In the previous volume, we learned that chromium-plating films are free from hexavalent chromium, but chromate films consisting primarily of chromic acid contain hexavalent chromium. We also learned that the higher content of hexavalent chromium results in superior anticorrosive properties.
A chemical process of forming a film, chromate treatment for example, is called chemical conversion coating. The films formed by this process are referred to as chemical films. Conventionally, chromic acid was used in this process.
The representative example is chemical conversion coating for enhancing adhesiveness of aluminum alloy or magnesium alloy. These metals are both active and known to form natural oxide films in the air. Thus, if they are coated directly, they fall off easily because of their poor adhesive performance. In addition, as a post-plating process, chromate treatment has been applied to surface treatment steel plates produced in large quantity.
Various methods have been researched and developed in order to eliminate hexavalent chromium in these products. Such attempts are classified into mainly two directions.

(1)Trivalent chromate film

Since trivalent chromium is not regulated as hexavalent chromium, most manufacturers would try the familiar method of forming chemical films using trivalent chromate solution containing chromium compounds. Currently, applying trivalent chromium films is a mainstream treatment after galvanizing. [Table 1] is an example of a product distributed by a certain manufacturer.

[Table 1] Comparison of hexavalent chromium and trivalent chromium chemical films

(2)Non-chromium compounds

Industrial use of magnesium has become popular quite recently, when mobile phones were rapidly spread across the world. Using chromium-free chemical films has been a common practice for aluminum, which is used for beverage cans (coffee and beer, for example) and construction materials.
Possible metallic salts as an alternative of chromium include transition metals, such as Ti (titanium). Zr (zirconium), Mn (manganese), Mo (molybdenum), V (vanadium), Co (cobalt), and rare-earth elements like Ce (cerium).
However, their anticorrosion properties are not as good as chromic acid materials when they are used by themselves. In actual applications, they are mixed with chelating agents like tannic acid, colloidal silica, hydrosoluble polymer, or organosilane compounds.
The ultimate goal of surface treatment processes is to eliminate chromium from the processes. Eventually, use of trivalent chromium will become obsolete.