• The protective properties of the rust layer on high-Cr weathering bridge steel evolve in a periodic 'ascending–constant' pattern, not continuously ascending.
• Chromium addition promotes the formation of FeCr2O4 in the rust layer, which acts as a barrier against chloride ion penetration.
• The study integrates atmospheric corrosion monitoring (ACM) sensors and big data mining to reveal rust layer evolution dynamics in simulated tropical marine environments.
• These insights provide a scientific basis for designing advanced high-Cr weathering bridge steels with enhanced corrosion resistance.