|Teguh Endah Saraswati||- Department of Chemistry, Sebelas Maret University
- Research group of Plasma Science and Technology, Sebelas Maret University
|Kartiko Nugroho||- Department of Chemistry, Sebelas Maret University
- Research group of Plasma Science and Technology, Sebelas Maret University
|Miftahul Anwar||Department of Electrical Engineering, Sebelas Maret University|
In coating technology, many coating methods have been developed to enhance metal’s resistance to corrosion. This study demonstrated the use of a simple, easy, and economical manual thermal flame spray method to create an anticorrosion coating layer composed of ball-milled wood charcoal (a readily-available source of carbon) and titanium dioxide (TiO2). The coating materials were sprayed and melted by passing them through a flame; they were then deposited onto the surface of metal substrate of structural steel (SS) 400. We observed that the surface layers of the substrate sprayed with the wood charcoal–TiO2 mixture contained deposits such as titanium carbide, as a result of the carbothermal reduction reaction between TiO2 and the carbon present in the wood charcoal, and other iron carbide and titanium-iron oxide compounds. This deposit layer may explain why this substrate also exhibited a microhardness of more than twice that of the uncoated substrate. The coated substrate shows a darker shade than the uncoated substrate. The observation under optical microscopes shows that the uncoated metal substrate has a rougher surface with many more voids than the coated substrate. The coated surface has a water contact angle of ~107o indicating that a hydrophobic surface can be maintained. The coated substrates also demonstrated greater corrosion resistance to both acid and water, with the wood charcoal–TiO2-coated substrate demonstrating the best performance; in fact, its corrosion rate was nearly three times slower than that of the uncoated substrate.
Ball milling; Charcoal; Coating; Flame thermal spray; Titanium dioxide
Most equipment made from iron-based materials has one major weakness: susceptibility to oxidation while in contact with free air. The oxidation process corrodes iron-based metals, thereby reducing equipment lifetime and increasing metal waste, which can in turn lead to ecological degradation, especially in agricultural areas and near water sources.
Many methods have therefore been developed to coat metal substrates with anticorrosion materials. There are two approaches: wet- and dry-coating methods. These terms refer to the medium in which the deposited material is processed. Wet-coating methods mainly involve electroplating (Lowenheim & Senderoff, 1964), electroless plating (Mallory & Hajdu, 1990), hot-dip galvanizing (Rahrig, 1995), and corrosion-inhibitor addition (Shetty & Shetty, 2017); dry-coating methods include, among others, vapor deposition (Pierson, 1999), thermal spray techniques (Davis, 2004; Pawlowski, 2008), brazing (Schwartz, 2003; Shapiro & Rabinkin, 2003), weld overlays (Hattori & Mikami, 2009), and peening and nitriding (Ariati et al., 2016).
Thermal spraying is an industrial coating process that consists of using a heat source and a coating material in a powder or wire form that is melted into tiny droplets and sprayed onto surfaces at high velocities. High-velocity oxygen fuel (HVOF) is a type of thermal spraying method which is appropriate for coating rocket nozzles for WC-Co deposition, and which provides high hardness and low porosity (Sofyan et al., 2010). However, HVOF equipment is expensive because the spray gun usually cannot be operated manually; automated manipulation is typically needed to access restricted surfaces on a complex structure due to the inflexibility of the gun. Therefore, alternative thermal spraying methods are needed, such as flame spraying. In this method, the heat is supplied from the combustion of fuel gas and oxygen; the resultant heat melts the powder or wire of the coating material. The melted material is then propelled onto the substrate at a rate that achieves high deposition quality. This method offers a cost-effective way to apply metallic and ceramic coatings in a less demanding environment. In addition, it allows for the use of a wide variety of metallic or ceramic coatings onto a broad range of component materials where excellent wear and impact resistance are required (Fauchais et al., 2010; Fauchais et al., 2011; Fauchais et al., 2014).
In addition to the coating method, it is also important to take into account the coating material. The oxidation process can be prevented through the use of anticorrosive coating materials which have sufficient hardness to prevent surface contact with the air—for example, carbides. Carbide layers can be produced using carbon-based material, and a wide variety have been successfully applied as coatings to metal substrates, including titanium carbide (TiC), which has high hardness, high chemical stability, and good electrical conductivity (Benoit et al., 2005; Li et al., 2008; Rahaei et al., 2012). TiC can be synthesized using carbothermal reduction (Sen et al., 2010), thermal plasma (Mohapatra et al., 2013), solid-state electrochemical (Osarinmwian et al., 2015), or high-energy mechanical alloying (Saba et al., 2016). In the flame spraying method, TiC is generally used as a powder feed together with other materials, such as ceramics made of simple oxides (e.g., Al2O3 or Cr2O3) or complex oxides (e.g., MgZrO3), nitrides (e.g., BN), borides (e.g., ZrB2), or halides (e.g., CaF) (Pawlowski, 2008).
TiC can be produced during the carbothermal reduction process from the reaction between titanium dioxide (TiO2) and carbonaceous materials at high temperatures (Swift & Koc, 1999); however, the production of carbonaceous-based materials using conventional equipment and methods is not economically viable for application on a broad industrial scale. Our study therefore sought to use a starting material, such as carbon precursors, which could lower production costs. One potential carbon precursor material is wood charcoal, which is less expensive and readily available due to its widespread uses as a liquid ink, a coal source, a raw material for pencils, and a water purification adsorbent (Ishimaru et al., 2007).
We therefore posited that wood charcoal could be used as the raw material for the production of carbide via carbothermal reduction reactions with metal oxides, e.g., TiO2, for use in creating an anticorrosive coating for iron-based metals. Carbothermal reactions can be achieved in either liquid (Zhong et al., 2011) or powdery solid (Sen et al., 2010; Rahaei et al., 2012) states. Although liquid is generally used to achieve homogeneous carbon-based suspensions or solutions, liquids and suspensions should be avoided during the carbothermal process for safety reasons due to the flammable nature of the organic solvent.
Therefore, to achieve a successful carbothermal reaction between wood charcoal and TiO2, mechanical processing is required for optimal results. Ball milling is a relatively inexpensive and effective powdery solid-phase mechanical processing method which provides not only a reduced-size material (Ohara et al., 2011), but also a homogeneous powder mixture even in its atomic and crystallographic phases (Fecht, 1995). After ball milling, the carbon from the wood charcoal will react effectively with the TiO2 to form carbides when treated with heat, for example by using the flame thermal method.
In the present study, we describe a modified manual flame thermal spraying method that uses a simple, operator-friendly design with low-cost initial materials of TiO2 and carbon present in the wood charcoal which supposedly reacted to produce titanium carbide coating layers. Overall, our results indicated that this method successfully deposited coating layers onto metal substrates.
An examination of the physical and chemical characteristics of the substrates treated with coating layers composed of wood charcoal with and without the addition of TiO2 indicated that the addition of TiO2 enhanced corrosion resistance, although wood charcoal alone was also an improvement over no treatment at all. Ultimately, we conclude that the TiC deposits produced with the wood charcoal–TiO2 coating using our proposed flame spraying method helped to protect metal substrates against corrosion, and we suggest that this method is feasible for future applications in home industries that produce metal-based products.
Our results indicated that using flame spraying to coat metal substrates with a mixture of ball-milled wood charcoal and TiO2 could potentially be useful to prevent metal corrosion. Our flame spraying method successfully deposited the coating materials onto the metal surface. Overall, the addition of TiO2 improved the physical properties of the metal substrate compared to the use of wood charcoal alone or to no treatment. After spraying the substrate with the wood charcoal–TiO2 mixture, the surface composite layers contained deposits such as TiC, which was a product of the carbothermal reduction reaction between TiO2 and the carbon present in the wood charcoal, and other iron carbides and titanium-iron oxides. The substrate’s morphological structure also became smoother after the wood charcoal–TiO2 treatment, because the sprayed material—comprised of tiny particles as a result of the ball milling process—filled the fibrous slots of the metal substrate. The wood charcoal–TiO2 coating also gave the metal substrate a higher liquid contact angle (>90o), as well as a microhardness nearly twice that of the uncoated substrate and a corrosion rate that was nearly three times slower than that of the uncoated metal substrates. We conclude that the formation of the carbide layer deposited by the wood charcoal–TiO2 treatment could account for the increased microhardness and, consequently, some of the increased resistance to corrosion.
This work was partly supported by Grants-in-Aid Research from the Ministry of Research, Technology and Higher Education under project No. 339/UN27.11/PL/2015 and 474/UN27.21/PP/2018.
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