Aluminum anodization is a well-known process of bettering the surface of parts and elements made out of aluminum alloys. Most often: manufactured with CNC machining but not exclusively. Such a process provides very good wear and corrosion resistance (dependant on the thickness of the coating) but – just like in any type of electroplating process – it has few types.
Type I and II of aluminum anodization is possible to perform at home (although still can be potentially dangerous due to the usage of acids) – to a limited extent.
On the other hand type III anodization (also called 'hard anodization') is a domain of highly specialized factories. What is a hard aluminum anodization, what features does it give to the surface of part and why is it better to order that process to the professionals?
What is aluminum anodization?
Aluminum anodization is an electrochemical process that creates on a surface a thin layer of aluminum oxide (mainly aluminum trioxide, Al2O3). Such a layer is achieved during the electrolytic process (bath) thanks to the influence of acids (chromium, sulfur) and electric current of voltage–intensity adjusted to the requirements of the particular process. The main goal of aluminum anodization is increased corrosion resistance – although aluminum alloys do not corrode, in some particular situations such a process can occur.
Naturally formed layers of aluminum oxide (that creates upon exposure to the atmospheric air) can be not sufficient enough for some applications (eg. contact with salt water). Artificially created anodic coating of aluminum trioxide provides much better protection. It is important in situations when parts are used in highly corrosive environments (eg. have contact with corrosive agents like salt water or abrasive cleaning substances).
On top of that – the layer achieved in the aluminum anodizing process has better mechanical parameters (like hardness or abrasion resistance) and can be permanently colored. Anodization can be executed on the alloys of aluminum and titanium – due to cost and most applications, more often we talk about the first metal. Stainless steel – as an alloy – cannot be subjected to that processing due to material parameters.
Parameters of a layer achieved in the anodization process – in that case: 'hard anodization' – are as follows:
- very good abrasion resistance,
- high hardness of surface (up to 550 HV) and perforation resistance (1500 HV for 50µm layer thickness),
- very good corrosion and wear resistance (up to 1500 hours of exposition in the salt chamber),
- increased resistance to thermal exposure (even up to 2000 ºC / 3632 ºF).
To put hard anodization in perspective, there are 3 types of that process:
- aluminum anodization type I – often called 'soft anodization' – the oxide layer is achieved upon exposition of metal surfaces on chromium acid and although rigid enough for many applications, leaves a darker surface. That type of process is perfect for parts with complicated surface structures,
- aluminum anodization type II – the most popular process that allows the creation of a layer of aluminum oxide on the surface of a part. Sulfur acid used in that process (in a low concentration) and an achieved layer allows coloring, which is why is often used for decorative anodization (eg. anodized cookware, like frying pans or bathroom accessories),
- aluminum anodization type III – often called 'hard anodization' – is a complicated but very effective technology for achieving a layer of aluminum trioxide on the surface of parts. It is used in many branches of industries that require high hardness and overall mechanical rigidity. To achieve such a layer in a hard anodization process is used sulfuric acid in high concentration. The current also differs from the type II process and changes during the electrolytic process. Such a layer is suited for very demanding applications like engine parts or military equipment.
Hard anodized aluminum: how is it achieved?
Surely hard anodization is the most demanding and complicated process of all 3 methods to achieve an aluminum trioxide layer. The process itself goes through many stages and requires in-depth knowledge of metallurgy (type of processed alloy), and expected results (eg. thickness of the oxide layer).
The hard anodization process can be divided into the following steps:
1. after the finalization of machining aluminum parts (eg. in the CNC machining process) objects selected for the anodization are washed and degreased. In some cases, these parts can be additionally chemically polished (to remove scratches or tool marks). The goal of that preparation is to achieve a clean and even surface for anodization,
2. before starting the electrolytic process parts are additionally rinsed, brightened, and again rinsed,
3. such prepared parts are ready for hard anodization. Elements are submerged in the electrolyte (solution of sulfuric acid) and exposed to a current of variable voltage and intensity – these parameters are dependent on the step of the anodization process,
4. after the anodization process parts can be subjected to additional processes like coloring – with organic pigments (SANDALOR) or during the following anodization (the thicker, the darker).
5. The last step of the hard anodization (and other types) process is sealing. It could be performed cold or warm.Applications of the hard anodization
Aluminum parts coated with a layer of oxide created during the hard anodization process are perfect for many demanding applications, especially when exposed to harsh and abrasive conditions.
Hard anodized aluminum parts are used in demanding industries like:
- automotive – a vast variety of aluminum parts are subjected to hard anodization to improve their rigidity and overall toughness,
- aircraft industry,
- military industry (eg. weapons parts or construction elements),
- shipbuilding (due to very good protection against salt water),
- medical (eg. surgical tools),
- home appliances (eg. hobs, ovens parts).
Why is it better to outsource hard anodization?
On the internet (discussion forums, YouTube) you can find many tutorials that answer questions like 'how to anodize aluminum parts at home. Such materials are focused on type I and II of the process – mainly due to the wider availability of accessories required for that process (chromic or sulfur acid in lower concentration) and the simplicity of the whole process.
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