After plating various metals on the surface of aluminum and aluminum alloy, the physical or chemical properties of the surface can be obviously improved. When aluminum and aluminum alloy are used as conductors, silver plating on the surface can improve the electrical conductivity of the surface or electrical contact parts; It is easy to weld and is plated with copper, nickel or tin on its surface; in order to improve its wear resistance, it is plated thick on its surface. In terms of decoration, most of them are electroplated enamel.
The surface plating of aluminum and aluminum alloys has been tried for a long time and has been used in actual production. However, there is an oxide between the aluminum and the aluminum alloy and the plating layer. The thermal expansion coefficients of the aluminum and the aluminum alloy and the metal plating layer are different, and the plating layer has pinholes and residual plating liquid, etc., which causes poor adhesion of the plating layer, peeling off even after long-term use, and even plating. Immediately after peeling off, in the field of surface treatment, the electroplating process of aluminum and aluminum alloy is still in the exploratory stage, and there has been no substantial breakthrough for a long time. So far, no perfect and mature technology has been formed. The poor adhesion of the coating is the plating quality and product qualification rate of aluminum and aluminum alloy is still the industry bottleneck.
2. Traditional aluminum and aluminum alloy plating
Aluminum and aluminum alloy can form a coating by electrolysis in the electrolyte, but the adhesion of the coating is not strong and easy to peel off. Therefore, the aluminum may be first plated in an aqueous solution containing a zinc oxide and then electroplated, which is either a zinc replacement method or a deposition method. It is also possible to first form a thin porous oxide film on the surface of aluminum and aluminum alloy by anodizing power supply for electroplating.
2.1 Conventional aluminum and aluminum alloy plating 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 electroplating 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 is: degreasing - washing - erosion - washing - pickling - washing - activation - washing - zinc dipping - washing - zinc removal - washing - secondary zinc dipping - washing - neutral nickel plating - washing - Subsequent plating.
There is also an anodized film using a wave instead of a zinc immersion process for subsequent plating.
2.2 Deficiencies in traditional pretreatment processes
1. Long process flow and many processes.
2. The process is complicated, the operation range is narrow, and the process parameters must be strictly controlled.
3. The scope of application of the process is not wide. Different grades of aluminum alloy pretreatment process can not be the same, and the pretreatment process must be adjusted according to the grade of the aluminum alloy.
4. Under the premise of strictly controlling the pre-treatment process, the qualification rate of electroplating products is very low, the pass rate of ordinary decorative electroplating is 85%-90%, and the pass rate of functional electroplating is 60%-70%.
5. The applicable life of each process solution is short and the treatment cycle is short.
Since the conventional pretreatment of aluminum and aluminum alloys agrees that the above deficiencies are common, they must be improved.
3. Improved general-purpose aluminum and aluminum alloy plating pretreatment process
Degreasing and alkali etching - water washing - pickling - washing - ash - washing - alkaline activation - zinc immersion - water washing - neutral nickel plating - water washing - subsequent plating.
Summary of practice
The new process is aimed at the actual defects of the current aluminum alloy plating, combined with the special physical and chemical properties of the aluminum alloy itself, and improved on the basis of the original process, the specific advantages are as follows:
1. The process is short, and the purity of the aluminum alloy surface is improved by selectively dissolving other metals of the aluminum alloy surface layer, which is an increase in plating adhesion.
2. The alkaline activation is used instead of the acid activation process to completely remove the silicon and silica gel remaining on the surface of the alloy during the pre-plating treatment, thereby effectively improving the adhesion of the coating.
3. After alkaline activation, it is directly immersed in zinc without washing, and it can avoid exposure to air in the air when it is washed with water and then immersed in zinc to form an oxide layer.
4. Only one simple zinc immersion can be used, and the plating adhesion is obviously better than the secondary zinc immersion and the complex multi-cyanide zinc immersion process.
5. The process is versatile and is used in almost all grades of aluminum alloy plating.
6. The pass rate of product plating is significantly higher than the traditional process. If the adhesion of the simple coating is evaluated, the yield of the product is close to 100%.
7. The process in actual production is simple and convenient.
8. The actual production of long-term large-scale gantry automatic production line proves that the improved new aluminum alloy plating pretreatment is very stable.
9. It is convenient to carry out process improvement on the basis of traditional aluminum alloy electroplating process.