4140 steel is not corrosion-resistant. It is a medium-carbon alloy structural steel with a low chromium content (0.8-1.1%), which provides some protection, but it cannot form a stable passivation film like stainless steel.

4140 steel is prone to corrosion in humid environments, and once corrosion occurs, it will rust like most steels that suffer the same damage. The corrosion rate is even faster when exposed to acids, alkalis, and salts. Therefore, surface treatments such as blackening, electroplating, and nitriding are required to improve its corrosion resistance.