Assessment of Monel 400 corrosion in hydrofluoric–hydrochloric acid condensation environments for condensing heat exchangers

Abstract

This study assesses Monel 400′s corrosion behavior in liquid and vapor phases in hydrofluoric (HF), hydrochloric (HCl), and mixed HF-HCl acid environments. Through mass loss testing and electrochemical measurements, the alloy’s degradation was evaluated under varying acid concentrations, temperatures, gas atmospheres (N₂, O₂), and metal ion contaminants (Cu²⁺, Fe³⁺). Electrochemical measurements (ASTM G59/G102) and mass loss (ASTM G31/G1) were used for corrosion testing. The findings demonstrate that corrosion rates are considerably increased by vapor-phase exposure, especially in combined HF-HCl vapors (up to 0.8 mm·y⁻¹), because of combined fluoride and chloride chemical attack. Improved anodic dissolution and decreased passivation are confirmed by electrochemical data, particularly when oxidizing conditions (O₂) and Cu²⁺ and Fe³⁺ ions are present. Significant surface deterioration, dealloying, and the development of corrosion products rich in fluoride and chloride were discovered by SEM/EDS investigations. The addition of CuCl₂ and FeCl₃ changed the surface chemistry and increased localized corrosion. Similar degradation trends were shown by industrial validation in a condensing heat exchange unit employing Monel 400, where concentrated acidic condensates promoted rapid material loss and corrosion deposits consisting of chlorides. The findings emphasize the limitations of Monel 400 in HF-HCl systems, notably under vapor-phase and oxidizing conditions, and propose mitigation measures such as alternate Ni-Cr-Mo alloys, protective coatings, and oxygen exclusion to improve durability in harsh fluorinated environments.