Metallic coatings (I): processes
Electrolytic or chemical coatings, also called galvanotechnics, are based on the electrochemical installation of thin layers of metal on the surface of a piece submerged in the solution of metal ions or electrolytes. Quite pure chemicals such as salts and metals are used for this purpose. If the appropriate technologies are used in this process, the metals used are completely glued to the parts, without just causing damage to the environment. However, in companies that use coverage processes in the Autonomous Community of the Basque Country, the situation can improve. In fact, the possibilities of waste and wastewater reduction in electrolyte coating processes are currently high.
Electrolytic coatings
The electrolytic coating basically consists of converting the metal of the anode into metal ions. Once these dissolving ions are dispersed, they are deposited in the cathode, that is, in the part that will be coated, forming a metallic layer on its surface.
The specific characteristics of the coatings will depend on the components of the electrolyte used. The quality of coverage is achieved only through the existence and close monitoring of concrete and sustainable working situations. In order to minimise the generation of a contaminated electrolyte bath, it is important that electrolytes be stable over the long term, which involves continuous monitoring of the concentrations of the basic compounds, physical conditions and organic and inorganic contamination. Also, the maintenance and cleaning of the bathroom is essential to eliminate pollutants.
The most common electrolytic coating systems are copper, nickel plated, chrome plated and zinc plated. The copper electrolytes most used in the Basque Country are those based on cyanides, with hardly any additives. Cyanide copper is the first multi-layer coating to provide protection against rust. Usually applied on load and/or steel.
Nickel coatings are a very suitable base for most decorative coatings with chrome, brass, silver, gold, etc. Its good anticorrosive properties give it great utility in the manufacture of locks and taps, as well as in the manufacture of components and tools of the car. The most commonly used nickel electrolyte is called Watts, composed of chlorides, sulphates, boric acid and organic additives.
The most appreciated characteristics of chromium plating are its brightness, hardness and anti-corrosive ability. Two types are distinguished: decorative and hard. The first consists of applying chromium in thin layers and the second in thick layers. The main component of chromic electrolytes is chromic acid.
There are many types of zinc. High and medium concentration cyanides are the most traditional. These are highly tolerant to organic contamination and allow working without optimized pretreatment. On the other hand, zinc acids are spreading a lot, since in addition to forming high gloss layers, they are high performance. Alkaline zinc, although cyanide is not used in it, has many properties of cyanide electrolytes. In addition, its treatment cost is low.
Chemical coatings
Two types are distinguished: nickel and copper. The nickel chemical bath is composed of a nickel salt, a reducer thereof and a series of relatively weak fillers. The use of modern electrolytes allows a long bathing life.
Chemical copper electrolytes contain very stable complexes such as tartrate, quadrol and EDTA. These complexing agents must be recovered or treated with appropriate technologies so that the wastewater treatment system does not work very badly.
Subsequent treatments: Chemical and lacquered passivation
Chromic passivates are often used to prevent oxidation of the covered surface. This process is very interesting for the zinc-plated parts, since it is not a very noble metal, the zinc coating gradually oxidizes. There are different passivation baths according to components, temperature and pH. The most common are yellow and blue. Most chromic passivates are used on the basis of chromic acid.
The electrolytic lacquering of metallic parts in aqueous base is spreading more and more in the current market, both for decorative finishes of high added value (silver, brass and gold), as a substitute for electrolytic coatings of high cost or great technical difficulty (gold or bronze). Lacquer baths, on the other hand, have a great maintenance work, since it is necessary to ultrafiltrar the bath if you want to avoid the accumulation of organic acids.
Desmetalado
For the removal of coatings is used the desmetalado. These processes can be both electrolytic (anodic) and chemical. The former are composed similar to electrolytes and the latter contain violent complexing components that can cause problems in the treatment of wastewater.
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