The excellent corrosion protection offered by zinc coatings or galvanizing derives from both the low natural corrosion rate of zinc coupled with its ability to extend protection to adjacent exposed steel areas, an effect known as cathodic protection. The coating also exhibits strong adhesion to the underlying steel surface due to its unique metallurgical bond that, together with the inherent toughness of a metallic coating, provides superior resistance to mechanical damage. The combination of these features results in a very durable coating, enabling concrete structures to be more tolerant of variability in concrete quality and reinforcement placement.
The use of galvanized reinforcement is uniquely advantageous:
It offers excellent resistance to chloride salt attack and is unaffected by concrete carbonation.
Zinc’s cathodic protection inhibits corrosion at any minor coating discontinuity and also prevents ‘undercutting’ of the coating, confining any corrosion risk to the local area of exposed steel.
Zinc corrosion results in little accompanying volume change. Unlike with steel corrosion, there is no adverse impact on the surrounding concrete. Research shows that any corrosion products simply diffuse into the adjacent concrete, helping to fill micro porosity that further inhibits corrosion.
Steel reinforcement corrosion will initiate when the critical chloride threshold is reached. The industry proposed critical chloride threshold for black steel reinforcement is 0.06% by weight of concrete, based on a 20% chance of corrosion initiation. Galvanized reinforcement can tolerate chloride concentrations well above this level due to the stability of the passive calcium hydroxyzincate film which forms on the galvanized bars surface, and while there is no universal agreement, a literature review on the subject shows the chloride threshold of galvanized steel to be 2 – 6 times higher than black steel. In general, a very conservative value for the critical chloride threshold for galvanized reinforcement is considered 2 to 2.5 times than for back steel.
The chemical stability of the calcium hydroxyzincate film at neutral pH ensures that hot dip galvanizing has a very low corrosion rate over a wide range of pH values (pH 6 – 12.5). Because of this, HDG remains stable as the pH level of the concrete drops to near neutral levels (pH 7) over time due to carbonation. Conversely black bar is only stable in a small range (pH 11.5 – 13.2) and will begin to corrode once the pH level of the concrete drops below 11.5. Hence galvanized rebar can completely avoid the effects of carbonation.
Reinforcement Corrosion: Cause and Effect
Bare (or black) steel reinforcement bars rely entirely on protection provided by the surrounding concrete. However, concrete permits the passage of chlorides from sea salts or other corrosive substances to the rebar because of its natural permeability, and also through cracks and expansion joints. Even carbon dioxide from the air will eventually result in rebar corrosion.
Corrosion can be managed by reducing concrete permeability through optimal water/cement ratios; appropriate compaction and curing conditions; the use of concrete impregnation methods or membrane-type concrete coatings; and by providing a good depth of concrete cover over the rebar.
All of these measures can delay the corrosion of rebar, but not prevent it. The use of galvanized rebar has real benefits in improving the safety and reliability of reinforced concrete, even when the measures described above are used.
There are two basic types of coatings: barrier and sacrificial. Most coatings can be classified as barrier because they provide basic protection from air and water penetration to the steel they are covering. Sacrificial or zinc coatings offer barrier protection, but also provide a secondary line of defence if the barrier coating is damaged as the zinc sacrifices itself or corrodes preferentially before the steel.
In durability planning, deterministic chloride diffusion models based on Ficks Second Law are commonly used to predict the durability of steel reinforced concrete structures. Variants of these models include Fick’s Second Law modified with an age factor (Bamforth, 2004), Luping and Gulikers Diffusion Model (2007) and the Time Weighted Average Diffusion Coefficient Model (RMS, 2018). All of these models predict the time to corrosion initiation based on site specific variables such as the surface chloride levels (environmental exposure), concrete age, chloride diffusion coefficients, type and content of supplementary cementitious materials.
A technical note on Chloride Threshold Modelling of galvanized steel is now available which shows that regardless of the deterministic chloride diffusion model used, exposure environment, concrete mix or cover, the time to corrosion initiation for galvanized rebar is 2-10 times greater than that for black steel reinforcement, dependent on site specific variables.
The GAA has developed an interactive durability model, based on Luping and Gulikers Chloride Diffusion Model, which allows the user to estimate the time to corrosion initiation for both black and hot dipped galvanized reinforcement for user specified conditions, including chloride levels (environmental exposure), concrete age, chloride diffusion coefficients, type and content of supplementary cementitious materials and thickness of cover. The model also plots the chloride profile versus concrete depth for the user specified service life and offers default input variables if the user doesn’t have project specific variables available.
The technical note, durability model and guidelines for the use of the model are available here or to ask about free training on the use of galvanized rebar click here and fill out your details or simply phone us on (03) 9654 1266.
A Sustainable Material
Material specifiers and product engineers in key end-use markets such as building, construction, and transportation are increasingly interested in selecting materials that have the best environmental profile while meeting traditional cost, quality, and technical performance criteria.
Measuring the impact and resource requirements associated with zinc production against the impact and the benefits of using zinc during other stages in the product life cycle show zinc as a very sustainable material. The environmental footprint of galvanized coatings has also been documented and the GAA now has available an industry-wide Environmental Product Declaration.
Galvanizing can extend the life of steel and concrete structures to well over 100 years, enabling huge conservation of natural resources by reducing the waste inherent with premature end-of-life. Energy savings are also accrued through minimized maintenance and upkeep. The end-of-life recycling of zinc-coated steel also adds to this conservation because energy requirements for re-melting steel and recovering the zinc are less than those required for producing the original metals.
The zinc and galvanizing industries understand that environmental and sustainability programs are integral to their future and are committed to updating the already favourable life-cycle information.