Types of Coating
There are several of attacks of the surface of metals or materials such as: Air, Hot gases, Liquids, Micro-organisms, Chemicals and Mechanical contact. Therefore metals or materials must be protected from these attacks. There are several types of coating methods such as following:
- Metallizing
- Galvanizing
- Chemical vapor deposition (CVD)
- Paint coatings
- Thermal spraying
Metallizing is the general name for the technique of coating metal on the surface of objects, it contains the process variants of Electroplated metal coating and Electroless metal coating Metallic coatings may be decorative, protective or functional. Techniques for metallization started as early as mirror making. Usually, the parts are carried out in aqueous medium. The metallic salt of the metal to be deposited, dissolved in solution, ionises and the metal cation is then discharged to form the metal itself, using and external voltage source. Electroplating is the process of plating one metal onto another by hydrolysis. There are also specific types of electroplating such as copper plating, silver plating, and chromium plating Plating non-metallic objects grew rapidly with introduction of ABS plastic. Because a non-metallic object tends to be a poor electrical conductor, the object's surface must be made conductive before plating can be performed. The plastic part is first etched chemically by a suitable process, such as dipping in a hot chromic acid-sulfuric acid mixture. The etched surface is sensitised and activated by first dipping in tin(II) chloride solution, then palladium chloride solution. The processed surface is then coated with electroless copper or nickel before further plating.
Electroplating allows manufacturers to use inexpensive metals such as steel or zinc for the majority of the product and then apply different metals on the outside to account for appearance, protection, and other properties desired for the product. The surface can be a metal or even plastic .In this process the cathode would be the piece to be plated and the anode would be either a sacrificial anode or an inert anode, normally either platinum or carbon (graphite form). Sometimes plating occurs on racks or barrels for efficiency when plating many products
Galvanizing is a process by which a layer of zinc is applied to a ferrous metal to prevent corrosion. The process most-commonly refers to hot-dip galvanizing, where a piece of steel is dipped into a bath of molten zinc. The zinc adheres to the steel and immediately reacts with oxygen in the air to form a very strong zinc oxide layer, which prevents corrosion of the steel below. The galvanized steel can be painted to achieve a specific colour. When galvanized metal is welded, the weld and exposed steel must be coated with a special zinc paint to prevent the joint from rusting since the galvanized coating has been removed.
Chemical vapor deposition (CVD) is a chemical process for depositing thin films of various materials. In a typical CVD process the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile byproducts are also produced, which are removed by gas flow through the reaction chamber. CVD is widely used in the semiconductor industry, as part of the semiconductor device fabrication process, to deposit various films including: polycrystalline, amorphous, and epitaxial silicon, SiO2, silicon germanium, tungsten, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics. The CVD process is also used to produce synthetic diamonds.
Paint is the most commonly used material to protect steel. Paint systems for steel structures have developed over the years to comply with industrial environmental legislation and in response to demands from bridge and building owners for improved durability performance. Previous five and six coat systems have been replaced by typically three coat alternatives, and the latest formulations have focussed on application in even fewer numbers of coats, but with increasing individual film thickness. Examples of this are epoxy and polyester glass flake coatings that are designed for high build thickness in one or two coat applications, and single coat high build elastomeric urethane coatings, up to 1000μm thick
Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame).Thermal spraying can provide thick coatings (approx. thickness range is 20 micrometers to several mm, depending on the process and feedstock), over a large area at high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition. Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites.