An influencing factor I can think of is that a dirt road will kick up debris such as small rocks, which will in turn potentially chip any passivation layer (chr

An influencing factor I can think of is that a dirt road will kick up debris such as small rocks, which will in turn potentially chip any passivation layer (chrome treatment, paint, etc.) on a metal component. This could accelerate corrosion by allowing moisture ingress if subsequently driven through puddles. Specifically, chipped paint is doubly bad as moisture can ingress and then flow underneath the paint, trapping it in there and not letting it dry out, so it continues to corrorde.