How to coat platinum on titanium?

Plating Process of Platinum on Titanium Substrate

The electroplating process of platinum on titanium involves the following steps:
Pretreatment of titanium substrate → Electrocleaning → Water rinsing → Activation → Distilled water rinsing → Brush electroplating of platinum → Distilled water rinsing → Blow drying.

 

1.Pretreatment of Titanium Substrate

 

Currently, there are various types of platinum plating solutions that can be used on many substrates. However, platinum plating on titanium is challenging because titanium is highly prone to passivation. The passive film on the surface prevents strong bonding between the coating and the substrate, making it difficult to achieve a well-adhered platinum layer. Therefore, pretreatment is necessary to remove the passive film and form an active layer-a titanium hydride (TiH₂) film. This hydride layer forms quasi-metallic bonds with both the titanium substrate and the platinum coating, ensuring good adhesion.

 

Etching Process

 

The purpose of etching is to remove the passive film on the titanium surface. This is typically done using a high-concentration nitric acid-hydrofluoric acid system at room temperature for 5–10 minutes.

 

Activation Process

 

The purpose of activation is to generate an active film on the titanium surface. After activation, the titanium surface forms a titanium hydride (TiH₂) layer, which appears gray-black. Due to the overlapping energy bands between the titanium substrate, TiH₂, and the platinum coating, quasi-metallic bonds are formed, ensuring strong adhesion. Only after activation can the titanium sheet be immersed in the plating solution for platinum deposition.

 

2. Electroplating Process

 

Aqueous Electroplating

 

Aqueous electroplating of platinum is the most widely used method. The plating solutions can be categorized into acidic and alkaline types.

Alkaline platinum plating solutions include:

 

P-salt plating solution (dinitrodiammine platinum as the main salt)

Potassium hexahydroxyplatinate-based strongly alkaline plating solution

Acidic platinum plating solutions include:

Sulfamic acid-based platinum plating solution

DNS (dinitrosulfatoplatinate) sulfate plating solution

 

(1) Sulfamic Acid-Based Plating Solution

 

This solution produces bright, thick platinum layers with fine crystallization due to the complexing effect of sulfamic acid, which enhances cathode polarization.

 

(2) DNS Plating Solution (Strongly Acidic)

 

This solution allows for bright, thick platinum deposits at lower temperatures. Since no gas is released during plating, the risk of pinholes or porosity is minimized. However, the current efficiency is low, and the preparation of dinitrosulfatoplatinate is more complex than P-salt.

 

Molten Salt Electroplating

 

Molten salt platinum plating began in the 1930s. In a molten cyanide bath, the passive film on refractory metals can be dissolved, and since the electrolyte is oxygen-free, the coating adheres strongly. The resulting deposit is ductile and stress-free.

Electrolyte preparation:

A mixture of 53% NaCN and 47% KCN is pre-melted in a ceramic crucible.

When the temperature exceeds the melting point by 50°C (~550°C), two platinum electrodes are inserted, and electrolysis begins.

Once the platinum ion concentration reaches ~0.3%, plating can commence.

Operating conditions:

Cathodic current density: 30–300 A/m²

Current efficiency: 65–98%

Argon gas protection is required except during melting.

This method produces thick, bright, stress-free platinum coatings, but the process is complex, environmentally hazardous, and expensive, making it unsuitable for large-scale applications.

 

Brush Plating

 

Brush plating originated in Europe in 1899 as an off-tank repair plating method. Initially, a cotton-wrapped anode was dipped in plating solution and rubbed over defective areas to deposit a repair layer. Early brush plating was slow, had poor adhesion, and produced thin coatings. However, after decades of development-with specialized plating pens, ampere-hour-controlled power supplies, and high-concentration plating solutions-brush plating became a widely adopted electrochemical deposition method.

Advantages of Brush Plating:

High metal ion concentration → Fast deposition rate (5–15 times faster than conventional plating).

Low hydrogen embrittlement, high hardness, low porosity, controllable thickness → Usually requires no post-machining.

Simple equipment, easy operation, on-site applicability → Ideal for repairing worn or oversized parts.

Due to these advantages, brush plating is primarily used for repair applications rather than decorative plating.