Aluminum Finishing Company
 
What is Anodizing?

Anodizing is a process which converts metal surfaces to their corresponding oxide, i.e. Aluminum, Magnesium, Titanium, etc. Not all metals can be anodized and of those that can be anodized Aluminum gives the most desirable final result.

This conversion of the metal to metal oxide (Al to Al Oxide) is produced electrochemically in the presence of some an acidic solution and electricity. Where, parameters like amperage per surface area, temperature of the solution, and concentration of the acid used, are important to this process.
This conversion of the metal to metal oxide (Al to Al Oxide) is produced electrochemically in the presence of some acidic solution and electricity. Where, parameters like amperage per surface area, temperature of the solution, and concentration of the acid used, are important to this process.


Anodic Coating:

When aluminum is extruded, fabricated, cleaned, etc., it immediately forms a layer of aluminum oxide on it's surface(s). If this naturally made layer of aluminum oxide is left undisturbed it will protect the aluminum from further oxidation until the layer is broken through. This layer is so thin that almost any physical or chemical contact will break through the coating. In practice, to enhance the wear and tear, corrosion and cosmetic properties of aluminum, manufacturers have deployed the anodizing process. The anodizing process produces layers and layers of aluminum oxide on each other. Anodizing should be performed in an acidic environment. The acid solution helps to dissipate the heat, conduct electricity and most importantly, dissolve some of the aluminum oxide molecules in the intact anodized layers to make fresh Aluminum available for further reaction. In this manner, the acid and electricity keep going deeper and deeper to reach the pure aluminum and make more aluminum oxide. The end result of this process is an anodic coating that can be imagined having a spongy structure. By controlling the anodizing process, one can achieve anodic coating with the desired thickness and density. The most important parameters are:

    Aluminum Alloys
    Temperature of the solution
    Amperage (Voltage)
    Concentration of the acid
    Agitation of the solution

Anodized Coating on aluminum as it is formed has a strong affinity for absorbing many chemicals (dyes, oils, coffee, etc.) into it's spongy structure, where there are chemically active sites which attract other chemicals. This absorptive property can be good and bad. The positive is that you can color the coating by using dyes or use the anodizing on the aluminum as a primer for painting. On the downside, colored and clear coatings can absorb stains or corrode when in service. To solve this problem, manufacturers have developed methods that render these chemical sites inactive. These methods use metal compounds of nickel and cobalt as catalysts in hot deionized water to change the properties of the anodic coating. After this process, the anodic coating should not absorb any material in or bleed any dye out. This process of rendering the anodic coating inactive is called sealing. 

The aluminum oxide produced under controlled anodizing conditions can be from near zero thickness to several mils (1mil = 0.001 inch). Anodized coating has several desirable properties which enhance the use of aluminum in many industries. The anodized coating is a very good electrical insulator for which they can be used in capacitors. Main uses for anodizing are:


1) Cosmetic
2) Corrosion prevention
3) Wear and tear

Number 1 & 2 above use a process called conventional anodizing (could be clear or colored).
Number 3 above requires the Hard Anodizing process.


Types of Anodizing:

Type I:       Chromic Acid Anodizing, primarily used as a primer for painting on aluminum. 
                          Used in airspace industry
Type II:      Sulfuric Acid Anodizing, with Class I (clear) & Class II (dyed).
                          Also known as conventional anodizing.
                          Used for cosmetic and corrosion prevention, in a small degree wear
                          and tear prevention. Architectural and Engineering industries.
Type III:     Sulfuric Acid Anodizing, Hard Anodizing; can also be dyed in a limited way.
                          Used primarily in engineering industries.


Note: Other types of anodizing processes exist which use different chemicals such as phosphoric acid or oxalic acid or a combination of other chemicals. 


Some interesting sites about everything pertaining to Aluminum:




Aluminum Finishing Company
615 W. Ransom St, Kalamazoo MI 49007
Tel. 269-382-4010, Fax. 269-382-3613
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