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. When it comes to China hot dipped galvanized pipe, where freshly galvanised steel is exposed to pure water (rain, dew or condensation), in an oxygen deficient environment, the water will continue to react with the zinc and progressively consume the coating. The most common condition in which white rust occurs is with galvanised products that are nested together, tightly packed, or when water can penetrate between the items and remain for extended periods.
Generally, pure water (H2O) contains no dissolved salts or minerals and zinc will react quickly with pure water to form zinc hydroxide, a bulky white and relatively unstable oxide of zinc. Steel conduit consists of a thin coating of zinc fused to a steel substrate. This combination provides material that has the mechanical properties of steel enhanced with the corrosion resistance of zinc. However, once the galvanised surface has been attacked and the zinc hydroxide compounds have formed, it is desirable to remove the oxide products from the surface because: 1) their presence inhibits the formation of stable carbonate based oxides; 2) they are unsightly.
With regard to cold rolled steel pipe, re-passivating the galvanised surface is another very important treatment for steel pipe in service. Specifically, 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 China steel tube manufacturer in China, we would like to provide a number of simple steps that can greatly reduce or eliminate the formation of white rust as follows for your inference:
• Keep the packed work dry
• Pack the items to permit air circulation between the surfaces
• Stack the packed items to allow water to drain out
• Treat the surface with proprietary water repellent or barrier coatings to prevent moisture contact with galvanised surface
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Post time: Aug-20-2018