Cold rolled steel (CRS) is a form of low-carbon steel that has undergone a shaping process at or near room temperature. This cold working enhances the material’s strength, improves its dimensional accuracy, and gives it a smooth, bright surface finish. The simple answer to whether cold rolled steel rusts is yes. Like all carbon steels, CRS is highly susceptible to corrosion when exposed to the elements.
Why Cold Rolled Steel Rusts Quickly
Cold rolled steel is fundamentally an iron alloy, and the iron content makes it inherently vulnerable to the electrochemical process known as oxidation. Rust, or hydrated iron oxide, forms when iron atoms react with both oxygen and water.
Unlike hot rolled steel, which is processed at high temperatures, the cold rolling process removes the protective layer called mill scale. Mill scale is a bluish-black surface layer that temporarily shields the underlying metal from the atmosphere.
When CRS leaves the mill, it has a clean, bare metal surface that is highly reactive and immediately exposed to environmental moisture and oxygen. This lack of a barrier means corrosion can begin almost instantly. For this reason, manufacturers often apply a temporary rust-preventive oil before shipping to delay “flash rusting” during transport and storage.
Environmental Factors That Speed Corrosion
The presence of moisture is the primary driver, but the exact speed of the reaction is heavily influenced by the atmosphere surrounding the steel. Rusting is an electrochemical process, and water acts as the electrolyte, facilitating the transfer of electrons necessary for oxidation to occur.
High relative humidity is a particularly significant factor; corrosion can begin when the humidity level is sustained between 70% and 80% and the temperature is above freezing. Temperature fluctuations also play a role, as rapid changes can lead to condensation, forming a thin but persistent film of water on the steel surface.
Corrosive agents significantly increase the speed of deterioration. Salt, such as the sodium chloride used for de-icing roads or found in coastal air, acts as a powerful electrolyte, enhancing the water’s conductivity and accelerating the formation of rust. Pollutants like sulfur dioxide and nitrogen oxides in industrial areas can dissolve in moisture on the steel surface, forming mild acids that aggressively attack the metal. The presence of these chemicals means that cold rolled steel in urban or maritime environments will corrode far faster than the same material in a clean, dry, inland setting.
Strategies for Long-Term Rust Protection
Applying a robust protective barrier is necessary for long-term durability. The most fundamental strategy is to ensure the steel is clean and dry and to store it in a climate-controlled environment whenever possible. For any application where the steel will be exposed to the elements, an external coating is the most reliable solution.
A common and effective approach is applying a multi-layer paint system, which begins with a specialized primer formulated to adhere to bare metal and inhibit rust. This primer creates a strong bond, followed by a durable topcoat that provides UV resistance and an aesthetic layer. Powder coating offers an even tougher, thicker barrier; this process involves applying dry powder that is then cured under heat to form a hard shell of vinyl, epoxy, or nylon over the steel.
For maximum longevity in highly corrosive environments, galvanization is a superior method. This process involves coating the steel with a layer of zinc, which acts as a sacrificial anode. The zinc is more reactive than the iron in the steel, meaning it will corrode first, sacrificing itself to protect the underlying steel even if the coating is scratched. This method can protect the steel for decades, depending on the environment.
Another temporary but effective option, particularly for stored parts or tools, is the application of rust-preventative oils or waxes. These products contain corrosion-inhibiting additives that chemically bond to the steel surface, creating a water-repellent film that blocks oxygen and moisture. Regular reapplication is necessary to maintain the integrity of the protective layer.