Electrochemical Potential Differences in Multilayer Nickel Plating:The Lifeline of Corrosion Protection Performance

1. Introduction to Potential Difference in Multilayer Nickel Plating

A multilayer nickel plating system typically consists of semi-bright nickel, high-sulfur nickel, bright nickel or pearl nickel, and a nickel seal layer, forming either a triple-layer or quadruple-layer nickel structure.

The formation of potential differences between layers is mainly due to variations in sulfur content and differences in the microstructure of each nickel deposit.

The functional mechanism of the potential difference is to promote lateral corrosion propagation rather than vertical penetration, thereby protecting the integrity of the underlying layers or the base material.

In practice, some electroplating companies test the sulfur content of nickel deposits once or twice per year to evaluate coating purity and performance, including:

Corrosion resistance

Ductility

Internal stress

Potential difference

Chromium coverage capability

However, it should be noted that potential difference cannot fully represent the sulfur content of the nickel layer.

2. What Is Potential Difference in Multilayer Nickel Plating?

The potential difference in a multilayer nickel system refers to the intentional creation of a stable and properly directed electrochemical potential difference between adjacent nickel layers.

Typically, this includes:

The potential difference between semi-bright nickel and bright (or pearl) nickel

The potential difference between bright (or pearl) nickel and high-sulfur nickel

The potential difference between the nickel seal layer and bright (or pearl) nickel

Potential Difference Between Semi-Bright Nickel and Bright (Pearl) Nickel

Standard requirement:
To ensure excellent corrosion resistance, the semi-bright nickel layer must have a more positive potential than the bright (or pearl) nickel layer.

In other words:

Semi-bright nickel is more noble (more inert)

Bright (or pearl) nickel is more active (more negative)

Typical range:
The potential difference is usually controlled within 100–200 mV.

In simple terms:
Like a battery system, the semi-bright nickel acts as the positive electrode, while the bright (or pearl) nickel acts as the negative electrode.

When corrosion occurs, current flows from the negative layer (bright or pearl nickel) to the positive layer (semi-bright nickel), thereby protecting the underlying metal.

Potential Difference Between Bright (Pearl) Nickel and High-Sulfur Nickel

High-sulfur nickel plating is specifically designed for applications requiring very high corrosion resistance. This process increases the sulfur content of the nickel layer to approximately 0.1–0.2%.

High-sulfur nickel is mainly used to form quadruple-layer nickel systems. Since modern triple-layer nickel systems (semi-bright nickel, bright or pearl nickel, and nickel seal) already meet most corrosion resistance requirements, high-sulfur nickel is less commonly applied in practice.

The required potential difference between high-sulfur nickel and bright or pearl nickel is generally −15 to −40 mV.

Potential Difference Between Nickel Seal and Bright (Pearl) Nickel

The required potential difference range for the nickel seal layer is relatively wide, typically 20–90 mV. This is intended to meet the salt spray performance requirements of trivalent chromium systems, where a higher potential difference can provide improved CASS test performance.

A suitable potential difference directs corrosion to occur within the bright nickel layer

If the potential difference is too low or becomes negative, the nickel seal layer will corrode, leading to collapse of the chromium layer

If the potential difference is too high, it may cause accelerated corrosion of the bright nickel layer

3. Why Is This Potential Difference Necessary?

The fundamental principle of multilayer nickel systems is based on sacrificial anodic protection and lateral corrosion control, with the ultimate goal of significantly enhancing the corrosion resistance of the substrate.

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