Technical Info Sheet #1
When hot-dip galvanized steel is exposed to the atmosphere it results in the formation of ‘zinc patina’. This ‘patina’ gradually reduces the luster of the zinc surface and increases its life expectancy.
WHITE SPOTS ON THE ZINC!?
It happens with some regularity that the still “fresh” galvanized steel no longer looks shiny, but has a white haze and/or white spots. Often the customer will be startled by this appearance and make comments. The client has doubts about the durability or thinks the galvanizing was not done properly. Colloquially, this phenomenon is also known as “white rust” or “white rust.” This publication gives a brief explanation of the how-and-why and in what way it can be prevented or repaired.
WHY IS THIS HAPPENING?
After hot-dip galvanizing in the factory, the surface of the zinc layer soon wants to react with oxygen to form zinc oxides. Zinc is a relatively base metal and, like many other metals, will want to return to its lowest energy level; that of oxide. The protection against corrosion of the zinc layer occurs only after a few days through the formation of a so-called zinc patina layer by the action of CO2 from the atmosphere. The formation of the patina layer is clearly visible. The initially shiny zinc layer becomes slightly duller and slightly grayer. The formation of this important additional protective layer takes up to 12 months after galvanization and depends on the relative humidity of the air. The thickness of this patina layer is less than ¼ micrometer.
WHAT IS ZINC PATINA?
The patina consists of an insoluble, well-adhering and protective top layer. This layer is mainly composed of basic zinc carbonate. Stagnant water on the galvanized surface impedes the formation of this layer, causing whitish patinas to form very quickly, composed mainly of basic zinc oxide and basic zinc hydroxide. These powdered products do not show good adhesion. This phenomenon usually occurs when rainwater or condensation water remains present on the metal surface. This is the case, for example, when this water is trapped between stacked pieces or sheets during storage or transportation. These stains due to damp storage are mainly aesthetically annoying.
what does the international galvanizing standard say?
The International Galvanizing Standard EN ISO 1461 ‘Hot-dip galvanizing applied
coatings on iron and steel objects – Specifications and test methods’ states about these white storage stains that they should not be a reason for rejection, provided the coating thickness remains above the minimum value indicated. For special applications, it can be agreed in advance between the parties that white rust is not allowed (under the heading of aesthetic galvanizing). The English term for these white stains is Wet Storage Stain.
DO THESE SPOTS REMAIN VISIBLE?
When exposed to the atmosphere, the white spots will gradually disappear.
The protective film of alkaline zinc carbonate will then be able to form gradually. On the other hand, when the hot-dip galvanized steel is to be painted (duplex system), it is absolutely necessary to remove the white rust to allow proper adhesion of the paint layer.
HOW TO REMOVE WHITE RUST?
Should you want to improve the appearance of the material right away, there are a number of ways to remove stains caused by damp storage:
In the latter two cases, do not forget to rinse generously afterwards with clean tap water. This is especially true for materials that are not sprinkled after installation.
HOW TO PREVENT STAINS FROM DAMP STORAGE?
Proper storage and transportation prevents white rust in most cases. Hot-dip galvanized steel should be stored and transported in such a way that a film of water never remains on the steel. Also, a film of moisture should never become trapped between two metal surfaces. Here are some recommendations:
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 1
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 1: General design principles and corrosion resistance.
TECHNICAL DATA SHEET 10
Corrosion resistance of hot-dip galvanized steel
TECHNICAL DATA SHEET 25
Zinc patina: origins and protection
