Corrosion protection: an integral part of the fabrication process for steel structures
When it comes to choosing materials, steel, because of its exceptional physical and mechanical characteristics, is the most popular amongst designers. It can be used to rapidly build economical, durable, and safe structures. The steel used to make hot-rolled sections, which can themselves be indefinitely recycled, is made from recycled scrap. It is particularly suitable for machining and shaping. However, it tends to react with agents in the atmosphere to form stable thermodynamic bonds – ferrous oxides and/or salts.
The ability of steel to revert to its natural or original state is called corrosion. In layman’s terms: steel rusts!
Suitable corrosion protection for long-lasting steel
The primary quality of steel - i.e. its ability to retain its physical and mechanical strength, or in the case of a steel structure, its load bearing capacity - is generally long-lasting and is only compromised when corrosion reduces its cross section to such a degree that safety is adversely affected.
The service life of a structure depends on the rate of reaction between the steel and its environment. These reactions depend on the nature and concentration of the corrosive agents present.
Corrosion protection of structures must therefore be considered as intervening in this process in order to prevent the reaction or greatly reduce the rate.
Large numbers of steel structures, some of them a hundred years old, which have benefited from the combination of a suitable corrosion protection treatment and regular maintenance, show impressively the potential of steel. However, these types of structures, requiring high maintenance costs, are no longer acceptable today.
New requirements govern the design of the steel structures of the twenty-first century:
• Environmental protection policies enforced at both the national and international level have led to a considerable reduction in the amount of corrosive load in the atmosphere over the last twenty years. This has resulted in a noticeable drop in the average corrosion rate of steel and zinc as well as improved resistance of coating systems.
• The design of steel structures is substantially improved through the effectiveness of welding procedures and the vast range of rolled sections available thus contributing positively to ensure that corrosion protection systems have long service lives. Maintenance and renovation costs continue to fall because the amount of exposed surface area has been greatly reduced and ease of access improved. The ability of new coatings to resist the stresses in their surroundings has been increased considerably. New inexpensive coating methods are employed in fabrication shops and on construction sites.
• Zinc baths have sufficient capacity and are large enough to allow widespread hot-dip galvanisation of steel structures involving large components.
• Combining the hot-dip galvanisation technique with adapted paint systems for zinc coatings - so called duplex systems – renders maintenance unnecessary with, in most cases, the corrosion protection not requiring any attention for the entire service life of the structure.
• Corrosion protection is becoming an integral part of the fabrication process for steel structures.
Text: ArcelorMittal Commercial Sections
Images: ArcelorMittal Europe - Long Products, ArcelorMittal, ArcelorMittal Europe - Flat Products