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Anodizing successfully combines science with nature to create one of the world's best metal finishes.
It is an electrochemical process that thickens and toughens the naturally occurring protective oxide. The resulting finish, depending on the process, is the second hardest substance known to man, second only to the diamond. The anodic coating is part of the metal, but has a porous structure which allows secondary infusions, (i.e. organic and inorganic coloring, lubricity aids, etc.)
Anodizing Definitions and Methods
While the chemical anodizing process remains the same for all applications, the mechanical methods vary according to the two physical types and shapes of metals used:
Appearance options and quality are improved through the use of dyes and special pretreatment procedures. This makes the aluminum look like pewter, stainless steel, copper, brushed bronze or polished brass and can also be colored with brilliant blues, greens, reds, and many varieties of metallic gold and silver.
The unique dielectric properties of an anodized finish offer many opportunities for electrical applications.
The surface of the aluminum itself is toughened and hardened to a degree unmatched by any other process or material. The coating is 30 percent thicker than the metal it replaces, since the volume of oxide produced is greater than that of the metal replaced.
The resulting anodic coating is porous, allowing relatively easy coloring and sealing.
Hard Anodizing is a term used to describe the production of anodic coatings with film hardness or abrasion as their primary characteristic. They are usually thick by normal anodizing standards (greater than 25 microns) and they are produced using special anodizing conditions (very low temperature, high current density, special electrolytes). They find application in the engineering industry for components which require a very wear resistant surface such as piston, cylinders and hydraulic gear. They are often left unsealed, but may be impregnated with materials such as waxes or silicone fluids to give particular surface properties.
Batch and Coil Anodizing
Batch and coil anodizing are accomplished in five carefully controlled, calibrated, quality-tested stages:
1. Cleaning. Alkaline and/or acid cleaners remove grease, and surface dirt.
3. Anodizing. The anodic film is built and combined with the metal by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications.
4. Coloring. Coloring is achieved in one of four ways:
Here is detailed information comparing two of the most common coloring processes used: (note - these two types of processes will not produce identical colors; both can be overdyed. Source: Aluminum Anodizers Council Technical Bulletin #1-94, issued January, 1994.) See below.
5. Sealing. This process closes the pores in the anodic film, giving a surface resistant to staining, abrasion, crazing and color degradation.
Quality control. Throughout the entire anodizing process, AAC members monitor the process and quality of the product. The application of electrical power and color is preprogrammed and verified on all batches and coils.
This quality control ensures uniformity to end product specifications for film thickness, density, abrasion resistance, corrosion resistance, color uniformity, fade resistance, reflectivity, image clarity, insulative properties, adhesion and sealing.
In many cases, AAC members use Statistical Process Control (SPC) methods to meet rigorous quality assurance standards.
Comparison of A32/A42 and A34/A44 Colored Aluminum Anodic Finishes