11 April 2016

What is anodizing | Types | Why anodizing | Anodizing Sequence

Anodizing is an electrochemical process that converts the metal surface into a decorative, resistance to corrosion, durable, anodic oxide finish. Mostly the aluminum is ideally suited for anodizing but also some other nonferrous metals, such as magnesium and titanium also can be anodized. 

Anodizing is a surface treatment process to improve surface roughness, toughness, and surface quality.

The Anodizing process changes the microscopic texture of the surface and the crystal structure of the metal near the surface. The anodic film is made by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed and combined with the metal. The thickness and surface characteristics of the coating are strictly controlled to meet the specification of the end product. 

The process is called anodizing because in this process forms the anode on an electrode by the electrical circuit.

Types of anodizing

  • Chromic acid anodizing, low voltage process, chrome-free process. 
  • Conventional room temperature sulphuric acid anodizing. 
  • Hard coat anodizing, done in sulphuric acid at temperatures close to the freezing point of water. 

Why anodizing?

  • A very thin coating. 
  • Extremely durable, hard, abrasion resistance, and long-lasting. Coating lasts indefinitely.
  • Some types of anodizing have colors that are fade-resistant in sunlight nearly indefinitely. 
  • Excellent corrosion protection. 
  • Environmentally friendly surface finish.
  • Good electrical insulator. 
  • Inexpensive competitive with powder coating and paints. 
  • It can be readily recycled. 

Typical Anodizing Sequence

The anodizing process is not completed in a single anodizing tank but includes pretreatment steps before anodizing and post-treatment after it. A typical and perhaps the most common sequence would be:
  • Clean
  • Rinse
  • Etch
  • Rinse
  • Desmut
  • Rinse
  • Anodize
  • Rinse
  • Neutralize
  • Rinse
  • Dye
  • Rinse
  • Seal
  • Rinse
In this sequence, the cleaning tank would be a non-etch alkali cleaner to remove soils by using ultrasonic agitation in the cleaning tanks.

Most commonly, the etch process would be caustic soda, but it could be an acid etch like ammonium bi-fluoride. Etching dissolves aluminium so that it can be minimized or skipped for some alloys, leaving the other alloying materials behind. 

Aluminum for mirrors or reflectors is brightly dipped in a very strong nitric-phosphoric bath to give a mirror finish.

Definitely, bright dipping is not just another tank. It is one of the real nasties in surface finishing and before attempting to specify it or use it, you should see installations that do bright dipping to understand ventilation and secondary containment issues. 

The desmut step attempts to dissolve on the surface of the parts of the grey-black alloying ingredients like silicon, copper, zinc, and magnesium. This step is sometimes referred to as de-oxidizing, a misnomer widely accepted. In the desmut step, the constituents will depend on the alloying ingredients that need to be removed. 

Trying to neutralize sulfuric acid through a dip of sodium bicarbonate or a dip in dilute nitric acid is fairly common. 

Dyes are usually heated and can be organic dyes relatively similar to fabric dyes, or they can be inorganic metallic salts, often applied with the help of A.C, especially for architectural work. Electricity, giving the name two-step anodizing. The processes can also be combined by applying an inorganic dye and then overdyeing with an organic dye.  

Sealing is the step of swelling at the top of the honeycomb-like anodizing pores to lock in the dye and lock dirt out. Sealing is an independent science, with older approaches such as steam or D.I. boiling. Water used as well as newer processes of mid-temperature such as nickel acetate and low-temperature seals such as nickel fluoride. 

For military work, chromic acid sealing may still be specified for best corrosion resistance and color matching.

The D.I is often the last step in the process of water rinse to minimize problems with staining.


This is a brief overview of the chemistry of the anodizing process. The process can encounter many difficulties in an industry if care is not taken to ensure that concentration and temperature solutions are controlled. After each stage, thorough rinsing of the work is performed to ensure that it enters the next process in the correct condition. It also ensures that solution contamination is kept to a minimum from one stage to the previous stage. Another aspect not covered by this industry is that of quality control. Even in small plants, chemists are employed to constantly check and monitor the conditions of the solutions and make recommendations/adjustments. Furthermore, the thickness of the film, its density, and the color quality is frequently checked.

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