Hot rolled steel (HRS) is a fundamental material in construction and manufacturing, created by processing steel billets at temperatures above the recrystallization point, typically above 1,700°F (927°C). This high-temperature process makes the steel highly malleable, allowing it to be formed quickly into shapes like structural beams, plates, and frames. Since the process involves iron being exposed to the atmosphere at extreme heat, HRS is highly susceptible to corrosion. The surface condition created during manufacturing is the primary reason for this vulnerability.
The Role of Mill Scale
The unique characteristic of hot rolled steel is the presence of a dark, flaky surface layer known as mill scale. This scale forms as the newly rolled steel cools down and the surface iron reacts with oxygen in the air, creating a mix of iron oxides. Initially, this bluish-black layer may seem protective, providing a temporary barrier against the atmosphere.
However, mill scale is brittle and non-uniform, meaning it frequently cracks, chips, or flakes off when the steel is handled or bent. When the mill scale remains partially attached, it acts as a cathode because it is electrically conductive and less reactive than the base steel beneath it. Any small break in the mill scale exposes the underlying, more reactive steel, setting the stage for accelerated corrosion.
Rust Mechanism and Rate
Rust forms through an electrochemical process that requires the simultaneous presence of iron, oxygen, and moisture. When water lands on the surface of hot rolled steel, the exposed base metal becomes the anode, while the surrounding mill scale acts as the cathode in a localized corrosion cell. This condition results in galvanic corrosion, where the base steel rusts preferentially and rapidly at the point of the breach. This localized attack often leads to deep, concentrated pitting corrosion rather than uniform surface rust.
The rate of corrosion is heavily influenced by the steel’s environment. High humidity and moisture significantly accelerate the reaction by serving as the electrolyte in the corrosion cell. Exposure to saltwater or industrial air pollutants, which contain chlorides or sulfur compounds, further speeds up the rusting process. Temperature fluctuations can also increase the corrosion rate by promoting condensation.
Preparing and Protecting Hot Rolled Steel
Preventing rust requires completely removing the mill scale before applying any protective coating. Since the scale will inevitably lift and take the coating with it, thorough surface preparation is mandatory for long-term protection and adhesion.
The most common and effective removal method is abrasive blasting, often called sandblasting, which physically strips the scale and provides a rough surface profile for better coating adhesion. Chemical methods like acid pickling use hydrochloric or sulfuric acid baths to dissolve the scale quickly. While pickling is fast, it requires careful neutralization afterward to prevent further corrosion.
Once the clean, bare steel is exposed, a multi-step protection process must be implemented.
Long-Term Protection
- Applying a zinc coating through hot-dip galvanizing offers superior, long-lasting protection for permanent outdoor applications.
- Painting the steel, which requires an industrial primer specifically formulated for bare metal, followed by a durable topcoat.
- Using powder coating, which is applied electrostatically and baked onto the surface to provide a tough, aesthetically pleasing, and highly resilient finish.
Temporary Protection
For temporary protection during storage or transport, a simple coating of oil or a specialized rust inhibitor is applied to keep moisture away from the surface.