One of the most widely used methods to counter the degradation of a materials mechanical properties to due to corrosion is to use more material. If the speed of uniform corrosion is known (mm per year), then it is possible to calculate the required thickness of the material, so that it can serve its purpose during its expected lifetime. The Steel Bridge in Portland (USA) is just one example where this method is used. Protective coatings are also often applied in order to slow down the corrosion of the construction material even more.
Anodizing aluminum is the ultimate technique for enhancing the corrosion resistance of automobile and aircraft parts, creating nanoporous templates for nanotechnological applications and making scratch resistant casings for electronic devices (cell phones, laptops etc). Anodizing is basically a 3 step process, that consists of a pretreatment for preparing the metal surface, anodizing for creating the anodic oxide layer and sealing the pores for enhancing the corrosion resistance. In the following steps youll need to use protection like gloves and goggles in order to keep yourself safe and also to prevent the contamination of the metal that will be anodized. Anodizing should also be performed in a very well ventilated room – the acidic vapours will cause severe damage to lungs during a prolonged exposure. These vapours also quickly corrode vulnerable parts in nearby electronic systems – so dont keep any valuable devices in the anodizing room.
Before starting with the pretreatment i recommend attaching the aluminum piece on a holder (aluminum wire for example) made out of the same aluminum alloy – that way youll prevent the contamination of the metal piece in the next steps as you no longer need to touch it. This holder will later also serve as the electrical connection in the anodizing process.
In order to remove the organic contamination, soap and water can be used for cleaning away most of the dirt. After rinsing the aluminum piece with deionized water, the final cleaning needs to be done with acetone. Before anodizing however, we need to remove the natural aluminum oxide layer from the metal surface. For that purpose the aluminum piece is dipped into a sodium hydroxide solution for a short time. The bubbling of hydrogen indicates that the oxide layer has been removed and that sodium hydroxide is reacting with aluminum. After removing the natural oxide layer the aluminum piece is rinsed with distilled water. If you have freshly polished the aluminum substrate, then you can likely skip the chemical cleaning and can go straight to anodizing – that saves you a lot of money. Just be sure you dont contaminate the freshly polished surface.
For anodizing a two-electrode setup is used, where the anodizable aluminum piece is the anode. The aluminum object is connected to the positive lead (usually red). The negative lead is connected to the cathode which can be made of stainless steel. Both electrodes need to be seperated and parallel to each other. However, in order to get a uniform oxide layer all over the anodized plate, i recommend using a stainless steel bath as a cathode instead. The bath should match the size and shape of the anodizable substrates.
The anodizable substrate needs to be completely immersed into the electrolyte before starting the process. The porosity and thickness of this oxide layer depends on the electrical parameters, type of electrolyte, its temperature and anodizing time. For example hard – scratch resistant oxide layers are done with type III anodizing in sulphuric acid, at near freezing temperatures and with lower current densities.
In our experiment we used a 10% sulphuric acid solution, a current density of 2 A / dm2 and the anodizing time was 30 minutes. Since the total surface area of the substrate was 3 dm2, the anodizing current was set to 6 ampers. The temperature of the solution was around 22C at start but it had significantly increased when we measured it again after anodizing. So if there is a need to use higher current densities for anodizing, i recommend using a cooling bath around the anodizing bath. In industrial processes a constant anodizing temperature needs to be ensured.
During the anodizing process a nanoporous oxide layer is generated at the cost of aluminum and this alters the appearance of the aluminum piece. These pores are so small, that they are only visible with a powerful scanning electron microscope. For making corrosion resistant coatings, these pores need to be completely sealed and there are several ways to do it. One such method is hydrothermal sealing, which is basically keeping the anodized substrate in boiling water. As a result aluminum oxide is partially turning into aluminum hydroxide, which takes up more space and seals the small pores. Another popular method is dipping the freshly anodized substrate into paint, which is immediatelly sucked into the pores. This significantly increases the metals corrosion resistance and also gives it an awesome appearance.
The nanoporous aluminum oxide can also be used as a template in nanotechnologial applications. In our case we electrochemically deposited silver into these pores and then removed the aluminum oxide with sodium hydroxide. As a result we got silver nanowires with well defined length and diameter.
Anodizing is quite easy and with some practice, its a powerful technique for treating aluminum objects for personal or commercial purposes.