Effect of Hydrogen as fuel for continuous annealing of AISI 316L stainless steel—annealing and pickling tests

This work explores the replacement of natural gas with hydrogen as the combustion fuel in free-flame continuous heat treatment furnaces for stainless steel strip, focusing on its influence on oxide scale formation and removal in AISI 316L steel. The higher water vapor content of hydrogen combustion atmospheres may alter the oxide scale’s composition, thickness, and adherence, potentially affecting downstream descaling and pickling processes. To assess these effects, both cold- and hot-rolled AISI 316L samples were annealed and oxidized in 100% CH4 and 100% H2 combustion atmospheres under three different thermal cycles. The resulting oxide layers were analyzed through SEM, EDX, and GDOES to determine their morphology and chemical composition, followed by comparative evaluation of the descaling behavior and minimum pickling times. Results indicate that annealing in both methane and hydrogen fumes modifies the scale of hot-rolled materials, leading to oxidation and spallation. Hydrogen fumes promote spallation, primarily forming Fe3O4 and Fe2O3 powder, while chromium depletion remains unaffected by the atmosphere. For hot rolled material, annealing in hydrogen fumes tends to reduce or leave the pickling time unchanged and to increase the shot blasting weight loss compared to natural gas. In cold-rolled 316L, oxide thickness increases with temperature and is constantly greater in hydrogen fumes due to water vapor-induced chromium evaporation. Iron oxide clusters observed at high temperatures suggest the beginning of breakaway oxidation, though not fully developed under the tested conditions. Annealing in hydrogen fumes similarly reduces or does not influence the pickling time and does not affect overall weight losses. These findings support the potential of hydrogen as a sustainable fuel alternative without detrimental effects on surface quality or processing efficiency.