Anodizing

By Michael MacLeod

Lockdown driving you crazy? Calling all artisists, machinists, jewellers and hobbyists, why not start a new hobby of anodizing aluminium items for decorative looks or for corrosion and abrasive properties?

A step by step DIY book detailing the equipment and chemicals needed. Includes lots of safety advice.

Free software included.

• Language: English
• Publisher: Michael MacLeod
• Release date: Oct 19, 2021
• ISBN: 9781005898977

Michael MacLeod

Michael MacLeod was born and educated in Benoni, South Africa. After completing his military training as an infantry soldier at the 2nd South African Infantry Battalion in the deserts of Walvis Bay, Namibia and seeing action in the ‘Border War’, he became an aircraft maintenance engineer and worked for one of the world’s leading international airlines before retiring. He has travelled South Africa extensively. Among his other interests are designing and building of aircraft, electronics, clock making, woodworking and is also a part time inventor.

Book preview

Chapter 1

What is aluminium and why anodize it

What is Aluminium? It is a chemical element, symbol Al, that is found naturally on Earth and is the most abundant metal on our planet. It makes up roughly 8% of the crust on the surface of the earth. It is very versatile; making it the second most used metal after steel. It is used in a variety of ways such as in the construction of aircraft, automobile parts, buildings, as well as many products in our home. It is too soft to use in its purest form, so it is alloyed with various metals to give it strength.

The various grades of aluminium are denoted by a four digit number, the first signifying the major alloying element. The rest of the numbers specify the various minor additives.

Anodizing1

Aluminium is an extremely reactive metal. We are familiar with the action of the environment on iron. It oxidizes quite readily to produce iron oxide - or rust as it’s more commonly known. Aluminium is even more reactive than iron and as such will rust more readily. This corrosion shows up as a white powdery layer on the surface and will eventually eat through thin sheet, especially in coastal regions due to the salt air. On thicker material it manifests as pitting on the surface and leads to metal fatigue.

We make use of this oxidizing effect in anodizing by forcing a corrosive layer onto the surface of the part we’re anodizing and then sealing it so that no further corrosion takes place. We can even dye this thin protective layer to give the item a nice decorative look. A by-product of anodizing is that it also produces a hardened surface, giving it some useful properties.

Basic theory

Anodizing is so named because the item being treated is now the anode, whereas in normal electroplating the  item is the cathode.

Drawing1

Simply put, anodizing is the process of creating an oxide layer on the surface of the part by running a current through it while submerged in a bath of sulphuric acid. This layer is very thin, varying from 12.7 microns to 25.4 microns, (half a thou, 0.0005 to one thousands of an inch, 0.001). 

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Initially the oxide layer forms inside the metal skin creating a barrier and only then does it start to grow onto the surface of the part and looks like a beehive honeycomb under a microscope. The barrier that first forms becomes an insulator which does not allow current to pass through. It is this property that is used in the manufacture of aluminium electrolytic capacitors. 

Unlike the powdery oxide that forms naturally on aluminium, this new coating is very hard, giving the item good abrasive properties as well as protection from corrosion. The honeycomb lattice structure allows us to dye the aluminium for a nice decorative look due to the pockets that are formed, which we then fill with dye.