1. What Determines Corrosion Risk in Steel Buildings?
Corrosion risk depends on more than whether a building is located indoors or outdoors. The exposure environment, moisture retention, structural detailing, and accessibility of the steel all influence how a protection system should be designed and maintained.
1.1 Exposure Environment
Steel structures can be exposed to:
- Atmospheric moisture and humidity
- Rain and condensation
- Chlorides and salt spray
- Industrial pollutants
- Persistent wet conditions
- Immersion or contact with soil in specific applications
The severity of the exposure should be considered when selecting a corrosion protection system.
Rather than classifying a project simply as rural, urban, or coastal, engineers can use the corrosivity categories defined by ISO 12944-2 when evaluating atmospheric exposure. The standard also addresses environments involving immersion in water and contact with soil.
A coastal building, for example, may experience greater chloride exposure than a similar building located inland. Exposure can also vary between individual members depending on their location, detailing, ventilation, drainage, and ability to retain moisture.
1.2 Moisture Retention
Water that remains on a steel surface for extended periods creates conditions that promote corrosion.
Areas requiring particular attention include:
- Horizontal surfaces where water can accumulate
- Closed or poorly ventilated spaces
- Crevices and narrow gaps
- Overlapping components
- Connections that trap moisture
- Areas where dirt and debris can accumulate
Effective corrosion protection begins before a coating system is selected. Good detailing can reduce the conditions that allow corrosion to develop in the first place.
1.3 Accessibility and Maintenance
A steel member that can be easily inspected and recoated presents a different maintenance requirement from one located in an inaccessible or concealed area.
Designers should consider whether important surfaces can be:
- Inspected
- Cleaned
- Repaired
- Recoated
This is particularly important for connections, concealed spaces, and areas exposed to repeated moisture.
A corrosion protection system should be evaluated not only for its initial performance, but also for how realistically it can be maintained throughout the building's service life.
2. Corrosion Protection Starts with Structural Detailing
A protective coating cannot compensate for fundamentally poor detailing.
Good structural detailing reduces opportunities for water and contaminants to remain on the steel surface and makes the protection system easier to apply, inspect, and maintain. ISO 12944-3 addresses design considerations that support durable coating performance.
2.1 Provide Effective Drainage
Structural details should allow water to drain rather than accumulate.
Where practical:
- Avoid horizontal ledges that retain water
- Provide suitable drainage paths
- Avoid unnecessary pockets and recesses
- Consider how water will leave enclosed or partially enclosed spaces
The objective is to minimize prolonged wetting of steel surfaces.
2.2 Avoid Difficult-to-Protect Crevices
Narrow gaps and inaccessible surfaces can be difficult to prepare, coat, inspect, and maintain.
Where possible, detailing should minimize:
- Narrow crevices
- Overlapping surfaces that retain moisture
- Inaccessible internal surfaces
- Details that prevent proper coating application
A protection system is only as effective as the surfaces that can actually be prepared, coated, inspected, and maintained.
2.3 Consider Ventilation and Enclosed Sections
Enclosed or poorly ventilated spaces can retain moisture and condensation.
Where hollow or enclosed structural components are used, the design should consider:
- Internal moisture
- Drainage
- Ventilation
- Fabrication requirements
- The selected protection system
For components intended for hot-dip galvanizing, suitable appropriate vent and drain openings may be required so that molten zinc can enter and leave enclosed sections safely and effectively.
These requirements should be considered during fabrication design rather than added after the steelwork has been completed.
2.4 Consider Dissimilar Materials
When different metals are placed in contact in the presence of an electrolyte, galvanic corrosion can become a consideration.
Material compatibility and connection detailing should be reviewed where steel interfaces with other metals, particularly in wet or aggressive environments.
The appropriate solution may involve suitable material selection, separation between dissimilar metals, drainage improvements, or other project-specific detailing measures.
3. Main Corrosion Protection Systems
Once the exposure conditions and structural detailing have been considered, an appropriate protection system can be selected.
Common approaches for steel buildings include:
- Hot-dip galvanizing
- Protective paint systems
- Duplex systems combining galvanizing and paint
- Other metallic coating technologies for specific applications
No single system is universally the best choice. The appropriate solution depends on the environment, structural configuration, fabrication process, appearance requirements, expected durability, maintenance strategy, and project specifications.
3.1 Hot-Dip Galvanizing
Hot-dip galvanizing protects steel by applying a zinc coating to the steel surface.
The zinc coating provides a physical barrier and can also provide sacrificial protection to exposed steel under suitable conditions. If a localized area of zinc is damaged, the surrounding zinc can provide electrochemical protection to the exposed steel.
Hot-dip galvanizing can be used for a wide range of fabricated steel components, including:
- Secondary structural members
- Purlins and girts
- Bracing systems
- Fasteners
- Smaller fabricated steel components
- Other exposed steelwork where the process is suitable
Project specifications may reference standards such as ISO 1461:2022 or ASTM A123/A123M for fabricated steel products, while fasteners and other specific products may be governed by their own applicable product standards.
Advantages of Hot-Dip Galvanizing
Important advantages can include:
- Factory-applied protection
- Good resistance to atmospheric corrosion
- Sacrificial protection from the zinc coating
- Consistent coating coverage when properly fabricated and processed
- Reduced dependence on field-applied painting
- Suitability for many exposed steel components
Galvanizing can be particularly attractive where regular repainting would be difficult or where maintenance access is limited.
Galvanizing Design Considerations
Galvanizing is not simply a coating process added after fabrication. The steel should be designed and fabricated with the galvanizing process in mind.
The selected structural steel grade can also be relevant to the galvanizing process, particularly where steel chemistry influences coating characteristics.
Relevant considerations may include:
- Vent and drain holes for enclosed sections
- Component dimensions
- Steel chemistry
- Fabrication details
- Welding
- Handling after galvanizing
- Connection details
The suitability of galvanizing for larger structural assemblies also depends on the capacity and process limitations of the galvanizing facility.
3.2 Protective Paint Systems
Paint systems protect steel primarily by separating the steel surface from the surrounding environment.
A typical multi-coat system may include:
Primer → Intermediate Coat(s) → Topcoat
The exact system depends on the exposure environment, required durability, surface preparation, coating technology, and project specification. ISO 12944-5:2019 provides guidance on commonly used protective paint systems and their selection for different environments and surface-preparation conditions.
Paint systems can be used for:
- Primary structural frames
- Columns and rafters
- Secondary members
- Fabricated steel components
- Architectural steelwork
- Projects where appearance and color are important
Why Surface Preparation Matters
A coating system does not begin with the first layer of paint. It begins with the steel surface.
Surface preparation can affect:
- Coating adhesion
- Coating performance
- Corrosion resistance
- Coating durability
- The ability to achieve the specified protection system
Depending on the steel condition and specified system, preparation may involve cleaning, removal of contaminants, removal of corrosion products, abrasive blast cleaning, or other suitable preparation methods.
ISO 12944-4 provides a framework for types of steel surfaces and surface preparation, while AMPP/SSPC standards provide widely used requirements and practices for specific surface-cleaning methods and preparation levels.
Coating Application and Inspection
After surface preparation, the coating system should be applied according to its specification.
Important factors can include:
- Environmental conditions during application
- Compatibility between coating layers
- Application method
- Recoat intervals
- Dry film thickness
- Surface condition
- Inspection requirements
Dry Film Thickness (DFT) measurements can be used to verify whether the applied coating meets the specified requirements.
The required thickness should not be treated as one universal value. It depends on the selected coating system, exposure category, durability requirement, and applicable specification.
3.3 Duplex Protection Systems
A duplex system combines:
Hot-Dip Galvanizing + Organic Paint Coating
The zinc coating provides metallic protection while the paint system adds an additional barrier and can provide a different surface finish.
A duplex system can be considered where the project requires enhanced durability, a specific appearance, or additional protection in a demanding environment.
The paint system must be compatible with the galvanized surface and applied using an appropriate surface-preparation and coating procedure.
Duplex protection should be treated as a coordinated system rather than simply applying paint over galvanized steel without preparation.
4. How Should a Corrosion Protection System Be Selected?
There is no universal answer to the question:
Is galvanizing better than painting?
The appropriate choice depends on the complete project.
| Consideration |
Hot-Dip Galvanizing |
Paint System |
| Protection mechanism |
Zinc barrier + sacrificial protection |
Primarily barrier protection |
| Factory application |
Typically factory-applied |
Commonly applied in controlled shop conditions; field application may also be required |
| Field touch-up |
Requires an appropriate repair procedure |
Can generally be repaired using the specified coating system |
| Appearance |
Characteristic galvanized finish |
Wide range of colors and finishes |
| Maintenance access |
Can be advantageous where repainting access is limited |
Future inspection and recoating should be considered |
| Large fabricated members |
Subject to facility capacity, dimensions, fabrication, and process limitations |
Generally flexible for large structural assemblies |
| Color requirements |
Limited unless additionally coated |
Highly flexible |
| Duplex option |
Can be combined with paint |
Can form the organic coating layer of a duplex system |
| Surface preparation |
Governed primarily by the galvanizing process and any subsequent coating requirements |
Critical to coating performance |
The selection should consider environment, structural detailing, member dimensions, fabrication sequence, appearance, expected durability, maintenance access, and project specifications rather than comparing galvanizing and painting based only on initial cost.
5. Corrosion Protection During Fabrication and Erection
Corrosion protection can be damaged after the steel leaves the coating facility.
For this reason, protection should be considered throughout fabrication, transportation, storage, and erection.
5.1 Fabrication
During fabrication, attention should be given to:
- Welded areas
- Cut edges
- Surface contamination
- Sharp edges where relevant to the specified coating system
- Areas requiring subsequent touch-up
- Compatibility between the protection system and fabrication procedures
The selected protection system should be compatible with the actual fabrication sequence.
For galvanized components, fabrication details should also accommodate the requirements of the galvanizing process.
5.2 Transportation and Storage
Steel members can experience coating damage during:
- Lifting
- Stacking
- Packing
- Loading
- Transportation
- Unloading
Members should be handled and stored in a way that minimizes mechanical damage and prolonged exposure to standing water.
This is particularly important for coated steel components transported long distances before erection, where inadequate packing or storage conditions can damage the protection system before the structure reaches the site.
5.3 Erection and Site Modification
During erection, protective coatings can be damaged by:
- Lifting equipment
- Temporary supports
- Bolted connections
- Site welding
- Cutting
- Drilling
- Grinding
- Accidental impact
Damaged areas should be evaluated and repaired according to the applicable coating or galvanizing repair specification.
For galvanized steel, repair procedures can follow standards such as ASTM A780/A780M, where applicable. The standard addresses repair methods for damaged or uncoated areas of hot-dip galvanized coatings, including damage associated with fabrication, welding, cutting, shipping, and erection.
6. Inspection, Repair, and Maintenance
Corrosion protection is not finished when the steel leaves the fabrication shop.
A durable protection strategy also requires inspection and maintenance throughout the building's service life.
6.1 Inspection
Inspection may include:
- Visual examination
- Identification of coating damage
- Detection of rust or corrosion products
- Checking areas exposed to persistent moisture
- Coating thickness measurements where appropriate
Inspection frequency should reflect the environment, protection system, condition of the steelwork, and project maintenance requirements.
Particular attention may be required at connections, drainage points, exposed edges, damaged coating areas, and locations where moisture or contaminants can accumulate.
6.2 Local Repair
Localized coating damage should generally be addressed before corrosion develops further around the damaged area.
Depending on the protection system, repair may involve:
- Cleaning the damaged area
- Preparing the exposed steel
- Restoring the specified protection system
- Verifying the repaired area
For galvanized components, the repair method should be compatible with the original zinc protection system and the applicable repair specification.
6.3 Long-Term Maintenance
A corrosion protection system should be considered together with the expected maintenance strategy.
A system that is inexpensive initially but difficult to inspect or repair may not provide the best long-term value.
Project decisions should consider:
Initial protection + inspection + repair + future maintenance
rather than initial coating cost alone.
This whole-life approach can help project teams select a protection system that is practical not only at the time of construction, but throughout the intended service life of the building.
