Hazard of uneven thickness of plating

- Jan 24, 2019-

In many cases, how to evaluate the effect of a part's electroplating process depends mainly on whether the surface is smooth, dense, and whether the thickness of the coating is uniform. Therefore, it is always desirable to have a uniform distribution of the thickness of the plating on the workpiece during electrodeposition. When the total amount of metal deposited on the workpiece is the same, if the thickness distribution is not uniform, many defects are caused.

For example, the problem of unevenness of the thickness of the galvanized layer is a problem that cannot be ignored in the manufacture of the threaded fastener, which is related to the screwing property of the thread and the quality of the coating. The thickness of the electroplated zinc layer has a great relationship with the quality of the galvanized layer, mainly in:

(1) The corrosion resistance of the galvanized layer depends on the thickness of the coating and the exposed environment. The harsher the conditions of use, the thicker the coating is required, and the corrosion resistance of the entire coating depends on the weakest part of the coating, including the smallest local thickness. The corrosion resistance of the place.

(2) Where the zinc layer is too thick, it is prone to rickets such as roughness, nodules, and shedding.

(3) Where the zinc layer is too thin, the gloss is poor, dark, foggy, and easy to expose in the process of light extraction and passivation.

The following summarizes the effects of uneven coating thickness:

For the anodic coating, the thin layer of the coating cannot withstand sacrificial corrosion and the substrate will be rusted. However, after a part of the part is rusted, it is unqualified, and the excessive thickness actually forms a waste of the plated metal. If it is to ensure that the thinnest part does not rust, the average thickness can be greatly increased, and the plating processing cost is increased.

For the cathodic coating, the porosity of the thin coating is high, and it is easy to produce point rust, and then the rust point is increased to form contiguous rust. Compared to the anodized coating, the coating is thin and rusted faster. For local anti-seepage and carburizing coatings, the pores are easy to form pores and lose protection. If the thickness is uniform, the porosity of each part is not much different, and the overall corrosion resistance is improved.

For bright plating, the thin layer of the coating has a small cathode current density and poor lightness and flatness, which deteriorates the overall appearance.

Alloy electrodeposition is characterized by different alloy compositions at different thicknesses, or uneven appearance, or inconsistent corrosion resistance.

The physical and mechanical properties of the coating at different thicknesses are different (such as brittleness, internal stress, etc.). If it is subjected to machining processing such as press forming after plating, the coating is too thick and often has poor machinability (peeling, cracking, powder shedding, etc.).

Regardless of corrosion resistance, appearance, and machinability, it is desirable to improve the thickness uniformity of the plating layer. It is difficult to apply hard chrome to parts with precise dimensional requirements. Sometimes users do not need to grind after plating. It is difficult to do it; sometimes, in order to ensure that the thinnest part meets the final size requirement, when the thickness uniformity is poor, the average thickness must be greatly increased. This is not uncommon in production. In order to make the thickness of each part of the coating on the part as close as possible, it is necessary to understand the factors that affect the uniformity of the thickness distribution.

In the electroplating process, current density and time, temperature, main salt concentration, anode area, bath agitation and other factors will have an impact on the uniformity of the thickness of the coating. Therefore, the electroplating plant needs to pay more attention to the electroplating process. All the links in the process are strictly controlled to ensure the quality of the workpiece is stable and the thickness of the coating is uniform.