Anodizing is the process of creating an oxide layer over a metal surface. Including gold, many colors can be applied to this layer.
It improves the part's abrasion resistance and provides a pleasing texture.
Gold anodizing is an electrochemical process in which metal gold or dyes are applied on the surface of a base metal like aluminum, titanium, or zinc. It's a finishing operation to enhance the metal's texture, corrosion resistance, and durability.
This article discusses various methods, workflow, and applications of gold anodization.
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Gold Anodizing Explained
What is Anodizing?
Anodizing is an electrochemical process that develops a porous oxide layer over the base metal.
You can fill this porous oxide layer with real gold or dyes. In most cases, dyes are used to reduce the cost of production.
The following methods are used for gold anodizing.
Electrolytic coloring is an electrochemical process. It uses an electrolyte solution with gold nanoparticles and a pair of electrodes.
It provides a good-quality golden color to the metal surface.
The material surface needs to have a large pore size on the oxide layer for proper absorption of the gold particles.
You can increase the pore size of the oxide layer through the hard anodizing method.
It is similar to the standard anodizing technique, but the difference comes in the thickness of the oxide layer.
Hard anodizing provides an additional 15 - 100μm thicker oxide layer compared to the standard anodizing technique.
You can perform the hard anodizing operation on aluminum alloys having copper and magnesium in their composition.
To further enlarge the pore size of the oxide layer, you can use sulphuric acid or phosphoric acid as electrolytes.
The pore diameter also depends on the anodizing voltage. Usually, for alloys of aluminum, it is above 100V.
Depending on the aluminum alloy, the desired pore structure is obtained using different electrolytes and voltage combinations.
In the immersion coloring technique, the hard anodized part is dipped into a hot bath solution of dissolved gold nanoparticles and then boiled in deionized water.
Here golden-colored dyes extracted from iron oxides are used. It provides a yellowish-brown tint.
As the size of the golden nanoparticle increases above 100 nm, it appears in golden color.
This is because it is under the wavelength of visible light (400 - 700nm). The wavelength of metallic gold is 576.69 nm.
Hence, to stain the oxidized surface with large golden nanoparticles you'll need to hard anodize the base metal.
Using the Interference of Light
Titanium has properties similar to that of aluminum. When anodized, it forms a thin, transparent oxide layer.
Because of its thickness variations and transparent nature, thin-film interference occurs, causing the oxide layer to show a wide range of colors like rustic brown, silver, gold, and many of its tinges.
Two out-of-phase half wavelengths reflected from the inner and outer layer of the oxide layer give a complementary color.
The figure below shows the colors corresponding to the voltage (marked on each part) during anodizing titanium.
Benefits of Gold anodizing
Gold-anodized parts have a longer life compared to gold-plated parts. This is because of its high resistance to corrosion and abrasion.
Since the coating is integral to the surface, it doesn't add much weight to the part but increases its thickness.
Also, gold anodizing a metal part is a cost-effective alternative to making parts using gold.
DIY Gold Anodizing: How to Do
Choose an aluminum alloy with manganese content to get the best results for a DIY gold anodizing project.
When they are worked with sulphuric acid as an electrolyte solution, it produces an oxide layer having a golden hue.
Also, when you adopt the hard anodizing process, the size of oxide pores will be large enough to hold gold nanoparticles.
- Prepare an alkaline solution (neutralizer) to reduce the pH value of the sulphuric acid. Add 2 cups of baking soda to 4 liters of distilled water to obtain the solution.
- Clean the workpiece thoroughly in a bath of water to which three tablespoons of sodium hydroxide is added.
- Rinse the part using distilled water.
- Prepare the electrolyte by adding 1 part of acid to 4 parts of water.
- Connect the negative power cable to the scrap metal, which acts as the cathode, and the positive terminal to the specimen, which acts as the anode.
- Place the part in the electrolytic solution.
- Turn on the power supply, begin with a lower voltage value and slowly increase it to about 70 to 100V for the hard anodizing process. Continue the process for about an hour.
- In the meantime, prepare the solution of the dye in distilled water.
- After the anodizing process, remove the part and rinse it with water.
- Place the part in the hot dye bath for about 15 minutes. The longer the duration of rest, the stronger the color.
- After dyeing, the color is sealed by placing the part in a hot water bath for about half an hour.
- Remove the part from the hot bath and let it cool at room temperature.
- Clean all the equipment used for the process using baking soda solution to remove dirt and other organic compounds.
Alternative Methods of Gold Plating
Apart from anodizing, metal parts can also be color coated through electroplating.
It is a finishing operation that deposits a layer of metal coating over a base metal. This layer prevents the metal from corrosion and abrasion.
In the electroplating process, the metal to be coated is dissolved in the solution. When electricity is passed, it gets deposited over the negatively charged part.
This technique allows gold to be plated on sterling silver, nickel, brass, and copper.
The first step of gold plating a part is to prepare the surface.
Before plating, the surface has to be polished, etched or sandblasted to remove all dirt and contaminants.
Otherwise, it will lead to poor adhesion of the gold coating, i.e., a non-uniform coating.
After surface preparation, clean the part thoroughly with water.
Once rinsed in water, apply a thin nickel plating onto the part. It improves the adhesion of the gold coating to the base metal.
Once again, rinse the part to remove any residues from the previous step.
Connect the power terminals to the experimental setup.
The negative terminal of the power cable is connected to the part which acts as a cathode, and the positive terminal to the gold solution (anode).
The negatively charged part attracts the positively charged gold ions, slowly covering the part with a layer of gold.
The thickness of gold plating depends on the duration of the electroplating process and the amount of current passed through the solution.
Different Types of Gold Plating
The table below briefly describes the types of gold plating used in commercial applications.
|Gold plated||Electroplating of gold ions onto a base metal.||Tarnishes when exposed to chemicals.|
|Gold Vermeil||Thick gold plating on a sterling steel.||Offers better features than gold plated. |
Continuous care is needed for its longevity
|Gold Filled||A thick layer of gold applied on the base metals is rolled under high temperatures.||The coated layer has better wear resistance..|
Applications of Gold Anodizing
Gold anodizing applications are limited to parts that don't need to withstand extreme forces and temperatures.
The following table highlights the thickness of the gold layer required for each application.
|Application||Gold Plating Thickness|
|Electronic industry - connectors, PCB fingers||0.8 microns|
|Medical industry- dental dentures, medical instruments||6 - 8 microns|
|Musical Instruments- mouthpieces||1 micron|
The common application of gold anodizing is in making imitation jewelry. But, these ornaments are most commonly electroplated rather than anodized.
Many prefer electroplated gold jewelry over anodized ones because you can control its composition and, thereby, the quality of gold in karats.
However, electroplating doesn’t improve material properties. Hence it is confined to the making of jewelry.
You can apply different thicknesses of gold plating based on the duration of electroplating, the voltage applied, and the concentration of gold on the electrolyte.
Gold anodized parts offer a better appeal and superior metallic properties when compared to other colored parts.
The coating forms an integral part of the base material, improving its material property and providing longevity.
Nanoparticles of gold deliver a wide variety of reflective properties, and your choice of particle size will define the shade of gold you get.
So at first, you'll have to do some test runs to find the right composition.
Frequently asked questions
Is gold anodized products better than gold-plated ones?
Yes, gold-anodized products are better than gold-plated ones because of their durability and ability to re-anodize when the coating fades.
Why is gold-plated jewelry preferred over gold anodized ones?
Gold-plated jewelry is preferred over gold anodized ones because it shows a bright shiny yellow color that looks similar to gold. Gold anodized jewelry has a light yellow color that doesn't look like real gold.
Is anodizing a coating?
Yes, anodizing is a coating process that applies a protective oxide layer over a metal surface. Unlike other coating methods like painting, in anodizing, an electrochemical process is used to apply the coat