Analysis of the causes of uneven thickness of electrolytic nickel plates

The most fundamental cause of uneven thickness lies in the uneven distribution of the electric field. During electrodeposition, the actual thickness at any point on the surface is primarily determined by the current density at that point, while the current distribution is primarily determined by geometric factors such as the shape of the part, its position relative to the anode, and the way the part is placed on the plating rack. The current density is higher at the protrusions and the area closest to the anode because the distance between the anode and the cathode is shorter and the resistance to current flow is smaller; on the contrary, the recesses and areas that are blocked or facing away from the anode have increased resistance and smaller current. These changes in current density necessarily result in thicker plating being formed at the protrusions.

The most typical phenomenon is undoubtedly the "edge effect." Due to the concentration of electric power lines in the edge area of ​​the cathode plate, there is a significant difference in current resistance between the center and the edge of the current field. In particular, the thickness at the edge is thicker than at the center. As the plating area increases, the edge effect becomes more serious. In actual production, the ratio of edge thickness to center thickness usually reaches 2.5 to 4 times, and the deposition area naturally forms a concave cross-section. Parts where the nickel thickness exceeds the range are also often concentrated at the edges of the plate.

In addition to the "primary current distribution" determined by geometric factors, electrochemical factors also have an important impact on thickness uniformity. Process parameters such as cathode current density, electrolyte conductivity, cathode-anode spacing, anode plate area, and electrolytic cell size will all affect the uniformity of the electrodeposition thickness distribution. Among them, the peak current density has a particularly significant impact on thickness uniformity. As the peak current density increases, the uniformity will significantly decrease.

The electrolyte formula and the selection of additives are key variables that directly determine the deposition behavior. Due to different current parameters, electrolyte formulas, temperatures and additives from different manufacturers, there are large differences in the internal quality of electrolytic nickel plates. Adding appropriate additives to the electrolyte is an effective strategy to solve the problem of uneven coating deposition. However, if the concentration distribution of various additives such as brighteners and wetting agents in the electrolyte is uneven, it will directly change the local polarization behavior, expand the difference in deposition overpotential, and then form a thickness deviation. The throwing ability of nickel plating solutions is generally at the low end of the positive range. Lowering the current density, increasing the conductivity of the solution, increasing the distance between cathodes and anodes, and appropriately increasing the pH value and temperature can improve the throwing ability to a certain extent.

The circulation and flow field distribution of the electrolyte cannot be ignored. The circulating flow of electrolyte can make the distribution of nickel ions in the electrolytic tank more uniform, thereby improving the quality of the finished electrolytic nickel. Uneven flow rates, temperature field differences, or concentration fluctuations may cause the nickel ion mass transfer velocity to deviate at different locations, further exacerbating thickness unevenness.

Electrolytic nickel uses the initial electrode piece as the substrate for electrodeposition growth. The quality of the initial electrode piece directly determines the uniformity of subsequent deposition. When a seed plate electrolyzer is used to prepare the starter plate, each seed plate in the same electrolytic cell is connected in parallel, and the current is freely distributed on each seed plate with a large difference, resulting in uneven thickness of the starter plate. If the surface treatment process of the starter plate is improper, or there is an oil stain or oxide layer on the surface, the nucleation barrier will be inconsistent, the local nickel ion reduction rate will be different, and finally, the thickness will be uneven on the entire board.

The continued stability of the process cannot be ignored either. Interruptions in the electrolysis process or current fluctuations can easily cause stacks (or delaminations) of varying sizes to appear inside the plate, and most of the delaminations occur near the thickness of the original starting electrode piece where nickel electrodeposition begins. When nickel is deposited on the cathode, hydrogen is easily evolved, and micropores are formed inside the electrolytic nickel plate. These internal defects are difficult to detect with the naked eye, but when serious, they can have a negative impact on product consistency and subsequent processing.

4. Industry improvement direction

In response to the above problems, the industry has developed a number of effective improvement measures. In terms of electric field control, methods such as the pictographic anode method, the addition of anode baffles, and the addition of dispersants to the electrolyte can effectively improve the uniformity of current distribution. The auxiliary cathode enables a more uniform nickel deposit thickness by preventing excess charge build-up. By changing the distribution of power lines and blocking or weakening the power lines at the edge of the cathode plate, the "lipless" preparation process can also effectively control the thickness uniformity of electrolytic nickel thick plates.

In terms of process optimization, finely controlling the current density, optimizing the additive ratio and purification conditions, and using multiple cathode plates to produce simultaneously to balance the cathode and anode areas can effectively alleviate edge effects and significantly improve thickness uniformity. In addition, the use of ultrasonic flaw detection technology to sort out delamination defects and subsequent heat treatment is also an important means to ensure stable product quality.

Conclusion

The uneven thickness of the electrolytic nickel plate is not caused by a single factor but is the result of the interweaving of multiple factors such as electric field distribution, electrolyte chemistry, flow field dynamics, and operation management. A deep understanding of these mechanisms is the key to systematically improving the quality of electrolytic nickel products and will also lay a solid foundation for upgrading electrolytic nickel products to high quality and high added value.