The overview on the previous page shows some of the many possibilities for the composition of the zinc patina layer. The basis is that zinc oxide (Zincit) changes under the influence of the atmosphere. Within a few days, a zinc patina layer forms in the form of hydrozincite. Then other factors come into play, such as whether the atmosphere is dominated by sulfates or by chlorides. After several months, other compounds form, depending on changing (climatic) conditions.
The rate at which these compounds form depends in part on whether the surface is exposed to rain or not. In rural areas, the zinc patina layer forms faster on rain-exposed surfaces and slower on surfaces that are covered. This is because rain washes corrosion-initiating compounds from the surface.
The following parameters are important in estimating the duration of protection of the zinc coating at a given location:

• Humidity (%)
• Rainfall (mm/year)
• Average temperature (°C)
• Chloride content (Cl)
• Sulfur dioxide content (SO₂).
• Irrigated surface/non-irrigated surface

Chloride levels often immediately think of salts brought from the North Sea by onshore winds. However, salts used for ice control also play an important role inland. Research has shown that up to 40% by weight of these salts can precipitate up to 100 meters from the road. However, these road salts can be more aggressive than those carried by air from the sea. In fact, the higher content of magnesium salts in seawater inhibits zinc corrosion, sometimes as much as 2.5 to 9 times slower than road salt.
Until the late 1980s, sulfur dioxide (SO₂) played a major role in zinc corrosion. Today, thanks to stricter environmental measures, the amount of SO₂ in the atmosphere has been greatly reduced. As a result, chloride content has now become the most important factor. Especially in coastal areas of Iran and South America, the highest zinc corrosion rates are measured, caused by chlorides. However, in the Benelux, with its temperate maritime climate, sea salts have little influence on the formation of the zinc paten layer.
Zinc carbonate plays a crucial role in slowing down zinc corrosion by forming a poorly soluble zinc paten layer. The CO₂ content in the atmosphere is responsible for this. Zinc is the only metal in which this carbon dioxide plays an important role.
Non-sprinklered surfaces sometimes behave just like sprinklered surfaces, especially when there is high humidity combined with adequate air exchange. For sprinklered surfaces, it is important that rainwater can drain easily. When puddles form in corners or cavities, for example, zinc corrosion proceeds differently due to the localized formation of a more permeable zinc patina.
In the case of “fresh” hot-dip galvanized steel that is placed indoors or exposed to direct emissions of combustion gases, zinc corrosion can occur more quickly than expected. For example, the indoor atmosphere may contain formaldehyde, a substance harmful to the formation of the desired zinc patina layer. Formaldehyde is released from building materials such as panels, flooring, furniture, paint and upholstery, and evaporates continuously. Under certain conditions, this can lead to acid formation on the zinc surface, such as formic acid or acetic acid.