(1)Processing history of materials

Iron and steel materials have been used in a wide variety of industrial fields. They are usually formed into intended products through such metal processing as listed below and then subject to plating.

Therefore, steel materials greatly vary in the state of metal surface according to the type of steel and the type and history of metal working as mentioned in the previous series of tutorial. Consequently, appropriate surface treatment methods should be applied for these factors.
In the application of electroplating, especially important indicators include the adhesion properties of plating to the material to be plated and the finished state of the plated surface. Important key points are chemical pretreatment (e.g., degreasing, acid treatment) for the former and mechanical pretreatment (e.g., mirror polishing, satin finish) for the latter.

(2)Dehydrogenation treatment of high-carbon steel

Additionally, one of the important points specific to steel materials is post-treatment after plating. Where acid pickling, zinc plating, or industrial chrome plating is applied to carbon steel, hardened steel, or other materials, which contain 0.4% or more carbon, atomic hydrogen is produced on the surface of the materials during the treatment and the hydrogen atoms are occluded in the steel microstructure. If the product is used without removal of hydrogen, it offers high hardness but becomes very brittle and then defined as a defective product. This is referred to as hydrogen embrittlement.
In order to prevent this phenomenon, heat treatment is conducted within fourhours after plating to remove the hydrogen from the material (baking process). A typical case is described below.

[Table] Conditions for hydrogenation

Therefore, the chromate treatment after zinc plating is applied following the baking treatment.