Precious metals are well-known for their aesthetic appeal, but there’s so much more to them than their looks. Precious metals, which include gold, silver and platinum, are widely used in engineering and industry. While precious metals possess an attractive luster that makes them visually appealing, they also hold a great deal of utilitarian value — just a few of the precious metal family’s properties are chemical stability, electrical conductivity and ductility. One of the most notable features of precious metals, however, is their ability to resist corrosion.
At Sharretts Plating Company, we can direct you toward the right precious metals to help prevent corrosion in your next project. Read on to learn more about corrosion in all its forms and precious metal uses in creating corrosion resistant products.
What Are Precious Metals?
Precious metals are most typically defined as naturally-occuring metals that are relatively rare and less chemically reactive than other metals. Ductile and lustrous, precious metals are often used for aesthetic purposes and are commonly used as bullion. However, precious metals are also highly valued for their chemical stability, electrical conductivity and corrosion resistance.
The precious metals family is made up of three primary subgroups:
- Gold and gold alloys: Gold is an extremely inert precious metal and will not oxidize in the presence of normal environmental conditions. An extremely soft and ductile metal, pure gold is often used as a lining or electrodeposited coating in electronics. Gold can also be alloyed with other metals to increase its strength, allowing it to be used in freestanding components in the aerospace and electronics industries.
- Silver and silver alloys: Silver has the highest thermal and electrical conductivity of all metals, and it is also easily alloyed with other metals, creating thermally stable alloys for industrial applications. Since it is less expensive than other precious metals, silver is a popular alternative for those seeking to reduce costs. Silver plating is most often found in the automotive, electronics and electricity generation industries.
- Platinum metals and alloys: The platinum group of metals is extremely rare and includes platinum, palladium, rhodium, ruthenium, iridium and osmium, which all occur naturally in the same ore. These silver-white metals are extremely inert and stable, with some variations in malleability and strength. All maintain their stability at high temperatures, making them ideal for the automotive and aerospace industries in various applications.
Any of these metals can be electroplated onto a surface to lend some of their properties to the product. At SPC, we offer rack and barrel electroplating for precious metals.
What Causes Corrosion?
Corrosion, derived from the Latin word “corrodere” meaning “to gnaw to pieces,” is loosely defined as the deterioration of a material over time. In the context of metals, corrosion is defined as an electrochemical reaction between a metal and its environment, resulting in the deterioration of the metal from a high energy state to a low energy state. Usually, this occurs as atmospheric corrosion through a process called oxidization, where a metal reacts with the oxygen and water in the atmosphere. For example, iron naturally combines with oxygen and water to produce hydrated iron oxides, or “rust.” Exposure to acid or high levels of electrolytes may also result in chemical reactions that lead to corrosion, but atmospheric oxidization is the most common form of corrosion. Protection against atmospheric corrosion accounts for approximately 50% of all corrosion protection costs.
Additional factors affect the rate of corrosion, including:
- Temperature: Chemical reaction rates tend to increase with temperature, meaning that high-temperature materials in corrosive environments will tend to corrode more quickly.
- Surface finish: The surface finish of the material will determine the severity of corrosion — rough or dirty surfaces and small crevices can trap corrodents, causing the corrodent to act more quickly and aggressively.
- Electricity: Exposure to electrical currents may result in faster corrosion of a material over time due to the loss of electrons from the material.
Plating with appropriate materials, including precious metals, can help reduce the effects of the above factors on corrosion rates, effectively protecting the material from corrosion.
How Does Corrosion Appear?
Depending on the specific cause of the corrosion, in combination with the factors listed above, corrosion may appear in one of eight primary forms:
- Uniform attack: The most common form of corrosion, uniform attack is characterized by a uniform spread of corrosion over the surface of a material. While this form of corrosion accounts for the greatest amount of metal destruction by volume, it is one of the easiest forms of corrosion to manage with appropriate coatings.
- Galvanic corrosion: When two dissimilar metals are physically or electrically in contact, their proximity can create a flow of electrons, causing the less corrosion-resistant metal to corrode at a higher rate and the more corrosion-resistant metal to experience a lower rate of corrosion.
- Crevice corrosion: Corrosion will often occur more quickly in crevices in a material where the corrodent is trapped for long periods of time. Crevice corrosion is typically found at joints, holes and rivet points.
- Pitting: Pitting is a localized form of corrosion that results in holes forming in the material, and is one of the most destructive forms of corrosion, reducing structural integrity with only a small amount of structural weight loss. This is one of the hardest forms of corrosion to find and is comparatively unpredictable.
- Intergranular corrosion: Poorly-alloyed materials may form intergranular corrosion in certain conditions, resulting in corrosive attacks at grain boundaries that reduce the strength of the alloy.
- Selective leaching: In this form of corrosion, the corrosion functionally removes one element from a solid alloy, most commonly zinc from brass alloys.
- Erosion corrosion: Erosion corrosion is the increase in corrosion rates caused by movement between a surface and a corrosive fluid. This movement results in grooves, waves and holes in the surface of the material, usually appearing in the direction of movement.
- Stress corrosion cracking: Tensile stress reduces the amount of energy needed for corrosion to take hold in a material, and when tensile stress and a corrosive medium are both present, stress corrosion cracks may form. These usually appear as fine cracks in an otherwise unaffected metal surface.
Product design changes, including the addition of a corrosion-resistant coating, can effectively prevent the majority of the corrosion types listed above.
The Effects of Corrosion and Why It’s Important to Prevent
Corrosion can have serious consequences for the functionality and safety of products and can result in increased costs over time. Some of the most frequent effects of corrosion include:
- Efficiency loss: Corrosion can reduce the efficiency of a product, reducing the amount of functional material to a point where the product can no longer achieve standard functionality. On top of that, machines with corroded equipment may need to be shut down for repair, reducing the company’s overall efficiency.
- Safety concerns: Corrosion may reduce the structural or functional integrity of a material to the point where it collapses or fails, injuring operators and users. Additionally, holes caused by corrosion may result in toxic materials leaking into the surrounding area, causing a health or environmental hazard.
- Appearance: Rust and corrosion are aesthetically unappealing, and can cause onlookers to think less of a product or express concern over its safety.
- Repair costs: Corroded equipment and materials must be modified, repaired or replaced, but they’re not the only parts that may be affected by corrosion. Parts and equipment adjacent to the corroded material may get damaged as an indirect result of corrosion, further increasing repair costs and reducing design life.
All of the above factors can decrease profits for a company by increasing costs and reducing sales. Plating can help prevent corrosion, therefore decreasing these costs — while the additional cost associated with corrosion-resistant plating may present increased initial costs, these pale in comparison to the costs incurred by corrosion damage.
Why Are Precious Metals so Effective in Preventing Corrosion?
Almost all metals will corrode in the presence of regular environmental conditions. The exceptions are precious metals.
Precious metals aren’t just defined by their high value — precious metals are also known for their stability. The galvanic series sorts metals on a hierarchical scale from noble to active. Active metals tend to interact more with the environment, while noble metals are more stable. Metals in the precious metal family are on the noble end of this scale, meaning that they are more resistant to interactions with other chemicals, therefore more corrosion-resistant.
While precious metals are noble, the terms “precious metal” and “noble metal” are not synonymous. The definition of a noble metal varies based on the field of study — in atomic physics, for example, noble metals only include copper, silver and gold because they have completely filled d-subshells, which makes them more stable. Under the broadest definition, however, both groups include the metals gold, silver, platinum, palladium, rhodium, ruthenium, iridium and osmium, and the noble metals also include mercury, rhenium and copper. These last three elements are not corrosion resistant, which is why it’s essential to define precious versus noble metals clearly.
How to Prevent Corrosion With Precious Metals
One of the best ways to prevent corrosion is by plating with precious metals and their alloys. However, it is essential to choose the right type of precious metal for your application and choose an electroplating method and thickness appropriate for your purposes. The qualities you seek for your plating, as well as the application, will be important to consider in your decision.
The following are the most significant factors to consider for a corrosion-protective finish:
1. Coating Material
Precious metal coatings seal off the substrate from the environment, creating a smooth protective finish that prevents water and oxygen from reaching the base material. Because precious metals vary slightly in their electrochemical and physical properties, however, some are better for certain applications than others. This precious metals list provides the strengths of each element along with its most common applications:
- Gold: Gold is thermally and electrically conductive and highly stable as a corrosion resistant coating. It is frequently used in electronics but tends to be more expensive than other corrosion-resistant coatings.
- Silver: Silver is a less expensive plating material compared to the other precious metals and offers superior electrothermal conductivity. It is used in electricity generation and automotive applications.
- Platinum: Platinum has a high melting point but is thermally stable, making it ideal for high-temperature applications. Typical applications include the electronics, electricity generation and aerospace industries.
- Rhodium: Rhodium is the most electrothermally conductive of the platinum group and maintains high reflectivity, making it an ideal material for mirrors in high-temperature applications. Rhodium is most commonly used in aerospace and electronics applications.
- Ruthenium: Ruthenium is often used as an alloying agent, specifically for its acid resistance properties. The electronic industry primarily uses ruthenium in the production of electrical contacts.
- Palladium: Palladium is a popular alternative to gold. When alloyed with nickel, it is less brittle and more solderable. When alloyed with cobalt, it is more durable and electrically conductive. In general, this metal and its alloys are frequently used in the electronics industry to produce contacts on printed circuit boards.
Sharretts Plating Company can help you assess your project and determine the best plating material for your application.
2. Substrate Quality
The effectiveness of any type of coating depends heavily on how well the substrate is prepared for plating. If the substrate is pitted, cratered, dirty or otherwise defective, the coating will either form surface imperfections or will not bond properly to the surface of the substrate. SPC can help you assess the quality of your substrate and make any suggestions for improvements. We also employ a thorough preparation process for plating to ensure that the final product is of the highest quality possible.
3. Coating Thickness
The thickness of the precious metal coating has a significant effect on the durability and longevity of the final product. Thinner deposits may not adequately cover the substrate, allowing for corrosive molecules to breach the surface and attack the substrate below. On the other hand, coatings that are too thick may be unnecessarily expensive and negatively impact the functionality of the product. SPC’s specialists can discuss the best coating thickness range for your application, and may even suggest additional layers of non-precious materials to improve the efficacy of the final product.
Choose SPC for Precious Metal Plating
If you’re considering precious metal plating as an option for your next project to help prevent corrosion and keep repair costs down, Sharretts Plating Company can help you get started. With over eight decades of experience in the industry, we know the industry inside and out and have perfected our precious metal plating methods to ensure top quality every time.
Our plating experts can meet with your business to determine your needs and suggest the best plating options for you. We can meet with you for an on-site consultation to discuss your project from start to finish, answering any questions you have and helping you explore all of your options.
Ready to get started? Get your free, no obligation quote today! Please fill out this quote request form or give us a call at (717) 767-6702 to learn more about precious metal plating and how it can benefit your business.