Protecting Structural Steel from Corrosion

By: RK Steel

When it comes to choosing materials, steel is preferred among designers because it can be used to rapidly build economical, durable and safe structures. However, its downfall is the tendency to react with oxygen and water, causing corrosion.

Applying the correct protective coating to structural steel upon installation can give a steel frame a long life, limit corrosion and reduce ongoing maintenance needs. Older paint systems consisted of five or six coats but, in recent years, have been replaced by fewer coats of thicker film. These paints are composed of three main ingredients: pigments, binders and solvents. Each layer applied has a specific function.

Breaking Down the Layers

Primer is applied directly to clean steel, providing corrosion resistance and good adhesion to the subsequent layers. Intermediate coats help to build up the total film thickness; generally the thicker the coating, the longer the life of the steel. In addition, these layers decrease the permeability of oxygen and water, delaying moisture penetration and improving tensile strength. The finish coat not only gives the paint job the desired glossy appearance but also acts as the first line of defense against condensation, weather and sunlight. An additional stripe coat is often applied directly to welds, fasteners and external corners to bring the thickness in those areas to satisfactory levels.

Paint Application Methods

The method of application used can have a significant impact on the quality and durability of the coating. Airless spray is often used under controlled shop conditions. Brush, rollers and air spray are typically used for on-site application.

Temperature and humidity are the main conditions that affect the final paint job and can be more easily controlled under shop conditions. Air and steel temperature can affect how the solvent evaporates, as well as drying and curing times. Paints should not be applied when there is moisture present or if the humidity level is high. This can affect the overall paint application and drying time.

Metallic Coatings for Structural Steel

Hot-dipped galvanizing is sometimes applied to structural steel surface. It is a process that involves immersing the steel components into a bath of molten zinc, leaving the surface uniformly coated with more durability, abrasion resistance and increased protection. Since this requires a dipping process, there are obvious restrictions on the size of pieces that can be galvanized. However, when it can be used, it is a highly sustainable, producing minimal waste.


The primary quality of steel is its physical strength, or load-bearing capacity, which is generally long-lasting. It is only compromised by corrosion, adversely affecting the steel’s safety. The life of a structure depends on the rate of reaction to its environment, as well as the protective coatings applied. Corrosion protection must be applied from the onset to minimize this process, preventing or greatly reducing the rate of corrosion.