Wet Storage Stain and White Rust – Are They the Same Thing?

28 March 18

A common challenge that manufacturers and product designers have to face with regards to transportation and storage of galvanized steel sheets is white rust.

White rust is basically a surface condition that occurs in newly produced batches of galvanized steel. It is a result of the reaction between zinc (the metal is found as a coating on galvanized steel) and atmospheric moisture, where the latter oxidizes the former to produce zinc oxide and zinc hydroxide.

Both zinc oxide and zinc hydroxide are unstable chemical compounds that can penetrate deep inside galvanized steel sheet and cause it to corrode. Since the two compounds have a white-greyish appearance, the surface condition is referred to as white rust.

Normally, the zinc patina layer present on the surface of galvanized steel should be able to combat white rust. But since newly produced batches of galvanized steel sheets don’t have the patina layer (it typically takes 6-12 months for the patina layer to fully develop on a galvanized steel sheet), they are not protected against the action of white rust.

White rust can develop during transport or storage of galvanized steel sheets. Truck loaded stock coming in contact with spilled water or sheets being tightly stacked onto one another and stored in a humid environment, are some of the common scenarios that provide perfect conditions for white rust to develop.

Where the cause of white rust is improper execution of recommended storage protocols, the condition is referred to as wet storage stain.

Wet storage stain is the more insidious form of white rust, caused when two sheets of galvanized steel are tightly stacked against one another, with a water drop trapped between the two.

The zinc surface near to the middle of water drop gets a lower supply of oxygen, while the surface in contact with the edges of the water drop gets exposed to higher oxygen concentration environment. This setup is representative of a typical galvanic cell – the central area serves as an anode and the edge area as a cathode. It creates an electrolytic potential, thus causing zinc to corrode.

Quite often manufacturers fail to identify the issue of poor storage and the condition is allowed to exist. This prompts white rust (wet storage stain in this case) to seriously damage the galvanized steel sheet.

Manufacturers are therefore advised to store their stock of galvanized steel sheets in dry area and to leave enough room between the stacked sheets for air to flow freely. Regular visits to the storage site and periodic monitoring of the storage conditions is also recommended.

Protect your investment; protect your galvanized steel sheets from white rust and wet storage stain.

For further reading: The Benefits of Working with Prepainted Galvanized Steel Coil

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