Corrosion Protection of Steel: Galvanizing and Duplex Systems
Paint protects steel by hiding it from the environment: where the film stops, the protection stops. Zinc works in a completely different way, and grasping this changes every subsequent decision: it is consumed instead of the iron, so it goes on protecting it even where the steel has been left bare.
Overview
TAV. 00Steel corrosion is not a generic chemical phenomenon but an electrochemical process: it requires a metal, water and oxygen, and the moment two dissimilar metals are in electrical contact a hierarchy is established in which one is consumed and the other stays intact. All protection of structural steel is built on those two facts. Paints work as a barrier: they interrupt contact between metal and environment, and their effectiveness depends entirely on the continuity of the film, which is also their limit, because a scratch or an uncoated edge locally defeats the system. Hot-dip galvanizing works instead by sacrifice: zinc is less noble than iron, so in the presence of moisture it oxidises first and, while any remains, it protects even the areas where the coating is missing. That is why a scratched galvanized component does not rust in the scratch, whereas a painted one does so immediately. From these two logics comes the duplex system, that is, paint applied over galvanizing, which is not simply the sum of the two but a system in which each covers the other's weak point.
The site score
TAV. HThe detail of the craftthe holes that save the component
Designing the component so that it can be galvanized
Each element is checked as fitting the available bath, and vent and drain holes are provided in every closed section. Overlaps that are not continuously welded are avoided, steels with suitable composition are chosen, and the suspension points for dipping are defined.
«You do not galvanize a sealed tube: you make it explode. Vent holes are not a detail, they are safety.»site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The difference that decides everything
Paint covers, zinc sacrifices itself
The two great families of steel protection differ not in quality but in mechanism, and every practical consequence follows from that. Paint protects by barrier: it puts a film between metal and environment, and while that film is continuous it works very well. Its limit is precisely its principle: where the film is interrupted, by a scratch, a poorly coated edge or a site cut, protection there no longer exists, and rust starts at that point and travels beneath the paint, lifting it. Galvanizing protects by sacrifice: zinc is less noble than iron, so in the presence of moisture it oxidises first, and in doing so it also protects the surrounding bare steel. That is why a scratched galvanized component does not rust in the scratch, and why galvanizing is consumed uniformly and predictably instead of failing at one point. Hence a practical rule: where there will be site working, galvanizing forgives; paint does not.
A qualitative diagram of the two mechanisms; the choice of protection class and thicknesses depends on the environment's corrosivity category and the required durability, defined by sector standards and by the design.
Why combining them pays
Duplex: each covers the other's weak point
Putting paint over galvanizing does not simply produce two superimposed protections: it produces a system in which the two weaknesses cancel each other out, and that is why the resulting life clearly exceeds the sum of the two taken separately. Paint's weak point is discontinuity: one scratch and rust begins beneath. In a duplex system that scratch exposes zinc, not steel, so nothing irreparable happens. Zinc's weak point, by contrast, is consumption: it oxidises slowly but relentlessly, and in aggressive environments its thickness is used up in predictable times. In a duplex system the zinc is shielded by the film, receives far less moisture and oxygen, and its consumption slows appreciably. The result is a system with no sharp point of failure: it degrades slowly and legibly, which allows maintenance to be planned rather than endured. It is also how you obtain a colour without giving up durability.
A qualitative diagram of the synergistic effect; the size of the durability gain depends on the environment, the thicknesses and the paint system adopted, and must be assessed case by case.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QHow long does galvanizing last?
It depends almost entirely on the environment, and that is the most useful answer because it lets you reason without tables. Zinc is consumed by oxidation at a rate that depends on how long the surface stays wet and on what substances are present in the air: in a dry rural environment consumption is extremely slow and protection lasts many decades; in an urban environment it is faster; in industrial or marine environments, where sulphur compounds or chlorides are present, it can be several times faster. Since consumption is essentially linear with time, expected life follows simply: coating thickness divided by the environment's consumption rate. This is what makes galvanizing predictable in a way paint is not: there is no moment at which it fails, there is gradual, measurable consumption. It is also why, in the more aggressive environments, duplex is not a luxury but the economically correct choice: the paint reduces how much moisture reaches the zinc and extends the life of the whole system far more than it costs.
Can you weld or drill an already galvanized component?
You can, but you must know that you are accepting two distinct consequences, and that one of them concerns people's safety. The first is technical: the heat of welding vaporises the zinc in a band around the weld, leaving bare steel at a point that is often the most stressed part of the component. That area must be made good, and the correct repair is not just any paint but a zinc-rich product, which reproduces at least partly the sacrificial mechanism instead of merely acting as a barrier. The second consequence is a health one and should be taken seriously: the zinc fumes produced by welding are harmful and cause a specific condition well known among welders, so the work requires local extraction and respiratory protection. That said, the practical rule is simple and takes you back to design: everything that can be drilled, cut and welded before galvanizing should be done before. Site working should be reduced to the indispensable minimum, and the unavoidable operations should be priced together with their repairs.
Why did the components come out different colours?
Because they are not made of the same steel, even when the drawing says they are. The appearance of galvanizing depends decisively on the composition of the base material, and particularly on its silicon content: low-silicon steels give bright, shiny, relatively thin coatings; higher-silicon steels give thicker, grey, matt coatings. Since silicon varies from cast to cast even within the same commercial grade, two sections bought at different times can come out of the same bath looking markedly different. This is not a defect and does not indicate poor workmanship: on the contrary, the thicker, matt coating is often the one that will last longer. The problem is one of expectations, and it is managed in only one way: by saying so beforehand. If visual uniformity is a requirement, there are two workable routes, namely sourcing all the material from the same cast, which is not always possible, or planning a duplex system from the outset, where the paint gives the final appearance and the underlying variation does not show.
Galvanizing or stainless steel: how do you choose?
They are two different strategies and the choice is rarely decided by the price per kilogram. Stainless steel resists corrosion thanks to a passive oxide layer that regenerates itself in the presence of oxygen: it is protection intrinsic to the material, which is not consumed over time. Galvanizing is an added, sacrificial protection that is consumed and therefore has a calculable life. In most structural applications galvanized steel is the economically sensible solution, because it offers lives measured in decades at a fraction of the cost. Stainless becomes the correct choice in three situations: when the zinc would be consumed too fast, as in marine or heavily industrial environments; when the element will never again be accessible for maintenance or replacement, and therefore must last as long as the building; and when long-term visual appearance matters. What should not be done is mixing the two materials without thinking: a stainless bolt on a galvanized plate creates a galvanic couple that consumes the zinc precisely around the connection, and it is a mistake that shows a few years later.