What is Sulfuric Acid Anodizing?
Sulphuric acid anodising is an electrolytic process using electricity (DC current) and a sulphuric acid electrolytic solution causing oxidation of the aluminium surface to form a thin “harder” film of aluminium oxide under controlled conditions.
The sulfuric acid anodizing process involves immersing a metal part in a sulfuric acid solution while applying a direct electrical current.
Sulfuric acid anodizing is an electrochemical process used to create a protective layer of aluminum oxide on the surface of aluminum, titanium, and other metals.
The process involves immersing the metal parts in a sulfuric acid electrolyte solution (the chemical portion of the electrochemical process) while applying a direct current (the electrical part of the electrochemical process).
The part acts as the anode (or source for a negative charge), while a cathode (the source of positive charge that attracts electrons) made of lead or stainless steel is also immersed in the solution.
When a direct current is applied, a controlled oxidation reaction occurs and forms a layer of aluminum oxide on the surface of the metal.
While the thickness is dependent on processing parameters, the anodized layer created by sulfuric acid anodizing is typically between 5 and 25 microns (0.0002 to 0.001 inches) thick.
This aluminum oxide layer is hard, durable, non-porous, and provides excellent corrosion resistance so it’s ideal for use in a wide range of applications, including aerospace, automotive, and construction.
Sulfuric acid anodizing can also be used to produce a range of colors on the surface of the metal. This is achieved by using dyes or pigments to fill the pores in the anodized layer.
The resulting colors are stable, long-lasting, and ideal for decorative or branding purposes.
How Does Sulfuric Acid Anodizing Work?
Sulfuric acid anodizing is one of the most widely used types of anodizing for aluminum and is typically done at room temperature, although elevated temperatures can be used to accelerate the process. The process requires several steps:
- Cleaning: The first step is to clean the metal to remove any dirt, oil, or other contaminants to remove any foreign substances on the surface of the metal that can interfere with the anodizing process.
- Acid Etching: Once the metal is clean, it is typically etched in an acid solution to create a rough surface that helps the oxide layer adhere to the metal.
- Anodizing: The metal is immersed in a sulfuric acid solution and connected to the positive terminal of a power supply. When the power is turned on, a current flows through the solution and causes the oxidation reaction on the surface of the metal.
- Sealing: After the anodizing process is complete, the metal is rinsed with water to remove any excess acid, then immersed in a sealing solution to close the pores in the oxide layer and improve the durability of the finish.
The thickness of the oxide layer can be controlled by adjusting the voltage and time spent in the anodizing bath.
Thicker oxide layers are typically more durable and resistant to corrosion, but can also have a rougher surface texture.
Thinner oxide layers are smoother and more aesthetically pleasing, but may not offer as much protection against corrosion.
Sulfuric acid anodizing is an environmentally friendly process that produces no harmful byproducts. The electrolyte solution can be recycled and reused, which reduces waste and lowers the cost of finishing.
The resulting anodized layer is also environmentally friendly, as it is non-toxic and anodized parts can be recycled.
How To make Sulfuric Acid Anodizing Bath
The question sometimes arises of whether the sulfuric acid anodizing bath is made up of volume or by the weight of sulfuric acid.
The bath is always made up by the weight of sulfuric acid, not by volume. If the bath is made up by volume, it will give results that are almost twice the concentration that is wanted.
This is because 93% sulfuric acid the most commonly used concentration weighs almost twice as much as water: 15.3 lb/gal for the sulfuric vs. 8.35 lb/gal for water.
Below is the formula for calculating the amount of sulfuric acid to use for the anodizing bath makeup. This formula is for using 66o Baume’ (93%) sulfuric acid.
If you are not using 93%, you will have to adjust the formula according to the weight of the sulfuric acid being used.
The weight can usually be found on the container label. The numbers in these formulas are rounded off.
The concentration of the anodizing bath should be expressed in grams per liter (g/l). Note that 1% by weight sulfuric acid is approximately 11 g/l. The metric calculation to make up a 15% by weight, 165 g/l, 100-liter bath is:
- 100 liters of water weighs 100,000 grams (1 liter of water weighs 1000 grams)
- 15 x 100,000 = 15,000 grams of 93% sulfuric acid needed
- 93% sulfuric acid weighs approximately 1834 g/l (15.3 lb/gal ÷ 785 liters/gal = 4.04 lb/liter. 4.04 lb/l x 454 g/lb = 1834 g/l ).
- 15,000 grams of sulfuric acid ÷ 1834 g/l = 8.2 liters sulfuric acid
So, 100 liters of 15% (weight) sulfuric acid is made up using approximately 8 liters of 93% sulfuric acid and 92 liters of water. Always add acid to water; never add water to concentrated sulfuric acid!
The English calculation for making up 100 gallons of 15% (wgt), or 165 g/l, solution of sulfuric acid is:
- 100 gallons of water x 8.35 lb per gallon = 835 lbs
- 15 x 835 = 125.25 pounds of 93% sulfuric acid needed
- 125 lb sulfuric acid ÷ 3 lb/gal = 8.2 gallons of sulfuric acid
So, 8.2 gallons of 93% sulfuric acid added to 92 gallons of water will make 100 gallons of 15% (wgt) sulfuric acid.
When making up a completely new anodizing bath, be sure to add enough aluminum sulfate to have approximately 2 g/l of aluminum in the bath as a “buffer” to help protect parts from burning in the new anodizing bath.
Materials Needed to Perform Sulfuric Acid Anodizing
The materials you’ll need to anodize are pretty basic and not too expensive. If you think you’ll be doing this somewhat regularly, it makes sense to set up a home anodizing station.
Here’s what you’ll need:
- Sulfuric acid
- Distilled water
- Several tanks (containers) to hold the liquids
- A cathode
- Aluminum wire (titanium will also work)
- Degreaser
- Lye
- Acid neutralizer
- A power source
- Dye (if you want a colored part)
You will also need a well-ventilated area to set up your anodizing station, as well as personal safety gear (eye protection, gloves, respirator, etc.).
In addition to the necessary materials above, you may also want to purchase some optional items to make your work easier. These include the following:
- An agitator for the acid bath
- Scotch-Brite pads for cleaning parts before anodizing
- Ping pong balls (put in the tank to prevent acid mist)
- Cheap tea kettle to heat dye
- Fish tank heater to regulate tank temperature
- Cooking thermometer for checking temperatures
How Is Sulfuric Acid Anodize Different From Other Types Of Anodize?
It is particularly suited for applications where hardness and resistance to abrasion is required. However, where parts are subjected to considerable stress, (such as aircraft parts), the possible presence of the corrosive acid residue is undesirable.
The porous nature of sulfuric acid films prior to sealing is used to particular advantage in the production of colored surface finishes on aluminum and its alloys.
The porous aluminum oxide absorbs dyes well, and subsequent sealing helps to prevent color loss in service.
Although dyed anodized films are reasonably colorfast, they have a tendency to bleach under prolonged direct sunlight.
Some of the colors are: Black, Red, Blue, Green, Urban Grey, Coyote Brown, and Gold. Parts can be treated chemically or mechanically prior to anodizing to achieve a matte (non-reflective) finish.
Benefits of Sulfuric Acid Anodizing
Sulfuric acid anodizing produces a uniform protective layer across the entire surface of a metal part and it’s possible to achieve a high degree of precision and repeatability in the anodizing process. This layer of aluminum oxide provides several benefits including:
- Improved corrosion resistance: The anodized layer is highly resistant to corrosion, making it ideal for use in applications where the metal will be exposed to harsh environments or corrosive substances.
- Increased durability: The anodized layer is hard and durable, which helps to protect the metal from wear and tear, extend its lifespan, and reduce the need for maintenance.
- Enhanced appearance: Sulfuric acid anodizing can produce a range of colors on the surface of the metal, which is useful for improved aesthetics and branding purposes.
- Electrical insulation: The anodized layer is an excellent electrical insulator, so it’s useful in applications where electrical conductivity must be controlled.
- Non-toxic and environmentally friendly: Sulfuric acid anodizing is an environmentally friendly process that produces no harmful byproducts and the resulting anodized layer is non-toxic and can be recycled.
Overall, sulfuric acid anodizing is a versatile and effective process for enhancing the durability and corrosion resistance of aluminum and other metals, which makes it a popular choice in a variety of industries.
Limitations of Sulfuric Acid Anodizing
While sulfuric acid anodizing is a widely used and effective finishing method, there are several limitations to this process, including:
- Limited Color Options: Sulfuric acid anodizing is generally limited to a range of natural metallic colors, such as silver, gold, and bronze. While it is possible to dye the oxide layer to create other colors, the range of colors is limited.
- Surface Imperfections: The anodizing process highlights surface imperfections, such as scratches, dents, or blemishes, so isn’t well-suited for applications requiring a perfectly smooth surface.
- Limited Thickness: The thickness of the oxide layer that can be achieved with sulfuric acid anodizing is limited to around 25 microns or less. While this can provide good corrosion resistance, it may not be sufficient for high-stress or high-wear applications.
- Brittle Coating: The oxide layer created through sulfuric acid anodizing is relatively brittle, which means it may crack or chip when subjected to significant stress or impacts.
- Environmental Concerns: The sulfuric acid used in the anodizing process can be corrosive and dangerous, and proper safety precautions must be taken. Additionally, the disposal of the acid solution can be environmentally harmful if not handled properly.
Sulfuric Acid Anodizing Applications
- Military weapons
- Optical components
- Medical components
- Mechanical hardware
- Decorative applications
- Computer & electronic enclosures
- Hydraulic valve bodies
Type II anodizing is used in a variety of industries, including aerospace, automotive, and electronics. In the aerospace industry, it is used to provide corrosion resistance to aluminum parts and components, as well as to enhance their appearance.
In the automotive industry, sulfuric acid anodizing is used to provide a decorative finish on aluminum wheels, trim, and other components.
In the electronics industry, it’s used to provide a protective coating on electronic components and printed circuit boards.
Sulfuric acid anodizing is a widely used process for enhancing the properties of aluminum and its alloys, along with other metals such as titanium and magnesium.