Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
This process is called anodizing because the workpiece forms the electrolytic cell anode electrode. Anodizing increases resistance to corrosion and wear and provides better adhesion for primers and paint adhesives than pure metal. Anodized films can also be used for several cosmetic effects, either with thick porous coatings that can absorb dyestuff or with thin transparent coatings that add interference effects to reflected light.
Anodizing is also used to prevent abrasion of threaded parts and to manufacture dielectric films for electrolytic capacitors. Anodizing films are most commonly used to protect aluminum alloys, although processes also exist for titanium, zinc, magnesium, niobium, zirconium, hafnium and tantalum. Metal of iron or carbon steel is broken down by oxidation under neutral or alkaline microelectrolytic conditions; i.e., iron oxide (actually iron hydroxide or hydrated iron oxide, also known as rust) is formed by anoxic anode pits and a large cathode surface, these pits concentrate anions such as sulphate and chloride, accelerating the corrosion of the base metal. Carbon flakes or nodules in cast iron or high carbon steel (high carbon steel, cast iron) can cause electrolytic potential and interfere with coating or electroplating. Ferrous metals are usually anodized electrolytically in nitric acid or by treatment with red smoky nitric acid to form solid black iron oxide (II, III). This oxide remains conformal, even if it is applied to the wiring and it is curved.
The anodizing changes the microscopic texture of the surface and the crystal structure of the metal near the surface. Thick coatings are usually porous, so a sealing process is often required to achieve corrosion resistance. For example, anodized aluminum surfaces are harder than aluminum, but have low to medium wear resistance, which can be improved with increasing thickness or by using suitable sealants. Anodized films are usually much stronger and better adhesive than most types of paints and metal coatings, but are also more brittle. This reduces the likelihood of them cracking and peeling from aging and wear, but makes them more susceptible to thermal stress cracking.
Also, our company provides hard anodizing services –
is a dense anodized aluminum oxide coating applied by converting a properly purified and oxidized aluminum alloy component into an oxide film using a suitable electrolyte, usually sulphuric acid, cooled to almost freezing point, and applied voltage above 100 volts at applied current density of 24-36 amps per square foot.
Why is hard anodizing different from other types of anodizing?
Anodizing aluminum is usually divided into three types. Type I is anodized chromium acid (or chromium alternative), which is very thin, about 2 microns. Type II is a common anodized layer of sulfuric acid, which can be decoratively painted almost any color, and its thickness is from 3-8 microns. Hard anodizing is type III, which is performed under more severe process conditions, resulting in a harder, denser, thicker and more abrasion resistant coating. The thickness of the hard coating can vary from 20-50 microns, but depends on the specific alloy to be anodized.