Amid a global wave of stimulus programmes and resultant construction activity, the International Zinc Association (IZA) deemed it opportune to demonstrate the proven performance of zinc to protect steel from corrosion in the built environment.
In a webinar on August 12, IZA Africa Desk manager and corrosion specialist Simon Norton illustrated the chemistry of zinc as it applied to corrosion protection of steel, whether for steel structures or steel reinforcement in concrete structures.
If left unprotected, steel will corrode in any environment, but even more rapidly, for example, in Gauteng and Mpumalanga, with its high levels of atmospheric sulphur pollution, owing to industrial activity, or in coastal areas with the tendency for chloride corrosion from sea spray and sea mist.
This excellent transition metal protects steel by providing a physical barrier to the corrosive atmosphere and also provides cathodic protection for the underlying steel usually by means of hot-dip galvanizing.
Norton elaborated that zinc acted as a sacrificial anode on steel and steel would not corrode until the zinc coating had been consumed.
The barrier protection is based on the development of the zinc patina. Norton described how the zinc patina first develops with exposure to the oxygen and moisture in the air which reacts to form zinc hydroxide. This then reacts with carbon dioxide, which is ever-present in the atmosphere, particularly in industrialised areas, forming a barrier layer of tightly adherent and insoluble zinc patina.
Considering that there are a variety of zinc coatings available, it is important that coatings are specified according to the conditions to which the coating will be subjected and the expected design life.
Steel reinforcement in concrete structures corrodes by two main mechanisms: carbonation, which is a decrease in the pH of the concrete owing to exposure to carbon dioxide in the air, and by chloride-induced corrosion.
The pH of freshly poured concrete is generally higher than 12.5 and, because of its porous nature, is open to carbon dioxide penetration, which, in turn, decreases the pH of the structure with time.
A lower pH makes steel rebar more susceptible to corrosion and, ultimately, leads to tensile stress. Unchecked tensile stress inevitably causes spalling and cracking of concrete and renders a structure likely to collapse.
Zinc coated or galvanized rebar passivates at high pH through the formation of a protective layer of calcium hydroxyzincate and actually becomes less susceptible to corrosion as the pH of the concrete decreases owing to carbonation.
Norton added that galvanized rebar also withstood the effects of chlorides from coastal exposures and only showed internal stress signs over a long timeframe, making it a suitable addition for marine structures.
There is a threshold amount of chloride build-up before black rebar will corrode and concrete will crack, but galvanized rebar has a threshold that is four times higher.
Discussing how to hot-dip galvanise rebar and why it was a superior solution to black or uncoated rebar, IZA independent consultant Martin Gagné presented case studies showing the proven performance of zinc-coated rebar over 50 years.
General galvanizing, or hot-dip galvanizing, refers to the galvanizing of fabricated or manufactured steel items. The fabricated steel item undergoes surface preparation including degreasing, pickling and fluxing before being dipped into a molten zinc bath. The process is easily applied to reinforcing steel. Zinc-coating on the rebar from this process should be at least 85 μm.
Further, Gagné pointed out that about six-million tonnes of rebar were produced in Africa each year, mostly in Egypt, which translated into a potential zinc market of 300 000 t. “This is a very interesting opportunity for hot-dip galvanizers.”
For designing reinforced concrete structures using hot-dip galvanized rebar, he advised that all procedures were the same as for uncoated black steel rebar and that installing galvanized rebar involved the same handling procedures as black rebar and should not pose any issues for repairing or touching up on site.
Galvanized rebar is typically used in precast structures, bridge decks, transportation structures, coastal structures, water treatment plants, parking garages, balconies and foundations.
Gagné pointed out some examples from around the world where concrete structures had been corrosion- and maintenance-free for 50 years, notably bridges in Canada exposed to road de-icing salts in winter, and the Sydney Opera House located on the sea front in Australia.
Closer to home, in Sea Point near Cape Town, the new Sea Wall has sections constructed with galvanized rebar that have remained corrosion-free since 1990.
Terry Smith, a specialist advisor on hot-dip galvanizing, unpacked the coating of smaller items in more detail during the webinar.
He said the industry offered four types of metallic zinc-coated fasteners, including centrifuge (hot-dip spin galvanizing), mechanical plating (peen plating), sherardizing and zinc electroplating.
Smith also touched on the durability of metallic zinc coatings (life of a metallic zinc coating is proportional to its thickness) and novel bolt uses for protection against vibration and increased security. He stated that thinly coated zinc electroplated bolts most often prematurely corrode when used on a hot dip galvanized structure.
Centrifuge hot dip galvanizing takes place when suitably precleaned fasteners are loaded into perforated steel baskets, dipped in molten zinc and following a metallurgical reaction, are placed in a centrifuge where rapid spinning and quick braking removes excess zinc. Nuts are galvanized as blanks for oversize tapping after coating, to accommodate the additional coating thickness. A zinc coating thickness on bolts of 80 µm is common.
Both mechanical zinc plating and sherardising are performed in a rotating barrel, the former at room temperature (mechanical bond) and latter at about 350 ºC (metallurgical bond). The processes use different impact media, are quite slow and result in a coating of up to about 50 µm, although >30 µm requires over sizing the nuts. Both coatings offer improved corrosion control over zinc electroplated fasteners and high strength steel does not suffer from hydrogen embrittlement.
Zinc electroplating, on the other hand, involves the deposition of zinc onto a steel substrate by means of chemicals and electrical current. The process involves the formation of an electrolytic cell, consisting of the cathode – the component – and the anode – the metal being used for plating, this supplying zinc into the electrolyte.
Zinc electroplated fasteners have a coating thickness of less than 10 µm and are most commonly supplied by hardware and chain stores.
The two most important factors when choosing a metallic zinc coating are: a correctly set out specification, indicating the required corrosion control based on the environment at hand, and timeous ordering of the said fasteners.
In conclusion, the panellists agreed that metallic zinc was the unsung hero of structural integrity in a modern built society.