Page 163 - Corrosion Engineering Principles and Practice

138 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 139

by far the most widely used for the production of a great variety of
applications. There are many reasons and processes to anodize a part.
Following are a few considerations that may help choose the type
and process:

• Appearance: Products look finished, cleaner, better and last
longer.
• Corrosion resistance: A smooth surface is retained while
weathering is retarded. Useful for food handling and marine
products.
• Ease in cleaning: Any anodized product will stay cleaner
longer and are easier to clean when needed.
• Abrasion resistance: The treated metal is tough. It is harder
than many abrasives and is ideal for tooling and air cylinder
applications;
• Non-galling: Screws and other moving parts will not seize, drag,
or jam while wear in these areas is diminished. Gun sights,
instruments, and screw threads are typical applications.
• Heat absorption: Anodizing can provide uniform or selective
heat absorption properties to aluminum for the food-
processing industry.
• Heat radiation: Anodizing is used as a method to finish
electronic heat sinks and radiators.
Anodizing Process
The anodizing process illustrated in Fig. 5.43 consists of a few steps
through which a part is submitted sequentially by being dipped in
baths to achieve the following goals:

• Pretreatment: Cleaning is done in a nonetching, alkaline
detergent heated to approximately 80°C. This process removes
accumulated contaminants and light oils.
• Rinsing: Multiple rinses, some using strictly deionized water,
follow each process step.
• Etching (chemical milling): Etching in a sodium hydroxide
solution to prepare the aluminum for anodizing by chemically
removing a thin layer of aluminum. This alkaline bath gives
the aluminum surface a matte appearance.
• Desmutting: Rinsing in an acidic solution to remove unwanted
surface alloy constituent particles not removed by the etching
process.
• Anodizing: Aluminum is immersed in a tank containing an
acid electrolyte in which direct electric current is passed
through the electrolytic cell between the aluminum part
serving as the anode and the tank polarized as the cathode.