Coatings pictured above (left to right): Zinc-Nickel, Phosphate & Oil, Zinc with Trivalent Chromium, Zinc Flake
Before the 1960s, the fastener industry relied on cadmium and hexavalent chromium (also known as hexachromium) to protect bolts, screws, studs, and other essential components from corrosion. These coatings offered incredible protection, particularly in harsh environments and applications.
But there was just one problem; they were toxic.
After a few years of research, cadmium and hexachromium were found to pose significant environmental and public health risks. Now, as industrial safety standards and environmental regulations evolve, fastener manufacturers are transitioning toward safer coating alternatives.
The Problem with Cadmium Fastener Coatings
Cadmium had previously been a popular fastener coating due to its exceptional corrosion resistance, even in saltwater environments (think marine vessels or offshore oil rigs). It also offers great lubricity, which can help prevent fastener damage like thread galling. Unfortunately, the coating was too good to be true.
While cadmium still has its uses in some aerospace, marine, and nuclear industries, its use has significantly declined and is highly regulated.
Cadmium Health & Safety Risks
Cadmium is now classified as a carcinogen, meaning prolonged exposure can increase the risk of cancer. Workers who handle cadmium-coated fasteners or even inhale cadmium dust during manufacturing are at risk of severe respiratory, kidney, and/or bone damage.
In addition, cadmium can leach into soil and water and cause environmental harm.
Regulatory Pressures
Due to its toxicity, regulatory bodies began strictly limiting the use of cadmium. The European Union’s Restriction of Hazardous Substances (RoHS) did so via their 2000 End of Life Vehicle (ELV) Directive, and the United States’ Environmental Protection Agency (EPA) followed suit shortly after. This forced engineers and manufacturers to find a different solution with similar corrosion resistance.
Take 2: Hexavalent Chromium Fastener Coatings
The initial solution to cadmium’s health and safety risks involved switching to zinc fastener coatings. Zinc provided excellent corrosion resistance, but the issue wasn’t entirely solved because hexachromium was commonly used as a passivating layer over zinc to provide even more protection, and it too was hazardous.
Hexachromium Health & Safety Risks
Like cadmium, hexachromium is highly toxic. It is recognized as a human carcinogen and can cause severe respiratory issues like lung cancer, skin ulcers, and eye damage in workers who are exposed to it. Also like cadmium, it can contaminate soil and groundwater, leading to environmental damage.
Fun fact: hexachromium because infamous following its depiction in the movie Erin Brockovich, which highlighted its harmful impact on public health in Hinkley, California.
Regulatory Pressures
For all the reasons listed above, hexachromium was also heavily regulated very quickly. The U.S. EPA defined it as hazardous in 2006 and slowly phased out its use. It is also restricted under the RoHS directive.
4 Safe Coating Alternatives for Cadmium & Hexachromium
Fortunately, several safer fastener coating solutions have emerged that provide similar, if not better, performance in terms of corrosion resistance and durability. As the industry moves away from cadmium and hexachromium, it’s turning to these options that meet modern environmental and health standards.
1. Zinc-Nickel Plating
One of the most popular cadmium alternatives is zinc-nickel (short for zinc-nickel electroplate). This coating provides excellent corrosion resistance, making it suitable for demanding environments like marine and automotive applications.
Zinc-nickel coatings typically consist of an 8-14μm layer of 12-16% nickel alloy that’s topped with a .06-.15μm trivalent passivate layer and a .5-4μm layer of top coat. The additional layers enhance the overall protection compared to pure zinc-nickel.
Zinc-nickel coatings are receiving more recognition now than ever. The U.S. military and other government entities are really pushing it. Here’s why:
- Up to 10x more corrosion resistance than conventional zinc — up to 1000 Neutral Salt Spray hours!
- Great thermal stress and wear resistance, especially in applications with moving parts.
- Safe for the environment and human health, as we’ve mentioned.
- Reasonable cost, falling somewhere between the moderate to moderately-high price category, which is worth it for the performance and safety.
Do keep in mind that zinc-nickel plating can put fasteners at risk for hydrogen embrittlement, which is a legitimate concern, but can be eliminated by baking the parts after plating.
2. Zinc with Trivalent Chromium
Zinc coatings treated with trivalent chromium passivation have emerged as a leading hexavalent chromium alternative. Unlike hexavalent chromium, trivalent chromium (also known as Cr3+) is far less toxic and presents minimal health and environmental risks.
In addition to being safer, zinc with trivalent chromium offers comparable corrosion resistance and has become the preferred choice for many industrial fastener applications, particularly in the automotive and electronics industries.
Just like zinc-nickel plating, this type of coating is electroplated so the risk of hydrogen embrittlement is present here too.
3. Organic Zinc Flake Coating
Organic zinc flake coatings, such as Magni and Geomet, have gained popularity as a highly effective and environmentally friendly option for fastener protection. These coatings contain zinc flakes suspended in an organic binder, which provides exceptional corrosion resistance without the need for electroplating or hazardous chemicals like cadmium or hexavalent chromium.
One of the key advantages of organic zinc flake coating is its ability to offer high levels of corrosion protection even with thin layers, reducing the overall weight of the fasteners. These coatings are also free of hydrogen embrittlement, making them ideal for high-tensile fasteners that must maintain their strength and integrity under stress.
One minor disadvantage of zinc flake coating is that it needs time to cure, either by baking or at room temperature, before the parts can be put into the field. This cure time can take up to seven days for the coating to be fully effective.
4. Phosphate and Oil Coating
For applications where extreme corrosion resistance isn’t necessary, phosphate and oil coatings offer a cost-effective and simpler solution. This coating method involves applying a layer of phosphate to the surface of the fastener, followed by an oil coating that provides lubrication and mild corrosion resistance.
While phosphate and oil coatings don’t offer the same level of protection as the other options mentioned in this article, they are widely used in less corrosive environments, such as indoor applications or equipment that doesn’t encounter harsh elements.
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