Anodized screws are metal fasteners with enhanced oxidation for color coding, wear and corrosion resistance. Aluminum is the most common type due to its natural resistance to corrosion. Anodized screws are used in structures exposed to natural weather conditions and in high-stress metal parts. The surface coating is an oxide of the base metal, making it durable but prone to cracking in highly alkaline or acidic environments. Anodized products are also common in the jewelry industry and have a lower coefficient of friction, making them easier to clean. There are two main types of anodizing processes: hard anodizing and mass anodizing, both using sulfuric acid.
An anodized screw is a special type of metal fastener in which the natural oxidation on the metal surface has been enhanced for a variety of reasons. Anodized products can be treated in this way to color code them as in their use in surgery in the medical field and to provide a higher level of resistance to wear and corrosion. The most common type of anodized metal as of 2011 for an anodized screw is aluminum due to its natural resistance to corrosion compared to that of harder metals such as steel and iron. Other types of metals that can be processed into an anodized screw for special applications include magnesium and titanium screws used in high-stress metal parts such as in aircraft frame design.
Anodized screw is often used for the assembly of structures exposed to natural weather conditions, along with other types of these anodized fasteners such as anodized bolts. The surface coating for most anodized products is an oxide of the base metal itself, with the aluminum having an anodized surface of aluminum oxide, Al2O3. These coatings are very durable compared to other types of corrosion resistant surfaces because they are an integral part of the underlying molecular structure of the metal itself which is strongly bonded to it. The disadvantage of an anodized screw is that the surface is prone to cracking in highly alkaline or acidic environments. Because the anodizing process also creates a very thin surface layer usually 2 microns to 25 microns deep, it can become brittle and crack or flake with age and wear.
Anodized products are also quite common in the jewelry industry where a layer of titanium oxide is surface coated over other metals such as nickel. This is because titanium can come into contact with the internal or external surface of the body through jewelry or medical implants without causing any type of adverse reaction, while other commonly used metals such as nickel can cause allergic reactions in some individuals. Titanium anodized screw fasteners are therefore often used to hold medical devices and implants together.
An added benefit of an anodized screw or other fastener is that the surface has a lower coefficient of friction than the base metal, known as a wear surface, which allows moving parts to operate with less likelihood of jamming. One major application that takes advantage of this feature is in gun sights and screw thread controls, where the screw is used to fine-tune a setting on a machine part or instrument cluster. The low level of friction also makes an anodized screw easier to clean of grease and debris when coated in these parts over time.
There are two main types of anodizing process, including hard anodizing or hard coat anodizing and mass anodizing, both of which use sulfuric acid. Hard anodizing is done by applying an electric current in an electrolytic bath, usually to aluminum parts larger than screws, and this type of coating can be stronger than case-hardened steel when completed. Batch anodizing is done on batches of small parts with odd shapes such as an anodized screw or rivet, where they are coated together in large quantities in plastic or titanium baskets. The sulfuric acid in both processes serves to oxidize the surface layer of the metal and can be used simultaneously to give it a texture or color. One of the anodized products where these effects are most evident are non-stick cookware and kitchen utensils.
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