The International Zinc Association (IZA) Africa has emphasised the essential role of zinc-based coatings in extending the service life of steel across multiple industries. Used for decades to counteract both aqueous and atmospheric corrosion, zinc coatings protect steel through three distinct yet complementary mechanisms: barrier protection, cathodic or sacrificial protection, and the formation of inhibiting corrosion products.
Barrier protection isolates the steel substrate from corrosive agents and ensures that the zinc layer corrodes first. However, barrier action remains effective only as long as the coating is of sufficient coating thickness and bare steel is not exposed. However, in cases where the coating is scratched or suffers limited damage, zinc continues to protect the steel through galvanic action.
Since zinc is less noble than steel, it preferentially corrodes at these exposed sites, ensuring that the underlying substrate remains intact. Over time, zinc corrosion products such as zinc carbonate and zinc hydroxide form adherent films that act as inhibitors, further reducing the rate of corrosion and extending the durability of the system.
Hot dip galvanizing remains the most widely applied zinc coating process for structural steel applications. In this method, steel is cleaned, pickled, fluxed, and immersed in molten zinc at temperatures between 435°C and 455°C. The process forms a tightly bonded series of iron-zinc alloys that envelops the steel entirely, including recesses and internal cavities.
Batch hot dip galvanizing produces relatively thick coatings, often exceeding 85 µm, which ensures long, maintenance free service lives for steel used on bridges, electrical pylons, rebar in concrete, and other heavy structural steel components.
By contrast, continuous hot dip galvanizing of steel sheet, performed on high-speed lines at integrated steel mills, produces thinner coatings in the range of 15 to 20 µm per side. It is normal to apply an organic primer and topcoat over the galvanized layer which gives a very good product. In South Africa we have Chromadek Ultim and Bluescope Colourbond Ultra as examples of this product.
Uncoated continuously galvanized sheet products are highly suitable for applications such as automotive body panels, roofing materials, and consumer goods. Coating thickness remains the critical determinant of service life, as galvanized steel corrodes in a linear fashion over time.
In highly corrosive marine environments, Galfan has proven to be a particularly effective coating. This zinc–aluminium eutectic alloy, comprising 95% zinc and 5% aluminium, with trace mischmetal, was designed to provide a zinc rich coating without the formation of brittle intermetallics at the steel interface.
The resulting two-phase microstructure corrodes according to a parabolic rate law, slowing down over time and providing nearly double the operational life of conventional continuously galvanized coatings. Galfan is widely applied to wire, strand, rope, and tubing where formability and long-term performance are critical.
Another widely used zinc alloy coating is Zincalume, which consists of 55% aluminium, 43.5% zinc, and 1.5% silicon. Supplied mainly in sheet form, Galvalume is applied to roofing, cladding, and other building profiles. Its corrosion protection arises from the complementary roles of zinc and aluminium.
The zinc rich regions corrode first, protecting exposed steel, while the aluminium remains largely unaffected until the zinc is depleted, at which point aluminium oxidation products form a durable barrier layer. This dual mechanism provides extended protection, but the performance of Zincalume depends heavily on coating thickness, environmental corrosivity, and the quality of any top-coat systems. In South Africa, the material is sold under trade names such as Zincalume, Colourbond, and Colourplus.
“Engineers and specifiers must carefully distinguish between coating systems when designing for durability,” cautions Simon Norton, Director, IZA Africa. Batch hot dip galvanizing provides vastly superior life expectancy compared to thin electroplated bolts and fasteners, which fail rapidly in aggressive environments. “Always specify bolts to be hot dip galvanized,” urges Norton.
Duplex coating systems, where organic coatings or UV stabilised powder coatings are applied over galvanizing, offer enhanced performance and aesthetic options, but must be designed with care to avoid premature failure through coating under film corrosion or coating delamination.
“Zinc coatings represent a comprehensive engineering solution to the problem of steel corrosion,” explains Norton. “By employing barrier protection, galvanic action, and inhibiting corrosion products simultaneously, zinc extends the service life of steel by decades. Proper specification and application of these coating systems is fundamental to reducing life-cycle costs, conserving resources, and ensuring sustainable infrastructure across Africa,” he concludes.
