Analysis of main points for maintenance of zinc-nickel alloy electroplating process for auto parts

The zinc-nickel alloy coating has high corrosion resistance when the nickel content is 8%~15%. After the corrosion resistance far exceeds the plating The layer of zinc is 4-8 times that of pure zinc. The corrosion resistance of the coating is mainly related to the nickel content. To obtain sufficient nickel content, the control of process parameters is very important.

At present, many parts of automobiles are made of zinc-nickel alloy parts. According to the requirements of the TL244 standard of Volkswagen, the corrosion resistance of the coating is the strongest when the nickel content of the coating is between 12% and 15%.

For this reason, the alkaline zinc-nickel alloy process of German Seletar Company is introduced: 5.0~8.0 g/L zinc, 0.5~1.5g/L nickel, 100~140 g/L free sodium hydroxide, 35~50 mL/L, ZN51, 65~85 mL/L ZN52, 33~10 mL/L ZN5, 0.5~2.0 mL/L ZN54, 0.5~1.5 mL/L ZNS6, temperature 33~37℃; cathode current density 1.5 -2.5 A/dm2. The factors affecting the performance of the coating are summarized as follows for reference.

1 temperature

Alkaline zinc-nickel alloy plating solution has strict temperature requirements, which has a great influence on the distribution of zinc-nickel alloy in the coating.

Keep the temperature within 33~37℃, the plating solution is stable, and the nickel content of the plating layer is 12%~15%. If the bath temperature is higher than 37 ℃, the nickel ion co-deposition speed will increase sharply, the nickel ion content in the alloy coating is greater than 15% or more, the coating is bright silver-white, and the passivation film is colorless or blue-yellow after the rainbow passivation ; After the black passivation, a black iridescent passivation film will appear.

If the temperature is lower than 33℃, the zinc ion co-deposition rate will increase, the zinc ion content in the alloy coating will increase, the nickel ion content will be less than 12% or less, the passivation film will be light yellow or colorless, and light yellow passivation The film is easy to wipe off.

Therefore, the temperature of the plating solution must be strictly controlled during production, and automatic devices should be used to control it to ensure the stability of the plating solution.

2 Current density

The cathode current density is within 1.5 ~2.5 A/dm2, the color passivation effect of the coating obtained by using the upper current density is not good, and there will be blue or blue-yellow, and transparent passivation sometimes appears light yellow or brown, and The brightness is uneven and the contrast is great.

When the black passivation of the coating obtained with the lower limit current density is used, sometimes the passivation film has different color spots, insufficient brightness, uneven color, and sometimes the film is rough and dark, which is caused by the low nickel content in the alloy.

Therefore, the middle and lower current density should be adopted for color and transparent passivation; the middle and upper current density should be adopted for black passivation.

3 Main components of plating solution

The proper configuration of zinc ion, nickel ion and free sodium hydroxide concentration will affect the stability of the plating solution, the dispersing ability and the plating ability of the zinc-nickel content ratio in the coating and the color of the coating after passivation.

The zinc ion concentration is high, the zinc content in the coating is large, and the colorless or easy to fall off pale yellow will appear after passivation; and the high concentration will reduce the dispersion ability of the bright area of ​​the plating part, and the low concentration will lead to the nickel ion in the alloy. The content is large, and the passivation film is colorless or blue-yellow.

The nickel ion concentration is high, and the nickel content in the alloy plating layer is also high. If the nickel content in the plating layer is greater than 16%, especially in the high current density area, it will cause poor passivation. It should be adjusted by increasing the zinc ion concentration; nickel The ion content is low, and the nickel content of the coating is low. When the nickel content is less than 12%, the low current density area will get a dark brown passivation film when the yellow passivation is performed. The relationship between the nickel content of the coating and its corrosion resistance is shown in Figure 1.

As can be seen from Figure 1, the corrosion resistance is the best when the nickel content of the coating is 13%. The concentration of sodium hydroxide should be kept within the specified range. A low concentration will cause poor dispersion in the bright area and darkening of the passivation film in the low current density area.

During trial plating production, problems such as poor brightness of the coating, light yellow and nearly transparent color of the color passivation film occurred. At this time, the content of zinc and nickel ions are within the specified range, and the effect of increasing the current density is still the same. The analysis found that The zinc ion concentration is the upper limit, and the nickel ion content is at the lower limit. The ratio of the two is too different. Finally, the ratio is controlled within (6~10):1. The final coating alloy ratio distribution is normal, and the coating quality also meets the requirements.

During production, various main components cannot be directly added to the plating tank to prevent damage to the proportional relationship in the plating solution. Zinc is first dissolved in the zinc melting tank, and after the analysis is qualified, the filter is used to transport to the plating tank. The addition and adjustment of other components also need to be analyzed and then filtered into the plating tank through the zinc melting tank. Only in this way can the stability of the solution be ensured.

4 Passivation and post-treatment

When the zinc-nickel alloy coating passivation film shrinks during drying, cracks are not easy to reach the surface of the coating, which can delay the corrosion process and improve its corrosion resistance.

There are three main factors that affect the performance of alkaline zinc-nickel alloys after color passivation: the concentration of nickel ions in the plating solution, the pH value of the passivation solution, and the temperature of the passivation solution.

If the concentration of nickel ions in the plating solution is high, the nickel content in the plating layer will be large, which is not easy to passivate or the passivation film is colorless, and when the nickel content is greater than 15%, its corrosion resistance will be reduced.

When the pH value is higher than 2.1, the color of the passivation film is blue or blue-yellow. When the pH value is lower than 1.7, the color of the passivation film is dark and easy to wipe off; when the temperature is low, the passivation film is colorless or blue . The corrosion resistance of the zinc-nickel alloy coating before and after color passivation is shown in Table 1.

From Table 1, it can be seen that proper color passivation can significantly improve the corrosion resistance of the zinc-nickel alloy coating.

There are three main factors that affect the performance of the alkaline zinc-nickel alloy black passivation plating solution: the pH value of the nickel ion concentration passivation solution and the acid corrosion before passivation.

Control the nickel content in the plating solution to passivate between 13% and 15%, and obtain a black and bright passivation film with good adhesion and uniform color; when the pH value of the passivation solution is higher than 1.0, the color of the passivation film is Bright iridescent, the color of the passivation film is dark iridescent when it is lower than 0.8. Poor activation before black passivation, uneven spots will appear in the passivation film.

It can be seen from the above that only by controlling various parameters within the specified process range can the quality and protection requirements of the passivation film be met.