In its From the KETTLE series of articles, the Corrosion Institute of South Africa (CorrISA) highlights hot-dip galvanising. 

CorrISA highlights key aspects of a hot-dip galvanised coating by ensuring that customers or specifiers have a greater understanding of the coating so that it is not necessarily rejected or accepted for the wrong reasons.  

The following case studies illustrate the issues of wet storage staining and white rust respectively.  

Issue: Wet storage stain 

Wet Storage Stain and White Rust

Wet storage stain presents as a white voluminous deposit of zinc hydroxide. This typically occurs as a result of ‘nested’ or confined storage or high moisture levels around the stored articles. As such hot-dip galvanised articles may have moisture trapped between the closely packed items. 

Consumption of the protective zinc coating will continue as long as such conditions remain in place. 

Remedy 

Wet storage stain ceases when the cause is eliminated. If the coating thickness at the affected area is equal to or greater than the minimum required in the specification, it is not a cause for rejection, other than for aesthetic reasons. The latter is subject to discussion with the end user. 

To prevent staining, customers should exercise caution during the transport and storage of hot-dip galvanised components. 

Issue: White rust 

White rust is a white surface stain on a freshly hot-dip galvanised surface that is subjected to moist or wet and/or humid conditions.  

Similar to wet storage stain, white rust is also zinc-hydroxide, but it is not as voluminous. Rather, it is more of a superficial surface stain that is formed on freshly galvanised surfaces in the presence of humid/ moist conditions.  

Usually all items that are hot-dip galvanised are quenched in water containing a passivation solution to prevent excessive white rust forming over the short term. Freshly galvanised coatings reacting with the environment will show evidence of white rust until such time, as a stable zinc-carbonate film is formed on the coating surface.  

Remedy 

The occurrence of white rust ceases when the cause is eliminated. Superficial white rust will very rarely affect the lifespan of a general hot-dip galvanised coating. For aesthetic purposes, it can be excluded by prior arrangement with the galvaniser. Similarly, it can be appropriately removed.  

Where components are exposed to continuous rain for a number of days, most of the passivation would have been removed and white rust may develop. Customers are urged to exercise caution during transport and storage.  

CorrISA advises on the rejection or acceptance of wet storage staining and white rust of hot-dip galvanised coatings on site. Further information is also available at https://www.hdgasa.org.za 

Full acknowledgment and thanks go to https://www.corrosioninstitute.org.za/  and www.hdgasa.org.za for the information in this article. Originally published in Corrosion Exclusively Vol 8 Issue 1 2023.