Protect your products from Tarnish – How lacquering preserves the final finish.
The most common problem that manufacturers of metal products or metal plated products face is the tarnishing of the metal surface. This is also of huge concern to the Electroplating industry as it reduces the life of their plated finishes. The metal plating alters the physical properties of the underlying material, but if tarnished they can compromise the quality and performance in its intended use.
Tarnish is a thin layer of corroded metal that forms on the surface of the plated product. It is of a dull black or dark greyish colour and it can be scraped off. In the long run it can cause significant loss of metal and reduction in thickness of layer. The metal surface also becomes pockmarked and uneven.
The most common metals that are prone to tarnish are
- Sterling silver
While noble metals like Gold, Platinum and Palladium are not expected to tarnish, they are also susceptible to it, based on the underlying material they are coated on. This is the reason Gold or Silver is plated over a layer of Nickel plating to prevent any form of corrosion from interaction of various materials.
Factors that cause Tarnishing:
Tarnishing doesn’t just occur by itself. There are various factors that induce and intensify the tarnishing process. It can occur when,
- The metal plating interacts with the oxygen in the air, causing oxidation
- The metal plating is exposed to corrosive compounds like Hydrogen sulphide, Sulphur Dioxide etc…
- The metal plating is used in underwater applications.
- The metal plating is exposed to intermittent humid and dry conditions.
- The metal plating undergoes physical damage exposing the underlying material to the outside environment.
The electroplating industry turns to various solutions to prevent tarnishing in their finishes. To prevent the tarnishing of the metal plated surface, the following can be used:
A major portion of tarnishing occurs due to contact with atmospheric oxygen, and moisture content under humid conditions. These can be prevented from interacting with the surface metal by creating mechanical barriers between the environment and the metal. Materials like desiccants, absorb moisture and oxygen from the immediate surface, thus keeping the metal surface dry. Additionally protective wrapping is used to keep conditions airtight. However these are temporary solutions and are viable only for transit and storage. The product will be exposed to the environment during use, and are prone to tarnishing.
In the Nitrogen bagging process, the product is placed in a bag of pure nitrogen and stored. This cuts off exposure to any atmospheric oxygen and moisture. Again this method can be used only for protecting the product during transportation and storage.
The electroplating experts had to find a method to prevent tarnishing even when the product was in use, every day and exposed to a wide variety of elements. Thus they turned to lacquers.
Lacquers are hard finishes that are used in protecting a variety of surfaces like wood, plastics and metals. They are usually in liquid form and are coated on the surface and left to dry. Lacquers have been in use since ancient times, obtained from the sap of certain trees and from the secretions of the lac insect. They were mostly used to protect decorative finishes.
Modern day lacquers are made from a combination of chemicals similar in composition to natural lacquers. They can be manipulated to provide various clear or colored finishes, from matte to gloss textures. They are ideal for use on hard and smooth surfaces like metal plated products.
The lacquers are transferred on to the surface of the metal in 3 methods.
Paint on Lacquers:
The lacquer comes in liquid form and is painted onto the intended surface with a paint brush. This method is generally used to coat wood products. The resultant finish may not be as durable as expected.
Spray on Lacquers:
The lacquer is available in liquid form, with lacquer thinner as solvent thinner. It is sprayed onto the intended surface using a high pressure jet spray. This is used to get a hard, glossy finish that also gives the appearance of depth. It is primarily used in aesthetic finishes in the automotive industry.
The metal surface needs to be cleaned and smoothened to prepare for the spray lacquer. Presence of any foreign material can cause the lacquer to bubble up and lose its bonding. For coloured finishes you can mix spray lacquers with enamel paints. The lacquer is applied in a generous coating, preferably in a single quick movement. Spray lacquers dry off pretty quickly.
Electrophoretic lacquer is electrolytically deposited onto the metal surface. They are a special formulation of polymer resins with water as the solvent. They are also environmentally friendly compared to the other types of lacquers.
The set up consists of electrodes made of stainless steel with the product to be plated as the counter electrode. The lacquer is charged electrostatically. When a current is passed through this set up the lacquer is deposited on to the metal surface. The excess lacquer is washed off with deionised water. The product is then left to air dry or placed in the oven for the lacquer to achieve its required integrity.
Advantages of Electrophoretic lacquers:
The use of electrophoretic lacquers is the most preferred in the electroplating industry as it gives the best and desired finish when compared to other lacquers. There are also a number of associated benefits that make it the finish of choice.
- Electrophoretic lacquers have low quantities of Volatile organic solvents. They are dissolved in water. Not only does this make them environmentally safe, but the associated fire hazard is reduced considerably.
- The electrolytic process reduces the risk of exposure to the workers who are handling them. The paint on and spray on lacquers impose the risk of being inhaled by the workers which is a health hazard unless proper precautions are taken.
- They are easy to transfer and the transfer efficiency is easily over 95%, making it the most viable lacquering process. The film formed is of uniform thickness, provided there are no impurities in the electrolytic bath.
- The lacquer film formed has a smooth and even surface, making the overall quality of the product high. This protective film can prevent any form of tarnishing and high density lacquers offer protection against even physical damage.
- Electrophoretic lacquers are relatively low maintenance. In cases of decorative finishes that are handled on an everyday basis, they significantly extend the life of the product. In case of machine parts, they offer good protection to physical damage during transit and storage.
- Electrophoretic lacquers are easy to apply and the process is extremely economical.
- Electrophoretic lacquer coatings are popular for creating clear coatings. They are non-yellowing and preserve the physical appearance for the stipulated period of use.
- The electrophoretic lacquer coatings offer excellent UV resistance, thus preventing any discolorations or metal surface damage from exposure to UV light, either artificial or from exposure to sunlight.
- They also offer resistance to Salt spray and a wide variety of corrosive chemicals or low intensity.
- They can be air dried by hanging on a rack or oven cured. The curing temperatures required are relatively low, to the tune of 90 degree Celsius.
In case the products are already tarnished and need to be refurbished, the tarnish can be removed by:
Polishing with polishing agents or sand paper. This is useful if the tarnishing is minimal.
Abrasive blasting, with high pressure water wash and blasting medium like steel shot, glass beads etc. This process must be performed with caution as it can actually damage the surface if not done with precision.
Smart Microns offers both Spray lacquers and Electrophoretic lacquers as part of our industrial electroplating services. Depending on industrial use or decorative purposes, the lacquer density is adjusted. We also do stripping and electroplating services, with lacquer finishes. To consult with our experts on the best lacquer finish for your products and book our services, call us on 9884407200.