Precious Metals have been used for centuries now for their corrosion resistant properties and aesthetic. They have become essential for the production of engineered systems and “high-tech” devices, especially in the communication and electronic industry sectors.
Precious Metals also have widespread applications in the automotive and aerospace industries. These metals are rare and expensive but irreplaceable because of their properties, such as high positive electrode potentials, meaning they are relatively resistant to most aggressive environments. Due to their high cost, it is necessary to closely control the chemical processes in which they are used, such as chemical surface treatment technology, to make the processes efficient and economically viable.
The two most common types of precious metal plating are gold and silver.
Gold
Gold is considered malleable, practically indestructible, and completely recyclable, while also being virtually immune to the effects of water, air, and oxygen. This unique combination of properties makes it an essential component in many industrial, medical, and electrical applications. Gold is one of the most electrically conductive metals and is also an excellent conductor of heat or thermal energy.
Gold plating is useful for parts that need wear or corrosion resistance while still maintaining their flexibility. It has a high electrical conductivity, and it is non-reactive with other elements. Although it is the most expensive option for electroplating, it’s by far the best method for securing the value and quality of your product.
Gold plating is often used in electronics in order to add a corrosion-resistant electrically conductive layer to copper, usually in printed circuit boards and electrical connectors.
Silver
Generally, silver is a more cost-effective plating solution; it is cheaper than gold, and it also plates copper well. It has the highest thermal and electrical conductivity of any metal. Other attractive characteristics of silver are that it has the highest optical reflection, whitest color, and lowest contact resistance of all metals.
Issues that may arise with silver plating include galvanic corrosion and humidity. Silver plating is not recommended for applications that are subjected to humidity since it is prone to flaking and cracking, which could eventually expose the base substrate.
With the exception of luxury goods, the industries that utilize silver plating the most are the electronics (semiconductors and connectors, musical instruments, and bearings) and modern power generation (battery and solar).
Precious Metal Coating Process
The electroplating of precious metals such as silver and gold coatings can be done from a variety of aqueous electrolytes. Typically solutions of cyanides are used.
1. Pre-Treatment
The pre-treatment depends on the condition of the components that need to be treated and generally includes the following process:
- Mechanical pretreatment
- Degreasing
- Rinsing
- Activation, sometimes by making the substrate electrically conductive
- Rinsing
- Nickel or copper deposition
- Rinsing
2. Gold and Silver Plating – Deposition
The deposition of gold and silver is similar to other electrodeposition processes. Considering the cost of precious metals, deposition is done under very exact conditions (A/sec.m 2) so that the layers can be deposited with exact thickness. Special care needs to be taken in order to minimize the losses of noble metals during the post-plating rinsing process.
3. After-Treatment
Normally gold layers don’t need an after treatment. Sometimes the layer is gently polished with a soft tissue. Silver layers sometimes get coated with a thin layer of rhodium.
What is Electroplating?
Electroplating includes passing an electric current through a solution that’s called an electrolyte. This is done by dipping two electrodes into the electrolyte and then connecting them into a circuit with a battery or some other power supply. The electrolyte and electrodes are made from carefully chosen compounds or elements. When the electricity flows through the circuit that they complete, the electrolyte separates. At this point, some of the metal atoms it contains are deposited in a thin layer on the surface of one of the two electrodes, thereby completing the electroplating process. All sorts of metals can be plated in this way, including gold, silver, platinum, etc.
Electroplating is similar to electrolysis (it includes using electricity to split up a chemical solution). These are all examples of electrochemistry, which is a chemical reaction caused by producing electricity that gives industrially or scientifically useful end-products.
How Electroplating Works
First, we need to choose the right electrolyte and electrodes by deciding on the chemical reactions we want to create when the electric current is applied. The metal atoms that plate an object come from the electrolyte, so if we want to gold plate something, we need a gold-based electrolyte, and so on.
Next, we need to make sure the electrode we want to plate is entirely clean. If it’s not clean, the metal atoms from the electrolyte can’t form a good bond with the surface, allowing them to be easily removed with friction. Usually, the cleaning is done by dipping the electrode into a strong alkaline or acid solution or by connecting the electroplating circuit briefly in reverse. When the electrode is completely clean, atoms from the plating metal can bond to it effectively by adhering to the crystalline structure’s outside edges.
Now we’re ready for the most important part of electroplating. We need an electrolyte, two electrodes that are made from different conducting materials, and an electricity supply. One of the electrodes is usually made from the metal we want to plate, and the electrolyte is usually a solution of a salt of the same kind of metal. So, for example, if we’re silver plating a piece of brass, we need a silver electrode, a brass electrode, and a solution of a silver-based compound. Precious metals like silver and gold don’t easily dissolve, which means they require strong and dangerous cyanide-based chemicals to be made into electrolytic solutions. The electrode that will be plated is typically made from a more affordable metal or a nonmetal material coated with a conducting material like graphite. In either case, it needs to conduct electricity in order for the plating to occur.
We dip these two electrodes into the solution and connect them into a circuit, so the silver becomes the positive electrode, and the brass becomes the negative electrode. When we turn the power on, the silver sulfate solution splits into ions (atoms with too many or too few electrons). Silver ions (that are positively charged) are attracted to the negatively charged electrode and slowly deposit on it, creating a thin layer of silver plate. Meanwhile, sulfate ions (that are negatively charged) will get to the positively charged copper anode and release electrons that move through the battery to the negative electrode.
It takes some time for electroplated atoms to build up on the negative electrode’s surface. How long this will take depends on the strength of the electric current used and the concentration of the electrolyte. Increasing either of these factors can increase the speed at which electrons and ions move through the circuit, which will increase the speed of the whole plating process. As long as electrons and ions continue to flow, the plating process will continue.
If you are interested in precious metals plating or have questions about the processes we use, feel free to contact us.
