Before the 1960s, cadmium, also referred to by its elemental abbreviation “Cd,” was the go-to material for numerous industries. The blossoming aerospace industry and rapidly expanding military industry both sought cadmium for use in several key mechanical applications, but things changed when expanding research discovered several drawbacks of the material, both to public health and to the environment.
In response to the discoveries, as well as rapidly mounting government regulations, the industries benefitting most from cadmium had to start seeking out alternatives to the material. Extensive research and repeated trial and error brought forth a variety of alternative materials to use, each with its own unique qualities. Though not identical to cadmium, the alternative materials and processes serve as excellent replacements within the majority of applications.
To better understand cadmium and its alternatives, you should consider the positive and negative qualities of cadmium, as well as contemplate the safer alternatives to cadmium plating currently available on the market.
Why Use Cadmium in Connector Plating?
Cadmium was once widely used in coating applications throughout the automotive, aerospace and marine industries. The metal was primarily used as a finishing option for electrical connectors and connector hardware, including connectors for electrical cables in aerospace and military applications. Fasteners and other mechanical hardware were other common applications for cadmium.
The material was ideal for these situations and applications, primarily because of cadmium’s more unique qualities. Some of these attributes include the following:
- Thinner layers: Cadmium, as well as its alloys, can be electroplated in a very thin layer. This thin, even coating can be achieved on highly detailed or complex shapes, making it an ideal material for electrical connector coatings and other applications where extremely thin layers are necessary for facilitating tight connections.
- Corrosion resistance: Cadmium provides incredible resistance to corrosion compared to other metals. In most cases, corrosion resistance is tested with a constant barrage of salt spray. Cadmium can handle over 1000 hours of salt spray without exhibiting signs of rust, making it incredibly useful in applications where resistance to corrosive elements is key, such as naval applications. Cadmium also provides protection from galvanic corrosion, which is corrosion that occurs because two metals come into electrical contact with one another. This is an extremely important factor in aerospace and military applications.
- Lubricity: Cadmium provides a very low coefficient of friction, which means it moves easily against other surfaces, without much resistance or wear. This is especially important for applications where parts need to last for long periods without much maintenance, such as military or aerospace applications.
- Conductivity: Cadmium is a very electrically conductive material. This quality makes it useful in applications requiring electrical conductivity, such as electrical connector finishes. This conductivity also makes cadmium useful for shielding equipment from electromagnetic interference.
Because of the benefits, cadmium was considered a cost-effective and long-lasting material for various industries for decades. Even today, cadmium has its uses in the aerospace, marine and nuclear industries. The extent and frequency of these applications, however, have reduced significantly in recent decades, largely due to the less desirable qualities of the element.
Why Is Cadmium No Longer Desirable?
As with most materials and processes, we rarely discover the drawbacks until we’ve had a chance to use them. Cadmium plating was no exception. After years of research and observation, as well as advancements in knowledge and technology, experts discovered numerous hazards and weaknesses of cadmium, including the following:
- It sublimates in a vacuum: In hard vacuum environments, like space, cadmium is known to sublime, or transform from a solid to a gas. The products of sublimation can redeposit on equipment, damaging or interfering with essential processes. This is especially problematic in aerospace applications, where equipment is exposed to hard vacuums for long periods, and the slightest equipment malfunction can result in critical losses.
- It’s a known carcinogen: When released into the air in dust form, cadmium can be extremely hazardous to a person’s health. Cancer, lung disease, kidney failure and death are only a few of the possible outcomes of cadmium inhalation, and OSHA guidelines are now extremely strict when it comes to worker exposure to the material.
- It uses cyanide baths: The oldest method for depositing cadmium was onto product-involved baths filled with cyanide, a toxic chemical. Because of its hazardous qualities, working around the material involved the adoption of additional safety precautions, and disposing of used baths was both difficult and environmentally taxing.
For industries still using cadmium, cyanide baths have widely been replaced with sulfuric acid and neutral pH baths, both of which are much less toxic than their cyanide counterpart. Sulfuric acid plating baths are currently common in larger-scale industrial applications, and they are composed of three to five percent sulfuric acid, some grain refiner and a brightener. The neutral pH bath contains ammonium sulfate, ammonium chloride and either ammonium hydroxide or sulfuric acid, producing a neutral solution. Both of these options are much more environmentally friendly than the traditional cyanide bath.
- It’s not as useful in acidic environments: Cadmium is particularly susceptible to acidic environments. Cadmium’s touted corrosion resistance is practically nonexistent in acidic environments, making cadmium useless in certain applications.
- It whiskers: Like pure tin, cadmium tends to whisker, creating loose shards of material. The shards can potentially cause serious damage, as they can detach and lodge themselves into other machinery, causing shorts and other malfunctions. For military and aerospace applications, this can spell disaster.
For these reasons and more, the European Union restricted the use of cadmium via the 2000 End of Life Vehicle Directive, known more commonly as the ELV. The act banned the use of cadmium in the automotive industry in Europe. The United States’ EPA classified the element as a carcinogen shortly after, limiting its use in American industry as well. A few years later, the European Union listed cadmium as one of six hazardous substances to be restricted in their 2006 Restriction of Hazardous Substances directive, also known as the RoHS directive.
Since these legal actions and restrictions, other laws and directives in Europe and the United States have further limited the use of cadmium plating, among other heavy metals. As a result, the marine, automotive, aerospace and military industries have had to look into RoHS-compliant alternatives to cadmium, and plating companies have had to adjust their processes to meet international standards.
What Are the Alternatives to Cadmium?
Numerous hazards associated with the material and regulatory bans have forced interested industries to pursue other options despite cadmium’s unique characteristics. Such options include alternative materials, zinc-based and otherwise, and additional coatings.
Research and testing has shown that zinc alloys in particular are excellent alternatives to cadmium for plating. All zinc alloys are relatively corrosion resistant and can handle up to 370 inch-lbs of torque, making them relatively wear-resistant. Further qualities, including plating methods, depend on the specific zinc alloy.
Some of the most effective zinc alloys include:
- Zinc-Iron: This alloy is an excellent cadmium plating alternative, with the ability to deposit very evenly and at high thicknesses. In addition to being a relatively cheap option, zinc-iron offers excellent corrosion resistance compared to other options. The primary downside to this particular material, however, is the lack of lubricity, especially when compared to tin-zinc. The system used to plate this alloy is based strongly on alkaline zinc plating baths, with iron added to the mix. The percentage of iron in the mixture can vary anywhere from 0.4 to 0.8 percent.
- Zinc-Cobalt: This alloy is able to deposit in very even layers, and can be thickly deposited on a number of surfaces. Similar to zinc-iron, zinc-cobalt has a distinct disadvantage when it comes to lubricity. Zinc-cobalt is deposited in one of two processes, one that is almost identical to the zinc-iron process, and the other that is based on an electrolyte bath. In this second process, zinc and cobalt are added in a ratio of eight to one or ten to one, and use minimal additives. Both processes are efficient and result in a bright end deposit.
- Zinc-Nickel: A zinc-nickel alloy consisting of 10 to 14 percent nickel is an excellent alternative to traditional cadmium, primarily because of its corrosion resistance. A 0.3 millimeter thick deposit can resist over 500 hours of salt spray before exhibiting any sign of rust, and it retains its shape much more effectively than zinc by itself, or even zinc-iron or zinc-cobalt coatings. In addition to its corrosion resistance, zinc-nickel is also an extremely useful material for depositing thin layers. This makes it an ideal material for low-tolerance applications. Zinc-nickel is also the most ductile zinc alloy, making it an excellent choice for applications where forming or manipulation will take place later. Furthermore, zinc-nickel exhibits impressive tolerance to high temperatures and galvanic corrosion. This particular coating uses an acid plating process, developed by Boeing Corp. in 1992 for aerospace applications. Today, zinc-nickel plating is typically completed via rack electroplating or barrel plating, both of which are offered by Sharretts Plating Company.
- Tin-Zinc: This alloy is one of the most common cadmium alternatives beside zinc-nickel, and provides an environmentally friendly plating option with several distinct advantages. Consisting of around 70 percent tin and 30 percent zinc, this option is very corrosion-resistant, with a 0.3 millimeter deposit providing protection from a constant salt spray for more than 600 hours. This material is also very good for soldering and inhibits the growth of tin whiskers, one of the most common problems with the material. Like cadmium, tin-zinc is also resistant against friction and wear. The bath for this process is a nearly neutral pH bath, making it relatively easy to dispose of without negatively affecting the surrounding environment. This cheap and relatively easy plating process has made tin-zinc plating all the more popular in recent years.
These zinc options are very common replacement materials for cadmium, but they are far from the only ones. Aside from zinc alloys, some common cadmium alternatives include:
- Gold: This material provides excellent thermal conductivity, and it is particularly appropriate for applications requiring shielding or low residual magnetism. The primary issue with gold is the cost and relative softness of the material, which makes it unsuitable for certain applications.
- Passivated Stainless Steel: This material is preferred for hardware items like fasteners, primarily for its low cost and hardness. While it does not have the same properties as cadmium, passivated stainless steel is appropriate for some more basic applications where cost is more of a priority.
All of the listed plating options offer several advantages and share certain qualities with cadmium. However, many of the listed materials fall short in areas like corrosion resistance. To make up for this difference in resistance, many manufacturers use additional protective coatings, such as:
- Black coating for zinc-alloys: This trivalent chromium coating, designed for zinc, zinc-iron and zinc-cobalt, results in a protective coating that can triple the corrosion resistance of a zinc-based coating. The process for depositing this coating takes about 20 to 50 seconds, and it takes place at room temperature, resulting in an even black coat.
- Clear coating for zinc-alloys: Another trivalent chromium coating designed for zinc, zinc-iron and zinc-chromium, this protective coating provides the same resistance boost as the black coating without the color change. It can also be dyed to change color for custom orders.
- Clear coating for zinc-nickel: This coating is specifically designed for zinc-nickel, and it can increase the corrosion resistance of a zinc-nickel plating to over 1000 hours of constant salt spray protection.
- Chrome-free coating for zinc alloys: If the application at hand requires a chrome-free option, there are two options available for any zinc plating options. The process must be completed at high temperatures, however, due to the nature of the coating.
Who Can Provide Connector Plating Services?
If you are looking for a plating company experienced in the area of zinc alloy plating, or if you are simply looking for advice on which cadmium alternative you should use, Sharretts Plating Company can help!
Sharretts Plating Company is an RoHS-compliant company of surface treatment experts with over 80 years of experience in metal finishing services. Not only do we pride ourselves on the quality of the product we produce, but we also strive to be an environmentally friendly company, following all RoHS standards concerning materials and disposal techniques.
Since our founding in 1925, we at Sharretts Plating Company have dedicated ourselves to innovation and service. Not only do we hold several patents and continue to pursue advancements in the metal finishing field, but we also provide a consultative approach to metal finishing services, offering our knowledge and expertise to our clients. By pushing for discovery and offering our quality services, we can help our customers meet all of their goals, solidifying long-standing and mutually beneficial relationships with companies across all industries.