What is Chrome Plating and How Does it Work?

What is Chrome Plating?

Chrome plating is an electroplating process where a layer of chromium is deposited onto a metallic substrate. Chromium is a silvery-white metal with a subtle blue tint. When exposed to air, metallic chromium readily passivates, quickly forming an extremely thin, protective oxide film on its surface.

From an electrochemical standpoint, a chrome coating acts as a cathodic plating when applied to steel, aluminum, or aluminum alloys, but it is an anodic plating when applied to copper and copper alloys. Notably, a chrome finish maintains its original luster for extended periods in the atmosphere, offering excellent reflectivity and decorative appeal. Beyond aesthetics, it delivers high hardness, strong thermal stability, and superior wear resistance, making it an indispensable industrial coating.

Property	Typical Value
Density, g/cm3	6.9 ~ 7.1
Hardness, HV	400 ~ 1200
Melting Point, °C	1890
Specific Heat, J/(g · °C）	0.502
Linear Expansion Coefficient, 10-6/°C	7.4
Resistivity, μΩ · cm	14 ~ 67
Thermal Conductivity, W/m·K	69.8
Reflectance, %	70 ~ 72

Properties of Chrome Plating

A single layer of chrome plating inherently contains micro-cracks and is porous, which means it’s generally unsuitable for use as a primary corrosion protection layer on its own. To eliminate this porosity and enhance the protection of the base metal, plating systems typically employ multi-layer deposits, such as copper-nickel-chrome, multi-layer nickel-chrome, or double-layer chrome.

Chrome plating offers high hardness, which can range from 400 to 1200 HV depending on the bath chemistry and process conditions. Furthermore, it exhibits a remarkably low coefficient of friction, its dry friction coefficient is among the lowest of all metallic coatings. The combination of high hardness and low friction gives the chrome layer its exceptional wear resistance.

The coating boasts excellent chemical stability, remaining inert to bases, sulfides, nitric acid, and most organic acids. However, it can be dissolved by hydrohalic acids (like hydrochloric acid) and hot sulfuric acid.

Chrome plating also has good thermal stability. When heated below 500 °C, there are no significant changes to its luster or hardness. Oxidation and discoloration begin above 500 °C, and the hardness starts to decline when temperatures exceed 700 °C.

In the visible light spectrum, chromium’s reflectance is about 65%, placing it between silver and nickel. Since the chrome coating does not tarnish, it retains its high reflective capacity over long periods of use.

Types of Chrome Plating

There are numerous variations of the chrome plating process, classified here based on their primary application:

1. Decorative Chrome Plating

This is commonly referred to as decorative chrome. The coating is thin, bright, and aesthetically pleasing, typically serving as the outermost layer of a multi-layer system. For proper corrosion protection on a steel substrate, a sufficiently thick intermediate layer such as copper-nickel must be applied first. The final chrome layer is very thin, usually 0.25 to 0.5μm. A polished surface finished with decorative chrome achieves a silver-blue luster that resists tarnishing in the atmosphere. It is widely used for protecting and decorating parts in the automotive, bicycle, appliance, and consumer hardware industries.

2. Hard Chrome Plating

Hard chrome is valued for its extremely high hardness and wear resistance, significantly extending the service life of industrial components like cutting tools, drawing dies, molds, bearings, shafts, gears, and gauges. It is also used to restore the dimensional tolerances of worn parts. Hard chrome thickness generally ranges from 5 to 50μm, but can go as high as 200 to 800μm for highly demanding applications. Steel parts typically do not require an intermediate layer, though one may be used if specific corrosion resistance is required.

3. Opaque Chrome Plating

This chrome layer has a milky-white appearance and low reflectivity. It is characterized by good ductility, low porosity, and a soft color. While its hardness is lower than hard or decorative chrome, its high corrosion resistance makes it ideal for measuring tools and instrument panels. To boost its hardness, an extra layer of hard chrome can be applied over the opaque layer, a double-layer chrome system, combining the advantages of both types for parts requiring both abrasion and corrosion resistance.

4. Bore Chrome Plating

This process uses the chrome layer’s inherent micro-crack structure. After initial hard chroming, the surface undergoes a mechanical, chemical, or electrochemical etching process to deepen and widen the crack network. This leaves the chrome surface covered with wider channels that not only retain the benefits of hard chrome but also effectively store lubricating media, preventing dry running and significantly boosting the component’s anti-friction and wear capacity. It is commonly used for heavily loaded sliding friction surfaces, such as the cylinder bores and piston rings of internal combustion engines.

5. Black Chrome Plating

Black chrome produces a coating that is black with a uniform, pleasing luster, offering excellent anti-glare (light-absorbing) properties. Its hardness is relatively low, typically 130 to 350 HV, but its wear resistance is 2 to 3 times higher than bright nickel at the same thickness. Its corrosion resistance is comparable to standard chrome and is largely determined by the thickness of the underlying layer. It has good thermal stability, showing no discoloration below 300°C. Black chrome is frequently used for parts in aviation instruments, optical equipment, and as a protective, decorative finish for solar energy absorber panels.

Chrome Plating Process

The production of electroplated chrome carries environmental and safety risks due to the easy generation of chromic acid mist. Operators must wear personal protective equipment, and all wastewater and exhaust gases must be rigorously treated to meet discharge standards. The typical process flow includes pre-plating stress relief, degreasing and pickling, the actual chrome plating, and post-plating hydrogen embrittlement relief. Depending on the part’s type and requirements, certain steps may be omitted or modified.

1. Stress Relief: This step primarily targets high-strength steel components prone to internal stress. Parts are placed in an oven for low-temperature tempering to relieve residual stresses introduced during manufacturing, preventing deformation or coating spalling under plating conditions.
2. Degreasing and Pickling: This process involves thoroughly removing grease and organic contaminants using alkaline solutions, followed by rinsing with water. The part is then immersed in a dilute acid solution to remove oxide scale and rust, thereby activating the base metal surface.
3. Copper Plating: The copper layer serves as a primary undercoat, leveraging its leveling properties to fill minor surface imperfections and provide a robust transition layer for superior adhesion of the subsequent nickel layer.
4. Nickel Plating: The nickel layer provides the primary corrosion resistance within the chrome system, serving as the protective barrier against environmental attack.
5. Chrome Plating: The part is immersed as the cathode in a chromic acid-based solution, and a high current density is applied to electrodeposit the chromium layer. The process is highly sensitive to bath temperature and current density.
6. Hydrogen Embrittlement Relief: Since hydrogen atoms can penetrate the steel substrate during plating and cause brittle failure, this essential safety step involves immediately heating the component to a low temperature to drive out the trapped hydrogen, restoring the base metal’s ductility and toughness.
7. Drying: Finally, the parts are dried using hot air or a dehydrating oil to completely remove any residual moisture or solution from the part’s surface and the coating’s pores, ensuring final aesthetic quality and preventing secondary oxidation.

Selection Guidelines for Chrome Plating

When selecting chrome plating for a component, the following principles should be observed.

Decorative chrome is used for protective and decorative finishes on interior components in general or industrial atmospheric environments. It should always use a multi-layer nickel or copper-nickel coating as the intermediate layer.

Hard chrome is used as an anti-wear protective layer for moving friction components. It is often recommended to use a nickel or opaque chrome layer as an intermediate coating. For use in severe marine atmospheric environments, a post-plating pore sealing treatment is recommended.

Chrome plating is suitable for parts requiring:

• High reflectivity.
• Durable decorative brightness.
• High wear resistance.
• Restoration of worn component dimensions.
• Prevention of sticking or adhesion to materials like plastics and rubber.

Chrome plating should not be selected for the following components:

• Parts operating at temperatures exceeding 650°C.
• Parts operating in mineral oil.

About Getzshape

Getzshape delivers globally compliant, precision finished components, including hard and decorative chrome plating solutions. Adhering to standards like ASTM B177, we promise engineers superior wear resistance, high hardness, and dimensional precision for critical components worldwide. Contact us to start your projects.