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What Is White Rust? 

White rust is a white, chalky substance that can form on the surface of zinc materials, like galvanized steel. White rust can form when zinc is exposed to hydrogen and oxygen. This combination creates a zinc hydroxide, as opposed to iron oxide which is a common form of rust. While it can form on any zinc material or zinc-coated material, white rust is a frequent problem for galvanized steel.

How Does White Rust Form?

White rust forms when zinc is in an environment with hydrogen and oxygen. Most frequently, it occurs when a freshly galvanized material is place into contact with water or hydrogen dioxide. White rust is especially likely to form on newly galvanized material. This is because the new zinc coating has not yet had the chance to form the stable oxides, and the hydrogen and the oxygen bond with the pure zinc which forms zinc hydroxide. White rust can be common on a galvanized sheet that is being stored, as condensation can become trapped between the individual sheets, leading to the formation of white rust.

Conditions Determining White Rust Formation

There are several key factors that determine the speed of the white rust formation process:

• Storage method
• Storage place
• Air circulation
• Climate

Storage Method

The white rust problem is often exacerbated when packs of tube and pipe are stored flat, one on top of the other or “nested”.

Storage Place

Storing the steel outdoors, exposed to moisture and the other elements will always increase the likelihood of white rust formation.

Air Circulation

If the packs of steel are strapped together, this restricts airflow around the product, reducing the chance for moisture to evaporate and increasing the chances of corrosion.

Climate

In areas of high rainfall, like tropical and many sub-tropical areas, moisture can get trapped between individual surfaces and wet storage stains can result.
Also, when zinc coated steel is expose to the atmosphere with alternate wet and dry conditions, its surface will dull with time, as the “normal” corrosion mechanism of zinc produces a zinc carbonate layer.

Care and Storage Recommendations

To maintain the aesthetic appearance of zinc coated steel, barrier protection coatings such as paint or powder coating are required. This will help prevent the “normal” corrosion that can affect the product.

In terms of white rust specifically, end users need to take precautions when receiving the product and then when storing it, addressing each of the conditions that cause it, as outlined above.

Firstly, when it is delivered make sure any zinc-coated product is in adry condition and free of white rust. Deliveries should be rejecte if an inspection reveals signs of wet storage stain or if packs are wet or moist with condensation.

When making arrangements for the storage of your product consider the following factors:

  • Packs of zinc coated steel should be store undercover promptly, to ensure that they are not exposed to unnecessary moisture.
  • They should be store away from open doors, windows, or louvers to avoid exposure to moisture-laden air, salt contamination, and condensation.
  • It is important to maintain cleanliness and control the pH, corrosion, and bactericide inhibition levels in water-based cutting fluid systems used to cut packs of zinc coated steel tubular. Residual fluids and swarf left inside the pack will result in accelerated corrosion.

If the packs are kept in a dry place then the zinc corrosion rate will be very slow; the best location is inside and away from the elements completely. If the product becomes wet, white rust may result from the reaction of zinc, moisture, and oxygen.

If products are expose to water, the easiest way to avoid corrosion is to ensure that they can dry out. Wet packs must therefore be separate as soon as possible into individual lengths, to allow all surfaces to dry out thoroughly.

If there is no alternative but to store packs outside, with minimal protection from the elements, then it is essential that the pack be broken open and individual lengths stacked with plastic dividers between each layer, as shown in the diagram below:

White Rust method for storage

The corrosion rate of zinc coated steel stored outside, as per the above diagram, will be marginally higher than if the tube was stored inside, but not as fast as if the product is stored in pack form and stacked in a pile outside.

Bear the following additional recommendations in mind when deciding how to store your products:

  • Product stored outside should be provide with cover for protection from the rain; a low-cost under-roof shelter (i.e. roof and no walls) offers increased protection from moisture and increases the longevity of the zinc coating.
  • Use of tarpaulin is not advised as this will trap moisture and condensation and exacerbate the formation of the white storage stains.
  • Surfaces can be treate with proprietary water repellent or barrier coatings which prevent contact with moisture on galvanized surfaces.

It is easier to prevent white rust than to cure it!

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