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White rust

• Introduction
• White rust formation
• Avoiding white rust formation
• Treating galvanized surfaces affected by white rust
  • Light white rusting
  • Moderate white rusting
  • Severe white rusting
• Re-passivating the galvanized surface
• Conclusion

Introduction

One of the commonly encountered problems with galvanized coatings of all kinds is ‘white rust’ or ‘white storage stain’. It is manifested as a bulky, white, powdery deposit that forms rapidly on the surface of the galvanized coating under certain specific conditions.

White rust can cause considerable damage to the coating and is always detrimental to the galvanized coating's 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. Carbon dioxide in particular is a contributor to the formation of these stable oxides.

With newly galvanized steelwork, the zinc's surface has been subjected to little oxidation and is at its most vulnerable. For this reason, Industrial Galvanizers uses a chromate passivation in conjunction with its galvanizing operations to provide protection to the galvanized 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.

White rust formation

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. Where freshly galvanized 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 galvanized products that are nested together, tightly packed, or when water can penetrate between the items and remain for extended periods.

Avoiding white rust formation

There are a number of simple steps that can greatly reduce or eliminate the formation of white rust.
These are:

1. Keep the packed work dry
2. Pack the items to permit air circulation between the surfaces
3. Stack the packed items to allow water to drain out
4. Treat the surface with proprietary water repellent or barrier coatings to prevent moisture contact with galvanized surface.

Treating galvanized surfaces affected by white rust

Once the galvanized 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 and
2. They are unsightly.

The effect on the galvanized coating can range from very minor to extremely severe and various levels of remedial treatment are available to deal with white rust problems at the various levels at which they are likely to occur.

The following treatments are recommended to deal with white rust on galvanized products:

Light white rusting such as this may appear on newly galvanized work after periods of heavy rain. Because these trailer parts are well ventilated and well drained, the white rusting will be limited and is easily removed.

Light white rusting

This is characterized by the formation of a light film of white powdery residue and frequently occurs on galvanized products during periods of heavy rain. It is particularly evident on areas that have been buffed or filed during quality assurance operations. These treatments remove the passivated surface from the galvanizing and expose unoxidised zinc to attack from rainwater. Provided the items are well ventilated and well drained, white rust rarely progresses past this superficial stage. It can be brushed off if required but will generally wash off in service with normal weather. No remedial treatment is generally required for this level.

Moderate white rusting

This is characterized by a noticeable darkening and apparent etching of the galvanized coating under the affected area, with the white rust formation appearing bulky. The galvanized coating thickness should be checked to determine the extent of attack on the coating. In the majority of cases, less than 5% of the galvanized coating will have been removed and thus no remedial work should be required as long as the appearance of the affected area is not detrimental to the use of the product and the zinc hydroxide residues are removed by wire brushing. If appearance is unacceptable, the white rust affected area can be treated as follows:

1. Wire brush the affected area to remove all white corrosion products
2. Using a cloth pad wet with aluminum paint, rub the surface with the pad to apply a thin film of aluminum paint to the affected area to blend it with the adjacent unaffected galvanized surfaces.

Severe white rusting

This is characterized by very heavy oxide deposits. Items may be stuck together. Areas under the oxidized area may be almost black of show signs of red rust. A coating thickness check will determine the extent to which the galvanized coating has been damaged. Remedial treatment to reinstate the coating should be undertaken as follows:

1. Wire brush or buff the affected area to remove all oxidation products and rust if any.
2. Apply one or two coats of approved epoxy zinc-rich paint to achieve required dry film thickness of 100 microns minimum.

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.

Treatment of Galvanized Steel Damaged by White Rust

Galvanized sheet damaged by white rust generally cannot be restored to its original high luster appearance. The stain, depending on severity, irreversibly alters the surface characteristics of the zinc to varying degrees. Nevertheless, there are treatments that are helpful in improving the appearance, depending on the severity of the stain.

1. For less severe initial white rust, rub/brush the surface with a mixture of mineral oil and sawdust. The mild abrasive action may remove the stain, although this treatment is not of much help for advanced wet storage stain.
2. Another method for mild white rust is to use a household cleanser, like 409, and brush the surface then rinse with water.
3. If the stain is not too severe, it may be removed by washing with a 10% (by volume) acetic acid solution, followed immediately by a thorough rinsing with water to neutralize the surface. The removal can be assisted by the use of a stiff bristle brush. The sheets must be dry before restacking. This treatment may remove some of the metallic luster, even in non-stained areas.
4. For more severe staining, a solution of 5% (by volume) of phosphoric acid in water, with a wetting agent added, can be brushed onto the sheets. The sheets must be immediately well rinsed to neutralize the surface and then thoroughly dried. This treatment will remove some of the metallic luster, even in non-stained areas.
5. If the stain has progressed to dark grey or black in color, removal is probably not possible.
6. One method of restoring the protective value of the zinc coating, and improving the appearance of white rust damaged sheets, is to apply a good, color matched, zinc-rich paint. The surface must be thoroughly brushed, rinsed and dried beforehand. After two years or so, weathering will largely remove any difference in appearance between the zinc-rich paint and the galvanized surface.

Galvanizing - White Rust Prevention and Cure

Topics Covered

Background
White Rust Formation
Avoiding White Rust Formation
Treating Galvanized Surfaces Affected by White Rust
Light White Rusting
Moderate White Rusting
Severe White Rusting
Re-Passivating the Galvanized Surface
Conclusion

Background

One of the commonly encountered problems with galvanized coatings of all kinds is ‘white rust’ or ‘white storage stain’. It is manifested as a bulky, white, powdery deposit that forms rapidly on the surface of the galvanized coating under certain specific conditions.

White rust can cause considerable damage to the coating and is always detrimental to the galvanized coating's 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. Carbon dioxide in particular is a contributor to the formation of these stable oxides.

With newly galvanized 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 galvanized 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.

White Rust Formation

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. Where freshly galvanized 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 galvanized products that are nested together, tightly packed, or when water can penetrate between the items and remain for extended periods.

Avoiding White Rust Formation

There are a number of simple steps that can greatly reduce or eliminate the formation of white rust. These are:

• 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 galvanized surface

Treating Galvanized Surfaces Affected by White Rust

Once the galvanized surface has been attacked and the zinc hydroxide compounds have formed, it is desirable to remove the oxide products from the surface because:

• Their presence inhibits the formation of stable carbonate based oxides and
• They are unsightly

The effect on the galvanized coating can range from very minor to extremely severe and various levels of remedial treatment are available to deal with white rust problems at the various levels at which they are likely to occur.

The following treatments are recommended to deal with white rust on galvanized products:

Light White Rusting

This is characterized by the formation of a light film of white powdery residue and frequently occurs on galvanized products during periods of heavy rain. It is particularly evident on areas that have been buffed or filed during quality assurance operations. These treatments remove the passivated surface from the galvanizing and expose unoxidised zinc to attack from rainwater. Provided the items are well ventilated and well drained, white rust rarely progresses past this superficial stage. It can be brushed off if required but will generally wash off in service with normal weather. No remedial treatment is generally required for this level.

Moderate White Rusting

This is characterized by a noticeable darkening and apparent etching of the galvanized coating under the affected area, with the white rust formation appearing bulky. The galvanized coating thickness should be checked to determine the extent of attack on the coating. In the majority of cases, less than 5% of the galvanized coating will have been removed and thus no remedial work should be required as long as the appearance of the affected area is not detrimental to the use of the product and the zinc hydroxide residues are removed by wire brushing. If appearance is unacceptable, the white rust affected area can be treated as follows:

• Wire brush the affected area to remove all white corrosion products
• Using a cloth pad wet with aluminum paint, rub the surface with the pad to apply a thin film of aluminum paint to the affected area to blend it with the adjacent unaffected galvanized surfaces.

Severe White Rusting

This is characterized by very heavy oxide deposits. Items may be stuck together. Areas under the oxidized area may be almost black and show signs of red rust. A coating thickness check will determine the extent to which the galvanized coating has been damaged. Remedial treatment to reinstate the coating should be undertaken as follows:

• Wire brush or buff the affected area to remove all oxidation products and rust if any.
• Apply one or two coats of approved epoxy zinc-rich paint to achieve required dry film thickness of 100 microns minimum.

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% sulphuric 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.

Source: Industrial Galvanizers Corp. For more information on this source please visit Industrial Galvanizers Corp

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