Why can't most aluminum die castings do the anodizing of sulfuric acid?

【China Aluminum Industry Network】 Anodizing is the use of electrochemical methods. In an appropriate electrolyte, the alloy part is used as the anode, stainless steel, chromium, or the conductive electrolyte itself is the cathode, under certain voltage and current conditions, The anode is oxidized to give the surface of the workpiece an anodized film. According to the type of the electrolyte and the nature of the film layer, it can be divided into anodizing of sulfuric acid, chromic acid, oxalic acid, mixed acid, hard and porcelain; according to the dyeing performance of various anodized films, only the oxide film obtained by the anodizing of sulfuric acid is suitable Dyeing; Others such as oxalic acid, porcelain anodic oxide film can be colored, but the interference color is serious; chromic acid anodic oxidation film or hard oxide film can not be colored; comprehensive said, to achieve the purpose of anodizing, only There is anodization of sulfuric acid.

First, sulfuric acid anode oxidation of aluminum alloy material restrictions:

1, the presence of alloying elements will reduce the quality of the oxide film, under the same conditions, the oxide film obtained on pure aluminum is thicker, the hardness is higher, the corrosion resistance is better, and the uniformity is better. Aluminum alloy materials, in order to obtain a good oxidation effect, to ensure the content of aluminum, usually, not less than 95% is preferred.

2, in the alloy, copper red oxide film will damage the electrolyte quality, increase oxidation defects; silicon will make the oxide film gray, especially when the content exceeds 4.5%, the impact is more obvious; iron due to its own characteristics, in After anodization, it will be in the form of black spots.

Second, the main die casting aluminum alloy and the composition of the main components:

Commonly used die-cast aluminum alloys can be divided into three major categories; one is aluminum-silicon alloy, which mainly contains YL102 (ADC1, A413.0, etc.), YL104 (ADC3, A360), and the other is aluminum-silicon-copper alloy, which mainly contains YL112 ( A380, ADC10, etc.), YL113 (3830), YL117 (B390, ADC14) ADC12, etc.; third, aluminum-magnesium alloy, mainly including 302 (5180, ADC5, and ADC6).

For the aluminum-magnesium alloy, it can be oxidized and painted. This is an important feature distinguishing it from other alloys; but in comparison, there are also some drawbacks.

1. The anodic oxide film has a dual nature, with large pores and uneven distribution, and it is difficult to achieve a better anti-corrosion effect;

2. Magnesium has the tendency of hardening and brittleness, reducing elongation and increasing hot cracking, such as ADC5, ADC6, etc. In the production, due to its wide solidification range and large shrinkage tendency, it often produces shrinkage and cracks, casting properties. Very poor, therefore, there is a greater limitation in the scope of its use, the structure of a slightly complex workpiece, simply not suitable for production;

3. The aluminum-magnesium alloys commonly used in the market are difficult to produce transparent protective film because of their complex composition and low purity of the aluminum. When the sulfuric acid is oxidized, it is difficult to produce a transparent protective film.

In summary, it can be seen that the commonly used die-cast aluminum alloy is not suitable for anodizing with sulfuric acid; however, not all die-cast aluminum alloys can achieve the purpose of oxidation and coloring, such as aluminum manganese cobalt alloy DM32, aluminum manganese magnesium alloy DM6, etc. Die casting performance and oxidation performance are excellent, but because of the short time to enter the country, it has not been popularized.

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