Corrosion resistance

Corrosion resistance is one of the main reasons why NIPLATE® electroless nickel plating coatings are used in precision mechanics and in industrial applications exposed to aggressive environments.
The coatings consist of a nickel–phosphorus (Ni–P) alloy, which provides high chemical stability in the presence of corrosive agents that are typically critical for common metal alloys, such as marine and industrial atmospheres, or outdoor environments exposed to contaminants.

NIPLATE® coatings are also substantially insensitive to chemical attack by hydrocarbons and solvents, a feature that broadens their use in numerous industrial sectors.

Corrosion protection mechanism

The corrosion protection mechanism of NIPLATE® coatings is predominantly a barrier type.
The electroless nickel layer isolates the base material from the external environment, preventing direct contact between the metal and corrosive agents.

A key factor is the thickness uniformity typical of electroless nickel plating, which enables effective protection even on complex geometries, internal surfaces, and areas that are difficult to reach with other coating processes.

Corrosion resistance increases as coating thickness increases, because the growth of the layer progressively reduces the deposit’s intrinsic porosity. However, it is important to consider that the coating surface may exhibit local discontinuities, attributable to several factors, including:

• porosity or defects in the base material;
• non-metallic inclusions in the alloy;
• cracks or micro-defects in the coating;
• damage due to handling or subsequent machining operations.

These discontinuities can locally expose the base material, creating potential corrosion initiation sites in aggressive environments.

High-phosphorus electroless nickel coatings, in particular NIPLATE® 500 and NIPLATE® eXtreme, exhibit extremely low porosity even at limited thicknesses. Under typical industrial conditions, with thicknesses above 30 µm, these coatings can be considered free of through-porosity, providing particularly high corrosion protection.

Copper alloy protection

In the case of copper alloys, the protection mechanism partially differs from a purely barrier mechanism.
Copper has a more noble electrochemical potential than nickel; consequently, the electroless nickel coating also provides a form of cathodic protection of the base material.

This behavior results in particularly high corrosion resistance of copper alloys coated with NIPLATE®, even in the presence of local damage or coating discontinuities.
Thanks to this characteristic, electroless nickel plated copper alloys can be successfully used in marine environments, even under direct contact with seawater.

Accelerated corrosion resistance test in neutral salt spray

The method most commonly adopted in industry to evaluate corrosion resistance is the neutral salt spray test, performed in accordance with ISO 9227.

The test consists of exposing parts to a mist of a 5% aqueous sodium chloride solution, maintained at a temperature of 35 °C, for variable times, typically 96 or 480 hours. This environment accelerates corrosion mechanisms and allows a comparative assessment of coating performance.

A fundamental aspect in evaluating the results is the measurement of the corroded area, from which the Rp protection rating is determined according to ISO 10289.

Behavior of NIPLATE® coatings

NIPLATE® coatings show high chemical resistance to the salt spray environment and are generally not very sensitive to corrosion.
Protection is achieved by isolating the base material; therefore, any coating discontinuity represents a potential corrosion attack area.

Iron and aluminum alloys

For iron and aluminum alloys, salt spray corrosion resistance depends on multiple factors, including:

• coating type;
• deposited thickness;
• machining quality;
• metallurgical characteristics of the base material.

Given the complexity of the parameters involved, a reliable definition of corrosion resistance necessarily requires performing the salt spray test on the actual coated part.

Copper alloys

The behavior of copper alloys coated with NIPLATE® is significantly different.
Thanks to the electrochemical affinity between nickel and copper, corrosion protection is particularly effective even at relatively low thicknesses. By way of example, a brass component coated with 30 µm of NIPLATE® 500 can achieve an Rp protection rating of 9 even for exposures exceeding 1000 hours in salt spray.

Conclusions

The corrosion resistance of NIPLATE® coatings is a key factor in extending the service life of mechanical components in aggressive environments.
However, this performance does not depend exclusively on the coating itself, but on the interaction between base material, machining, deposited thickness, and operating conditions.

For this reason, corrosion performance should always be evaluated in the specific application context, through laboratory testing and field tests, in order to ensure component reliability in service.