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Chromium plating technology

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Hard chromium plating methods for various materials

Hard chromium plating on iron castings

Iron castings can be divided into three categories: gray cast iron, white cast iron and nodular cast iron. Gray cast iron is cooled with furnace after casting, and the internal stress of matrix is small, while white cast iron is cooled in air after casting, so the internal stress of matrix is large.

The common features of cast iron matrix are more pores, rough surface, and higher carbon and silicon contents. In fact, not all brands of cast iron are suitable for electroplating. For example, ht15-32 is not suitable for chromium plating. Ht21-40 and ht24-44 can be plated with chromium, but they should be polished before electroplating to make the surface roughness Ra value reach 0.63um or less. Due to the high carbon content and loose structure of iron castings, the pickling time should be short to prevent the precipitation of a large amount of carbon, resulting in the reduction of hydrogen overpotential and the negative deposition potential of chromium. When electroplating, only a large number of bubbles can be seen, and chromium can not be plated on the parts or locally. Therefore, if there is no rust on the casting, the pickling process can be omitted.

When electroplating cast iron, the bath temperature should be higher, which should be 60 ℃ or above. The anodizing time of gray and white cast iron is 3 ~ 5 seconds, and that of ductile iron can be extended to 15 seconds. The speed of switching on reverse current and positive current should be fast. First, use 100 ~ 200% higher current than normal current to impact for 3 ~ 5min, and then return to normal value. The impact current of nodular cast iron can be slightly smaller, about 50 ~ 75% higher than the normal value

If there are sandblasting conditions, try not to use pickling and reverse etching

In a word, the type of cast iron, the surface roughness of base metal, the shape and distance of anode, the time of anode treatment, the speed and size of starting current and other factors are the key to the success of hard chromium electroplating

Hard chromium plating on stainless steel

There are two kinds of stainless steel in electroplating

1. Non austenitic stainless steel: such as 2Cr13, 4Cr13, etc. This kind of stainless steel has high carbon content, and its crystal structure will change after heat treatment and quenching.

2. Austenitic stainless steel: such as 1Cr18Ni9Ti, cr14mn14ni, cr14ni14, etc. This kind of stainless steel has high nickel content

Stainless steel in the air or water will produce a strong anti rust oxide film, this layer of oxide film must be removed before electroplating, otherwise the adhesion of chromium layer is poor.

For the first kind of stainless steel, after pickling and washing, anodizing can be carried out for 1 ~ 2 minutes, and the time should not be too long. Then, the workpiece is turned into cathode, and the method of "ladder junction" is adopted, that is, the power is supplied for 1min (<; After that, the current density was gradually increased to the process current, and the whole process was completed in about 5 minutes;

For the second kind of stainless steel, because of its high nickel content, the surface is very easy to passivate, so generally do not use anodic treatment. This kind of stainless steel electroplating method is to use 10 ~ 15% sulfuric acid activation, charged (2 ~ 2.5V) in the bath, and then gradually increase the current to the process value

Hard chromium plating on other types of steel

1. High speed steel is similar to the first kind of stainless steel (non austenite)

2. After degreasing and derusting, molybdenum steel is impregnated first (the same method as austenitic stainless steel), and then stepped electrified (the same method as non austenitic stainless steel)

In a word, the thicker the coating is, the longer the anodizing time is.

Technical service of hard chromium plating

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