The following summarizes important parameters:

Zinc is a highly reactive metal. It exhibits a low corrosion rate only if a continuous passive film forms on the surface. A key requirement of corrosion control with galvanized steel is that the surface needs to remain in a soil environment that does not reduce or damage the protective surface film.

The following corrosion rates are experienced in different environments (Christofer Laygraf) :

1. Rural 2-3 micrometer/year
2. Urban 2-16  micrometer/year
3. Industrial 2-16  micrometer/year
4. Marine 5-8 micrometer/year

Galvanized steel corrosion products are typically white at the beginning, but under certain conditions may also take the form of a gray or black deposit on the metal surface. Accelerated corrosion of galvanized steel structures with resultant white rust and storage staining can occur when galvanized surfaces are held for extended periods in wet conditions immediately after the galvanizing process without passivation treatments. Corrosive compounds such as chlorides from marine and sulfur containing and acid producing atmospheres accelerate the formation of rust on the galvanized surface.  The following graph gives life expectancy as function of galvanized thickness.

However, localized conditions dominate and can present an environment that promotes accelerated corrosion beyond what the graph presents. This risk potential must always be considered for specific locations that may impact a enclosure’s corrosion behavior depending upon the presence or absence of acid rain, wind direction, time of wetness  and intensity of salt spray.

Galvanized steel enclosures are exposed to soil and grounding can also accelerate corrosion activity depending on soil chemistry, soil resistivity and the nature and surface area of the grounding materials. Structures next to and at substations have been observed to experience accelerated corrosion in low soil resistivity or otherwise corrosive environments and therefore should be considered for corrosion mitigation.

ASTM G90 should be considered for applications that require forming after galvanizing. Higher thicknesses may result in cracking of galvanized layer. It should be noted that in marine environments and where there is excessive chlorides galvanized steel without additional corrosion protection may not be the best choice due to chloride deposition, mechanical damage and presence of moisture. See the following photograph exhibiting accelerated corrosion after 10 years in service

Galvanized steel corrosion products are typically white at the beginning, but under certain conditions may also take the form of a gray or black deposit on the metal surface. Accelerated corrosion of galvanized steel structures with resultant white rust and storage staining can occur when galvanized surfaces are held for extended periods in wet conditions immediately after the galvanizing process without passivation treatments. Corrosive compounds such as chlorides from marine and sulfur containing and acid producing atmospheres accelerate the formation of rust on the galvanized surface.  The following graph gives life expectancy as function of galvanized thickness.

However, localized conditions dominate and can present an environment that promotes accelerated corrosion beyond what the graph presents. This risk potential must always be considered for specific locations that may impact a enclosure’s corrosion behavior depending upon the presence or absence of acid rain, wind direction, time of wetness  and intensity of salt spray.

Typically and for best performance the steel should conform to the mechanical and chemical properties listed in American Society for Testing and Materials (ASTM) specification A572. We also recommend the maximum silicon content for steel substrate be 0.06 % to ensure an adequate free zinc and uniform galvanized finish. The mechanical strength requirements for structural performance, such as tensile strength, (assuming the inherent material strength remains constant), is then dependent upon the material cross sectional area.  If inadequate, tensile failures could occur at locations where corrosion has produced localized reductions in cross sectional areas and created stress raisers. Higher tensile strength steels have less ductility and toughness.  These steels are considered notch sensitive. Normal constructional steels would not typically be notch sensitive but high strength low alloy (HSLA) steels can be notch sensitive. Corrosion pitting can create the notch which then becomes the location of crack initiation. Pitting or reduced thickness areas due to corrosion can also initiate mechanical fatigue cracks.

Recommended Practice for Storage of Galvanized Steel to Prevent White Rust.

When making arrangements for storage consider the following factors:

• Packs of galvanized (zinc) coated steel should be stored undercover promptly, to ensure that they are not exposed to unnecessary moisture. Passivation should be verified. Vapor phase corrosion inhibitors (VCI) may be used to mitigate corrosion during long storage times.
• The items should be stored away from open doors, windows or open louvres to avoid exposure to moisture-laden air, salt contamination and condensation.
• It is important to maintain cleanliness and control condensation. Residual cutting fluids and cutting debris left on surfaces can result in accelerated corrosion.
• If the materials are kept in a dry place, then the zinc corrosion rate will be very low; the best location is inside and away from the elements completely. If the products become wet, white rust may result from the reaction of zinc with moisture and oxygen.

If galvanized members are exposed to water, the easiest way to avoid corrosion is to ensure that they can dry out. Wet packs must therefore be separated 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 corrosive 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.

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

• Product stored outside should be provided 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 a tarpaulin is not advised as this will trap moisture and condensation and exacerbate the formation of the white storage stains.

How To Repair White Rust

After white rust has been identified and documented the following treatments are recommended.

White rust, depending on its extent, volume, and porosity can be classified as light, medium or heavy. For medium-heavy white rust, the procedures outlined in ASTM A780should be used to repair the white rusted area. It is important to document and take photos with sufficient detail prior to and after repair procedures.

• Light white rusting– This is characterized by the formation of a light film of adherent stain residue, It is particularly evident on areas that have been buffed or filed during quality assurance operations or in mild corrosive storage conditions. It can be brushed off if required but will generally wash off in service with normal weather. No remedial treatment is generally required at this level.
• Moderate White Rust– This is characterized by a noticeable moderate white rust and apparent etching of the galvanized coating under the affected area, with the white rust formation being apparent on top. Prior to and after cleaning, the galvanized coating thickness should be checked to determine the extent of attack on the coating. In the majority of cases, 2 to 5% of the galvanized coating will have been removed and thus no remedial work should
be required if the remaining galvanized layer meets the minimum thickness requirement, and also as long as the appearance of the affected area is not detrimental to the use and the zinc hydroxide residues are removed by stiff bristle
• Heavy White Rust– This is characterized by a very noticeable heavy bulky white rust (Figure 2), with the white rust formation being apparent on top and some times dark zinc surfaces under the white rust. Prior to and after cleaning, the galvanized coating thickness should be checked to determine the extent of attack on the coating.