Definition of Coatings
Coating is the process whereby one material is used to continuously cover another surface. According to Dewo et al. (2015), coating is a surface modification approach that involves the addition of a new material onto the surface of a substance or element that is at risk of corrosion.
Formula: Rusting of Iron.
iron + oxygen + water ==> hydrated iron(III) oxide (RUST)
4Fe(s) + 3O2(g) + xH2O(l) ==> 2Fe2O3.xH2O(s)
Different Types of Coatings
The process of coating utilizes multiple films of coatings as primers and top layers. The purpose of each layer is particular; however, the layers often interact with one another (Özçam et al., 2016). The layers can comprise of metallic or non-metallic materials and are often effective in preventing corrosion. The type of coating used to prevent corrosion is often dependent on the metal used. There are three types of coatings and they include: barrier coatings, inhibitive coatings, and galvanic coatings. Epoxy and coal tar epoxy are examples of coatings that are used in contemporary engineering projects. Epoxy resins are available as polymers and pre-polymers. Epoxy consists of epoxy resin and a polyamine hardener that have to be mixed before application. Coal tar epoide on the other hand is a coating that can be applied at high film thickness in one coat and used for diverse purposes such as petroleum storage tanks and heavy duty structural coatings among others.
Natural earth pigments, such as kaolin clay and magnesium silicate, are used to prevent corrosion. Opaque materials, such as titanium dioxide, are also used in some cases (Özçam et al. 2016). Wet paint prevents corrosion by its viscous nature, but is has a relatively lower opacity power, which discourages its use. Aluminum leaves and micaceous iron oxide increase the barrier thickness and force moisture around their plate-like shield (Dewo et al. 2015).
Cathodic protection is a method that is used for the protection of metals from corrosion. In cathodic protection, the metal that is more likely to suffer from corrosion is referred to as a sacrificial metal. This sacrificial metal acts as an anode and suffers corrosion in the stead of the protected metal. An external direct current power supply may be required to provide sufficient current in the case of extensive projects such as long pipelines (Dewo et al. 2015).
Important Properties of Coatings
The three important properties of coatings are adhesion, resistance to alkali, and water permeability. Adhesion refers to the stickiness of a coating. Good coatings should be well adherent onto the materials on which they are placed. Materials that are basic in nature are ubiquitous in the environment in the form of lime, cement and soap among others. It is for this reason that a good coating should have alkali resistant properties (Dewo et al., 2015). Water is responsible for corrosion of most substances and therefore, coatings should be resistant to the entry of water, that is, they should not be permeable to water. Different coatings have varying features. The thickness of a coating is always its most important characteristic. Ideally, the thickness of a coating can be microscopically assessed by polishing samples of the coated material and examining a cross section of the element. The adhesion property of a coating is also a critical feature in its performance. In most cases, studying a cross section of the coated material can establish its adhesion properties. Subsequently, a sample of the material is studied to establish the differences between the core material and the modified surface layer (Özçam et al. 2016).
Importance of Surface Preparation
Surface preparation is a critical step preceding the coating of a potentially corrosive surface. Surface preparation enables the achievements of a high-quality bond between the material and the substance used to coat it (Özçam et al., 2016). Surface preparation is also a major determiner of the durability of coated surfaces. As a result, materials that do not undergo adequate surface preparation often degrade quickly. Surfaces that are prepared properly provide an excellent foundation for any coating and extend the durability of the coating material. Furthermore, excellent surface protection enhances the aesthetic appeal of a material and protects them from stains and other surface irregularities that may be difficult to remove. Accordingly, surface preparation is necessary for surface sealing, which is the uniform absorption rate of the surface that is to be coated (Dewo et al. 2015).
Types of Surface Preparation
Different approaches are used to prepare surfaces for the application of a topcoat that prevents corrosion. Solubilisation is one of the methods used for surface preparation, particularly in the removal of soluble organic and inorganic particles. Water among other solvents is used for solubilisation. Emulsification is another method of surface preparation that formation of an emulsion with an insoluble contaminant on a surface. Etching is a form of chemical surface cleaning procedure whereas sand blasting is a form of mechanical cleaning of surfaces in preparation for coating. Solvent cleaning is a surface preparation approach that entails the removal of visible organic materials using materials such as solvent, vapour, alkali, emulsifying agents, and steam (Zhang & Serpe, 2015). Hand tool cleaning is a surface preparation approach that entails the manual removal of debris by clipping, scrapping, sanding, and wire brushing. Power tool cleaning employs tools, such as power grinders and power chippers, to remove loose paint and rust. The white metal blast cleaning method entails removing all traces of grease, dust, dirt, and mill scales that are visible to the naked eye. Brush-off-blast cleaning is also a surface preparation technique that removes all forms of visible dirt on the surface of an element without removing the mill scale, rust, and coating oxides (Özçam et al. 2016). High and ultra-high pressure water jets are used to prepare surfaces that are to be coated. This technique is used for surfaces like steel and other hard materials. Concrete is often prepared through the application of mechanical, chemical, and thermal methods whereas industrial blast cleaning removes both visible and stuck mill scale, rust, and coating residues (Dewo et al., 2015).
Types of Coating Failures and How They Happen
Disbonding is a word that refers to the loss of adhesion between the material that is used for coating and the surface that is coated. The causes of coating disbanding include but are not limited to failure of the coating substance to adhere, and attacks by chemicals. The hydrogen ion concentration of an environment is also a factor that determines the likelihood of coating disbanding; excessive acidity or alkalinity can cause coating disbanding. Mechanical damage to a surface may also result in coating disbanding. In most cases, the different types of coating failures lead to coating detachments. On the other hand, multiple factors are responsible for the failure of surface coatings (Zhang & Serpe, 2015). Blistering is a form of coating failure that comprises spherical bubbles that may form on the surface of the coating or between the layers of the coating substance.
According to Özçam et al. (2016), result from the ionic contamination of a surface before it is coated. It is common in areas where corrosion has set in and in places where sacrificial anodes are present. Through film breakdown is a form of coating failure that results from the inefficient coverage of a surface or the use of few films for thickness (Özçam et al., 2016). Edge breakdown is a form of coating failure in which the edges of a material stiffen and become round cut outs (Zhang & Serpe, 2015). The surface tension of the material, as well as the coating material, is responsible for edge breakdown. Weld deposition results from the accumulation of oxides from the welding process and poor cleaning of surfaces before coating (Dewo, et al. 2015).
How Can We Have Good Coatings?
Good coatings result from deliberate choices in the process of preparing materials for engineering projects. Primarily, good materials should be selected for construction works. Compromised materials are unlikely to hold up coating works effectively (Özçam et al., 2016). The process of preparing the surface to be coated should be meticulous to ensure the durability of the coating. Moreover, the type of coating selected should be appropriate and relevant, especially in regards to the uses of the coated material. Some coatings are inappropriate for humid environments whereas some degrade quickly while in dry environments. Furthermore, the manufacturer’s recommendations should be followed while applying the coatings (Dewo, et al. 2015).
Coating Analysis and Evaluation
Qualitative and quantitative methods are used to valuate and analyses coatings. Quantitative aspects of the coatings, such as porosity, thickness, and pore size distribution, can be evaluated using MATLAB image processing and Confocal Laser Scanning Microscopy (Dewo et al., 2015). Macroscopic examination of a surface can also be used in the qualitative analysis of a coating. Sometimes, defects, such as blistering and rusting, are conspicuous. The qualitative analysis of coatings often employs chemical and physical methods. Samples of coating elements may be qualitatively assessed using spectrometry and chromatography, among others (Özçam, et al. 2016).
References
Dewo, P., Dharmastiti, R., Magetsari, R., Salim, U. A., & Hidayat, L. (2015). The Effect of Sandblasting and Electropolishing on the Surface Roughness and Corrosion Rate of AISI 316L Stainless Steel. In Advanced Materials Research (Vol. 1123, pp. 192-195). Trans Tech Publications.
Özçam, A. E., Efimenko, K., Spontak, R. J., Fischer, D. A., & Genzer, J. (2016). Multipurpose Polymeric Coating for Functionalizing Inert Polymer Surfaces. ACS applied materials & interfaces, 8(8), 5694-5705.
Zhang, Q. M., & Serpe, M. J. (2015). Versatile Method for Coating Surfaces with Functional and Responsive Polymer-Based Films. ACS applied materials & interfaces, 7(49), 27547-27553.
