Cadmium plating is a surface finishing process that deposits a thin layer of cadmium metal onto steel or other substrates to protect them from corrosion. It remains one of the most common coatings for aerospace fasteners, valued for a combination of properties that few alternatives can fully match: natural lubricity, excellent compatibility with aluminum, and reliable corrosion protection in marine environments. Despite its effectiveness, cadmium is a known carcinogen, and its use is increasingly restricted outside of defense and aerospace.
How the Plating Process Works
Cadmium plating uses electrodeposition, the same basic principle behind chrome or zinc plating. The part to be coated is submerged in a chemical bath containing dissolved cadmium ions (typically from cadmium sulfate or cadmium cyanide). The part is connected to the negative terminal of a direct current power supply, making it the cathode. A cadmium anode sits in the same bath. When current flows, cadmium ions migrate through the solution and deposit as a thin, uniform metallic layer on the part’s surface.
Before plating, every part goes through thorough cleaning and pretreatment to strip away oils, oxides, and contaminants. This step is critical because even small surface impurities can prevent the cadmium from bonding properly, leading to flaking or uneven coverage. The plating bath also contains additives that control how evenly the cadmium deposits and how well it adheres.
After plating, parts typically receive a chromate conversion coating. This secondary treatment enhances corrosion resistance and gives the plated surface its recognizable color. Depending on the chromate type, the finish can appear bluish-clear, golden-yellow, or olive drab. The olive drab finish is especially common on military hardware.
Why Cadmium Is Hard to Replace
Cadmium’s value comes from several properties working together. First, it acts as a sacrificial coating: if the plating gets scratched or damaged, the cadmium corrodes preferentially instead of the underlying steel, continuing to protect the base metal even where the coating is compromised. This is the same principle behind zinc coatings, but cadmium outperforms zinc in salt water and marine atmospheres. A 25-micrometer-thick cadmium coating may only last about a year in a polluted industrial atmosphere, but its lifespan increases significantly in marine environments.
Second, cadmium has natural lubricity. Plated fasteners slide smoothly during installation, producing consistent torque values. This matters enormously in aerospace, where fastener tension needs to be precise and predictable. Third, cadmium is easy to solder, making it useful for electrical connectors and components. And fourth, it causes very little galvanic corrosion when in contact with aluminum, which is the primary structural material in aircraft. Many alternative coatings accelerate corrosion at aluminum contact points, creating a problem cadmium largely avoids.
Where Cadmium Plating Is Still Used
Aerospace and defense are the primary industries that still rely on cadmium plating. According to NASA’s Fastener Design Manual, cadmium is the most common plating material for aerospace fasteners. High-strength alloy steel bolts, nuts, and screws throughout aircraft structures are routinely cadmium plated because these fasteners need corrosion protection without any loss in mechanical performance.
Military connectors, landing gear components, and various hardware on naval vessels also use cadmium coatings. In these applications, the combination of salt spray resistance, lubricity, and aluminum compatibility is difficult to achieve with a single alternative coating. Commercial electronics and consumer products, by contrast, have largely moved away from cadmium due to regulatory pressure.
Hydrogen Embrittlement Risk
One significant concern with cadmium plating on high-strength steel is hydrogen embrittlement. During the electroplating process, hydrogen atoms can be absorbed into the steel’s microstructure. In ultra-high-strength steels, even concentrations as low as one part per million by weight can cause severe embrittlement, making the steel crack under loads it would normally handle easily.
To address this, plated parts undergo a baking step after plating. The standard practice calls for heating the parts to 190 to 220°C for an extended period, often around 24 hours. Research from the Defense Technical Information Center has shown that baking at 190°C effectively drives out the harmful diffusible hydrogen from steel, including hydrogen trapped at specific sites within the metal’s crystal structure. This baking step is not optional for high-strength applications. Skipping it or cutting it short can lead to sudden, catastrophic part failure with no visible warning.
The cadmium layer itself complicates hydrogen removal because it acts as a barrier, slowing the outward diffusion of hydrogen. Thin, porous cadmium coatings allow hydrogen to escape more readily during baking than thick, dense ones, which is one reason plating thickness and baking protocols are tightly controlled in aerospace specifications.
Health and Environmental Concerns
Cadmium is toxic to humans, with the two primary risks being kidney damage and lung cancer. Workers who inhale cadmium dust or fumes during plating, grinding, or stripping operations face the greatest exposure. Epidemiological studies have found increased kidney dysfunction at cumulative airborne exposures well below levels once considered safe, and at least four out of five occupational studies have found qualitative evidence of lung cancer risk.
OSHA’s permissible exposure limit for airborne cadmium has been set at 5 micrograms per cubic meter over an 8-hour workday, based on preventing both kidney damage and lung cancer over a full working lifetime. Researchers have noted that no clear “safe” threshold for kidney toxicity has been established, meaning even low chronic exposures carry some risk.
From an environmental standpoint, cadmium plating generates hazardous waste in the form of spent plating solutions and rinse water. Disposal is heavily regulated, and facilities that perform cadmium plating must follow strict containment and ventilation protocols.
Regulatory Restrictions
The European Union restricts cadmium in electrical and electronic equipment under the RoHS Directive. General use of cadmium compounds in consumer electronics is prohibited, though specific applications can receive temporary exemptions with defined expiration dates. These exemptions cover niche uses like certain quantum dot displays (exempted through 2027) and medical device repair in closed-loop systems. Aerospace and defense applications often fall outside RoHS scope entirely, which is one reason cadmium plating persists in those sectors.
The EU’s REACH regulation also restricts cadmium, and many European manufacturers have moved to alternatives even where exemptions technically allow continued use. In the United States, there is no outright ban on cadmium plating, but OSHA exposure limits, EPA waste regulations, and Department of Defense initiatives to reduce hazardous materials have all pushed the industry toward replacements.
Alternatives Gaining Ground
The leading replacement for cadmium plating is zinc-nickel alloy plating, often with a trivalent chromium conversion coating. The U.S. Navy’s DDG-1000 destroyer program, for example, has accepted trivalent chromium as an approved alternative to cadmium. Testing by the Naval Surface Warfare Center found that certain zinc-nickel multilayer coatings matched or exceeded cadmium’s corrosion resistance in accelerated salt fog and outdoor seashore exposure tests.
Not every alternative performs equally, though. Some commercial zinc-nickel formulations showed pinhole rusting across the entire coated surface within the first week of accelerated corrosion testing. The most promising systems use multiple layers, combining zinc-nickel with phosphorus or silicon dioxide additions, to build up robust corrosion barriers. Several of these multilayer systems demonstrated superior corrosion performance compared to standard cadmium plating in head-to-head testing.
The challenge with alternatives is replicating cadmium’s full package of properties. A coating might match cadmium’s corrosion resistance but lack its lubricity, or perform well in salt spray but cause galvanic problems with aluminum. Qualifying a new coating for aerospace use also requires years of testing and certification, which slows adoption even when the technical performance is proven. For now, cadmium plating remains specified on thousands of active aerospace and military part numbers, and full replacement across the industry is still years away.