Corrosion Resistance of Boronized, Aluminized, and Chromized Thermal Diffusion-Coated Steels in Simulated High-Temperature Recovery Boiler Conditions

Abstract

In this study, the high-temperature molten salt corrosion resistance of bare steels and steels with protective coatings, fabricated by thermal diffusion processes (boronizing, aluminizing and chromizing), were investigated and compared. Surface engineering through thermal diffusion can be used to fabricate protective coatings against corrosion, while alleviating issues around possible cracking and spallation that is typical for conventional thermal-sprayed coatings. In this regard, samples of low carbon steel and 316 stainless steel substrates were boronized, chromized, and aluminized through a proprietary thermal diffusion process, while some of the samples were further coated with additional thin oxide and non-oxide layers to create new surface architectures. In order to simulate the actual corrosion conditions in recovery boilers (e.g., from black liquor combustion), the surfaces of the samples sprayed with a modeling salt solution, were exposed to low-temperature (220 • C) and high-temperature (600 • C) environments. According to microstructural and X-ray diffraction (XRD) studies and results of hardness determination, the coatings with multilayered architectures, with and without additional oxide layers, showed successful resistance to corrosive attack over bare steels. In particular, the samples with boronized and chromized coatings successfully withstood low-temperature corrosive attack, and the samples with aluminized coatings successfully resisted both low-and high-temperature molten salt corrosive attacks. The results of this study conducted for the first time for the thermal diffusion coatings suggest that these coatings with the obtained architectures may be suitable for surface engineering of large-sized steel components and tubing required for recovery boilers and other production units for pulp and paper processing and power generation.2 of 36 materials and process features, low-melting temperature ash or smelt is produced. The produced smelt mainly consists of different salts, e.g., sodium and potassium chlorides, sulfates, and carbonates. At elevated temperatures, mostly greater than 500 • C, the aforementioned salts in certain combination likely become molten. In the presence of oxygen, the molten salts can be highly corrosive [2,3]. Combustion of the black liquor fuel also results in the formation of hot corrosive gases, such as SO 2 , CO 2 , Cl 2 , and some others [5].Generally, high-temperature black liquor corrosion occurs according to two main mechanisms, namely (i) high-temperature active oxidation and (ii) corrosion due to the formation of sulfidic and chlorine gasses and residual deposits of molten salts and their interaction with the steel surface [6]. In the former mechanism, which generally occurs in high temperatures (above 450 • C), the anions of the molten salt, such as chlorides (Cl − ), sulfates (SO 4 2− ) and sulfides (S 2− ), continuously diffuse into the oxide-metal interface and actively sustain the oxidation. On the other hand, in the latter mechanism, the presence of the an...

Sulfidation-Oxidation Resistance of Thermal Diffusion Multi-Layered Coatings on Steels

“…The test coatings were prepared according to the proprietary thermal diffusion technology established by Endurance Technologies Inc. (ETI) in Calgary, Canada. The coating process was described in detail in previous works [ 29 , 30 , 35 , 37 , 39 ]. The list of materials for sulfidation-oxidation testing is shown in Table 2 .…”

“…In order to evaluate the degradation process of the exposed samples, the structural examinations with a light optical microscope (LOM) MEIJI Techno 1M7200 (Japan, Saitama) (for materials’ cross-sections) and with scanning electron microscope (SEM) JEOL JSM-IT300LV (Japan, Tokyo) combined with X-ray energy dispersive spectrum analysis (EDS)—JEOL JED-2300 DRY SDD EDS detector (for materials’ surfaces) were carried out. The details of the cross-section’s preparation were reported in previous works [ 35 , 37 , 39 ]. The materials’ micro-hardness was determined according to ASTM E384-17 using the micro-tester Clark Instruments CM 400AT (Novi, MI, USA); the testing was conducted using a Knoop diamond pyramid at the 100 g indentation load (HK0.1).…”

“…Some reliable coatings are inapplicable for protection of the inner surface of long tubing and complex shape components. In contrast, the thermal diffusion surface engineering technology, specifically boronizing and aluminizing of steels, can be used to protect critical components, including long tubing, exposed to harsh corrosion conditions [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]. Recently, advanced coating materials obtained through the thermal diffusion process were considered and tested, for the first time, for high-temperature sulfidation applications with promising results after exposure in H 2 S-Ar flows at 500 °C [ 39 ].…”

Microstructural and Mechanical Properties of B-Cr Coatings Formed on 145Cr6 Tool Steel by Laser Remelting of Diffusion Borochromized Layer Using Diode Laser

“…Despite the fact that there are relatively few publications on diffusion layers containing boron and chromium, undoubtedly these chemical elements are worth paying attention in the context of introducing them to steel surface. The boronizing process may have a positive effect on durability improvement of tools for mining as well as for dies or rolls using in metal forming processes (rolling, punching, stamping, or forging) [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Diffusion chrome plating has a number of useful applications, such as increasing durability of tool surfaces in foundry processes and also in plastic forming processes [22][23][24][25].…”

Special Issue: “Coatings for Harsh Environments”

“…New concepts involving thermal diffusion coatings are being developed for boilers. Extensive corrosion testing of boronised, aluminised and chromised systems are being carried out to assess their applicability [9].…”