What is anodizing? Anodizing is the key to finishing parts made from aluminium and other metals. Learn how anodising works and why it is an important part of CNC machining and manufacturing.
Anodizing is a key step in the production of CNC machined aluminium parts. An electrochemical process in which an oxide surface layer is applied to a metal part, anodising gives the part added strength and a more attractive finish. While anodising is most commonly used with aluminium, other substrates can also be anodised, including magnesium and titanium.
Anodising ensures that a part will resist corrosion and wear from prolonged use and retain its cosmetic appearance under all conditions. In many cases, manufacturers send parts to a third party service provider that specialises in anodising, similar to heat treating, tempering or electroplating.
The most common types of anodising are Type I (chromic acid anodising), Type II (sulphuric acid anodising) and Type III, also known as hardcoat. Each type of anodising is ideal for different materials and has its own specific manufacturing parameters. Despite the specific advantages and disadvantages of each type, they all work in a relatively similar way.
In this article, we explain how the general anodising process works, the common manufacturing parameters and the key advantages and disadvantages for various applications.
The easiest way to understand how anodising works is to look at the name. To anodise a part, you connect it to the positive terminal of an electrical circuit, also called the anode. You then immerse 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, you place the negative end of the circuit, or cathode, against a metal electrode in the bath. When you send a voltage through the circuit, the negative electrode attracts positive ions (cations) from the part, and the aluminium part attracts negative O2 ions (anions) from the solution.
As the positive aluminium ions leave the surface of the part, it becomes porous and reacts with the negative O2 ions to form a layer of aluminium oxide.
Let’s break the process down into these simple steps:
If you’re familiar with electroplating, you may recognise that anodising is similar in many ways. Comparing the two processes certainly helps to understand anodising as a unique and important step in the manufacture of CNC aluminium parts.
Similar to anodising, electroplating involves placing the part in an electrolyte bath and applying a charge to it. The key difference is that in electroplating the part is negatively charged and becomes the cathode rather than the anode.
The anode is applied to a piece of the desired plating material, such as gold. As with anodising, this circuit allows positively charged ions to flow to the cathode (the part). These ions coat the part with a thin, uniform layer.
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