Yellowing of zinc-nickel alloy plating in the low current region is usually related to process parameters, bath composition and operating conditions. The following are a few possible causes of yellowing in the low current region:
1. Abnormal nickel content: If the nickel content in a zinc-nickel alloy plating bath is too high or unevenly distributed, yellowing in the low-current region may result because the higher nickel content is out of proportion to the alloy formed by the zinc.
2. Localized overpotentials: In areas of low current density, uneven charge distribution may result in localized areas of varying rates of zinc and nickel deposition, which in turn creates an uneven alloy composition that manifests itself as yellowing.
3. Incomplete passivation: If the post-treatment (e.g. passivation) is not performed correctly or is incomplete, it may result in an incomplete passivation film in the low-current region, which, in turn, may appear as a yellowing in appearance.
4. Plating solution pH: If the pH of the plating solution is not properly controlled (too high or too low), it may affect the quality of the plating layer, especially in areas of low electrochemical activity prone to uneven color.
5. Effect of organic additives: Zinc-nickel alloy additives can also affect the color of the plated layer. Excessive or inappropriate additives may form irregular deposits in low current areas, resulting in color changes.
6. Excessive plating time: In the low current zone, nickel deposition may be excessive if the plating time is too long, thus affecting the color and properties of the zinc-nickel alloy.
Optimization of plating parameters, such as adjustment of bath pH, temperature, agitation, current density, and type and concentration of additives, is often required to solve this problem. In addition, regular maintenance and monitoring of the plating bath to ensure uniformity of the bath composition and its distribution is also essential to prevent yellowing in low current areas. The importance of zinc-nickel alloy additives should also not be overlooked. Adding Bigely zinc-nickel alloy additives to the bath can effectively improve the whiteness of the plated layer, with good equalizing ability; and the plated layer has good protection and low hydrogen embrittlement properties, which makes it suitable for plating high-strength steels.
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