Chrome plating is a popular surface finishing technique used to improve wear and corrosion resistance. The shiny chrome layer not only protects, but also provides an attractive aesthetic finish. However, chrome plating does have temperature limitations that must be considered for optimal performance. Exceeding the recommended temperature range can compromise the integrity and benefits of this versatile metal coating.

Why Temperature Resistance Matters

The operating temperature range is an important consideration for any plated coating. Excessive temperatures can alter the properties and performance of the plating layer.

For chrome plating specifically, operating above the recommended temperature range leads to:

Loss of hardness and wear resistance
Decreased fatigue strength
Loss of corrosion protection
Possible damage to the substrate material

High temperatures accelerate diffusion between the chrome layer and substrate, which modifies the coating structure. Hard chrome is especially susceptible, losing its signature hardness rapidly above 400°C.

Considering the temperature resistance allows proper selection of substrate materials and operating conditions. This ensures the chrome layer maintains its intended properties of surface hardness, friction and wear reduction, and corrosion defense.

Acceptable Temperature Range

The maximum recommended operating temperature for hard chrome plating is 400°C (752°F). Up to this temperature, the as-plated hardness and fatigue strength remain largely unchanged.

Decorative chrome plating has a slightly lower maximum temperature of 370°C (700°F). Exceeding this will rapidly degrade the finish appearance and corrosion resistance.

However, the chrome layer does not instantly degrade when these temperatures are exceeded. Hardness and fatigue strength gradually reduce as temperatures increase above 400°C. The chrome finish remains functional up to approximately 800°C (1472°F).

Hardness Reduction

One of the main concerns when chrome plated parts are exposed to high temperatures is the drastic loss of hardness and wear resistance. Hard chrome can reach 900-1000 HV (68-77 HRC), providing a hard, wear-resistant surface. But this hardness significantly decreases when subjected to elevated temperatures.

Some key points on the hardness reduction of hard chrome plating:

At 200°C (392°F), approximately 10% loss of the as-plated hardness occurs.
At 300°C (572°F), hardness reduced by 25%.
At the maximum recommended 400°C (752°F), around 40% of hardness is lost.
Above 500°C (932°F), the hardness decrease accelerates rapidly.
At 650°C (1202°F), hardness drops below 500 HV (46 HRC).
At 800°C (1472°F), hardness is only 200-250 HV (19-23 HRC).

This demonstrates the importance of keeping chrome plated components below 400°C whenever possible. But parts can still operate with some hardness retention up to about 650°C.

Melting Point

Although the hardness suffers above 400°C, the chrome plating layer itself does not melt. Pure chromium has an extremely high melting point of 1907°C (3465°F).

The actual melting point of an electroplated chromium coating is slightly lower, around 1700°C (3092°F). But this is still well above the temperatures that degrade hardness.

So chrome plating will not melt off the substrate during high temperature service. It can withstand over 1000°C (1832°F) before reaching its melting point. However, the drastic loss of hardness and wear resistance still makes high temperatures unsuitable.

Thermal Shock Resistance

Thermal shock occurs when a material undergoes rapid temperature changes, experiencing thermal stresses. This can cause cracking or delamination of coatings.

The good news is chrome plating exhibits excellent thermal shock resistance. The chrome layer can withstand repeated, quick temperature spikes without damage.

This makes chrome suitable for applications that cycle between high and low temperatures. Common examples include:

Plastics processing molds exposed to molten plastics then cooled.
Kiln car and furnace parts seeing rapid heating and cooling.
Engine components alternately heated by combustion then cooled.

Rapid cooling from 400°C to below zero temperatures does not crack or delaminate a quality chrome plating layer. This thermal shock resistance allows it to withstand fluctuating temperature environments.

Effect of Substrate Material

The temperature resistance figures discussed apply specifically to chrome plated on a steel substrate. But the substrate material can alter the effective temperature range.

Chrome on aluminum provides slightly better high temperature hardness retention. On the other hand, plating over brass or bronze further lowers the maximum recommended temperature.

The thermal expansion coefficient mismatch between plating and substrate influences temperature resistance. Smaller differences in expansion reduce harmful stresses at temperature extremes.

Influence of Plating Thickness

Thicker chrome plating layers typically retain their hardness to higher temperatures. For example, a 75 micron layer may maintain hardness up to 500°C versus only 450°C for a 25 micron layer.

With thicker coatings, it takes longer for the harmful diffusion effects between chrome and substrate to penetrate the full depth. This extends the temperature range before softening occurs.

However, thicker platings require longer processing times. So thickness is a balance of temperature resistance needs and production efficiency.

Process Method Considerations

The specific plating process used also impacts temperature resistance. Main process differences include:

Plating bath chemistry – Different chromium plating solutions are formulated for specific properties.
Current density during plating – Higher densities produce harder deposits.
Post-treatment – Steps like heat treatment or machining affect the coating structure.

For example, a high-current density sulfamate nickel underplate covered by dual medium-high current hard chrome will maximize temperature resistance.

Effects of Exceeding Temperature Limits

It’s clear that chrome plating can withstand short-term temperature spikes above 400°C. But consistently exceeding the maximum recommended temperatures accelerates material damage:

Hardness and wear resistance decrease rapidly. Surfaces become more vulnerable to scratching, galling, and indenting.
Fatigue strength is lowered, allowing earlier crack initiation under cyclic stresses.
Corrosion protection suffers as the chrome layer slowly diffuses into the substrate.
Chemical resistance declines as the coating structure changes.
Load capacity reduces as surface hardness drops.
Coefficient of friction increases, resulting in greater drag and stick-slip.

For these reasons, chrome plated components give optimal performance when kept below 400°C in use. Allowing temperatures consistently above this will shorten service life.

Best Practices for High Temperature Applications

For situations where chrome plated parts will be exposed to high temperatures:

Select substrate materials like stainless steel that resist heat damage and thermal mismatch stresses.
Specify thicker chrome layers up to 250 microns to extend hardness retention.
Use high current density plating processes to maximize as-plated hardness.
Consider alternative surface treatments (ceramic, tungsten carbide) for extreme temperatures beyond chrome’s capability.
When possible, limit time at peak temperatures to avoid cumulative material degradation.

With careful temperature control and plating process selection, chrome can be suitable for more demanding elevated temperature applications. But the recommended 400°C limit should not be exceeded for continuous optimal performance.

Conclusion

Hard chrome plating provides the best wear, friction, and corrosion resistance up to 400°C.
Gradual hardness reduction occurs above 400°C, with an accelerated drop above 500°C.
The chrome layer itself does not melt until over 1000°C.
Thermal shock resistance allows chrome plating to withstand rapid temperature spikes.
Substrate material, plating thickness, and process method all impact temperature capability.
Exceeding temperature limits degrades hardness, fatigue strength, and other key properties.
With proper precautions, chrome plating can function for limited periods above 400°C.

Understanding chrome’s temperature resistance allows selecting appropriate applications to maximize service life. Keeping chrome plated components below 400°C provides optimal performance. But chrome maintains limited functionality up to 800°C for short-term exposure. Considering these factors ensures chrome plating’s high hardness, low friction, and wear protection benefits are maintained.