Originally Posted by Hondo6

4130 CroMo steel is typically between 0.8% and 1.1% chromium and between 0.15% and 0.25% molybdenum. That amount of chromium in a steel alloy does effectively nada to provide corrosion resistance. Both elements are added to improve the strength of the resulting alloy, not for corrosion resistance.

https://matmatch.com/learn/material/4130-steel

Unless coated with a preservative or provided with some form of corrosion-resistant coating - like nickel or chromium plating, Parkerizing, or bluing - untreated bare CroMo will definitely rust.