Titolo della tesi: Electroless metal plating for harsh environments
The state of the art of protective coatings for harsh environments, characterized by phenomena such as high-temperature oxidation, hot corrosion, and surface erosion, currently involves various deposition techniques.. The techniques primarily adopted for depositing these protective coatings are heavily biased towards plasma spray-based techniques and electrochemical techniques such as galvanic deposition. The aim of this work is not to propose a substitute for these methods but rather to propose the use of a deposition technique to be used in synergy with the currently employed techniques to maximize the efficiency of protective coatings in terms of substrate adhesion and operational performance. The technique proposed in this study is electroless plating, a branch of electrochemical deposition techniques that does not require the application of an electric field to the piece to be coated but takes advantage of being free from electric field accumulation phenomena and non-uniform thicknesses by using a reducing agent in the deposition bath. Electroless plating is a relatively young coating technique considering that the first observations of this method date back to Brenner and Riddell in 1946. To date, this technique has been widely studied and used in the field of protective coatings, but only the surface of its potential in various fields of application has been scratched. Specifically, it allows for adhesive, cohesive, and continuous coatings on the entire geometry of the piece to be coated, unaffected by electric field accumulation problems. Furthermore, it is a technique that allows for accessing internal channels and reduced surfaces, being a liquid-phase technique, unlike thermal spray techniques that can only achieve optimal coatings where the torch can reach. In this study will be introduce the specific challenges of hostile environments, particularly the protection of steam cracking plant pipes, the chemical sector, and energy production, such as turbojet turbines. The coatings applied in these environments mainly involve the use of nickel, cobalt, or nickel-cobalt-based alloys containing aluminum and chromium to provide adequate protection against high-temperature oxidation and hot corrosion. The use of reactive elements such as Zr, Si, Y, etc., further enhances the performance of these coatings by mechanisms involving better adhesion of the protective oxide scale, slower oxidation rate, the ability to mitigate the Kirkendall effect, and substantially longer coating life. Coatings that enhance these properties are referred to as MCrAl(Y, Zr, etc.), with M representing the base alloy. These coatings exhibit optimal corrosion and oxidation resistance performance. Currently, they are only deposited using overlay techniques, such as thermal spraying processes. However, these techniques have limitations in terms of adhesion, internal coating porosity, and the inability to coat internal structures such as holes and channels. Electroless plating can overcome these limitations due to the intrinsic properties of the technique, making it an elegant alternative worth considering. The main objective of this study is to obtain protective coatings containing Cr, Al, and reactive elements for future deposition of MCrAl(Y, Zr, etc.)-type coatings using electroless plating in synergy with other currently employed deposition techniques. However, it is not possible to directly obtain a Cr coating using an electroless bath because hexavalent chromium is carcinogenic and its use has been banned, and chromium(III) is too stable in aqueous solution to be reduced by a reducing agent. Therefore, an innovative approach is presented in Chapter 5 for the production of NiCr coatings using the electroless composite technique, which involves the direct addition of chromium particles (nanoscale and microscale) to the deposition bath to obtain composite coatings with a nickel matrix and dispersed particles. Subsequent heat treatments allow the formation of NiCr alloys, which were then characterized for oxidation at 800°C. The same approach proves ineffective when aluminum particles are added to the bath due to their high reactivity, leading to the precipitation of the bath. Therefore, a different approach was chosen, which involves introducing aluminum and reactive elements (in trace amounts) using a gas-phase coating technique, namely, diffusion aluminizing. This technique allows for the introduction of high amounts of aluminum into nickel-based substrates (usually superalloys) and can introduce small amounts of reactive elements during the aluminizing process. The technique used in this study, described in Chapter 6, is called slurry-based aluminizing and involves the use of a low-viscosity solution containing activating salts and metallic particles containing aluminum, which is sprayed onto the sample surface at room temperature, followed by high-temperature diffusion aluminizing. Specifically, the possibility of introducing Zr and Si as reactive elements into the process was investigated, and the performance of the obtained coatings was evaluated for oxidation at temperatures of 1000°C and 1093°C. The slurry-based aluminizing process was further expanded and optimized by investigating the effect of process parameters, such as slurry composition, spraying parameters, and heat treatment conditions, on the resulting coating properties. The aim was to achieve uniform and adherent coatings with controlled composition and microstructure for improved oxidation resistance. Then the focus shifted towards the deposition of protective coatings containing silicon (Si) and zirconium (Zr) using slurry-based aluminizing. The process parameters, slurry composition, and deposition conditions were optimized to obtain coatings with improved adhesion, composition control, and oxidation resistance. The resulting coatings were characterized for their microstructure, composition, and performance under high-temperature oxidation conditions. The present study aims to explore the application of the electroless technique in synergy with the slurry-based aluminizing technique to obtain nickel-based coatings containing chromium (Cr), aluminum (Al), and reactive elements such as zirconium (Zr) and silicon (Si). This approach combines the advantages of overlay (electroless) and interlay (diffusion aluminizing) processes. The electroless technique allows for the deposition of desired thicknesses of NiCr and NiCoCr coatings on various substrates with the required compositions. Subsequently, the slurry-based aluminizing process is employed to introduce aluminum and reactive elements (RE). This approach enables the production of NiCoCrAl(RE)-type coatings using electroless plating as a base, overcoming the limitations and costs associated with current thermal spray techniques used for such coatings' deposition. Moreover, using a Ni-based coating obtained through electroless plating as a base for the aluminizing process opens up the possibility of depositing these coatings on lighter and more high-performance substrates than the current Ni-based superalloys, while retaining the protective properties provided by the electroless plating + slurry-based aluminizing coating. Future studies will focus on synergistically combining the approaches investigated and developed in this study, which are currently attracting attention from various companies operating in the energy production sector.