Zinc Plating - Everything You Shall Know

2. Zinc Plating By Process

This part will comprehensively introduce the main methods and processes of zinc plating, including Hot-Dip Galvanizing, Electro Zinc Plating, Mechanical Zinc Plating, and Electroless Zinc Plating.

2.1. Hot-Dip Galvanizing

Hot-dip galvanizing involves immersing steel parts into molten zinc(melt approximately at 419°C), where an interfacial reaction forms a composite coating on the surface consisting of a zinc-iron alloy layer and a pure zinc layer.

2.1.1. Process Steps

Step 1: Pre-process (Key Step)

To remove oil, rust, and scale from the part’s surface for full contact between the molten zinc and the steel, which can prevent coating detachment.

* Degreasing: Clean oil stains with alkaline solutions such as sodium hydroxide solutions.

* Pickling: Remove rust and scale by hydrochloric or sulfuric acid solutions.

* Fluxing: Immerse the part in a flux, typically zinc chloride or ammonium chloride solution, to form a protective film, which can prevent re-oxidation before immersion in the zinc bath and promote zinc wetting.

Step 2: Hot-Dip Galvanizing

Immerse the pre-processed part in the molten zinc at 440-460°C for several minutes (adjusted based on the part’s thickness). Then a diffusion reaction would occur between the iron on the steel surface and the zinc, forming a Zn-Fe alloy layer topped with a pure zinc layer.

Step 3: Post-process

Get the part out the zinc bath, and then cool it by water or air cooling.  Passivate the part if required, to enhance corrosion resistance and finally trim it to remove excess zinc dross.

2.1.2. Key Characteristics

* Coating Thickness: Relatively thick(typically 50-150 μm), with an extremely strong bond between the alloy layer and the part (peel strength >10 MPa), making it resistant to detachment.

* Corrosion Resistance: Excellent, with a service life of 20-50 years in outdoor atmospheric environments such as highway guardrails and transmission towers, and even longer in freshwater or mildly corrosive environments.

* Cost: Lower cost per unit area compared to electro zinc plating, suitable for large-scale and large-sized components.

* Limitations: Lower surface smoothness, appearing as a silver-gray matte finish; inability to achieve refined coatings for complex precise parts; potential deformation issues due to high-temperature treatment (temperature control is critical for thin parts).

2.2. Electro Zinc Plating

Electro zinc plating utilizes the principle of electrolysis, where steel parts act as the cathode and zinc plates serve as the anode in an electrolyte solution containing zinc ions like zinc chloride or zinc sulfate solutions.

When an electric current is applied, zinc ions are reduced to metallic zinc at the cathode (the part’s surface), forming a uniform pure zinc plating. As the entire process occurs at room temperature, it is also known as cold zinc plating.

2.2.1. Process Steps

Step 1: Pre-treatment

More meticulous than hot-dip galvanizing, require to completely remove oil and scale from the part, and activate it with weak acid, to ensure the surface clean and prevent pinholes or pits in the plating.

Step 2: Electrodeposition

Suspend the part in an electrolytic tank, and control the current density(typically 1-5 A/dm²), temperature(15-35°C), and time, to make zinc ions uniformly deposit on the surface and then form a pure zinc layer.

Step 3: Post-process

The key step that directly impacts corrosion resistance and appearance.

* Passivation: Immerse the part in chromate or chromium-free passivation solutions (commonly for environmental compliance) to form a colored, blue, or black passivation film on the zinc layer, which would improve corrosion resistance by 3-5 times.

* Sealing (optional): Applies a transparent sealer like organic coating to further enhance corrosion resistance and gloss.

* Drying: Remove surface moisture to prevent rusting again.

2.2.2. Key Characteristics

* Coating Thickness: Relatively thin(typically 5-20 μm), with precise control such as localized thickening or thinning and high surface smoothness. The surface can appear bright silver or colored with excellent aesthetics.

* Corrosion Resistance: The base pure zinc layer has relatively weak corrosion resistance with outdoor service life of about 1-3 years, but it would be significantly improved after passivation.

* Applicability: Suitable for precision parts(such as bolts, nuts, and electronic connectors), and complex structural components, enabling uniform plating without deformation due to room-temperature processing.

* Limitations: The coating adhesion to the substrate is weaker than that of hot-dip galvanizing (peel strength approximately 3-5 MPa), and the cost per unit area is higher(not suitable for oversized parts).

2.3. Mechanical Zinc Plating

Mechanical zinc plating, also known as cold galvanizing(noting to distinguish it from electro zinc plating), is a physical deposition process used to press and cover zinc powder onto the part surface to form zinc plating through physical mechanical actions like impact and friction, without the need for electrolysis or high temperatures.

2.3.1. Process Steps

Step 1: Pre-process

Similar to hot-dip galvanizing, it requires degreasing and pickling to remove surface impurities and activating the part surface.

Step 2: Mechanical Deposition

* Sequentially add the following materials to the barrel in strictly controlled proportions(taking 100 kg of steel parts as an example):

Part: 100 kg (must ensure no sharp edges to avoid scratching the coating during impact).

Impact Media: Glass beads (diameter 1-3 mm, hardness 6-7 HRC), used at 1.5-2 times the mass of the parts, functioning to transmit impact force and prevent direct collision deformation.

Zinc Power: Atomized zinc powder (purity ≥98.5%, particle size 5-20 μm, high specific surface area for easy deposition). The amount is calculated based on the target coating thickness (e.g., for a 20 μm coating, approximately 1.2 kg of zinc powder is required; zinc density is 7.14 g/cm³).

Additives: Binder, such as polyvinyl alcohol or carboxymethyl cellulose, to temporarily adsorb zinc powder onto the part surface; activator, such as ammonium chloride or ammonium sulfate, to adjust the system pH to 5-7 and promote zinc powder dispersion; water, with solid-liquid ratio of 1:(0.2-0.3) typically, to control system humidity and avoid over-drying (caking) or over-wetting (adhesion).

* After starting the barrel, add zinc powder in 2-3 batches to avoid uneven coating from one-time addition. The entire process lasts 30-60 minutes and can be divided into three phases:

Initial Deposition Phase (5-10 min): The barrel rotates at low speed (10-15 rpm) and the glass beads gently impact the parts. Then the zinc powder preliminarily adsorbs onto the surface with the help of the binder, forming a 5-10 μm base layer.

Rapid Deposition Phase (20-30 min): Increase the barrel speed to 20-25 rpm to enhance impact force. Zinc powder is mechanically embedded into the base layer, rapidly increasing the coating thickness to 80% of the target value. And note to stir continuously to prevent localized accumulation especially in threaded holes or grooves.

Densification Phase (5-10 min): Reduce the speed to 10-15 rpm, and add a small amount of densifier such as ultra-fine zinc or aluminum powder. Then tightly bind coating particles with gentle impact to reduce porosity(final porosity ≤5%).

* Separation and Cleaning

After galvanizing, the barrel is stopped. Then separate the parts and glass beads by screening method(The glass beads are reusable but require regular cleaning to remove residual zinc powder). Rins the parts with clean water to remove residual additives and undeposited zinc powder and prevent white spots during subsequent drying.

Step 3: Post-process

Passivate the zinc plating anddry the parts to enhance the corrosion resistance.

2.3.2. Key Characteristics

* Low-Temperature Processing with No Deformation: The entire process occurs at room temperature, eliminating the risk of high-temperature oxidation or deformation.

* Dense Zinc Layer with No Hydrogen Embrittlement: Avoid the hydrogen embrittlement issue associated with electro zinc plating(where hydrogen atoms infiltrate the steel substrate during electrolysis, causing brittleness).

Environmental Friendliness: No electroplating waste liquid generated, only a small amount of cleaning wastewater, resulting in lower treatment costs compared to electro zinc plating.

* Limitations: The adhesion of the zinc layer is slightly weaker than that of hot-dip galvanizing and electro zinc plating, with moderate corrosion resistance(outdoor service life of 5-10 years). And it is challenging to achieve uniform coating on complex-shaped parts.

2.4. Electroless Zinc Plating

Electroless zinc plating is a type of chemical conversion process which utilizes autocatalytic reduction to deposit a zinc layer on the part surface without an external power source.

2.4.1. Process Steps

Step 1: Pre-process

During the pre-process all oil, rust and other surface pollutant should be removed and the part should be activated to create deposition condition.

* Degrease to remove oil from the surface.

* Pickle to dissolve surface oxide layers. And the pickling solutions vary by substrate.

For Steel substrates: Use 10%-20% hydrochloric acid (HCl) or 8%-15% sulfuric acid (H₂SO₄) solutions at 20-50°C for 5-15 minutes and corrosion inhibitors like urotropine must be added to prevent over-etching.

Aluminum alloys: Employ dilute nitric acid (HNO₃) or phosphoric acid (H₃PO₄) solutions to avoid strong acid-induced substrate corrosion.

Copper alloys: Utilize a mixture of dilute sulfuric acid and hydrogen peroxide for gentle oxide removal.

* Activate the surface to provide catalytic center for autocatalytic reaction. And activation methods differ by substrate.

For Steel substrates: For direct zinc deposition, use weak acid activation like 0.5%-1% dilute HCl for 1-2 minutes, or pre-plate a thin nickel layer (electroless nickel) before zinc plating to improve adhesion.

For Non-conductive substrates: Firstly, sensitize the part by immersing it in stannous chloride solution to adsorb Sn²⁺ ions. And then activate the part by transferring it to palladium chloride solution, where Sn²⁺ reduces Pd²⁺ to metallic Pd(palladium), forming catalytic nuclei to initiate zinc deposition.

Step 2: Electroless Zinc Deposition

Formulate electroless zinc plating bath and immerse the pre-processed part in the bath to deposit zinc plating.

Below is the bath formulation table.

Note for bath preparation:

• Use deionized water for preparation to prevent Ca²⁺and Mg²⁺ in water from reacting with complexing agents, which could compromise bath stability.
• First dissolve the complexing agents, then add the main saltto prevent Zn²⁺ hydrolysis, and finally incorporate reducing agents and additives.
• After preparation, adjust the pH to the target range by pH regulators, and heat to the reaction temperature(typically 60-90°C). Step

Step 3: Post-process

Passivate that part.

2.4.2. Key Characteristics

* Excellent Uniformity: The zinc plating can achieve uniform coverage even on complex geometries without “edge effects.”

* No Hydrogen Embrittlement or Deformation: The process requires no electrical current, eliminating hydrogen atom infiltration. The low-temperature operation can prevent deformation, making it suitable for precision and high-strength components.

* Limitations: High cost for plating bath and the bath’s lifespan is short; the zinc layer is thin and offers inferior corrosion resistance compared to hot-dip galvanizing; maintaining stability in batch production is difficult due to stringent operational requirements.

Hot-dip galvanizing offers the best long-term corrosion resistance but with a rougher appearance, making it ideal for outdoor structures.

Electro zinc plating provides a smooth and uniform finish, preferred for CNC machined and precision parts, though with lower corrosion resistance.

These two zinc plating methods are most used.