Inside the air handler, airflow passing through the evaporator coil carries trace amounts of chemicals. If your HVAC system is outside your home, there will be different mix of chemicals naturally and not-naturally occurring in the air compared to inside your home.
Generally speaking, the chemicals which can cause HVAC coil corrosion include fluoride, chloride, acetic acid, and formic acid.
Fluoride typically is found in municipal water supplies, while chloride can come from salt used for melting ice, detergents and other cleaners, carpet, and fabrics. Acetic and formic acids can come from adhesives, paints, plywood, and cleaners.
When these chemicals are in a high enough concentration in the air inside or outside, they can cause coil corrosion over time. Fluoride and chloride ions form pits and pinholes in the metal coil, while the acids create tunnels and pinholes in the HVAC coils, both of which can lead to refrigerant leaks.