Corrosion Behavior and Degradation Mechanism of High-Alloy Bainitic Gray Cast Iron Cylinder Liners for Methanol Engines in Formic Acid-NaCl Media.

Methanol is a promising alternative fuel for internal combustion engines; however, formic acid (methanoic acid, HCOOH) formed or enriched during engine operation can corrode cylinder liners and reduce cylinder liner-piston ring reliability. In this study, high-alloy bainitic gray cast iron cylinder liners were investigated in formic acid-NaCl media under three surface states: untreated substrate, quench-polish-quench (QPQ) treatment, and nitriding-oxidizing treatment. Static full-immersion tests were conducted in 5 and 10 vol% formic acid-NaCl solutions, and corrosion damage was evaluated by mass loss, mass-loss-equivalent average corrosion depth, and cross-sectional SEM-BSE observation. With increasing solution aggressiveness, all specimens showed increased corrosion depth and mass loss. For the untreated substrate, these values increased from 97.65 to 146.05 μm and from 929.8 to 1391.2 mg, respectively; the corresponding changes were 61.45-112.55 μm and 545.0-997.3 mg for QPQ-treated specimens and 55.25-119.00 μm and 488.7-1054.6 mg for nitriding-oxidizing-treated specimens. Under the lower-severity condition, nitriding-oxidizing and QPQ treatments reduced the mass-loss-equivalent corrosion depth by 43.42% and 37.07%, respectively. Cross-sectional observations indicate that flake graphite/bainitic-matrix microgalvanic coupling, formate-assisted dissolution, local degradation of modified surface regions, and defect-assisted electrolyte penetration jointly promoted inward corrosion. These results provide guidance for corrosion-resistant surface design of methanol-engine cylinder liners.