INTRODUCTION TO ANODIZING
BASIC STEPS IN ANODIZING
SEALING METHODS OF ANODIZED ALUMINUM
WHAT YOU NEED
SETTING UP AN ANODIZING SYSTEM
TEMPLATE SYNTHESIS OF NANORODS AND NANOTUBES
Title: Captain Corrosion Handbook of Anodizing
Author & Copyright: Maido Merisalu
Published by: Captain Corrosion OÜ, Tartu, Estonia 2017
ISBN 978-9949-9973-0-5 (print)
ISBN 978-9949-9973-1-2 (digital)
Since time immemorial an ancient nemesis has plagued mankind, hindering our journey towards the stars. It is a natural phenomenon known as corrosion, which is the degradation of materials properties due to the environment. In order to counter this problem, we have developed numerous methods that can significantly increase the lifespan of the created tools and devices. One of such methods is anodizing, which is used to convert the surface of certain materials into a corrosion resistant layer and hence enhance the performance of the material. The greatest advantage of this technique is the easy control over the process parameters and thus also control over the properties of the obtained layer covering the treated material. As a result, anodizing has proven to be a valuable technique for enhancing the visual appearance or resistance against corrosion and wear of certain metals but also for creating nano-scaled structures for unique applications in energetics and medicine.
The purpose of the Captain Corrosion Handbook of Anodizing is to provide the reader an overview of anodizing and its applications and an anodizing guide that can be used in do it yourself (DIY) projects, laboratory experiments or even for setting up an anodizing company.
A few words about us – Captain Corrosion OÜ is a University of Tartu spin-off company, founded by a materials scientist Maido Merisalu in order to help other companies and individuals in the field of corrosion and materials science. Due to our academic background, we also highly value education and therefore we have made many study materials available on our web-site that are aimed for educators, companies and students. Anyhow, if questions arise, when reading this book, or if you have ideas what improvements should be made for the next edition, then you are more than welcome to contact us through our website!
INTRODUCTION TO ANODIZING
Anodizing is an electrochemical technique that is used to improve the corrosion and wear resistance, as well as visual appearance or biocompatibility of anodizable metals such as aluminum and titanium alloys [1-16]. Recently, however, anodizing has also been used for creating nano-porous templates for specific applications in nanotechnology [17-20]. In this process the surface of the alloy is electrochemically oxidized, creating an anodic oxide layer, which is electrically less conductive (often insulating) and hinders further reactions of the metal substrate with the environment (Fig. 1). The created anodic oxide layer uniformly covers the whole anodized substrate and it usually has a thickness up to tens or even hundreds of microns, depending on the anodizing parameters. It must also be noted that the anodic oxide layer may contain nano-scaled pores, which allow the electrolyte to reach the metal substrate during the anodizing process. At the bottom of these pores, only a very thin nanometric oxide layer separates the metal substrate from the environment . Therefore, these pores are often sealed with various techniques in order to achieve a better protection against corrosion [6, 8, 11, 14]. When anodizing high-precision aluminum details, it is also important to know that, as a general rule, the dimensions of an anodized detail increases by about 50% of the thickness of the created anodic aluminum oxide (AAO) layer due to the volume increase caused by the oxidation of aluminum .
The thickness, porosity, hardness, corrosion resistance and other properties of the created oxide layer can be tuned by utilizing correct anodizing parameters (e.g. electrolyte, temperature, anodizing time, electric current, voltage etc.). Such a control over the properties of the AAO layer and many post-treatment possibilities make anodizing an extremely useful technique for a variety of applications.
Figure 1. Anodic aluminum oxide created by anodizing aluminum. In this process the surface of the aluminum alloy is converted into aluminum oxide that contains nano-scaled pores.
Numerous anodizing techniques have been developed over the last decades to improve the performance of aluminum and titanium alloys. Below we discuss some that are most commonly used in the industry;
Type I anodizing has long been one of the most favored techniques to enhance the corrosion resistance of aluminum alloys [1, 9]. It is carried out at 38°C, at a voltage up to 50 V in chromic acid (30-50 g/l), allowing to obtain AAO layers with a thickness up to tens of microns or more. The voltage is gradually ramped up (e.g. 1 V/min) and then kept at the final value (e.g. for 30-60 min). The obtained oxide layer is, however, relatively soft in comparison with the films created by using other electrolytes and can easily be mechanically damaged. Furthermore, chromic acid is toxic and carcinogenic, which has led to regulations that prohibit its use in many regions.
Type II anodizing is used to enhance the corrosion resistance or visual appearance of an aluminum alloy [1, 9]. The oxide layers obtained with this type of anodizing are highly porous and are therefore most suitable for filling / coating with paints. This anodizing process is carried out in sulfuric acid in a temperature window of 20-80°C. The suitable sulfuric acid concentration is usually in the range of 5-20%.
Type III anodizing (a.k.a. hardcoat anodizing) is used to enhance the wear and corrosion resistance of aluminum alloys [1, 9]. The process is carried out similarly to type II anodizing in sulfuric acid but at much lower temperatures – usually at <10°C but preferably close to 0°C. The obtained films are more wear resistant in comparison with those grown by type II anodizing since the films are less porous. For type III anodizing, the sulfuric acid concentration should be in a range of ~10%.
Boric-sulfuric acid anodizing is considered an alternative to type I anodizing as the obtained films (with proper sealing) have a comparable resistance against corrosion . Furthermore, in comparison with chromic acid, the boric-sulfuric acid mixture is far less harmful for humans and the environment. As the name suggests, this anodizing process is carried out in a mixture of boric acid and sulfuric acid. The concentration of boric acid should be <1% and sulfuric acid 5-20%.
Anodizing of titanium is normally done for coloring either for practical purposes (color coding) or simply for enhancing the visual appearance of the metal . This process is carried out in a borax, Na3PO4 or sulfuric acid solution and the obtained color depends on the thickness of the anodic oxide layer, which can be tuned by the anodizing voltage (Fig. 2). The coloring effect is explained by the constructive or destructive interference of the light reflected from the surface of the oxide layer and the surface of the metal below – reflection at specific wavelengths is reduced while that at other wavelengths is amplified at given thicknesses of the oxide layer. One of the practical uses for this is color coding which is the coloring of similarly sized or shaped titanium components (e.g. titanium implants) so that they can be quickly used without having to measure them every time. Coloring by anodizing is also often used in the case of titanium jewelry to enhance their aesthetic appearance.
Figure 2. Titanium rods anodized in a 5 g/l Na3PO4 solution at different voltages.
Plasma electrolytic oxidation (PEO) is mostly used to enhance the wear resistance of aluminum alloys or the biocompatibility of medical titanium [15, 16]. In this process, higher anodizing voltages are normally used to initiate a discharge through the created anodic oxide layer. Due to a very high localized current density, the region is heated up to a few thousand °C. As a result, the electrolyte in this region is turned into plasma and this leads to a small explosion. Such localized high temperatures and pressures cause the anodic oxide layer to become more crystalline. Note that in all the previous cases the anodic oxide layers were usually amorphous. In the case of aluminum alloys, the anodic oxide layer created by PEO contains alpha phase alumina, which significantly enhances the wear resistance of the alloy – far more than the AAO created by type III anodizing. In the case of titanium alloys, the anodic oxide layer created by PEO contains anatase and rutile phase titanium dioxide, which increase the wettability of the surface and therefore also the biocompatibility. The latter one is also improved due to the increase of roughness. Some of the main drawbacks of this type of anodizing are, however, related to higher power consumption, more waste (electrolyte becomes contaminated faster) and lot of toxic fumes (the acidic or alkaline electrolyte literally boils near the substrate).
Safety first! Anodizing is relatively simple but there is little room for errors as acids, water and electricity are involved in this process. Therefore, there are immediate and long-term dangers that need to be considered before starting with anodizing.
CHEMICAL RESISTANT GLOVES – The solutions used for anodizing and pre-treatments are normally acidic or alkaline and therefore it is important to protect your hands when carrying out the processes or when preparing the solutions. Note that in most cases cheap latex gloves can be used, when dealing with dilute conventional anodizing solutions. Using gloves also helps against the contamination of the substrates by finger sweat, which would locally hinder the formation of the anodic oxide layer and thus lower the quality of the coating. Note that dilute sulfuric acid that is commonly used in anodizing, is quite harmless to the skin in the case of short exposures and can be washed off with plenty of water. However, if not washed away and forgotten, then it can cause damage to the skin already within a few hours. Continuous exposure to sulfuric acid will also lead to other health problems over time. Anyhow, when using rubber gloves, be sure that you don’t touch anything else with these gloves (e.g. face, clothes, cell phone, consoles or other surfaces).
LABORATORY COAT – When dealing with anodizing, it is simply a matter of time (usually rather short time), before some of the acidic anodizing solution (e.g. sulfuric acid) ends up on the clothes and etches holes in it. A good way to counter that is to use relatively cheap laboratory coats, that can last quite long.
GOGGLES / SAFETY GLASSES – It takes just one small drop of acid or alkaline to cause severe damage to an eye. Therefore, the use of goggles is highly recommended when preparing the solutions or carrying out the pre-treatment or anodizing of metal substrates.
ELECTRICITY – During anodizing the operator will be dealing with relatively high electric currents and voltages that can be lethal. Therefore, it is necessary to have basic knowledge about electricity in order to not get electrocuted. Be sure that the power supply is always turned off and disconnected, when touching the setup. The anodizing bath needs to be insulated from the rest of the room and make sure the floor of the room is always dry. Situations must be avoided, where an electric circuit could be created that passes through the heart.
VENTILATION – Anodizing and pre-treatments must be carried out in a well-ventilated room. Try to spend as less time in that room as possible to minimize long-term effects. If you are planning to create the anodizing solutions yourself from concentrated acids, then you will also need a fume hood and experience in working with dangerous chemicals. Make sure that the rooms used for anodizing, pre-treatment or storage are ventilated separately from the offices. Note that in most cases humans cannot smell the fumes created during pre-treatment and anodizing processes, which makes them even more dangerous.
STORAGE – Solutions used for anodizing and metal pre-treatments must be kept in appropriate containers that do not leak and can withstand the chemicals that are kept in them. These chemicals also need to be kept in a storage room that is well ventilated.
WASTE DISPOSAL – Pre-treatment and anodizing chemicals can often be used multiple times but eventually they become less efficient and need to be disposed. If the quantities are small (few liters), then the dilute acid and alkaline solutions can be neutralized and poured into the sewer. However, at higher quantities it becomes more difficult and then a disposal service is required. Note that you should definitely NOT pour acidic or alkaline solutions to the sewer as it is harmful for the environment and it would significantly shorten the lifespan of the pipes.
BASIC STEPS IN ANODIZING
The anodizing process consists of three stages: pre-treatment, anodizing and sealing of the pores. Each of these stages needs to be carried out properly as all of them affect the quality of the final surface.
Pre-treatment of the aluminum substrates is required to achieve a reproducible clean surface for the next stage. This is usually achieved by polishing the metal, which is followed by cleaning (e.g. with water and acetone) from organic and inorganic contamination and finally by chemical treatment in NaOH and then in HNO3. After the chemical treatment, the sample is also rinsed with water in order to avoid the contamination of the anodizing solution. The purpose of the chemical treatment is to remove the natural aluminum oxide layer along with other contamination (which was not possible with mechanical treatments and cleaning) but also to expose near-surface intermetallic particles. It should also be noted here that in some cases the chemical treatment in NaOH and HNO3 is not necessary, if the surfaces are properly cleaned and freshly polished prior to the anodizing step.
An example of complete pre-treatment procedure would look as follows;
1) Mechanical polishing
2) Cleaning with (pressurized) water and acetone
3) Immersing substrate into NaOH – (100g/l NaOH, 40°C, 1 minute)
4) Rinsing with (pressurized) deionized water
5) Immersing substrate into concentrated HNO3 for 1 minute
6) Rinsing with (pressurized) deionized water
7) Dry with pressurized air or preferably continue with anodizing right away
Anodizing of the pre-treated substrate is carried out next and the parameters (current, voltage, temperature, time) are selected based on the final application. In the case of sulfuric acid anodizing, using higher voltages and longer anodizing times usually results in thicker oxide films while having a higher temperature (room temperature or higher) of the electrolyte causes the AAO layer to be more porous, which is good for filling with paint later on in order to achieve an aesthetic appearance and good corrosion resistance. Using lower temperatures (<10°C) on the other hand results in a less porous AAO layer, which is more resistant to mechanical damage but also less suitable for painting. Using higher currents increases the growth rate of AAO but also heats up the electrolyte/metal interface and affects the porosity of the oxide layer.
Sealing of the pores in the AAO layer is the final step in the anodizing process and it is mostly necessary to achieve a long term protection against corrosion, improve the resistance against mechanical damage or to simply apply an aesthetic appearance. This can be done for instance by using hydrothermal treatment, paints, lacquers, Teflon or atomic layer deposition [6, 8, 11, 14]. It must also be mentioned here that some of the main alternatives to paints and hydrothermal treatment are dichromate, nickel acetate, nickel fluoride and Ce-Mo sealing techniques. The main reason to use these alternatives is generally to bring down the cost (e.g. less energy consumption) and increase corrosion resistance but each of them has their unique advantages and disadvantages. In the next chapter we cover some of these sealing methods in more detail.
SEALING METHODS FOR ANODIZED ALUMINUM
Hydrothermal treatment is one of the most commonly used methods to seal the pores in the AAO layer (Fig. 3) [6, 8]. In this technique the anodized substrate is essentially dipped into boiling deionized water. In this process aluminum oxide is converted into aluminum hydroxide, which takes up more space and as a result the nano-scaled pores become sealed. Such a sealing method is suitable for type III anodized substrates, where it can even slightly improve the wear resistance of the AAO layer. The general rule is that for every 1 μm of AAO layer, the substrate needs to be boiled for ~3 minutes.
Paints and lacquers are also very comfortable to use for sealing the pores in the AAO layer and they are especially effective in the case of type II anodizing, which creates a highly porous film. When dipped into a paint with appropriate viscosity, the paint is sucked deep into the pores and this ensures long term aesthetic appearance and corrosion resistance for the aluminum alloy. Note that by using warmer paints, it is possible to reduce the amount of air that remains trapped in the pores since during cooling the air takes up less volume and pulls more paint deeper into the pores, where it ultimately solidifies.
Figure 3. Hydrothermal sealing of the nano-scale pores in anodic aluminum oxide. In this process the anodic aluminum oxide is converted into aluminum hydroxide, which seals the nano-scale pores.
Atomic layer deposition (ALD) is a technique that allows deposition of thin films of a variety of materials by utilizing sequential self-limiting gas phase reactions [21 – 23]. The deposition process is carried out in a specially designed reactor (Fig. 4), where the reacting chemicals (precursors) enter the chamber one at a time and react chemically with the previously synthesized layer (Fig. 5).
Figure 4. An illustration of an atomic layer deposition reactor.
Due to the self-limiting nature of these reactions, only a thin layer of a material can be deposited during each precursor pulse and this allows uniform coating not only flat samples but also those that have a sophisticated three-dimensional shape. Therefore, ALD has also been intensively studied over the last few decades as a method for making thin (<100 nm) chemically resistant coatings for protecting other materials against corrosion [24 – 33]. The main problem of those coatings on aluminum alloys was poor resistance to mechanical damage and in some applications the level of protection against corrosion was not sufficient. To overcome these problems, the aluminum substrates can be anodized first to create a chemically homogeneous and nanoporous AAO sub-layer, which is then filled and coated by ALD with a chemically resistant material . The coatings prepared in such a two-step process have a thickness from a few hundred nanometers to a few microns (depending on the application) and can be applied on arbitrarily shaped precision details, without significantly changing their dimensions. Furthermore, these coatings have proven to provide long term protection against corrosion and since the coating is literally a very compact layer of nanostructured ceramic materials, it is also wear resistant.
Figure 5. Atomic layer deposition of TiO2 from TiCl4 and H2O, using N2 as carrier and purge gas. Color coding: Ti, O, Cl, H.
WHAT YOU NEED
A conventional anodizing setup consists of a power supply, an anodizing bath and the metal substrate that is anodized. However, a real system that gives more control over the process, is a bit more sophisticated and below we will discuss the purpose of the main components (Fig. 6);
POWER SUPPLY – It is the heart of the anodizing system as it drives the electrochemical process that converts metallic aluminum into aluminum oxide. The power supply is connected to the anodizing bath, which act as a cathode (-) and to the anodizable metal substrate, which is the anode (+). Note that if you switch the connections due to an accident, then your metal substrate will not anodize (it will be the cathode) and instead you will rapidly corrode your stainless steel bath as it becomes the anode. Anodizing is carried out by using specific electrical parameters (current, voltage and regime) and it must be possible to alter them on the power supply. For instance, Type II and Type III anodizing (in sulfuric acid) usually require a limiting voltage up to 20 V while for making a dielectric pore-free AAO layer in borate/tartrate solutions, you might need hundreds of volts. Higher voltages are also required for coloring titanium or for doing plasma electrolytic oxidation. Another critical parameter is the maximum output current of the power supply, as it determines how big substrates it is possible to anodize. Note that in Type II and Type III anodizing the limiting current density can be at least in the range of 15 A / dm2 or more and if the substrates are large, then a power supply with sufficient power output is required. Furthermore, it is also important to use appropriate wires that can withstand the maximum current over a longer period of time without heating up. Anyhow, anodizing at a fixed limiting voltage and current is often sufficient for beginners but in order to gain further control over the properties of the created AAO layer, it is useful to be able to utilize anodizing regimes, where the voltage or current is changed over time. For instance, the limiting voltage is sometimes raised over time (e.g. 1 V per minute) and then kept at a constant value in the end (e.g. at 20 V). The limiting voltage / current can also be applied to the substrate in short pulses. So based on the application and budget, a power supply is selected which has sufficient current and voltage output and which can be operated either manually or by computer. Note that computer controlled or programmable power supplies allow utilization of sophisticated anodizing regimes but they are also more expensive.
Figure 6. An illustration of a basic setup used for anodizing.
ANODIZING BATH – The anodizing bath has several major roles in the system. First, it acts as a cathode, which is necessary to carry out the electrochemical process. For that purpose, the bath needs to be made out of a conductive material. Second, the anodizing bath is filled with the electrolyte (the anodizing solution), which is often a 10-20% sulfuric acid solution. This means that the anodizing bath has to be able to withstand this corrosive (etching) environment. Furthermore, in the case of larger baths, the mechanical properties also start to play a bigger role as the baths will be filled with tons of electrolyte. Therefore, in most cases the anodizing bath is made out of stainless steel, which performs relatively well in a dilute sulfuric acid. Finally, the anodizing bath needs to have a suitable shape and size. Namely, the surfaces of the anodizable substrate should be as parallel as possible with the walls of the anodizing bath, while never touching them during the anodizing process. Having more electrolyte in the bath keeps, however, the temperature more constant and doesn’t need the replacement of the electrolyte as often. When the anodizing bath is large, then it is also wise to plan ahead how the anodizing solution will be replaced.
METAL SUBSTRATE & CONNECTIONS – The anodizable metal substrate acts as the anode (+) in the system and therefore it needs to be properly connected to the power supply. The anodizable metal always needs to be separated from the anodizing bath. If the whole substrate needs to be anodized, then it must be completely immersed into the electrolyte but in that case the connections that hold up the substrate, must be made out of a similar (e.g. aluminum) alloy. Make sure that these holders are electrically connected to the substrate – this can be ensured by removing the oxide layer from the holder prior to anodizing.
ANODIZING SOLUTION – The proper electrolyte is chosen according to the anodizing process that will be carried out. In most cases the solution is acidic but in some cases it can also be alkaline. For instance, Type II and Type III anodizing are done in a sulfuric acid solution (5-20%) and dependently on the temperature it is possible to obtain AAO layers that have a different porosity and therefore different corrosion resistance and mechanical properties. Note that the properties of the AAO layer strongly depend on the quality of the electrolyte, especially the content of Cl- ions in the solution. Therefore, it is highly recommended that deionized water is used instead of tap water for making the anodizing solution. Anyhow, titanium is normally anodized in borax, Na3PO4 or sulfuric acid solution (about 1-10% concentration window) but it is also possible to do it in Coca-Cola as it contains phosphoric acid. However, the quality of the obtained oxide layer obtained with Coca-Cola is inferior to those obtained with borax, Na3PO4 or sulfuric acid.
TEMPERATURE CONTROL – The anodizing temperature directly affects the porosity of the created AAO layer. During anodizing the electrolyte tends to heat up and therefore, it is critical to control the temperature in an anodizing process. Having more electrolyte certainly helps to keep the process more stable but often it is not sufficient to provide a constant temperature over a longer period of time. An efficient way to counter that is to apply system around the anodizing bath (e.g. water cooling) that regulates of the bath temperature. It is most critical when attempting to do Type III anodizing, where the temperature must be kept low. In laboratory or home/garage conditions, however, a constant room temperature can easily be achieved by having an external bath around the anodizing bath with sufficient amount of water. If it is necessary to bring the temperature close to 0°C, then we recommend using pre-cooled (kept in fridge) anodizing solution and having an ice-bath surrounding the anodizing bath. The temperature of the electrolyte must be measured before and after the anodizing process.
SETTING UP AN ANODIZING SYSTEM
When all the preparations have been made and the necessary components acquired, it is time to assemble the anodizing system (Fig. 6). Place the stainless steel anodizing bath (and the temperature regulation system) on an insulating ground or table, which also doesn’t corrode when coming into contact with sulfuric acid. When dealing with larger electrolyte quantities, it is also wise to have a floor drain for emergencies (e.g. anodizing bath starts to leak). The power supply should also be placed a bit further away on a table or wall, where it can safely be turned off in the case of accidents.
Next, a dipping system needs to be assembled that allows you to easily dip the anodizable metal into the electrolyte and extract it after the anodizing process. Note that the connections that are exposed to the electrolyte need to be made out of a similar aluminum alloy as the substrate. When anodizing aluminum in DIY/laboratory experiments, the connection can be an aluminum wire, which is held up with a lab stand. It is important that the lab stand/dipping system and the wire/connector are insulated from each other. The pre-treatment and post-treatment of the substrates should also be done nearby as this reduces the risk of contaminating the substrates and ensures a higher quality of the final product. In the next step the bath is filled with the appropriate electrolyte and warmed / cooled to the desired temperature (e.g. 0°C) with the help of the temperature regulation system. Note that if the electrolyte is prepared inside the anodizing bath, then it is important that it is thoroughly mixed (in larger baths it will take significantly longer). Next, the cleaned and pre-treated substrate is attached to the connector (e.g. wire) and dipped into the electrolyte. As a site note – it is highly recommended that the cleaning and pre-treatment of the substrate is done after it has been attached to the connectors as this would significantly reduce the risk of contamination. Finally, when the anodizing bath (cathode) and the anodizable substrate (anode) are properly connected to the power supply, the anodizing process can be started.
TEMPLATE SYNTHESIS OF NANOSTRUCTURES
The easy control of anodizing parameters allows precise tuning the diameter, length and shape of the pores in the AAO. Note that normally the diameter of the pores in AAO is in the range of 10-20 nm in the case of type II and type III anodizing (excluding holes left behind after the removal of intermetallic particles in the case of aluminum alloys). This makes anodizing suitable for making unique templates from pure aluminum for creating nano-scaled structures like rods, tubes or even “forests” that contain a vast amount of oriented rods or tubes [17 – 20]. The initial pure aluminum substrate can either be a plate, foil, wire or a thin film deposited on another material. Note that in order to produce a higher quality AAO, the first thin AAO layer is often removed by etching and then the substrate is anodized again. After the aluminum has been anodized, another material is deposited into the nano-scaled pores in the AAO matrix and the latter one is afterwards removed by etching. There are of course many different approaches how create nanostructures with the help of AAO templates but in this book we discuss a few cases, where it can be done by utilizing electrochemical deposition or atomic layer deposition (ALD);
Nano-scaled rods can be synthesized by completely filling the pores in the AAO template with a desired material that can later on withstand the removal of the template by etching in an alkaline solution (Fig. 7). Both electrochemical deposition and atomic layer deposition can be used for that purpose, depending on the desired material and the properties of the AAO template. For instance, if the AAO template is a completely oxidized sheet, then it is non-conductive and cannot be used for electrochemical deposition. In that case the only option is to completely fill the pores by ALD with a suitable material (e.g. metallic Pt or ceramic TiO2) at sufficiently long pulse times. When the AAO matrix is removed in an alkaline solution (start experimenting with 0.1-1 M NaOH), the material that was deposited into the pores will become free and go into the solution. Note that the pH of the solution might need to be adjusted to avoid the etching of certain materials and also ultrasonification may be required to separate the nanorods from one another. Anyhow, if the AAO template is on an aluminum substrate of sufficient thickness, then an electrical connection can easily be made with the aluminum. The oxide layer at the bottom of the pores is very thin and therefore it is possible to electrochemically deposit the suitable material (e.g. Ag, Cu, etc.) into the pores. The growth will start from the bottom of the pore and fill the whole pore until it reaches the surface – at that point the process needs to be stopped. Next, the AAO matrix can be removed with an alkaline solution again.
Figure 7. Template synthesis of nanorods. In this process an AAO template is created first by anodizing aluminum. Next, the pores are completely filled with another material by electrochemical deposition or by atomic layer deposition. Finally, the AAO matrix is etched away with an alkaline solution.
Nano-scale tubes are created by growing a thin layer of a desired material on the walls of the pores in the AAO matrix and then removing the matrix by etching. However, it is also possible to create multi-walled tubes by growing a thin layer of Al2O3 on top of the first layer and then deposit another layer of a desired material. When dipped into an alkaline solution, the AAO matrix and the Al2O3 layer between the two other layers will be removed and as a result a double-walled nanotube is created (Fig. 8).
Figure 8. Template synthesis of double-walled nanotubes. First, a nanoporous AAO matrix is created by anodizing. Next, the first layer of desired material is grown by ALD, which is followed by growing a separating layer of Al2O3 by ALD and then the synthesis of the second layer of desired material. Finally, the AAO matrix and the Al2O3 separating layer are removed by etching.
“Forests” of oriented nanorods or nanotubes can be created in a similar manner but an additional step is necessary. Namely, in the first steps, the desired material is still deposited into the pores of AAO either by electrochemical deposition or by ALD . In the next step, the surface of the AAO is covered with a suitable chemically resistant supporting layer, which is well adhered to the previously deposited material. Note that an alternative is the deposition of a thin film of aluminum on another material and anodizing it there. Anyhow, in the next step, the AAO matrix is removed and the nanorods or nanotubes remain in the same location where they were grown (Fig. 9, 10). The supporting layer has to be thicker to be mechanically durable and also with appropriate electrical parameters (conductive, semi conductive or insulating). Such a layer can be grown with a variety of techniques such as magnetron sputtering, pulsed laser deposition, chemical vapor deposition or others. It may also be necessary to apply another supporting layer later on by utilizing even more robust methods (e.g. glue).
Figure. 9. Template synthesis of a forest of nanorods or nanotubes. After the desired material has been deposited into the pores of AAO, a supporting layer is deposited on top of the structure. The AAO matrix is then removed by etching.
Figure 10. An illustration of a “forest” of oriented nanorods and nanotubes.
How thick is the anodic oxide layer? This is the most commonly asked question by companies and individuals that provide anodizing services. Furthermore, often it is also necessary to evaluate the mechanical properties, porosity or even biocompatibility of the anodic oxide layer. In this chapter we will discuss different methods that can be used to find answers to these questions.
The thickness of an anodic oxide layer can be measured with a microscope that has a sufficiently high resolution. For that purpose, a precise cross-section is made first in a site on the anodized substrate that is away from holes or edges. This is important because near the edges or holes, the thickness of the oxide layer may greatly differ from the thickness measured at the center of the substrate. In the case of thicker oxide layers (tens or hundreds of microns), the cutting is normally done mechanically and the quality of the freshly cut surface may be enhanced by polishing. The cross-section can then easily be studied with an optical microscope as the oxide layer and metal substrate would be clearly distinguishable. This is the most cost-efficient way to study the thickness of anodic oxide layers. However, if the oxide layer is thin, then mechanical preparation may not be suitable and the resolution of optical microscopes is insufficient. In this case the cross-section can be done with a focused ion beam (FIB) and then studied afterwards with a scanning electron microscope (SEM) . In a scanning electron microscope, the surface of the substrate is scanned row-by-row with a very narrow (e.g. 1 nm) beam of accelerated electrons. The signals created at the sample surface by the primary electrons are gathered with appropriate detectors and an image of the surface is created by the computer. The ion beam is created with an ion gun that is mounted to a scanning electron microscope. This technique is most suitable for anodic oxide layers that have a thickness from 0.01 μm (10 nm) to 20 μm but the characterization costs are significantly higher. However, in some cases FIB can be avoided if mechanical sample preparation can produce a cross-section surface of sufficient quality for SEM studies and this would bring down the cost. In the case of thinner anodized aluminum foils or AAO wafers, the samples can also be simply bent, broken or cut with scissors for studying the cross-section with SEM.
The pores in an anodic oxide layer can have a diameter in the range of tens of nanometers and this means that they are invisible for a conventional optical microscope. Therefore, they need to be studied with a modern high-resolution scanning electron microscope that has a resolution of ~1 nm. An alternative method for studying pores is an atomic force microscope (AFM), which uses a very sharp tip to scan across the surface . With AFM it is possible to create a three-dimensional recreation of the sample surface, where the pores appear darker as the cone-shaped probing tip would not be able to penetrate very deep into the pores during the measurement and interact with the pore walls. In order to study the shape and distribution of the pores in an anodic oxide layer, however, it is necessary to make a cross-section with FIB and image it with SEM. Furthermore, if atomic resolution is required, when studying the interior of an anodic oxide layer, then it can only be done with a transmission electron microscope (TEM), which uses high energy accelerated electrons that can penetrate through a thin (<100 nm) sample. However, for TEM studies, a sufficiently thin lamella must be made first by utilizing a SEM-FIB dualbeam system and this makes it one of the most expensive techniques.
The mechanical properties (e.g. hardness) of an anodic oxide layer can be evaluated by nanoindentation or microindentation techniques, depending on the thickness of the oxide layer [36, 37]. In both cases, an indenter tip of specific design (e.g. 3 or 4 sided pyramid), usually made of diamond, is forced into the tested substrate. Obviously in the case of softer materials, at the same loading force, the tip penetrates deeper in comparison with harder materials. Once the penetration depth and the contact area of the tip and the material are known at a certain force applied for the tip, it is possible to calculate the hardness of the material. Note that the hardness of an anodic oxide layer also depends on its crystallinity which can be studied further by Raman spectroscopy or TEM to develop even harder coatings.
Testing the adhesion of a coating (e.g. paint) to an anodized surface is quite easy and affordable even for beginners as it can be done with a tape (we recommend a power tape) bought from a local store. Adhesion testing with tapes should be carried out both on unharmed surfaces but also on deliberately scratched substrates. By scratching a cross on a painted surface before testing, it is possible to gain information about the behavior of the coating in the case of defects that are caused later on in the real application by the end consumer or the environment.
The biocompatibility of anodized substrates is mostly relevant for medical titanium (e.g. Grade 1-5 titanium) that is used for making implants. Although the titanium dioxide created during the anodizing process is known to be biocompatible, the seller of the final product (e.g. titanium dental implant) has to make sure that the product meets the requirements of ISO 10993 . Anyhow, when developing a new biocompatible material, it is reasonable to start testing with human cells (e.g. bone cells if the implant will be inserted into bone) and if the tests are successful then continue with pigs and eventually (when the researchers are absolutely convinced that the tested substrate is biocompatible) with human volunteers.
Often the main purpose of anodizing is to enhance the corrosion resistance of a material and this can be tested by exposing the anodized substrate to a corrosive environment that is similar or more aggressive than in the real application. Here are some of the main techniques that can be used for that purpose;
Corrosion testing by immersion is one of the easiest methods to evaluate the corrosion behavior of a material or coating. In this test, the substrate is immersed into a corrosive medium for a longer period of time. For instance, a test can be carried out in an acidic 5% NaCl solution at pH 3 and room temperature for 1000 h. The substrate is photographed and often weighed before, during and after the test. The corrosion behavior of the sample can then be explained by the change of appearance according to the photographs or the change in mass. In the case of stable corrosion products, the mass should increase over time and if the corrosion products are removed, then the mass of the tested substrate should be lower than before the test.
Salt spray testing is performed in specially designed chambers that precisely control the properties of the corrosive environment (e.g. humidity, pH and temperature). Testing at higher temperatures can significantly speed up the corrosion process and give valuable feedback about the performance of a coating or material much faster than immersion tests. Furthermore, the better control over the parameters of the corrosive environment and continuous exposure to fresh electrolyte, makes the tests more reliable and comparable with tests done by others in similar conditions (e.g. testing according to ISO 9227) . Similarly, to immersion testing, the substrates are weighed and photographed before and after the tests (sometimes also during the test).
Electrochemical tests are mainly used to obtain quick information about the corrosion behavior of materials and coatings. Electrochemical tests usually last only a few hours and give similar results as immersion or salt spray testing that take over several months. This makes electrochemical testing suitable for identifying possible candidates from many test-substrates in a reasonable timescale before proceeding with the more time and money consuming salt spray testing. An electrochemical test is carried out in a specially designed cell, which is filled with an electrolyte (corrosive environment). The three electrodes that are used to carry out the measurement are the sample (working electrode), a counter electrode (Pt wire or graphite rod) and a reference electrode (e.g. saturated calomel electrode). One of the most commonly used electrochemical tests is the measurement of a corrosion potential (CP) over a certain period of time. In the case of metal substrates, the CP value shows the activity of the metal (e.g. cathodic potentials show that the metal is active while anodic potentials show that the metal is passive). For coated substrates on the other hand, the stability of CP over a few hours may indicate that no defects are present in the coating. Rapid fluctuations in the CP (e.g. changes of 50 mV or more) however, may refer to corrosion processes that take place in the defects in the coating. Another useful method for accelerated electrochemical corrosion testing is linear sweep voltammetry (LSV), in which the potential applied to the tested substrate is linearly changed over time and the resulting currents are measured. The measured currents at different potentials represent the reactions (e.g. corrosion) that take place on the surface of the tested substrate. As an example, for testing coated aluminum alloys, the suitable parameters for LSV in a 0.1 – 1 M NaCl solution are as follows; starting potential -1 V vs. SCE (saturated calomel electrode), end potential +2 V vs. SCE, and a scan rate 1 mV/s. When testing a coated aluminum substrate in these conditions, then a sudden increase of current density (e.g. over 1 mA/cm2) at a certain potential (pitting potential) would indicate a formation or presence of a defect in the coating. If no sudden increase of current is detected during the polarization scan and no defects can be observed on the surface after the experiment, then the coating contains no defects. If the pitting potential is shifted to more anodic (positive) values for some substrates in comparison with others, then it indicates that the corrosion resistance has been significantly improved. Coated substrates should also generally exhibit lower currents when testing by LSV.
Setting up an anodizing system for laboratory experiments, DIY (do it yourself) projects or commercial purposes is relatively easy and the most important thing is to simply get started. Once you have learned the basics of anodizing with the help of this book (and with some trials and errors), you start to see the endless possibilities that present themselves and specialize in a desired direction. Note that some of the techniques discussed here require expensive scientific equipment that is not available for beginners but nowadays we have internet and there are many companies that provide with these services.
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 ISO 10993
 ISO 9227