Whether you work in the automotive industry or are an architect designing tall buildings, you’ve heard of the anodizing process before. You may have come across anodized tools as a dentist, or in the business of food preparation equipment. Anodized tools and devices have a wide variety of uses, such as the marine industry, home furniture, and parts for sports equipment. If cycling is your passion, for example, then you are definitely familiar with it. Anodizing is, however, mostly used in the aerospace and electronics industries, where certain parts need to be ‘corrosion-proofed.’ There are many processes that various metals used in those industries can be exposed to, and since anodizing can be used for many different metals, ranging from aluminum to titanium, it’s usually the best way to protect any given aluminum surface. Among these, aluminum is the most versatile. Therefore, we are going to discuss the key components to anodizing aluminum parts. We will touch on topics such as sealing anodized aluminum, bright dip anodizing, etching, voltage and time, and we will go through and compare the different types of anodizing processes such as hard coating.
The Process of Corrosion
Before defining anodizing, we need to discuss some natural processes and understand why some of them need to be annulled through the application of anodizing. It’s common knowledge that untreated iron will develop rust if it’s left exposed to air. Rusting is a chemical process called corrosion, where a refined metal tends to turn into a more stable form like oxide, hydroxide, or sulfide. Corrosion degrades useful properties of materials and affects its appearance, strength, and other characteristics related to its usefulness. While it’s mostly the exposure to moisture in the air that affects the materials, sometimes corrosion can develop if a material has come in contact with certain substances. To counter the way materials behave when in contact with air and other substances, methods have been developed to reduce the reactivity of exposed surfaces for increased corrosion resistance.
The Process of Passivation
One of the most frequent methods used is called passivation, and anodizing is one form of electrolytic passivation. Passivation is a process that ultimately renders a material more resistant to the environment, which makes it less prone to corrosion, abrasions and increases fatigue life. The material becomes passive by creating an outer barrier of aluminum oxide through anodizing. This light coat of anodized aluminum is a barrier against corrosion that, apart from adding to the object’s strength and durability, also preserves its appearance. We can define anodizing as a process of increasing the film thickness of the natural oxide layer on the anodized surface of metal parts. It’s a chemical conversion process that penetrates into the aluminum component and changes its stability.
Benefits of Anodizing
Initially, we mentioned corrosion protection as one of the more common reasons why certain parts, tools, and machinery are being anodized. The most commonly used anodic oxidation coatings in industrial anodizing provide electrical and thermal insulation. This layer of aluminum oxide consists of hydrated aluminum oxide that is considered impervious to corrosion. However, the benefits of aluminum anodizing do not end there. It can also add integral color to your object and act as a protective coat against aggressive chemical agents used for cleaning.
Apart from building resistance to wear and corrosion, the process also improves adhesion of paint primers and glues, compared to applying paint to bare metals. It’s also very useful in preventing galling of threaded components. Galling is a form of wear caused by adhesion between two sliding surfaces. It’s a combination of friction and adhesion, followed by slipping or tearing the crystal structure beneath the surface. Aluminum will gall rather easily, which is why anodizing is universally applied for its protection.
Process Equipment, Rectifiers and Chillers
If you were wondering what kind of equipment is needed to achieve the full process of anodizing, you may be surprised that there are small anodizing kits made for in-home use. However, these “anodize aluminum at home kits” are meant for very small projects, and they still require in-depth knowledge to ensure safe application. If you’re interested in purchasing one for yourself to put a nice finish on a bicycle part, you will need to talk to the sellers and perform thorough research online. We recommend that you visit one of the more prominent anodizing businesses that will have the experience to help you. These businesses usually have large industrial buildings filled with anodizing tanks and equipment meant for anodizing various aluminum and titanium parts and materials.
The machinery can be made up of several automatic anodize tanks for aluminum anodizing. These anodize tanks have integrated CNC machines that help the process. Both small and large anodizing systems need to have multiple tanks for completing the anodize process. A typical anodize line would involve a minimum of the following: Alkaline clean, rinse, etch, rinse, deoxidize, rinse, anodize, rinse, dye, rinse, seal, rinse, hot water dip. Normally, there should be two rinse tanks between each process tank and deionized water should be used. Also, ventilation systems are needed above the acid and etch tanks.
The chemical bath used in the anodizing process will produce a significant amount of heat. That heat needs to be eliminated to successfully anodize a part, which means that vital measures must be put in place to keep temperatures at an optimal level. Chillers are used to even out the metallic oxide deposition, and the cooling process is helped by either water-chillers or air-chillers. Chillers use temperature sensors and valves to maintain the required temperature.
In order to achieve the anodizing process, you will need to apply current to the electrolytic solution. The system that controls the current is in a rectifier. The anodizing process is a delicate one, so it needs both chillers and rectifiers to keep the temperature and voltage at optimal levels. Also, the thickness of the anodized layer will greatly depend on the amount of time the object spends in the tank connected to the electrical current.
Timing and Voltage
The process is best explained in Larry Chesterfield’s two-minute read on anodizing current density vs voltage: ‘’In the anodizing process, it is the current (amperage) that builds the anodic coating. As the coating builds up on the parts, it impedes the flow of current to the parts. If you fix the current throughout the anodizing cycle, the time required to build the desired coating thickness can be predicted.[…] By using amperage control (“anodizing by current density”) the amperage is fixed for the entire anodizing cycle, thus providing a “constant current density” for the entire length of the cycle.’’ He offers a few mathematical formulas that will help you calculate the adequate voltage and time that need to be used in relation to the size of the object being anodized.
It’s now time to tackle the chemical part of this electrochemical process. We’ve discussed the chemical baths being utilized in these special tanks, along with the large machines taking part in the process. We have not yet discussed the components of the chemical baths. If you wish to perform anodizing at home, you will need to know the formula for calculating what goes into the solution. Before we discuss the mathematics, take a moment to look at the formula in its written form: 2H2O + Elect ➝ O2 + 2H2 ; 2Al + 3O2 ➝ 2AlO3
The anodizing solution that is most often used for anodizing aluminum is sulfuric acid. Before beginning the process, one should calculate the liters or gallons of solution needed, while taking into account the recommended ratio of acid in the anodizing bath (15% to 18%). Keep in mind that this formula applies differently to different kinds of sulfuric acid. We’ll give an example of an estimation:
Gallons of H2SO4 required for a 500 gallon 15 percent by weight anodizing bath = 100 x .15 x 0.625 x (500/100) = 47 gallons.
Structure of the Oxide Coating
If you’ve wondered why anodizing is among the top surface finishes, we will let anodizing.org explain: ‘’The anodic oxide structure originates from the aluminum substrate and is composed entirely of aluminum oxide. This aluminum oxide is not applied to the surface like paint or plating but is fully integrated with the underlying aluminum substrate, so it cannot chip or peel.’’
As the aluminum part is immersed in the chemical bath and the current is running through it, the aluminum atoms are slowly extracted from the surface of the metal. While this happens, what we refer to as ‘pits’ are formed on the surface structure. The voltage applied impacts the size of these pits, and they form a porous structure that allows for secondary processes, such as coloring and sealing.
Other characteristics depend on the voltage, as well. The thickness of the layer of aluminum oxide coating greatly depends on the voltage the bath is subjected to and the time the object spends submerged in it. Higher voltage increases the speed of oxidation, and the duration of the process affects the coating. Other aspects, like temperature and characteristics of the electrolyte, influence the rate of dissolution of the metal surface.
Abrasion resistance is one of the crucial goals of anodizing. However, we are now discussing how the size and density of pores or pits affect said goal. Higher density and smaller pits mean greater resistance to abrasions. Certain other characteristics can affect the integrity of the coating, as well.
Types of Anodizing
The most commonly referred to types of anodizing are Type II sulfuric acid anodizing and Type III hard coat anodizing. This is because one (Type II) is known widely as ‘regular’ anodizing, and the other (Type III) as ‘hard’ anodizing. So what became of Type I, and are there any other processes that are not included in the original distinction? We are here to break them up for you evenly and make them more understandable.
Chromic Acid Type I Anodizing
Type 1 chromic acid anodize became less popular due to some safety regulations regarding chromium and air-pollution with which other newer acid types didn’t conflict. This is the oldest type of anodizing, and it forms softer anodized films, much thinner than the regular or hard anodized finishes. Its thinner coated oxide coating is usually expressed in microinches, from 20 to 100 at most. However, although thin in nature, it still provides equal corrosion protection and wear resistance when properly sealed. Chromic acid type I can’t be used for color anodizing, as it presents light grey in color and absorbs less color when dying. Its application isn’t for decorative purposes, since it’s mostly dyed black and used to make non-reflective surfaces in the aerospace industry, or it is used on components for precision machines.
Boric-Sulfuric Acid Subtype I Anodizing
This subtype is an alternative to chromic Acid Anodizing, as it bypasses the environmental concerns. It is covered by MIL-A-8625 and mostly used for corrosion protection and paint adhesion. It’s also more energy-efficient than its chromic anodize counterpart.
Sulfuric Acid Type II Anodizing
The most common type of anodizing is Type II, or regular anodize. Type II has the most widely applied solution containing sulfuric acid, providing a moderately thick coating. It’s also a much harder finish than chromic anodize. Its porous surface is conducive to color surface finishes on aluminum and aluminum alloys. These color anodizing effects can produce colors like Black, Red, Blue, Green, Urban Grey, Coyote Brown, and Gold. Prior to anodizing, the products can be treated to achieve a matte look. Dyed anodized films don’t lose color quickly, especially if they have been properly sealed. Sulfuric acid type II anodizing is overall the least expensive process, and it provides a wide variety of anodizing aluminum alloys and is available in many colors. On top of that, its waste treatment is far easier to control than that of chromic anodizing.
Sulfuric Acid Type III Hard Anodizing
Type III anodizing, also known as hard coat anodize, is also achieved with sulfuric acid based electrolyte, but the product is a much thicker, denser aluminum oxide coating. This process is meant for components that are subject to extreme wear or highly corrosive environments. A more durable coating is needed in things like blast shields, hinge mechanisms, valves, and joints. Type III is applied for enhanced electrical insulation.
Other Types of Anodizing
- Phosphoric acid: phosphoric acid anodizing improves performance in high-humidity environments. It’s also known as the Boeing Process, and is applied largely in the aircraft industry.
- Titanium anodizing: it’s predominantly used for coloring titanium and looking for stress cracks in the base material. It is mostly used in the medical community, as well as aircraft and spacecraft parts. It’s highly adhesive for dry film lubricant and paint.
- Architectural anodizing: this anodizing process is used in the industry where both appearance and durability are equally important. If you need a layer of aluminum oxide on a component that is going to support the exterior of a building permanently, architectural anodizing of aluminum is the only way to go.
- Bright dip anodizing: This aluminum anodizing process uses phosphoric and nitric acid to give a shiny, almost mirror-like appearance. It works best on 5000 and 6000 series aluminum. The harder the aluminum oxide coating, the brighter it will be. It’s popular in the production of car parts and flashlights. Aerospace Metals specializes in bright dip anodizing.
Etching and Sealing
Etching is a pretreatment process used to achieve a premium visual appeal. It’s vital that all the dirt, grease, and oil be removed from the surface of the metal to leave a clean outer layer that will convert into the anodized coating of aluminum. There are two types of etching used to eliminate surface defects. One uses caustic soda, and the other, acid etch, treats the surface with fluoride-based ions and eliminates the imperfections.
The process of sealing is one of the final steps in anodization, and according to many, it is the most important one. Sealing provides maximum corrosion resistance but minimizes abrasion resistance. Three of the most common types of sealing are hot deionized water sealing, mid-temperature sealing, and cold sealing. Types of sealants used vary based on the anodizing process at hand, some of which will require boiling deionized water. Others, such as those containing dye colored parts, will not require this step. Cold sealing provides the highest quality product but is likely to be slightly more complicated to perform.
As you can see, anodizing has an incredibly wide application throughout many industries. Without it, planes wouldn’t be as safe, space travel would be nearly impossible, and our living and working spaces would be less durable and sleek. The process can vary in its complexity and cost, but the final product performs and looks better than the original component, by far. From the small business owner of a key shop to companies developing parts for military-grade aircraft carriers, many can benefit from using anodized components.
If you are interested in having your parts anodized, please contact us at www.aerospacemetalsllc.com