Two misconceptions about nickel-free plating
Error 1:Nickel-free plating is called nickel-free plating.
Error 2: Plating that uses a copper-tin alloy instead of nickel plating is called nickel-free plating.
The correct definition of nickel-free plating
Plating with a copper-tin alloy of 3 microns or more instead of nickel plating of the same thickness to obtain a coating with corrosion resistance comparable to or better than that of nickel plating is called nickel-free plating.
Why choose nickel-free plating?
Since nickel can cause allergic reactions to human skin and blood, modern environmentally friendly plating has replaced nickel plating with copper-tin alloy plating.
Advantages of nickel-free plating
The copper-tin alloy plating has an excellent low electric displacement and a good density of plating, which makes it more corrosion resistant than nickel plating when the thickness is the same as nickel plating.
Comparison between nickel-free and nickel plating processes (for example, gold plating of zinc alloy die-cast jewelry) – Nickel plating process:
Substrate → pre-treatment → alkaline copper → coke copper → acid copper → nickel plating → pre-gold plating → gold plating – nickel-free plating process:
Substrate → pre-treatment → alkaline copper → burnt copper → acid copper → bright brass tin → thick white copper tin → pre-gold plating → gold plating Remark:
The nickel plating time is usually 5 minutes and the thickness of the layer is about 3 microns with some corrosion resistance.
Bright brass tin gives a bright mirror finish, detailed structure, enhanced bonding, deep plating ability, and corrosion resistance.
The 3-micron thick white brass tin (plating time approx. 6-7 minutes) further enhances the deep plating ability and corrosion resistance of the layer, giving better protection than nickel plating, which is a standard nickel-free plating process.
The unreasonable nickel-free plating process
Substrate → pretreatment → alkaline copper → burnt copper → acid copper → ordinary white copper tin (1-2 minutes) → gold plating Remark:
Ordinary white copper tin has no brightness, resulting in poor product gloss.
The deposition rate of ordinary white copper and tin is slow, about 1 micron for 5 minutes, which cannot play the role of wear resistance and corrosion resistance, so it is easy to discolor after gold plating.
The influence of the process on the performance of the plating layer (zinc alloy material):
The wear and corrosion resistance of the plating obtained by straight white copper-tin on acid copper is not good.
The corrosion and wear resistance of the plated layer obtained by plating brass tin on acid copper followed by white copper tin is significantly improved.
Cross-sectional metallographic analysis of nickel-free plated layers by different processes:
The acid copper layer and the white copper-tin layer penetrate each other, resulting in a loose and uncompacted plating structure that is easily discolored by corrosion.
The brass-tin layer penetrates into the acid copper layer to form a tightly structured protective layer, blocking the acid copper layer and the white copper-tin layer from penetrating each other, resulting in a structurally dense white copper-tin layer.
The brass-tin layer penetrates into the acid copper layer to form a tightly structured protective layer and interpenetrates with the white brass-tin layer to form a denser protective film, improving the overall corrosion resistance of the coating.
Neutral salt spray test comparison.
Note: The above data is obtained from experiments based on actual substrates and actual plating layers, which may lead to slight deviations due to different substrates or processes.
