Low oxygen environments in biomass gasification and the presence of chlorine in feedstocks can influence the corrosion rate of steel by affecting the formation of protective oxide scales. The effect of KCl on the high-temperature corrosion of low-alloyed steel (13CrMo4-5) under low oxygen partial pressure is investigated by KCl salt spray (0.1 mg·cm−2) and exposure to 3 vol% H2 + 30 vol% H2O + Ar (balance) at 500°C for up to 168 h. Specimens without KCl salt are exposed for reference. Specimens are characterized after exposure by mass change, SEM/EDS, and XRD. KCl-deposited specimens exhibit about 30% lower mass gain after exposure compared to non-sprayed specimens. Their scale shows a porous innermost layer and a denser layer on top. No Fe or Cr chlorides are identified. The specimens without salt exhibit a similar two-layered scale, with a porous inner Fe-Cr oxide, followed by a denser and thicker Fe-oxide above. KCl could potentially protect the surface from further degradation by physically covering the specimen, altering the scale morphology, and forming a less permeable barrier, hindering the transport of species through the scale. 

The highly corrosive behavior of ammonium chloride (NH4Cl) is known to originate from its intrinsic hygroscopic nature. Refining and co-processing process units for biofuels are often exposed to highly concentrated NH4Cl solution either resulting from water vaporization from an HCl and NH3 containing solution when the temperature increases or salt deposition from vapor phase when the temperature drops. For the latter case, once it reacts with water vapor present in the process, the deposited NH4Cl salts forms a concentrated and corrosive solution through deliquescence (unlike in conventional refinery). Consequently, severe pitting and localized corrosion are reported in hydrogenation and fluid cracking units. Therefore, studying corrosion resistance alloys (CRA) compatibility in the presence of concentrated NH4Cl salt solution is crucial to mitigate corrosion in the process. This paper reports the experimental evaluation of corrosion resistance of different CRAs, based on corrosion rate and stress corrosion cracking, exposed to saturated NH4Cl solution in the temperature range of 130-220°C. 

The highly corrosive behavior of ammonium chloride (NH4Cl) is known to originate from its intrinsic hygroscopic nature. Refining and co-processing process units for biofuels are often exposed to highly concentrated NH4Cl solution either resulting from water vaporization from an HCl and NH3 containing solution when the temperature increases or salt deposition from vapor phase when the temperature drops. For the latter case, once it reacts with water vapor present in the process, the deposited NH4Cl salts forms a concentrated and corrosive solution through deliquescence (unlike in conventional refinery). Consequently, severe pitting and localized corrosion are reported in hydrogenation and fluid cracking units. Therefore, studying corrosion resistance alloys (CRA) compatibility in the presence of concentrated NH4Cl salt solution is crucial to mitigate corrosion in the process. This paper reports the experimental evaluation of corrosion resistance of different CRAs, based on corrosion rate and stress corrosion cracking, exposed to saturated NH4Cl solution in the temperature range of 130-220°C. 

Firing waste wood in thermal power plants can lead to furnace wall corrosion due to corrosive elements such as chlorine, heavy metals, and alkali metals present in the fuel. This study investigates the influence of lead and chlorine on furnace wall corrosion of a low alloyed steel (16Mo3) and a nickel-based alloy (Alloy 625) during two field exposures using an air-cooled probe. Two two-week long test campaigns firing two different waste wood fuels (higher and lower lead and chlorine content) were carried out, exposing samples having metal temperatures in the interval 350-400 °C. The corrosion rates were determined using thickness loss measurements. The samples were examined using SEM/EDS/WDS and XRD techniques, to characterize the morphology and composition of the corrosion products. The findings suggest that higher lead and chlorine content in the fuel results in a higher corrosion rate for both materials; aggravated further above 370 °C. On 16Mo3 samples, iron oxides and chlorides, and chlorine-rich compounds are observed. Pits are observed on Alloy 625 samples, filled with nickel- and chromium-containing oxides mixed with corrosive species. The presence of lead compounds (e.g. lead molybdate) in connection to pits suggests active participation of lead in the corrosion process above 370 °C. 

Firing waste wood in thermal power plants can lead to furnace wall corrosion due to corrosive elements such as chlorine, heavy metals, and alkali metals present in the fuel. This study investigates the influence of lead and chlorine on furnace wall corrosion of a low alloyed steel (16Mo3) and a nickel-based alloy (Alloy 625) during two field exposures using an air-cooled probe. Two two-week long test campaigns firing two different waste wood fuels (higher and lower lead and chlorine content) were carried out, exposing samples having metal temperatures in the interval 350-400 °C. The corrosion rates were determined using thickness loss measurements. The samples were examined using SEM/EDS/WDS and XRD techniques, to characterize the morphology and composition of the corrosion products. The findings suggest that higher lead and chlorine content in the fuel results in a higher corrosion rate for both materials; aggravated further above 370 °C. On 16Mo3 samples, iron oxides and chlorides, and chlorine-rich compounds are observed. Pits are observed on Alloy 625 samples, filled with nickel- and chromium-containing oxides mixed with corrosive species. The presence of lead compounds (e.g. lead molybdate) in connection to pits suggests active participation of lead in the corrosion process above 370 °C. 

Aggressive corrosion can occur when firing waste or bio-based fuels, due to the presence of high concentrations of heavy metals, alkali metals, and chlorides. These deleterious compounds deposit on furnace walls and can form mixtures that can rapidly accelerate corrosion. The effect of salts containing lead had not been studied extensively at temperatures lower than 400 °C in nickel-based materials. This study investigates the effect of the individual salts PbCl2 and KCl and their mixture on the high temperature corrosion of alloy 625 at 340 °C and 380 °C. Samples of alloy 625 were covered with individual salts or a salt mixture and exposed to high temperatures in an atmosphere of synthetic air, 20-vol% H2O, and 100 ppm HCl. The results show that the presence of individual salts does not induce observable corrosion attack on alloy 625 after 168 h at any tested temperature. The salt mixture did cause a severe corrosion attack at 380 °C, observed after 24 h of exposure. It is suggested that the salt mixture induces the formation of lead chromates that may prevent or disrupt the formation of a protective chromia scale. It is believed that a key part of the mechanism is the formation of eutectic melts by the interaction of the scale with the salt mixture. Thermodynamic equilibria calculations show that the first melting temperature of PbCl2 and KCl salt mixture after reaction with oxygen can be as low as about 382 °C, and even lower (357 °C) if chromates are present. 

Three FeCrAl alloys (APMT, EF100 and EF101) from Kanthal® and the reference Ni-Cr Alloy 625 were used as weld cladding materials on tube shields in the evaporator tube bank of a waste-fired combined heat and power plant. For each alloy type, the overlay welded tube shields were placed in both roof and floor positions within the evaporator for 6 months. The metal-loss rate, the microstructure and hardness of the overlay welds before and after exposure and the corrosion products were analysed. The results showed higher metal-loss rates in the welds placed in the roof position, confirming heterogeneities in the evaporator bank environment. Alloys were ranked from higher to lower erosion–corrosion resistance as follows: APMT ≈ Alloy 625 > EF101 > EF100. The analysis of the corrosion attacks showed a significant variation among the alloys, from a primarily homogeneous corrosion attack on APMT to intergranular corrosion in EF100 and pit formation in EF101. 

High amounts of lead in waste/recycled wood fuel are known to be a contributing factor to the increased corrosion often related to this type of fuel. In combination with potassium, usually present in the fuel, low-melting point salt mixtures between lead chloride (PbCl 2) and potassium chloride (KCl) are expected to form. The purpose of this study is to investigate reactions in the mixed salt of PbCl 2 and KCl and its interactions with carbon steel P265GH and its oxide. Laboratory exposures were performed in an isothermal tube furnace with a salt mixture of PbCl 2/KCl (50/50 mol%) put on steel samples. The test duration was 24 hr at either 300°C or 340°C in an atmosphere of 100 ppm HCl and 20 vol% H 2O in synthetic air. After exposure, the salt mixture consists of distinct areas of KCl and PbCl 2 but also the compounds K 2PbCl 4 and KPb 2Cl 5. A general observation is that the oxide thickness increases with temperature and that areas with Pb/K-mixed salt are frequently found in close connection to more corroded areas. Often the more lead-rich phase KPb 2Cl 5 is located closest to the corrosion product indicating its importance for the corrosion.

Corrosion of furnace wall tubes is a problem often caused by the use of corrosive fuels. The relatively high contents of lead, zinc, alkali metals and chlorides in these fuels are believed to contribute to the corrosion. Initial corrosion as a function of lead content in a wood-based fuel was studied for three materials: 16Mo3, 304L and Alloy 625. The materials were exposed for 8 h in a laboratory combustion test rig at a position resembling furnace wall conditions. The metal temperatures investigated were 350 and 400 °C. Increasing the lead content in the fuel or the temperature accelerated the corrosion rate of 16Mo3. It is proposed that lead and lead oxides in deposits react with iron chloride to form lead chloride, which when combined with alkali chlorides results in a very corrosive deposit containing low melting salt mixtures. Negligible corrosion was observed for 304L and Alloy 625.

In this work, three different stainless steels (304L (CrNi-18-8), 253 MA®(CrNiSi-21-11), Kanthal® A-1 (FeCrAl)) and a reference low-alloyed ferritic steel (16Mo3 (Fe0.3Mo)) were exposed in a commercial biomass gasifier for three periods of 9 min, 580 and 1054 h in the temperature range 350–500 °C. Biomass is a fuel with generally higher amounts of chlorine and lower amounts of sulphur compared to coal and there is a current lack of data on materials performance in such environments. A high level of zinc sulphide was observed on the surfaces of all materials after exposure. It is argued that zinc plays a key role in capturing sulphur in this environment, thus preventing iron from sulphidation. Some incorporation of sulphur in the oxide scale was observed for Fe0.3Mo and CrNi-18-8. CrNiSi-21-11 showed some internal oxidation and pitting was observed for the FeCrAl material. All four materials showed acceptable performance with low total metal loss.