What is anodizing?

Anodizing is a key step in manufacturing aluminum CNC machined parts . An electrochemical process that involves coating a metal part with an oxide surface layer, anodizing gives the part additional sturdiness and a more attractive finish. While anodizing is most common with aluminum, other substrates can be anodized, including magnesium and titanium. 

Anodizing ensures a part can resist corrosion and wear and tear from prolonged use, and that it retains its cosmetic appearance under any conditions. In many cases, manufacturers send parts to a third-party service provider specialized in anodizing, similar to when you need heat treatment, tempering or electroplating.  

The most common types of anodizing are Type I (Chromic Acid Anodize), Type II (Sulfuric Acid Anodize) and Type III, also known as Hardcoat. Each type of anodizing is ideal for different materials and serves its own distinct manufacturing parameters. Despite each type’s specific advantages and drawbacks, they all work relatively the same way. 

How does anodizing work?

The simplest way to understand how anodizing works is in the name. To anodize a part, you connect it to a positive terminal of an electrical circuit, also called an anode. You then submerge the part in an acidic electrolyte bath solution. This solution contains chemical compounds, such as sodium phosphate, which fill the bath with positive and negative ions. 

Once the part is submerged and secured to a hanger so it doesn’t move around, you apply the negative end of the circuit, or cathode, to a metal electrode in the bath. When you send voltage through the circuit, the negative electrode attracts positive ions (cations) from the part, and the aluminum part attracts negative O2 ions (anions) from the solution. 

When positive aluminum ions leave the part’s surface, it becomes porous, reacting with the negative O2 ions to grow a layer of aluminum oxide. 

Let’s break the process down into these simple steps:

  • Turn the part into an anode with a positive charge

  • Turn the metal plates into the cathode with a negative charge

  • Submerge both in the acidic bath

  • Apply voltage

  • Subtract aluminum ions from the part, creating pores

  • Attract oxygen ions to the part’s surface

  • The reaction forms a hard, corrosion-resistant aluminum oxide layer


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