How can titanium be electroplated?

Titanium and its alloys are very active, and a passivation film will quickly form on the surface of them in both air and water. Therefore, it is very difficult to electroplate directly on titanium and its alloys, and the bonding force of the coating is very poor. This paper introduces several commonly used methods to improve the bonding force of coatings on titanium and its alloys.

 

 

  I. Introduction

Titanium and its alloys have the characteristics of high strength, light weight and corrosion resistance, so they are widely used in various fields. Especially in the fields of electronics, aerospace and other fields, its advantages have been brought into full play. However, because the surface of titanium alloy is easy to bite and has poor welding performance, in order to fully play its role, it must first be plated with a layer of other metals on its surface, such as nickel, copper, chromium, gold, etc. This not only brings into play the advantages of high strength and light weight of titanium alloy, but also improves the shortcomings of its surface that is easy to bite and difficult to weld.

Titanium is a very active metal, but usually titanium and its alloys show stable properties, because titanium has a strong affinity with oxygen and nitrogen in the air, and a dense passivation film is easily formed on its surface . In order to obtain a coating with better bonding force on the titanium surface, the passivation film must be destroyed first. However, the passivation speed of titanium is too fast, and the passivation film will be regenerated as soon as it is removed, which brings great difficulties to electroplating. There are generally two methods commonly used at present: one is to strengthen the pre-treatment control, that is, to find a way to destroy the passivation film on the surface of the titanium alloy, and not to regenerate an excessively thick passivation film before electroplating, and then to regenerate the passivation film on a relatively fresh surface. The other method is post-treatment, usually through high-temperature treatment, so that the coating with poor bonding force can be combined with the substrate to form a strong metal bond through diffusion at high temperature.

2. Common methods to improve the bonding force of coatings on titanium alloys

(1) Pre-plating treatment method

(1) Sand blasting method

As we all know, sandblasting can remove the passivation layer on the metal surface, roughen the surface, strengthen the mechanical bite force, and thus improve the bonding force of the coating. Experiments have shown that sandblasting is equally effective for electroplating on titanium. The author once compared the bonding force of sandblasted and non-sandblasted pure titanium test pieces after electroplating, and found that the bonded force of sandblasted treatment is much better. However, sandblasting will increase the stress of the workpiece, and some workpieces with high precision requirements are not suitable for sandblasting.

(2) Transition membrane method

①Hydrogenated membrane

When electroplating the surface of industrial pure titanium, a mixture of HCl (500ml/L) and TiCl3 (10-20ml/L) was used to activate the formula and some additives were added to obtain a coating with good adhesion. After analysis, they found that a layer of gray-black film would be formed on the surface of titanium treated with this activation solution. Electroplating is performed on this layer of film to obtain a coating with good adhesion. Further analysis using X-ray diffraction and photoelectron spectroscopy found that the main component of this layer of film is TiH2, which forms certain metal bonds with the titanium substrate and the coating respectively, ensuring the binding force requirements.

② Fluoride film

The titanium alloy was activated by using the mixture of NaCr2O4 (250ml/L) and HF (20ml/L), and a coating with good adhesion was also obtained. After analysis, it is found that a layer of fluoride film is formed on the surface of the titanium alloy treated with this activation solution, which is similar to the hydrogenated film, and has a good bonding force with the titanium alloy substrate and the subsequent coating.

(3) Transition metal layer method

① Zinc dipping method

A layer of zinc metal was obtained on the titanium surface by dipping zinc twice, followed by electroless nickel plating and gold electroplating. The coating obtained by this method was heated at 180°C for 1 hour, and then quenched in water. No peeling and blistering was found.

A thin layer of zinc was first deposited on the surface of the titanium alloy, and then plated on it, and a coating with good adhesion was also obtained.

② Immersion nickel plating method

The coating formed by the replacement reaction of titanium and nickel has good bonding force with the substrate, and electroplating is performed on it, and the coating has good bonding force with the substrate.

(2) Post-plating heat treatment method

Heat treatment is an effective method to improve the bonding force of the coating. At high temperature, there will be obvious mutual diffusion between the coating and the substrate, and a metal bond will be formed between two different metal atoms, so as to achieve the purpose of improving the binding force.

Cu/Ni was electroplated on titanium alloy (Tc4), and then heat treated in air and vacuum at 540°C for 3 hours. After the assessment, it was found that the bonding force of the coating was significantly improved.

The results show that after heat treatment, the interface between the coating and the substrate forms a diffusion layer mainly composed of solid solution or intermetallic compound. Using XRD to analyze the interface between the coating and the substrate after heat treatment, it is found that there are Ni3Ti, NiTi, NiTi2, etc. in the diffusion layer. intermetallic compound. The thickness of the diffusion layer after heat treatment is not a key factor for the improvement of the bonding force, and the improvement of the bonding force mainly depends on whether a metal bond is formed between the coating and the substrate. If the gap between the coating and the substrate does not increase due to thermal expansion and contraction, and the solid solution or intermetallic compound in the diffusion layer can destroy the integrity of the passivation film and other non-metallic films existing between the coating and the substrate, then The coating and the metal can easily diffuse each other and form a metal bond, thereby improving the bonding force of the coating.

3. Conclusion

There are many ways to improve the bonding force between the coating on the titanium alloy and the substrate, which can be roughly divided into two categories. One is to remove the passivation layer on the surface of the titanium alloy through pre-treatment, and cover the surface with a transition layer that has a good bond with the substrate, and then electroplate on this transition layer. The other is to make the atoms between the coating and the substrate diffuse each other through post-plating high temperature treatment to form intermetallic compounds or solid solutions, so as to achieve the purpose of improving the bonding force. If you combine pre-processing and post-processing, the effect will be better.