Breakdown mechanism of γ-Al₂O₃ on Ni₂Al₃ coatings exposed in a biomass fired power plant

Highlights

• In-plant testing of Ni₂Al₃ coated superheater tubes were performed in a biomass-firing power plant for one year.

• In the protective areas, a 30–50 nm γ-Al₂O₃ layer was present on the surface and no Al depletion was observed below the oxide layer.

• At locations with corrosion attack, no alumina scale was present and an amorphous K–O surface layer with a thickness of 200–250 nm was found.

• γ-Al₂O₃ was broken down by reaction with KCl(g) resulting in potassium aluminate and volatile AlCl₃ was formed by the migration of Cl.

• A molten salt consisting of KCl-AlCl₃ can lead to alumina fluxing and further penetration of corrosive species into the coating.

Abstract

A Ni₂Al₃ coated tube was exposed in a wood firing power plant for 7100 h. The exposure resulted in protective behaviour in most areas, while corrosion attack occurred in local areas. In areas exhibiting protective behaviour, a 30−50 nm thick γ-Al₂O₃ layer was found, and a 200−250 nm K–O rich amorphous layer was identified in the corroded area. A corrosion mechanism is suggested: γ-Al₂O₃ was broken down by reaction with KCl(g) resulting in formation of potassium aluminate. Migration of Cl through potassium aluminate resulted in formation of volatile AlCl₃. KCl-AlCl₃ then led to alumina fluxing and further corrosion attack.

Introduction

The combustion of biomass in thermal power plants has continued to increase in the past years, as combustion of biomass is overall CO₂ neutral and therefore beneficial for limiting greenhouse effects. Among the biomass resources, straw and wood chips with a high availability are attractive fuels. In contrast to firing fossil fuels, the utilization of biomass presents considerably greater corrosion-related challenges for hot components such as superheater tubes. The corrosion of superheater tubes occurs mainly due to the formation of deposits that contain a high content of potassium and chlorine [1].

The high temperature corrosion mechanisms resulting from the presence of potassium and chlorine have been extensively studied and described in the literature [[2], [3], [4], [5], [6], [7], [8], [9], [10]]. In the past, the presence of aggressive chlorine was always considered as the main cause of corrosion in the biomass-fired power stations. The corrosiveness of Cl-containing species in biomass firing environments is usually explained by the active oxidation mechanism [[2], [3], [4], [5]]. According to this mechanism, chlorine is able to pass through the oxide scale via small cracks or defects to the metal/oxide interface, where metal chloride is formed. At superheater operation temperatures, metal chlorides become volatile and therefore migrate outwards towards the oxide/gas interface [6,8]. These chlorides are converted to metal oxide and chlorine is released at higher partial pressures of oxygen. The released chlorine will then facilitate further corrosion, forming a cyclic corrosion process [9,11]. In recent years, it has been highlighted that alkali metals can play an important role in the breakdown mechanism for otherwise protective chromium containing oxides. When potassium is present in the flue gas, often as KCl, it reacts with chromia to form potassium chromate while releasing chlorine [[4], [5], [6],12].

While there are many approaches to control the corrosion in a biomass fired power plant, the use of either high alloy materials or protective coatings may help to mitigate the corrosion and increase the lifetime of superheaters. Among coatings described in the literature, nickel aluminide coatings have shown good resistance against KCl induced corrosion in laboratory setups. In the study by Li and Spiegel [2] on the corrosion resistance of a Ni-Al alloy and Fe-Al alloys in a KCl-air atmosphere at 650 °C, the Ni-Al alloy exhibited good corrosion performance, while extensive corrosion attack occurred on the Fe-Al alloys. Kiamehr et al. [13] investigated the corrosion behaviour of pack cemented Ni₂Al₃ and Fe-Al coatings with KCl deposit in air for 168 h at 600 °C. It was reported that no attack occurred on Ni₂Al₃ coatings, while other coatings experienced different degrees of attack. Dahl et al. [14] evaluated the performance of pack cemented Ni₂Al₃ coatings with KCl deposit in a gas flow of 40 % H₂O + 5% O₂ + N₂ at 600 °C for 168 h. After exposure, the Ni₂Al₃ coating showed the lowest corrosion rate compared with 304 steel and pure Ni. It was also found that slight corrosion reactivity occurred on the coating surface and resulted in formation of nodules that were rich in K, Al and O. The study of the nickel aluminide coatings reported in the literatures mainly contributed to the understanding of the corrosion performance and the analysis of the corrosion product in the similar laboratory environment. However, the actual corrosion performance and the formation of oxide scale of the coatings in the real power plants is not clear.

Because of the good performance of the pack cemented Ni₂Al₃ coatings in the laboratory, testing was done from our previous study in-plant in two biomass fired boilers [15]. The coating consisted of an outer Ni₂Al₃ layer on top of a Ni layer and then the substrate steel tube. The coating was formed by Al pack cementing pure Ni that was electrodeposited onto the steel tube. In the actual boiler environment, the corrosion mechanisms are caused by complex factors including fuel specifications and resulting flue gas and deposit composition, flue gas flow and temperature fluctuations. In one plant with 540 °C outlet steam temperature, the coating descaled from TP347H tubes due to frequent thermal-cycling, which made it difficult to assess the chemical protectiveness of the coating [16]. In the second plant, which ran at lower outlet steam temperature (approximately 520 °C) without thermal cycling the coating was well adherent to the Esshete 1250 tubes and showed protective behaviour in most areas and some local failures after 1 year [15]. After two years exposure, the outer Ni₂Al₃ coating had failed completely around the circumference of the steel tube, and the underlying nickel was being attacked resulting in corrosive species diffusing through to react within the nickel layer [17]. However, the identification of the surface oxide scale and the degradation mechanism of the coatings were not studied and clarified in the previous study. The present study is dedicated to in-depth characterization with modelling and thus understanding of the chemical degradation mechanisms leading to the failure of the Ni₂Al₃ coating. This has general implications for the potential use of alumina forming materials in biomass fired boilers.