Corrosion without a chance
Rain contains – in addition to its natural chemical constituents – a large number of other elements and compounds, many of which are of industrial origin and promote corrosion and thus rust formation in iron and steel. The hot-dip galvanizing and organic coating reliably counteract this for decades through the duplex process. Thus, metal buildings, canopies and even Bicycle racks are preserved for a long time. However, one should steer clear of a common misconception: “The thicker the coating, the greater the protective effect.”
With duplex process against rust
Exposed to air and rain, untreated steel components would rust after a short time. Rust in general occurs when metals react with oxygen from the ambient air under the influence of water and form oxides and hydroxides on their surfaces. Salts dissolved in water accelerate this process of corrosion, as can be observed particularly well on ships or ferrous construction elements near the coast.
To prevent the corrosion of iron and steel (base materials), the duplex process is used by coating steel with a protective layer (passivation). The steel is coated with a thin alloy layer of zinc to protect the underlying metal.
Walk through fire
Hot-dip galvanizing is the process of introducing steel into a molten zinc bath. Depending on the suitability of the object to be galvanized, this is done either piece by piece (discontinuous hot-dip galvanizing) or continuously (continuous hot-dip galvanizing or Sendzimir galvanizing).
Piece galvanizing offers the advantage of complete corrosion protection, which also covers cut edges and hollow sections. In addition, the achievable zinc layer thickness here is double to triple.
But here, too, less is sometimes more: “The thicker the zinc coating, the better the corrosion protection”. Sounds logical, but this leads to spalling of the protective layers, so that the exact opposite is achieved: Instead of providing particularly strong protection, the corrosion process is unintentionally accelerated.
The DIN EN ISO 1461 standard applies to piece galvanizing. Suppliers who work according to this standard point to many years of practical experience, which proved a protection period of 50 and more years.
Galvanized metal surfaces form the so-called white rust under the influence of air and moisture, which can be stopped by further chemical passivation.
The painting of metal serves two purposes at once: Paint provides (additional) corrosion protection and allows components to appear in a completely new light through a variety of possible colorings.
Every painting process begins with thorough pre-cleaning of the material, since grease, oil and dirt particles hinder the paint application. This can be done either by pretreatment and cleaning agents or by using mechanical methods, such as grinding, dry ice cleaning, etc., with which production-related impurities can be removed well and in an environmentally friendly manner.
The cleaned metal parts are then primed and coated with a wet or powder paint. Very good abrasion resistance and corrosion resistance values can be achieved with highly developed wet coatings, so-called high-solid coatings. At the same time, the coating application is low-emission, which benefits the environment.
With a two-coat paint structure consisting of a two-component primer and a two-component high-solids topcoat, corrosion protection can be achieved in accordance with DIN EN ISO 12944-6 of corrosivity category C3 with a protection duration (class) medium. Corrosivity category C3 medium provides 5 to 15 years of corrosion protection in urban and industrial atmospheres with moderate sulfur dioxide exposure or in moderate coastal climates. If higher corrosion protection (> 15 years) is required, other coating structures must be selected.
The most important summarized
A complex topic with a lot of input. Finally, we’ll bring it back to the point:
For particularly long-lasting protection, hot-dip galvanizing is combined with paint systems. Whenever you talk about hot-dip galvanizing followed by a coating, it is called “duplex”. The protection times of the individual processes do not add up, and the corrosion protection achieved is even significantly higher.
Incidentally, the corrosion protection of the outer skin of our cars has been based on this basic principle since the mid-1980s.