Most of the metals, especially steels, need special surface treatments for the desired optimum properties. There are various techniques for surface treatment methods but the most common one is the surface coating. Steels require further surface coatings for enhanced corrosion resistance, wear resistance and surface hardness. The coating methods differ for different applications. Therefore, classification of the coating methods can be very complicated. However, most common surface coating techniques can be classified as below list;
- Electrochemical Coatings
- Vapor Deposition
- Conversion Coatings
- Thermal Spraying
These techniques can be applied to different requirements of applications. Thus these different processes have various advantages and disadvantages.
Galvanizing is the most common surface coating technique which is used almost everywhere in the industrial applications. The main mechanism behind the galvanizing is the galvanic protection of the base steel. In nature, elements are settled due to their affinity to reduction or oxidation. A determinative array is called EMF series are constructed where potential different between hydrogen and different metals are calculated individually. Therefore, more noble metals are located at the top of the EMF whereas more active metals are located at the bottom. These more active metals tend to corrode more than the noble metals because of their huge potential difference. When two different metals are coupled, the corrosion rate of the more noble one decreases readily where the corrosion rate of the more active one increases. Thus the active metal protects the noble metal from corrosion by lowering its corrosion speed. In galvanization process, the noble metal is the base steel and the active metal is the coated zinc layer. When the zinc layer gets mechanically injured or scratched, the zinc layer acts as the anode material and the base steel acts as the cathode material so the zinc layer lowers the corrosion rate of the base steel and protects it from corrosion. In the galvanizing process, a multilayer coating occurs. These multilayered form usually consists of intermetallic compounds. Galvanizing process can be mainly separated into two parts which are surface cleaning and zinc coating. At the start of the process, a surface cleaning is compulsory. Steel coils are transported between long distances and these long distances can be dangerous for the steel. Thus coils are greased with different type of oils to protect the surface of the steel during these long transportation intervals. The oil covered surface of the steel coils must be cleaned with different type of alkali solutions to maintain the perfect removal of the dirt and different type of oils. A proper cleaning ensures a good adhesion between the molten zinc and the steel surface. The cleaning part divided into three sections; caustic cleaning, pickling and fluxing. In caustic cleaning part the surface of the steel is treated with a hot alkali solution which removes common dirt and oils. In the pickling section the surface rust and scales are removed by using a hydrochloric acid solution. Finally, in fluxing part, the surface oxides are removed and protected from the further oxidation risks. The cleansed steel sheet is immersed in a zinc bath for coating. The bath composition must contain at least 98% pure molten zinc. The optimum bath chemistry is determined by the ASTM A123 standards. The immersed sheet is removed from the bath when the steel sheet’s temperature approaches a near value to bath temperature. During the removal of the sheet, the excess zinc is detracted from the sheet by channels which flows pressurized air through the surface of the sheet steel. The desired coating thickness is generally determined by the chemical composition of steel, surface properties of the steel and cooling rate.
Electrochemical coatings use the electrostatic properties of different metals which can be either in solution as ions or can be as sheet metals. The electrochemical coatings are divided into two parts; electrodeposition methods and electroless coatings.
- Electrodeposition Method
The electrodeposition method is a very common technique for not only coating of the metal surfaces but also extraction of the metals. Electrodeposition also known as electroplating. The technique basically comes from the famous Faraday’s Law. Faraday states that the mass of any substance which is either deposited or liberated from a surface is proportional to its atomic mass and the amount of current passed from the system. Electroplating systems consist of anodes cathodes and electrolytes. A direct current is passed from the system for liberation or deposition of the ions. Deposition process can be done by using electrorefining processes, electrowinning processes or molten salt processes. Different metal ions can be coated through the base metal which are used as cathode in the system. In refining processes, an impure metal is used as anode and the base metal is used as cathode. The impure metal anodically polarized by an internal direct current force which causes a liberation of the desired metal ions. Thereafter, the liberated metal ions in the solution are deposited on the base steel which acts as cathode. The electrowinning process is very similar with the refining process but the only difference is the inert anode material used in the system. The desired ions already located in the solution and there is no need to separate them from a source metal. Ions again deposited onto base metals surface which acts as cathode material. The molten salt technique is unique, because some metals can’t be coated by using the general refining or winning techniques. The metals which located below manganese in EMF series, cannot be coated or produced via using these general techniques. The only electrometallurgical route is the use of a molten salt bath. In molten salt processes, the technique is similar with refining and winning but the bath consists of a molten salt composition of the desired coating ion.
Figure X. Schematic of electroplating method. Retrieved from: Electrophoretic Deposition (EPD). (2020).28 January 2020, https://www.mtm.kuleuven.be/onderzoek/Ceramics/old-info/EPD
- Electroless Coating Method
The main difference of electroless coating from electroplating is the absence of an external electrical power. In these method, the desired surface is first cleaned and after immersed in a solution. The surface is etched by the ions in the solution and ions located into the surface. Moreover, the desired coating ion is replaced with the located etchant ions by the difference of potential. Usually nickel atoms are coated by using these method.
Vapor Deposition Methods
The vapor deposition methods utilizes vaporization and condensation processes for making thin film layers. The desired metal first evaporated and after condensated onto the target surface. Vapor deposition methods divides into two groups; chemical vapor deposition (CVD) and physical vapor deposition (PVD).
PVD method can create an ultra-thin films from 0-20 micrometers. The ionization or atomization process can be done by physical evaporation of the substance or plasma sputtering. In physical evaporation method, the metal substance is first evaporated at high temperatures from 1000 to 2000 °C and condensated under highly pressurized vacuum environment. In the plasma sputtering method, the coating material is emitted from the surface by accelerating different ions to substances surface. Accelerated ions impacts to the surface of the substance and desired coating atoms are ejected from the surface and deposited onto the desired surface. These process is held under an electrostatic system where the desired substance is used as cathode. An argon atmosphere is utilized as the medium where argon gas is ionized and ions are accelerated through the desired metal substance and ejects them from the surface. The use of plasma sputtering method enables a high adhesion between the base surface and the coating substance because of the high kinetic energy of the substance ions.
Figure X. A basic representation of PVD technique. Retrieved from: Forschungszentrum Jülich – Employees at IEK-1 – PVD/CVD. (2020). 28 January 2020, https://www.fz-juelich.de/iek/iek-1/EN/Expertise/Duennschichttechnologien/PVD_CVD.html?nn=511174
CVD technique also uses the gaseous phase of the desired metal substance which can be condensated onto the target base metal surface. In CVD technique the substrate is heated at high temperatures over 850 °C. These high temperature process restricts the material selection because in this process only materials with high melting temperature can be used. First, the precursor substances are introduced into the hot reaction chamber where target surface located. The precursors vaporizes and are absorbed onto the target surface. Adsorbed substances chemically react with each other and leave behind the desired coating metal on the surface of the target. The desired metal coats an impervious thin layer on the surface of the base metal. CVD method is commonly used in coating of hard materials such as nitrides, carbides or borides.
Conversion Coatings usually consist of spraying or dipping the desired material through the additional substance. Conversion coatings develops the corrosion resistance of the surface by the additional substances such as chromium or phosphate. Moreover conversion coatings also develops the adhesion properties of the surface for the further painting applications. Conversion coatings can be classifies as chromating, phosphating and anodizing. In chromating process the base metal is dipped into a solution with chromium ions. Chromate coatings have self-healing properties. The hexavalent chromium leaches slowly when it’s in contact with a liquid or moisture. The dissolved hexavalent chromium ions are adsorbed by the defects and, finally, form a passive film that consists of insoluble trivalent chromium. In phosphating method, metal surfaces are dipped into phosphoric acid solution or solutions are sprayed onto the metal surfaces. A phosphate layer are formed on the surface of the base metal which eases the well adhesion of further painting applications. The anodizing method is different from the chromating and phosphating methods because in anodizing method, the protective film is formed by the base metal. Some metals (e.g. Ti, Al) can form a protective oxide film on their surfaces. When the target surface anodically polarized by and external electrical source, surface starts to corrode and forms a protective oxide film. This impervious layer protects the base metal from further corrosion risks.
In thıs method, the substances are heated and are sprayed onto the desired surface. The coating thickness can be higher than the other common methods. The heating of the substances are usually done by an external electrical source which can make an arc or plasma.