Improvement of pretreatment process for electroplating of aluminum and aluminum alloy

I. Introduction

After the surface of aluminum and aluminum alloy plating various metals, can significantly improve their physical or chemical properties of the surface, while aluminum and aluminum alloy conductor is made, a silver plating on its surface can increase the electrical conductivity or surface contact points; such that Aluminum is easy to weld, and its surface is plated with copper, nickel or tin; in order to improve its wear resistance, the surface is plated with thick crusts. In terms of decoration , it is mostly electroplated .

Aluminum and aluminum alloy surface electroplating has been tried for a long time and has been used in actual production. However, the presence of oxide, the different thermal expansion coefficients of aluminum and aluminum alloys with metal plating, plating with a plating solution remaining factors pinholes and the like, resulting in poor coating adhesion, peeling even in prolonged use between aluminum and aluminum alloy plating and the plated After the flaking immediately, in the field of surface treatment , the electroplating process of aluminum and aluminum alloys is still in the exploration stage. There has been no substantial breakthrough for a long time, and no perfect and mature process has been formed so far. The poor adhesion of the coating is the plating quality of aluminum and aluminum alloys and the product qualification rate is still a bottleneck in the industry.

2. Traditional aluminum and aluminum alloy plating

Aluminum and aluminum alloys can be electroplated in the electrolyte to form a coating, but the coating is not strong enough to peel off easily. Therefore, aluminum can be deposited in an aqueous solution containing zinc oxide compounds before electroplating. This method is either zinc replacement or deposition. It is also possible to obtain a thin porous oxide film on the surface of aluminum and aluminum alloys through anodizing power supply before electroplating.

2.1 Conventional aluminum and aluminum alloy electroplating process

The aluminum and aluminum alloy electroplating process consists of three parts: pre-plating treatment, electroplating, and post-plating treatment. Pre-plating treatment is a key process related to the quality of electroplated products. Its main purpose is to remove the grease on the surface of aluminum and aluminum alloys, and naturally form oxide films and other contaminants.

The general general process flow is: degreasing-water washing-corrosion reduction-water washing-pickling-water washing-activation-water washing-primary zinc dipping-water washing-dezincification-water washing-secondary zinc dipping-water washing-neutral nickel plating-water washing- Subsequent electroplating.

There are also subsequent electroplating after using wave anodized film instead of zinc immersion process.

2.2 Deficiencies in the traditional pretreatment process

1. The process flow is long and the procedures are many.

2. The process is complex and the operating range is narrow. All process parameters must be strictly controlled.

3. The scope of application of the process is not wide, the pretreatment process of aluminum alloys of different brands can not be the same, and the pretreatment process must be adjusted according to the aluminum alloy brand.

4. Under the premise of strict control of the pretreatment process, the pass rate of electroplating products is very low, the pass rate of ordinary decorative plating is 85%-90%, and the pass rate of functional plating is 60%-70%.

5. The application life of each process solution is short, and the treatment cycle is short.

As the traditional pretreatment of aluminum and aluminum alloys agrees that the above deficiencies generally exist, they must be improved.

3. Improved pre-treatment process for general-purpose aluminum and aluminum alloy electroplating

Degreasing and alkaline etching two-in-one-water washing-pickling-water washing-ash removal-water washing-alkaline activation-zinc immersion-water washing-neutral nickel plating-water washing-subsequent electroplating.

4. Practice summary

The new process is based on the actual defects of current aluminum alloy plating, combined with the special physical and chemical properties of aluminum alloy itself, and improved on the basis of the original process. The specific advantages are as follows:

1. short process, to mention the purity of the alumina surface of the metal alloy by other selective dissolution of the aluminum alloy of the surface layer, the coating adhesion is improved.

2. First live chemical arts acid substituted with an alkaline activator to completely remove remaining on the surface before the plating process of the alloy of silicon and silica, improve coating adhesion.

3. Directly immersed in zinc without water washing after alkaline activation, which can avoid the formation of oxide layer when exposed to the air when washed with water and then immersed in zinc again after acid activation.

4. Only one simple zinc immersion is needed, and the bonding strength of the coating is obviously better than the secondary zinc immersion and the complex multi-component cyanide zinc leaching process.

5. The versatility of the process is wide, and it is used in the pretreatment of aluminum alloy plating of almost all brands.

6. The one-time pass rate of product electroplating is significantly higher than the traditional process. If the bonding strength of the pure coating is evaluated, the pass rate of the product is close to 100%.

7. The process operation in actual production is simple and convenient.

8. The actual production of large-scale large-scale gantry automatic production line for a long time has proved that the improved new aluminum alloy pre-treatment is very stable.

9. It is convenient to improve the process based on the traditional aluminum alloy electroplating process.