Aluminum is the most commonly anodized metal, but it’s far from the only one. Titanium, magnesium, zinc, niobium, and tantalum can all be anodized as well. The process works on these metals because they each form a stable, adherent oxide layer when exposed to an electrolytic bath. Metals like steel and iron cannot be anodized because their oxides are porous, flaky, and chemically unstable.
Aluminum: The Most Widely Anodized Metal
Aluminum dominates the anodizing world for good reason. When placed in an acid electrolyte and subjected to electrical current, it naturally forms a dense, protective aluminum oxide layer that bonds tightly to the surface. This oxide is extremely hard, corrosion-resistant, and porous enough to absorb dyes for coloring. Dozens of aluminum alloys can be anodized, including common series like 1100, 3003, 5052, 6061, 6063, and 7075.
Not all aluminum alloys respond equally well. The 6000 series (alloyed primarily with magnesium and silicon) produces the most consistent, attractive results and is the go-to choice when appearance matters. The 2000 and 7000 series, which contain more copper and zinc, are less predictable. NASA documented coating failures on spacecraft hardware where Type II anodizing on these alloys flaked and spalled, particularly after extreme temperature cycling. If you need a reliable black anodized finish, 6000 series aluminum is the recommended starting point.
Three Types of Aluminum Anodizing
Aluminum anodizing is standardized under the MIL-A-8625 specification, which defines three main types based on thickness and performance:
- Type I (Chromic Acid Anodize) produces a thin coating with minimal dimensional change. It’s used on tight-tolerance aerospace parts like aircraft skins where fatigue resistance and weight savings are priorities.
- Type II (Sulfuric Acid Anodize) creates a thicker, more porous layer ranging from about 5 to 25 microns. This is the most common type, offering a good balance of corrosion protection, dye absorption for coloring, and cost. It’s used on everything from consumer electronics housings to aerospace mounting brackets.
- Type III (Hardcoat Anodize) is the heaviest option, typically 40 to 60 microns thick, sometimes reaching 100 microns. It produces an extremely dense, wear-resistant surface engineered for parts exposed to heavy abrasion, friction, or harsh environments. Landing gear components, weapon systems, and industrial machinery parts often use hardcoat anodizing.
Titanium: Color Without Dyes
Titanium anodizing works differently from aluminum in one striking way: it produces vivid colors without any dyes. The process grows a transparent oxide layer on the surface, and the thickness of that layer determines which wavelengths of light interfere and reflect back to your eye. By controlling the voltage, you control the color precisely.
At 10 to 15 volts, titanium turns bronze or brown. Between 17.5 and 27.5 volts, it shifts through dark purple into light blue. Yellows and golds appear around 50 to 60 volts, pinks at roughly 62.5 volts, rich purples return at 65 to 75 volts, and vivid blues and teals show up between 77.5 and 85 volts. At the top of the range (87.5 to 100 volts), greens and yellow-greens emerge. One notable limitation: true red is physically impossible through this thin-film interference process.
Beyond aesthetics, titanium anodizing plays an important role in medicine. Titanium is already biocompatible and corrosion-resistant, but anodizing it creates a porous, rough surface that promotes osseointegration, the process by which bone tissue grows directly onto an implant. Anodized titanium is used in dental implants, bone plates and screws, spinal cages, artificial heart valves, and cochlear implants. The oxide surface can also be structured into nanotubes that serve as tiny drug-delivery containers, releasing medication directly at the implant site.
Niobium and Tantalum
Niobium and tantalum are both refractory metals that anodize in much the same way as titanium. When voltage is applied, they form a surface oxide layer that refracts light into interference colors. This makes them popular in jewelry, particularly for people who need hypoallergenic options, since neither metal causes skin reactions in most wearers.
Tantalum also has significant industrial applications. Its anodized oxide layer has excellent dielectric properties, which is why tantalum capacitors are widely used in electronics. The stable, thin oxide film acts as the insulating layer between the capacitor’s plates, allowing for compact, high-performance components in smartphones, laptops, and medical devices.
Magnesium
Magnesium is the lightest structural metal, which makes it attractive for aerospace and automotive applications where weight matters. Its biggest drawback is poor corrosion resistance, and anodizing helps address that. Three industrial anodizing methods exist for magnesium alloys, each producing a protective oxide coating that reduces corrosion and can improve surface hardness.
Researchers are also exploring anodized magnesium for biomedical implants. Magnesium alloys are appealing for temporary implants because the body can gradually absorb them, eliminating the need for a second surgery to remove hardware. Anodizing these alloys with nanostructured oxide layers has been shown to reduce bacterial adhesion, including MRSA, on the implant surface while also slowing the corrosion rate in body fluids. Alloys combining magnesium with small amounts of zinc, calcium, or strontium have shown particular promise.
Zinc
Zinc can be anodized using either alternating or direct current across a wide range of electrolytes. The resulting coatings are both decorative and protective. Zinc anodizing is less common than aluminum or titanium anodizing in everyday products, but it serves a role in specialized industrial and decorative applications where zinc or zinc alloys are the base material.
Why Steel and Iron Cannot Be Anodized
The common thread among anodizable metals is that they form a stable, tightly bonded oxide when exposed to electrolytic current. Steel and iron fail this test completely. When iron is subjected to an anodizing-style process, it forms iron oxides: red rust or black magnetite. These oxides are porous, flaky, and chemically unstable. They don’t bond tightly to the surface, they allow moisture to penetrate, and they continue to degrade over time.
The chemistry of the process itself also works against steel. Standard anodizing uses acidic electrolytes like sulfuric acid, which corrode steel rapidly rather than building up a protective layer. Alkaline solutions are an alternative, but they’re harder to control and can aggressively etch the metal. For steel parts that need surface protection, other treatments like electroplating, powder coating, or galvanizing are used instead.
Copper and brass also fall outside the anodizable category for similar reasons. Their oxides lack the stability, density, and adhesion needed to form a useful protective or decorative layer through electrolytic oxidation.