Causes and Treatment Methods for Corrosion in Steel Roof Structures

1. Types of Corrosion in Steel Structures

Based on the different forms of corrosion that occur in steel structures, corrosion can be classified into three categories: atmospheric corrosion, localized corrosion, and stress corrosion.

(1) Atmospheric Corrosion Atmospheric corrosion is one of the most common types of corrosion in steel structures and can significantly impact the overall stability and load-bearing capacity of the structure. The primary cause of atmospheric corrosion is the long-term exposure of steel structures to outdoor environments, where the metallic elements in the steel react with water and air in the atmosphere, leading to corrosion. Atmospheric corrosion in steel structures is a long-term, continuous process that gradually penetrates the interior of the structure, compromising its quality and safety.

(2) Stress Corrosion: Stress corrosion is a relatively unique form of corrosion characterized by sudden onset. It typically occurs when steel structures are subjected to specific stresses. There is a significant correlation between stress corrosion and the stability of steel structures; sudden fractures within the internal structure lead to corrosion.

(3) Localized Corrosion: Localized corrosion is also a common form of corrosion in steel structures, which can be further divided into crevice corrosion and galvanic corrosion. Specifically, crevice corrosion occurs during the construction and subsequent use of steel structures when cracks form on the surface, allowing moisture, air, and other media to penetrate the interior of the structure. Over time, this gradually leads to corrosion issues. Galvanic corrosion, on the other hand, is caused by the positive and negative potentials generated by different metallic elements within the steel structure. Generally, negative potentials are more likely to cause corrosion than positive potentials, and the resulting corrosion is more severe.

2. Classification of Steel Structure Corrosion Severity

(1) Mild corrosion: Spotty rust spots appear on the steel structure surface, with most of the paint intact;

(2) Moderate corrosion: Extensive rust spots appear on the steel structure surface, and the paint layer is chalking;

(3) Severe corrosion: The steel structure body exhibits flaking in layers, the paint layer has completely peeled off or chalked, and pitting appears on the surface of steel pipes.

3. Areas Prone to Corrosion and Causes

(1) Space structure supports, brackets, purlins, etc.
Causes: First, the surfaces of support and bracket components were not treated with shot blasting or sandblasting for rust removal; second, the use of ordinary alkyd or nitrocellulose anti-corrosion primers led to surface rusting; third, after welding space frame supports to space frame spheres, the weld beads were not thoroughly descaled.

(2) Screws Connecting Roof Panels to Purlins
Causes: Cold bridges are prone to form at screw holes, and their surfaces corrode more rapidly when exposed to dew; roof leaks cause the glass wool insulation to become saturated with water, leading to screw corrosion.

(3) Upper surfaces of steel structural members at skylight locations
Causes: Rainwater dripping onto the upper surfaces of steel structures due to leaks, causing prolonged exposure to a damp environment and resulting in corrosion; skylights and their connections to the roof have high thermal conductivity, creating thermal bridges that lead to condensation during colder temperatures. Dew drops falling onto the upper surfaces of steel structures result in prolonged exposure to a damp environment and subsequent corrosion.

4. Causes of Roof Condensation

(1) Insufficient Insulation Thickness: Insufficient thickness of the roof insulation layer creates a temperature difference between the inner surface of the building envelope and the warm, humid indoor air, thereby meeting the conditions for dew point formation. For example, if the primary and secondary purlins of a steel roof structure are concealed within the roof panels, the insulation thickness at the purlin locations is severely insufficient, creating thermal bridges in these areas.

(2) Insufficient Airtightness of Roof Construction Steel structure roofs typically use rock wool for insulation. If a well-sealed vapor barrier is not installed beneath the insulation layer, moist, warm air from inside the building will inevitably penetrate the insulation and reach the underside of the upper steel roof panels. The temperature difference will cause condensation to form on the underside of the steel panels, leading to water absorption and saturation of the insulation layer, which then fails. This accelerates heat exchange between the interior and exterior, creating a vicious cycle of condensation.

(3) Excessive Relative Humidity: Excessive relative humidity is a major factor causing condensation in swimming pool facilities. The higher the relative humidity, the greater the molecular pressure of water vapor, the higher the dew point temperature, and the smaller the temperature difference between the indoor temperature and the dew point temperature—making condensation more likely to occur.

(4) Inappropriate Ventilation Volume: Ventilation volume primarily refers to exhaust airflow. Theoretically, the exhaust airflow should equal the fresh air supply. When the exhaust airflow is high, the air conditioning load is also high. If the air conditioning capacity is insufficient, the indoor temperature will be lower; conversely, when the exhaust airflow is low, the fresh air supply is also low, making it impossible to effectively reduce indoor relative humidity. Therefore, controlling the indoor exhaust airflow allows for regulation of the required heating load, thereby reducing daily operating costs.

5. Methods and Measures for Corrosion Treatment of Steel Structures

(1) Corrosion Prevention and Maintenance Plan for Bearings: Bearings are the most critical joints in steel structures. All loads borne by the steel structure are transmitted from the bearings to the concrete columns (or beams). However, since bearings are often out of sight, their corrosion is frequently overlooked, creating the greatest safety hazard. To treat bearing corrosion, first remove any obstructions around the bearings, then use an angle grinder to clean the bearing balls, and the paint and rust from the flanges. Apply rust remover to the metal surface, rinse with water, then apply one coat of zinc-rich epoxy primer, two coats of epoxy micaceous iron oxide intermediate paint, and two coats of polyurethane topcoat. Apply grease to the bearing bolts and install protective caps.

(2) Corrosion Prevention and Repair Plan for Steel Structural Members Structural members are the primary load-bearing components of steel structures and constitute their most critical elements. Based on the degree of corrosion, they are classified into three categories: mild corrosion, moderate corrosion, and severe corrosion. For severely corroded members, a safety assessment must be conducted first; corrosion prevention treatment should only be performed after safety is ensured.

Assessment Method: Measure the thickness of the flaking oxide scale; determine the actual wall thickness of the members based on the steel structure construction drawings; establish a structural model based on the actual wall thickness of the members; use structural analysis software to verify the strength of the members; if the strength does not meet requirements, reinforcement measures should be taken.

1) For members with mild or moderate corrosion, the anti-corrosion treatment involves using a paint stripper to remove the topcoat and primer from the member; then applying a rust converter to the surface; applying one coat of zinc-rich epoxy primer, followed by two coats of epoxy micaceous iron oxide paint, and finally two coats of polyurethane topcoat.

2) For members with severe corrosion, the anti-corrosion treatment involves first using an angle grinder to remove paint and rust from the members, then applying a rust-removing paste to the metal surface, rinsing with clean water, applying one coat of zinc-rich epoxy primer, followed by two coats of epoxy micaceous iron oxide paint, and finally two coats of polyurethane topcoat.

(3) Solutions for Leakage and Condensation Issues in Steel Roofs Roof leakage and condensation are common problems in steel structures in cold and severely cold regions; they are major causes of accelerated corrosion in steel structures. The most common locations for roof leakage include: roof panel joints, the junctions between the roof and skylights, and roof gutters. When addressing roof leaks, first identify the cause and location of the leak, and then develop the most appropriate waterproofing plan based on the roof’s waterproofing configuration.

6. Safety Recommendations for Construction Entities or Building Users

(1) The maintenance and upkeep of steel-structured industrial buildings are closely related to the safety and service life of the steel structure; therefore, they must be given sufficient attention. Construction entities or building users should establish a regular safety hazard inspection system. Under normal circumstances, a comprehensive inspection and maintenance should be conducted once a year to ensure that the steel structure is not corroded, and to check for leaks or damage, thereby preventing roof cracks, water leakage, and damage to the insulation layer. After three years of use, the steel structure should be painted to prevent further deterioration due to rust.

(2) If leaks, damage, or loose or torn components are discovered on the building roof, they must be repaired or replaced promptly to ensure structural safety.

(3) When significant or major safety hazards are identified, they must be reported to the relevant authorities immediately. Use of the facility should be suspended, personnel evacuated, and safety inspections and assessments conducted promptly, followed by targeted remedial measures to ensure the safety of human life and property.