How to prevent galvanized steel pipe in use from white rust

Galvanized steel pipe generally has the rational cost effective in the current steel pipe market. White rust is a post-galvanizing phenomenon. Responsibility for its prevention lies in the manner it is packed, handled and stored prior to the galvanised product’s installation and use. The presence of white rust is not a reflection on the galvanised coating's performance, but rather the responsibility of all those involved in the supply chain to ensure that the causes of white rust are recognised and the risks of its occurrence minimized on newly galvanised steel.

Hot dipped galvanized steel pipe has the good corrosion properties of zinc, due to a thin film of corrosion product that forms on the surface of the metal. This tightly-adherent corrosion film is called a passivation layer. The formation of the passivation layer is essential to achieving longlife with a galvanized product. Zinc will naturally form a passivation layer when exposed to weathering over an extended period of time. Rain combined with air dryingwill, over the course of a year or more form a passivation layer on all areas of the zinc which do not come in contact with recirculating water. Thus, the exterior of evaporative equipment and all interior areas above the drift eliminators will naturally develop a passivation film. Furthermore, where white rusting has occurred and the item may be subject to continuing exposure that may propagate similar corrosion, re-passivating of the surface can be done by treating the surface with a solution of 5% sodium dichromate 0.1% sulphuric acid, brushing with a stiff wire brush for 30 seconds before thorough rinsing of the surface.

As a professional steel pipe manufacturer in China, we know that white rust may sometimes be interchanged with the term wet storage staining since they have a similar corrosion mechanism. Wet storage staining is typically a pre-construction problem where new galvanized steel sheet or parts are exposed to a wet or moist environment because of improper storage. Post-construction white rust is a problem where the fresh galvanized surface is not able to form a protective, non-porous basic zinc oxide and typically the surface is partially wetted or completely submerged in water. In both cases, the deterioration begins when a localized corrosion cell is formed. The activity of such a corrosion cell/pit, results in rapid penetration through the zinc coating to the steel.

Today, galvanized steel pipe is one common type of structural steel pipe widely used in construction. With newly galvanised steelwork, the zinc's surface has been subjected to little oxidation and is at its most vulnerable. For this reason, a chromate passivation should be used in conjunction galvanizing operations to provide protection to the galvanised coating during the ‘youth’ period of the coating. This passivation coating provides short term protection to the zinc to give the stable oxides time to form on the surface.