1.Introduction

During the electroplating process of spring workpiece, hydrogen permeation is easy to occur in the processes of degreasing, acid etching and tin plating.The results of light blister coating, peeling; In severe cases, hydrogen embrittlement occurs, leading to spring fracture.
The characteristics of spring workpiece are:the diameter of spring material is small, between 0.2 and 1.2 mm; The outer diameter of spring is not big, generally within 4mm.Therefore, it is particularly important to reduce the hydrogen leakage of the spring and avoid the broken spring. 

2.Degreasing

The oil removal of spring workpiece is mainly based on 95 × aviation gasoline cleaning.Through gasoline cleaning, the oil on the workpiece is basically removed.Then, chemical degreasing was carried out for 5-10 minutes.Finally, the anode electrolysis was carried out in the electrolytic degreasing solution.During electrolysis, Da was controlled within 2.5a/dm2 for 3-5min.In any case, cathodic electrolysis is not allowed to remove oil. 

3.Acid etching

When the spring workpiece is etched by acid, sulfuric acid is not used, but only hydrochloric acid is used.The concentration of hydrochloric acid should be strictly controlled, and thiophene or Urotropine should be added as corrosion inhibitor.The content of HCI in the solution is generally 100-140 g/L, and the content of corrosion inhibitor should be controlled at 2-3 g/L.When there is ash hanging on the spring workpiece or contact copper is removed, it shall not be removed in the mixture of chromic acid and sulfuric acid, but cleaned in the mixture of chromic acid and sulfuric acid.Specific formula; Cr03 140～180g/L，(NH4)so4:60～90g/L。

4.Tinning

It is generally believed that in order to prevent hydrogen permeation in tin plating of spring workpiece, acid tin plating electrolyte with current efficiency of 95% is used.However, for small workpieces, in order to obtain high-quality tin coating, alkaline tin plating process is preferred
(1) Copper plating is not easy to be too thick, and the copper coating can reach 2 microns.
(2) The time of copper and tin plating should be shortened as much as possible.
(3) When the workpiece enters the tin plating solution to start electroplating, it can be impacted by high current for 3-5min, DK=1.5-2.0a/dm2.In order to prevent the copper layer from oxidation, improve the binding force and slow down the hydrogen permeation, the copper layer is quickly covered with a layer of tin.

5.Dehydrogenation treatment

In the above processes, although there are some anti-hydrogen measures, hydrogen is more or less infiltrated into the inside of the spring workpiece and the coating.The workpiece must be dehydrogenated.The author has done such an experiment:take the same tin plated spring workpiece in the same electroplating basket, part of the workpiece is dehydrogenated, and the other part of a certain number of workpiece is not dehydrogenated.The brittle tensile test of these two kinds of spring workpieces shows that about 18-20% of the springs without dehydrogenation break.The spring after dehydrogenation treatment did not break.This shows the importance of dehydrogenation of spring after plating. 

Hydrogen removal is carried out in a constant temperature electric oven, which shall be equipped with a blower to make the temperature in the oven uniform.The temperature was controlled at 125-135c for 120-240min;

Last：Knowledge of powder metallurgy

Next：Properties and standards of chromic anhydride for chromium plating