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Steel Coatings

Steel Coatings

Steel is one of the most sustainable materials on Earth. It is 100% reusable, recyclable, easy to use and maintain. To complement the numerous benefits of steel and extend its life, suitable protective measures must be taken to ensure structural performance is not compromised over its design life.

Corrosion protection systems have been used for decades, but recent advances in coatings, and improved durability design guidelines and quality control, can extend the time to first maintenance and improve the sustainability of steel structures.

This is especially the case for New Zealand corrosion environments. New Zealand has a diverse climate and a very wide range of corrosion conditions that, if not designed adequately, will prohibit steel from reaching its full sustainability potential.

Achieving improved sustainability

Any steel structure exposed to a corrosive environment must be designed to provide optimum long-term performance with a minimal level of normal maintenance. Durability design will require either the use of self-protecting stainless or weathering steel, or conventional carbon steel with a corrosion protection system utilising an applied coating.

A sustainable design approach is recommended to determine the best solution, one that will achieve the most economic time to first maintenance based on the structure's performance and aesthetic requirements, design life and location. A well-designed structure, whether coated or uncoated, will minimise the initial material and energy inputs, provide cost savings from reduced future maintenance, provide health and safety benefits, and for coated structures, less on-site debris to be contained and disposed of.

The design service life for structures is typically 50 years for buildings and 100 years for bridges. The process of selecting an appropriate coatings system is always site specific, and will typically be surface specific where microclimate effects are important and different surfaces have different exposures.

Protection Mechanisms

There are three types of protection mechanisms used in coatings systems: sacrificial, passivating and barrier.

Sacrificial systems act by cathodic protection, through the use of a material that is more reactive to oxygen than steel. When both are exposed to oxygen and moisture, the more anodic material oxidises and protects the steel, which is cathodic. The most common sacrificial material used in coatings is metallic zinc, deposited on the surface as zinc metal spray, galvanizing or as a zinc-rich priming paint.

Passivating systems use chemicals such as zinc phosphate in the primer coat. This combines with any moisture that penetrates the coating system and prevents the formation of anodes.

Barrier coats work both by sealing the steel surface from air and water and electrically isolating the anodic and cathodic surfaces, preventing the electrochemical reaction from operating.

Coating Systems

There are a number of different coating systems available, including metallic coatings and paint coating systems.

Metallic coatings

Galvanizing: Galvanizing involves the formation of a metallic zinc coating onto steel. Galvanizing does not just adhere to the steel surface, but is metallurgically bonded to the base steel, forming an alloy layer between the surface zinc and the underlying base metal. Galvanizing is a tough coating system, providing high resistance to mechanical damage in transport, erection and in service.

Galvanized steel-painted: Sometimes it is desirable to provide a ‘duplex' coating system for steel that includes both galvanizing and a paint system. There are several reasons why it would be desirable to combine these materials: aesthetics, colour coding, safety markings, and acidic environments are just a few. The key to success of a duplex system is proper surface preparation and selection of a suitable paint system.

Metal spray coatings: Corrosion control using metal sprayed coatings requires the spraying of an anodic metal in a molten or semi-molten form onto an abrasive blast cleaned surface. This forms a metallic layer which protects the steel from corrosion by both barrier and sacrificial galvanic action. Arc spray and flame spray are the most common application methods and normally use zinc, aluminium and their alloys in the form of wire as the feedstock. The arc spray process is often preferred because it yields higher production rates and greater bond strengths. It can also be applied in the shop or on site.

Paint coating systems

There is a wide range of paint systems ranging from acrylic, alkyd, chlorinated rubber and epoxy, to moisture-cured urethane, zinc-rich primer and polyurethane. Most paint coatings are made up of four principal parts: pigments; non-volatiles (resins or binders); volatiles (organic solvents, water or the combination of both); and additives (specialty chemicals which make the coating function). All of the components of a coating interact to accomplish the purpose for which the coating has been designed.