Characteristics of aluminum and aluminum alloy

Plating on aluminum and aluminum alloy is much more difficult and complicated than plating on steel, copper and other metal materials, the main reasons are:

① Aluminum and aluminum alloy has a high affinity for oxygen, very easy to produce oxide film, and this layer of oxide film will be removed in a very short period of time to produce a new layer of oxide film, seriously affecting the bonding force of the plating layer.

② Aluminum electrode potential is negative, immersed in the plating solution is easy to have a positive potential with the replacement of metal ions, affecting the combination of plating.

③ The expansion coefficient of aluminum and aluminum alloy is larger than that of other metals, so it is not suitable for plating in the range of large temperature changes. Different expansion coefficients of aluminum and aluminum alloy and other metal plating will cause greater stress, which will make the bonding between plating and aluminum and aluminum alloy not firm.

④ Aluminum is amphoteric metal, soluble in acid and alkali, and unstable in both acidic and alkaline plating solution.

⑤ Aluminum alloy die casting has sand holes and air holes, which will leave plating solution and hydrogen, easy to bulge, and will also reduce the bonding between plating layer and base metal.

Aluminum alloy die casting can be plated, should be for the above reasons, in the plating before taking certain pretreatment measures. In addition to conventional degreasing, etching, out of the light, but also need to carry out special pretreatment, make a layer of transition metal layer or can conductive porous chemical film layer, in order to ensure that the subsequent plating layer has a good combination.

Aluminum alloy die casting currently commonly used methods are two: first chemical dip zinc, and then plating other metals; first anodic oxidation treatment, and then plating other metals.

Aluminum alloy die casting anodic oxidation treatment

Anodic oxidation treatment refers to the generation of a layer of oxide film with a certain thickness and special structure on the surface of aluminum and aluminum alloy under certain process conditions. This layer of oxide film has many pores, large pore size, good electrical conductivity, and strong combination with the substrate. When plating, metal particles are deposited in the membrane pores to improve the combination of plating. Anodic oxidation can be carried out in a variety of solutions, compared with the oxide film obtained by other anodic oxidation processes, phosphoric acid oxidation film presents a relatively uniform roughness, with a super-microscopic uniform concave-convex structure, the largest aperture diameter and the smallest electrical resistance, and if the metal is deposited on this surface, there are many nuclei formed, and the plating layer is uniformly detailed and has good adhesion. Therefore, phosphoric acid anodizing is the most suitable anodizing process for plating primer. The process specifications of commonly used phosphoric acid anodizing are as follows:

Phosphoric acid (H3P04) 300~500g/L Voltage 20~40V

Temperature 25 ~ 35 ℃ oxidation time l0 ~ 15min

Anode current density l ~ 2A / dm2

The porosity of the oxide film increases with the increase of phosphoric acid content and temperature, and decreases with the decrease of current density; the thickness of the oxide film decreases with the increase of phosphoric acid concentration. The thickness of the oxide film is only about 3μm. Because the oxide film is extremely thin, in the later plating, it is not suitable to use strong acid or strong alkaline plating solution, the general plating pH value should be 5 ~ 8. The solution should be stirred constantly when anodizing to prevent the local temperature from being too high. Aluminum and aluminum alloy parts after anodic oxidation by dilute hydrofluoric acid solution (0.5 ~ 1.0mL / L) activation, cleaning should be plating.