an RFQ.