Coatings have long been developed for protection from corrosion and erosion that is to secure the material from chemical and physical attacks. Corrosion issues are major in the diverse industrial applications that cause degradation and complete damage of components and systems in the processing and manufacturing plants and in the service life of various components.
Different technologies can be used for suitable surface protection in specific conditions. They are often differentiated by coating thickness: deposition of thin layers and deposition of thick films. Thick layers are commonly developed at atmospheric pressure have a thickness above 30 micro-m to some mm and are used for the functional performance and service life of a component depend on the thickness of security layer.
The coating technology can be categorised as wet and dry coating methods, the key difference is the medium in which the deposited material is processed. Earlier the coating methods include electroplating, electroless plating and hot dip galvanisation. Thermal spray coating is a some type of newer depositing technology
Thermal spray coating includes a group of coating processes in which metallic or non-metallic materials are deposited in a molten form to develop the coating. Thermal spraying processes are used at atmospheric pressure in air or in inert condition or vacuum. Use of thermal spray wire as a coating material is heated then melted accelerated by high temperature, high velocity gas stream towards a substrate. It impacts on the substrate in the form of a stream of droplets that are produced by melting of wires in the high energy gas stream. The drops flatten on the substrate and produce splats. The accumulation of several layered splats creates the coating.
Protection from wear and corrosion
Thermal spray processes are commonly used to spray coatings to protect from wear and corrosion and also from heat and for functional purpose. The choice of deposition process depends firmly on the expected coated properties for the use and coating deposition cost. Coating characteristics are determined by the coating material, the form in which it is offered, and by the set of factors used to serve the deposition process. Thermal spray coatings are normally featured by a lamellar structure and the real contact between the splats and substrate or the earlier deposited layers describe the coating characteristics. The properties of Monel 400 wire while using it as a thermal coating material add to the protection of substrate. The real contact area between 20 to 60% of the coating surface parallel to the surface increases the impact velocities of particles offered that the latter are not too much superheated or below their melting point. Therefore roughly the density of coatings increases from wire arc.
Use of Monel 400 as a thermal coating material, secures the substrate from localized corrosion. It creates sacrificial coatings for longer protection. It protects against atmospheric, marine corrosion and high temperature corrosion, oxidation, carburization, sulfidation, molten salt etc.
Corrosive wear occurs when the effects of corrosion and wear are combined, resulting in faster degradation of the material. A surface that is corroded or oxidized can be mechanically weakened and more possibly wear at an increased rate. Additionally, the corrosion materials like oxide particles that are dislodged from the material surface can act as abrasive particles. Stress corrosion damage results from the combined effects of stress and corrosion. At high temperature reactions with oxygen, carbon, nitrogen, sulfur, implementation of Inconel 625 wire as thermal spray coating material secures the substrate surface.
The corrosion protection of large steel structures in metallurgical, chemical, energy and other industries is a major challenge. The protection of surfaces exposed to moist atmospheres and seawater like ships, offshore platforms, seaports is even more challenging. In many cases, the surfaces that need protection are several thousands of square meters in size, requiring that coating costs are high with the traditional methods. With arc spraying methods , the cost can be controlled while receiving extreme protection in field conditions. Therefore thermal spray coatings are widely used in industry as the investment is significantly low and coating adherence is good with almost no heating of the material. Although coatings received by these methods are comparatively porous. This porosity can be decreased by shot peening right after spraying.