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Hard anodizing is a process that creates a wear and corrosion-resistant coating on metals. It involves placing the metal in an electrolyte solution and passing an electric current through it. Hard anodizing uses higher currents and weaker solutions than decorative anodizing, resulting in a thicker and harder coating. The coating is chemically stable, non-toxic, and insulating. Hard anodized parts are used in heavy wear industrial parts, medical prosthetic parts, and automotive components. However, anodizing reduces the fatigue strength of the metal and makes it thicker, which can be a problem for pre-drilled spaces.
Hard anodizing, also known as hard coating or type III anodizing, is a process used to create a wear and corrosion resistant coating on a variety of metals. Anodizing can be divided into two broad subcategories: decorative and hard anodizing. The main differences between the two are the thickness and durability of the coating and the exact process used to create it.
The anodizing process
Anodizing a metal part involves placing it in an electrically conductive liquid, usually an acidic solution, called an electrolyte. The circuits have a positive electrode (cathode) where the electrons enter and a negative one (anode) where they leave; in anodizing, the metal part becomes the negative electrode. When an electric current is passed through the solution, the action of the electrons leaving the circuit through the metal part results in the formation of a tough, corrosion resistant oxidation coating. The finish can be left as-is after this treatment or further enhanced with decorative dyes and other performance-enhancing additives.
The manufacturing process of hard anodized parts differs from decorative coatings in several ways. It uses generally higher electric currents and slightly weaker electrolyte solutions. The temperature of the electrolyte solution is also lower allowing for less distortion of precision parts and better coating adhesion. In general, the anodizing process is also considered to be relatively environmentally friendly and the by-products are recyclable.
Types of coatings
Hard anodize coatings are typically applied to heavy wear industrial parts intended for use in aggressive or highly corrosive applications. These coatings are typically much thicker and harder than decorative coatings and usually give parts a life approaching that of hardened or case-hardened steel. They also penetrate and better cover superficial imperfections such as fissures.
Generally, hard anodized parts have coatings exceeding 10 μm (0.01 mm or 0.0004 inch) with typical coatings exceeding 25 μm (0.025 mm or 0.001 inch). Decorative anodizing usually has coatings of less than 10 μm and, while durable, does not have the same outstanding wear characteristics as hard anodizing. Decorative or architectural treatments are commonly found on consumer items such as household cookware, electronic device casings, and ornaments.
Coated items usually have a dull dark gray finish, although this can vary depending on the metal of which the item is made and the composition of the electrolyte solution. This method can also produce a black finish and various shades of bronze. Decorative anodizing usually produces a lighter shade and can be made matte or glossy. Both types typically take dye well.
Benefits
One of the main reasons for anodizing metal is to make it more resistant to corrosion. The thick oxidized outer layer prevents the inner material from being exposed to moisture, oxygen and other factors that can cause the metal to disintegrate. Sealed items are even more resistant to corrosion and can often withstand up to thousands of hours of exposure to salt water spray.
The outer coating is also extremely hard, typically much harder than the original metal. In many cases, a hard, thick anodized finish can be as hard as tool steel. It is also very wear resistant which means it is often used for pistons and other sliding parts that often rub against each other. Because the oxidation layer is part of the metal itself, it won’t come off; the metal surface may be rough after anodizing, however, it may need to be ground to prevent the pieces from cracking.
Hard anodized metals are generally very insulating, meaning they don’t conduct heat or electricity well. This is especially useful for applications that require the part to be operated at elevated temperatures. The coating is also chemically stable and non-toxic.
Additional treatments
As with decorative coatings, hard anodized surfaces can be dyed, although, in most cases, remain so due to the purely functional nature of most of the parts involved. However, they are often impregnated with performance-enhancing additives such as Teflon® which improves the self-lubrication of the part. In some cases, they are also sealed in boiling distilled water or dichromate solutions to further improve their corrosion resistance.
Disadvantages
Metal that has been anodized has a much lower fatigue strength meaning it is more likely to crack when subjected to stress, although this can be improved if the item is sealed. However, sealing the item can reduce its resistance to abrasive wear, so whether or not a part is sealed often depends on its end use. Also, anodizing does not protect thinner metal objects from damage such as dents. The outer coating makes the metal part thicker, which can be a problem if screw holes or other spaces are pre-drilled.
Materials that can be anodized
While aluminum is by far the most common metal subjected to hard anodizing, other materials can benefit from the treatment, including tantalum, magnesium, and titanium. In all cases, the treatments give the parts exceptional resistance to wear and corrosion and can be dyed in almost any colour. Common uses for hard anodized parts include heavy commercial cookware and bakeware, medical prosthetic parts, and automotive components. The military is another major consumer of these products, as most hard anodized surfaces meet or exceed stringent military specifications.
