In addition to its natural chemical constituents, rain contains a large number of other elements and compounds – many of which are of industrial origin – which promote corrosion and thus the rusting of iron and steel. Hot-dip galvanising and organic coating reliably counteract this for decades. Metal buildings, shelters and bike racks are thus preserved for a long service life. However, one should refrain from a widespread misconception: “The thicker the coating composition, the greater the protective effect.”
Duplex against rust
Untreated steel components rust quickly when exposed to air and rain. Rust generally occurs when metals react with the oxygen from the ambient air under the influence of water and form oxides and hydroxides on their surfaces. Salts dissolved in water accelerate this corrosion process, as can be observed particularly well on ships or ferrous components near the coast.
In order to prevent the corrosion of iron and steel (base materials), one uses the duplex method by covering steel with a protective layer (passivation). The steel is provided with a thin layer of zinc alloy to protect the underlying metal.
Going through fire
Hot dip galvanising occurs when steel is immersed into molten zinc. Depending on the suitability of the object to be galvanised, this is done either piecewise (batch galvanising) or on a continuous belt (continuous hot-dip galvanising or Sendzimir galvanising).
Galvanising offers the advantage of complete corrosion protection in which sharp edges and hollow profiles are also fully coated. In addition, the achievable zinc layer thickness here is twice to three times that of electroplating.
Yet, even here, sometimes less is more: “The thicker the zinc layer, the better the corrosion protection”. Sounds logical, but this leads to flaking of the protective layers, so that just the opposite is achieved: instead of particularly strong protection, one accelerates the process of corrosion unintentionally.
The DIN EN ISO 1461 standard applies to hot-dip galvanising. Suppliers who work in accordance with this standard, evidenced by many years of practical experience, can demonstrate protection duration of 50 years or more.
Under the influence of air and moisture, galvanised metal surfaces form so-called white rust, which can be stopped with further chemical passivation.
Two different purposes are pursued when coating metal: paint provides (additional) corrosion protection and lets components appear in a completely new light via a multitude of possible colours.
Every painting process begins with thorough pre-cleaning of the material since greases, oils and dirt particles hinder the paint application. This can be done either with pretreatment and cleaning agents or deploying mechanical methods such as grinding, dry ice cleaning, etc., with which the production-related impurities can be effectively and environmentally compatibly eliminated.
The cleaned metal parts are then primed and provided with a wet or powder paint. Very good values with regard to abrasion resistance and corrosion resistance can be achieved using highly developed wet paints, so-called high-solid paints. At the same time, the paint application is low-emission, which benefits the environment.
Deploying a two-coat paint system consisting of a 2K primer and a two-component high-solid topcoat, corrosion protection based on DIN EN ISO 12944-6 of corrosivity category C3 with a medium protection duration (class) can be achieved. Corrosivity category C3 medium provides 5 to 15 years of corrosion protection in urban and industrial atmospheres with moderate sulfur dioxide exposure or in temperate coastal climates. If a higher corrosion protection (> 15 years) is required, other coating structures must be selected.
The most important things in summary
It is a complex topic with a lot of input. Summing it up in a nutshell; for particularly long-lasting protection, hot dip galvanising is combined with paint systems. Whenever one speaks of hot dip galvanising and a subsequent coating, this is called “duplex”. The protection durations of the individual methods are not just added together, the achieved corrosion protection is even much more long lasting.
Incidentally, the corrosion protection of the outer skin of our cars has been based on this basic principle since the mid-1980s.