It is well established that combustion of biomass and waste generates corrosive gases/deposits consisting of e.g., alkali salts [1]. At 600°C and below, this environment is known to destroy the primary protection of stainless steels (Cr2O3 or Cr-rich (Cr, Fe)2O3)) and FeCrAl alloys (Cr,Al)2O3), causing breakaway oxidation [2, 3]. This occurs rapidly, within hours or days, in this environment and the alloys will thereby be dependent on the scales formed after breakaway, the so-called secondary protection. The secondary protection is characterized by formation of outward-growing Fe-rich oxide and inward-growing spinel oxides, which composition depends on the alloying elements. In recent research, the concepts of good secondary protection (referring to slow-growing scale formed after breakaway) and poor secondary protection (referring to fast-growing Fe-rich scales) have been introduced to improve the understanding of oxide scale formation after the breakaway phenomenon [4].
Most of the published literature on corrosion protection at high temperatures has focused on the integrity and lifetime of the primary oxide, coupled to the effect of alloying elements, e.g., Cr/Al [4], Si [5], Mo [6]. However, previous research shows that the mechanisms controlling oxide growth kinetics after breakaway oxidation i.e., the secondary corrosion regime, differ from the primary corrosion regime when it comes to the role of the environment [4].
The effect of chlorine containing species on the primary corrosion regime (pre-breakaway stage) is well investigated. Alkali chlorides such as KCl, NaCl and PbCl2 have been reported to breakdown the primary oxide by forming alkali-chromate, the so-called chromate formation mechanism [7]. HCl has been shown to facilitate formation of metal chlorides through an electrochemical approach which involves the formation of chloride ions at the scale-gas interface and cations at the metal-scale interface via oxidation [8]. In other work, the role of Cl on the oxidation process has been attributed to its catalytic nature, which promotes the formation of volatile metal chlorides at lower pO2, which upon diffusion through the scale to regions of higher pO2 are converted to respective metal oxides, the so-called active oxidation mechanism [3]. On the other hand, some investigations have reported that the environment plays little or no role on the oxide scale formation beyond breakaway oxidation [9, 10]. Instead, the oxide growth kinetics has been explained by a diffusion-controlled mechanism, which is determined by the diffusivity of the different cations through the spinel [11].
However, despite the extensive research carried out to understand alkali-induced corrosion, there is limited knowledge on the impact of increased Cl load on the oxide scales formed after breakaway, i.e., on the secondary protection of stainless steels and FeCrAl alloys.
The aim of this study is to investigate the impact of an increased Cl load on the scales formed after breakaway on both good secondary protection and poor secondary protection. For this purpose, high-temperature corrosion studies were conducted of three alloys: SVM12 (martensitic stainless steel), Kanthal® APMT (FeCrAl alloy) and alloy 27Cr33Ni3Mo (austenitic stainless steel), in a laboratory environment containing H2O, KCl and HCl at 600°C. Characterization of oxide products were performed using scanning electron microscopy (SEM) in combination with energy-dispersive x-ray (EDX) spectroscopy.