Think About Zinc, When it Comes to Plating and Corrosive Protection
As a leading provider of quality bolts and fasteners for the automotive, military, agriculture, transportation and material handling markets, Auto Bolt sees its fair share of products that are plated. Whether it’s electroplating or an organic coating, we’ve “covered” them all.
One popular coating is electro zinc plating. The zinc is applied to steel and iron to protect against corrosion. Electroplating or electrodeposition is the process of coating a metal with a thin layer of another metal by electrolysis.
Electroplating is done in a water-based solution which contains the chemical compound to be coated (in this case zinc). The parts are immersed into the solution and electric current is passed through the solution. This causes the plating metal to precipitate out and be deposited on the part.
Zinc plating is popular for the following reasons:
- Least expensive of the plating metals
- Broad range of thickness
- Good Corrosion resistance
- Good appearance especially with chromate treatments
- Relatively non-toxic
Typically, zinc coatings for fasteners range in thickness from 0.12 to 0.47 mils (3 to 12 µm). Coating thickness is regulated by the amount of zinc charged to the plating barrel and the duration of tumbling time.
Chromate Treatments Available
Chromate treatments are available for zinc and other platings. Chromate improves the appearance of the zinc as well as providing a hard non-porous film. Chromate coatings come in a variety of colors with the most common being clear, blue, yellow (iridescent), black, and olive drab.
Chromates can be broken down by their impact on the environment. That is whether they contain hexavalent chromate or not. Hexavalent chromates contain Cr6, which is a hazardous material and carcinogen. Hexavalent chromate has been phased out by the automotive industry with other industries following suit.
Trivalent chromate or passivate contains Cr3, which is a safer and approved alternative.
Hexavalent versus Trivalent Chromate
Both chromates are both still commercially available. Depending on the end use, hexavalent chromates are still very popular.
Advantages of hexavalent chromate:
- Relatively low cost
- Hexavalent chromate is “self-healing” to scratches caused by part-to-part contact.
- Typically better corrosion resistance than most trivalent chromates
- Chromate colors generally indicate the level of corrosion resistance; the darker the color, the more resistance to corrosion.
- Compatible with most topcoats and sealers.
The advantages of trivalent chromates:
- Becoming more and more common place due to environmental mandates
- Meets REACH, ROHS, and other environmental mandates.
- Color of the chromate is less important to the performance. Color is more for product identification or cosmetic appearance.
- Although typically lower in corrosion resistance than hexavalent, there are many formulations available for increased corrosion resistance.
- Many topcoats and torque modifiers are available.
- Many of the chemical companies offer coating systems, which comprise of the zinc base, chromate, and topcoats. These coating systems are typically designed to meet OEM and Industrial specifications.