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.
Tag Archives: electrochemistry
How to Generate Hydrogen by Splitting Water
Hydrogen is the most abundant substance in the universe. It fuels the starts that light the nightsky. Hydrogen will also power the future of mankind as it is already used as car fuel and within this century even in fusion reactors.
As most of you know, a water molecule consists of one oxygen atom and two hydrogen atoms. So in order to get hydrogen, it is needed to split the water molecule. This can be done for example electrochemically where an electrical potential is applied between electrodes in a salt water. For a home experiment one can simply put a 9 V battery into salt water and watch how hydrogen bubbles start to form at the cathode. At the same time oxygen is generated at the anode but since the anode on the battery is usually made of steel, it will quickly corrode as it reacts with chlorine and oxygen. This causes the salt water to go brown. So instead, you may want to use electrodes instead that are connected to an external power source. If a DC voltage is used then especially the anode needs to be made from a chemically inert conductive material such as platinum which doesnt oxidize. At this anode oxygen gas can be collected. At the same time hydrogen gas is generated at the cathode and can also be collected. If DC voltage is used then the electrode at cathodic potentials will not corrode very quickly as oxidation cannot occur. However hydrogen damage may eventually destroy the electrode.
Hydrogen damage occurs when the small atomic hydrogen generated at the cathode moves into pores and cracks inside the electrode and combines with other hydrogen atom to form molecular hydrogen. The molecular hydrogen however is too large to diffuse through metal and starts building up inside the sealed crack or pore and pressure increases until it splits the material.
In order to produce as much gas as possible, the surface area of electrodes needs to be increased. Make the electrodes rough, multilayered or highly porous for greater surface area.
If AC voltage is used to split water, then corrosion is suppressed and for some time even stainless steel can be used as both electrodes.
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How to Renew Tarnished Silver
Silver, a valuable metal, has been used long time in making coins, tableware and jewelery. The metal surface however becomes tarnished within a few years and this is caused by corrosion as silver reacts with the surrounding environment.
The surfaces can be easily renewed with stuff available at home – all you need is water, aluminum foil, salt and soda.
Follow these steps to do it yourself:
1. Mix salt and soda into water
2. Wrap the silver price into aluminum foil
3. Put the wrapped silver piece into the solution and wait a few minutes
For a greater effect you might want to boil the water solution as chemical reactions occur faster at higher temperatures.
Why this simple technique works? Silver is usually tarnished as it forms silver sulfide. In order to remove sulfur from the silver, it is needed to put it into contact with a more active metal such as aluminum. The solution there provides with a path for sulfur to move from silver to aluminum.
How Anodizing Works
Anodizing is an electrochemical process where a thicker oxide layer is grown on the material. This is useful for improving an objects corrosion and wear resistance, manufacture nanoporous templates or give the material a decorative appearance. Not all materials can be anodized however as their oxides are not dense and hard but the technique has been widely used on aluminum, titanium, zinc, magnesium and their alloys.
The anodizing system consists of a power source, anodizing bath, electrolyte and anodizable material. The bath is usually made from a chemically resistant conductive material such as stainless steel and serves as a cathode. The anodizable material serves as an anode and is placed inside the anodizing bath with the electrolyte. Both the anode and the cathode need to be connected to the power source. The grown oxide layers properties depend on the material, used electrolyte, temperature and electrical parameters used for anodizing.
In order to produce a uniform oxide layer, the substrates are also treated before the process. The main problem is usually organic contamination of the surface, which prevents growth of the oxide layer. This is removed with organic solvents such as acetone. Often the thin native oxide layer is also removed via etching before the anodizing.