Abstract
A novel temperature gradient laboratory-scale corrosion test method was used to study PbCl2 migration, interactions with SiO2, NaCl, Na2SO4, KCl, K2SO4, or NaCl–KCl (50:50 wt %) and corrosion of carbon steel in waste-fired boilers. Two different steel temperatures (200 and 400 °C) were tested. The temperature in the furnace above the deposits was 700–800 °C. Exposure times of 4 and 24 h were used. The deposit cross sections were analyzed using SEM/EDXA. The results show that PbCl2 vaporized and condensed in the adjacent deposits. PbCl2 did not interact with SiO2 but caused severe corrosion. Deposits containing Na2SO4, K2SO4, and/or KCl reacted with the PbCl2, forming various new compounds (Na3Pb2(SO4)3Cl, K3Pb2(SO4)3Cl, and/or K2PbCl4). In addition, melt formation was observed with all alkali salt deposits. Visibly more Pb was found in deposits where reactions between PbCl2 and alkali salts were possible, i.e., Pb was observed to be bound to the reaction products. No measurable corrosion was observed with steel temperature at 200 °C, while steel temperature of 400 °C resulted in catastrophic corrosion. PbCl2 in contact with the steel surface lead to faster corrosion than K2PbCl4.
Highlights
Temperature gradients may play a vital role in PbCl2 induced corrosion by the means of local melt formation and proximity of a molten phase to the tube surface.[17]
The higher steel temperature (400 °C) was chosen to represent a typical superheater temperature used in waste-fired boiler units, while the lower temperature (200 °C) was chosen as a comparison temperature
K2PbCl4 was formed in deposits where either KCl or K2SO4 was present
Summary
Combustion of recovered waste wood (or recycled wood) is known to cause severe corrosion problems on furnace walls.[1−5] Waste wood’s tendency to increase corrosivity is caused by elevated concentrations of heavy metals, chlorine, and alkali metals (potassium and sodium) together with relatively low sulfur content.[6−9] Especially heavy metals are known to be very corrosive, because they decrease the first melting temperatures of pure alkali salt deposits and increase the risk of molten phase induced corrosion.[10,11]. Interaction of K2SO4 and PbCl2 and the formation of a caracolite-type compound K3Pb2(SO4)3Cl has been observed.[16] In the same publication, a novel gradient corrosion furnace method was used for the first time for Pb-containing salts. The novel testing method has been used earlier for alkali chloride-alkali sulfate mixtures, increasing the understanding of alkali chloride migration within boiler tube deposits and clarifying the importance of understanding the effects of temperature gradients on corrosion reactions.[19,20] Temperature gradients may play a vital role in PbCl2 induced corrosion by the means of local melt formation and proximity of a molten phase to the tube surface.[17]. Laboratory measurements have shown K2SO4 to react with PbCl2 and to form a caracolite-type mixture, K3Pb2(SO4)3Cl, which induces increased corrosion with carbon steel material.[16]. The higher steel temperature (400 °C) was chosen to represent a typical superheater temperature used in waste-fired boiler units, while the lower temperature (200 °C) was chosen as a comparison temperature
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