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Galvanized steel and white rust

• What is white rust?
• Why does zinc protect steel?
• Why is zinc susceptible to white rust?
• Re-passivating the galvanized surface
• Conclusion

What is white rust?

White rust is the surface corrosion that occurs on galvanize when the metal sheets get wet while tightly bundled (in coils, or in lifts of blanked sheets/roll-formed panels), and then are not allowed to dry. The continual wetness prevents the formation of a protective passive film on the zinc surface. The result of this prevention is a stained, discolored sheet that is virtually impossible to return to its original shiny metallic appearance.

The surface of galvanized coatings is almost 100% zinc. It is the durability of the zinc that provides the outstanding anti-corrosion performance for steel, yet zinc is a relatively ‘reactive’ metal. It is the stable oxides that form on the zinc's surface that determine its durability, and these oxides are formed progressively as the zinc is exposed to the atmosphere, air. Carbon dioxide in particular is a contributor to the formation of these stable oxides. Galvanized sheet must be protected from contact with moisture whenever the sheets are in close contact (coiled or bundled) and free airflow is not available to dry the surface to protect from white rust.

Why does zinc protect steel?

Zinc, by its very nature, is a “reactive” metal and tends to corrode quite readily when exposed to moisture. Why then does it protect steel when a zinc-coated sheet is exposed to rainfall?

When zinc corrodes in the presence of air and moisture it undergoes a series of chemical reactions, changing from metallic zinc on the surface to other chemical compounds. In air, newly exposed zinc reacts with oxygen to form a very thin oxide layer. In the presence of moisture the zinc oxide reacts with the water, resulting in the formation of zinc hydroxide.

Over time, and under the influence of cyclic weathering, the final corrosion product is zinc carbonate (formed by the reaction between zinc hydroxide and carbon dioxide in the air). Zinc carbonate is a thin, tenacious, compact, and stable (insoluble in water) film. When the surface is further exposed to rain or condensation, the protective film serves as a barrier between the moisture and the zinc underneath. This type of chemical layer is called a passive film. It is the presence of this passive film that slows down the reactivity of the zinc, thereby significantly reducing the corrosion rate of the zinc coating.

Typically, when low carbon steel corrodes, the corrosion products (iron oxide and/or iron hydroxide) do not form a continuous, protective, passive film. Instead, they tend to spall or develop cracks, which allows moisture and air continued access to the iron, continuing the corrosion reaction. The difference in oxide film-forming behavior between iron and zinc is the main reason that galvanizing extends the life of steel.

In most applications, the passive surface film that forms on zinc, while tenacious, is not totally protective, and continued corrosion does occur over time. However, because of the nature of the passive film, the corrosion rate is diminished substantially.

Why is zinc susceptible to white rust?

Zinc is very reactive metal as we have stated. It exhibits a low corrosion rate only because a continuous passive film forms on the surface. A key part of the corrosion mechanism is that the surface needs to dry in air in order to develop and maintain the passive layer. It is during the drying part of a rain cycle that the zinc carbonate passive film develops.

When galvanized sheet gets wet while still in coil form, or stacked in bundles at a roll-forming plant or jobsite, white rust can result. White rust is simply the chemical compound, zinc hydroxide, which forms when zinc is in contact with moisture. It does not convert to a zinc carbonate passive film because the tightly packed sheets are not freely exposed to carbon dioxide-containing air. The protective zinc carbonate film never gets a chance to form. Since the corrosion reaction continues to proceed as long as the surfaces are wet and starved for carbon dioxide, a large accumulation of zinc hydroxide can form. Zinc is a very reactive metal in the presence of moisture when conditions do not allow the passive film to form.

When white rust does occur, there is an actual loss of zinc coating, and some of the zinc that is intended to protect the coated steel product while in service is lost.

The extent of the damage is primarily dependent on:

• the exposure time to moisture,
• the temperature that is experienced during storage, and
• the presence of accelerating corrosive agents, such as chloride-containing salts.

Often, the amount of white rust appears to be quite heavy when, in fact, the amount of zinc corroded is small. This occurs because zinc hydroxide is somewhat flocculent, and builds up in the area of the wetness. If the application is not aesthetically critical, the galvanized coating should perform very well and meet the requirements and expectations of the end user. In most instances involving outdoor exposure, the white rust will disappear over time, as it is either washed off by rainfall or is converted to zinc oxide and then zinc carbonate.

The surface of the zinc coating in the area that experienced white rust is “etched” and no longer has the bright, reflective appearance of “as-produced” galvanized sheet. Removing the white rust will not eliminate the etched appearance. This is why, for applications where appearance is critical, galvanized sheet with white rust may not be acceptable.

There are times, albeit seldom, when the sheets have been wet for a long time; long enough that the amount of corrosion of the zinc coating can be severe. In these cases, the product may no longer provide the corrosion resistance desired for the application. In these cases the white rust may take on a dark grey or black appearance. When the stain on galvanize turns black it usually means that iron has become part of the corrosion product, i.e. enough zinc has been consumed to expose the steel substrate. Nevertheless, it takes a trained observer to determine whether or not the amount of corrosion that has occurred is severe or not.

The prevention of white rust is strongly influenced by the methods and practices used for shipment from the steel manufacturer to the customer. It is vital to prevent water intrusion and to use practices that minimize the tendency for condensation during transit and storage. It is especially important to maintain controlled temperature storage (sometimes even during transit), to prevent condensation that can occur if the temperature of drops and passes below the dew point.

Re-passivating the galvanized surface

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% sulfuric acid, brushing with a stiff wire brush for 30 seconds before thorough rinsing of the surface.

Conclusion

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

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