Zinc is an excellent metal for plating iron, steel, aluminum and other metals. It provides a high degree of corrosion resistance and a tough and durable surface. However, several types of zinc coating methods are used and zinc chromate plating provides a number of benefits.
What is Chromate?
When you see vehicles with bright, shiny chrome bumpers and wheels, you are seeing metallic chromium plate and this is not chromate. When we speak of chromate we are talking about a chromate conversion coating. Unlike the shiny finishes metallic chromium provides, a zinc chromate plating contains chromium and zinc and is used mostly as a primer which not only prepares the metal for finishing but provides superior corrosion resistance. In addition, it can greatly increase the wear factor of many parts. Chromates can also be created with magnesium, cadmium, silver, copper and aluminum.
History of Zinc Chromate Plating
Ford Motor Company used zinc chromate for primer coatings as far back as the 1920s. In the 1930s, the United States military and aircraft industry utilized this protective coating. Zinc chromate was used to protect aluminum from corrosion during the Second World War. This coating is light sensitive and when black pigment is included, it helps to protect against ultraviolet light exposure.
One of the best features of zinc chromate is its ability to provide “self healing” properties to metal. For example, if the coating is scratched, it literally sacrifices itself by creating a protective coating from its oxidation.
The Process of Zinc Chromate Plating
To create zinc chromate a “Cronak” process is used. Materials are dipped in a special solution of sulfuric acid and sodium dichromate at room temperature. The coating is soft at first, but begins to harden over time. It also becomes water repellent. Zinc chromate conversion is often used on aluminum for the aircraft industry. Aluminum which has not been treated is very difficult to paint or finish and the conversion process makes the finish adhere to the aluminum surface.
Applications
Zinc chromate is used in several applications today and its primary use is to inhibit corrosion. It is especially useful for aluminum alloy parts in automotive, commercial and military aircraft industries. In addition, a primer paint of zinc chromate is often sprayed on parts for corrosion prevention.
Zinc Chromate Plating and Toxicity
Zinc chromate has been found to be toxic. In fact, it is a known carcinogen and one should wear protection if exposed to the process. However, many modern zinc chromate plating companies are working hard to develop less toxic alternatives to improve public safety and comply with all regulations.
