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Galvanized Steel

Galvanized Steel

Galvanized Steel, Painted Galvanized Steel  and Failure Analysis

Galvanized steel is one of the most often specified materials for electric transmission assets such as lattice towers and poles, and especially for high-voltage transmission line structures and substation structures. This material has a long record of proven performance in moderately corrosive environments.

Galvanized steel structures are protected from corrosion attack due to both the barrier effect and the galvanic (sacrificial) action of zinc. The applied zinc coating typically does an excellent job protecting steel when located in moderately corrosive environments in which oxidizing conditions prevail. The quality of the galvanized product is most dependent on the practices of the galvanizing facility. There are wide variations in quality between galvanizers, and even for products from each galvanizer. The production of high-quality galvanized steel structures begins with the chemistry of the underlying steel as purchased by the galvanizer, as it determines the desired metallurgical reaction between steel and molten zinc. Next is the quality of the preparation of the steel, and the makeup and consistency of the zinc bath chemistry. Cooling rate is next. Often overlooked are the subsequent shipping, handling and storage, as these introduce conditions that may promote unfavorable surface reactions. The factors often associated with corrosion failure of a galvanized steel structure are improper thickness, excessive brittleness of the intermetallic alloy layer, general galvanizing failure, substrate surface preparation (especially if previously coated), storage conditions, installation damage, soil service conditions, or unsuitable coating selection for the soil or expected in-service atmospheric exposure conditions. Galvanized surface colors (different shades of gray) may be specified based on project site requirements and aesthetics. Chemical and electrochemical treatments may also be utilized to achieve specified project or asset coloring as may be required to achieve stakeholder acceptance. The following summarizes important parameters:

1) Barrier Protection

  • Isolates metal from the environment
  • Must adhere to the base metal
  • Must be resistant to abrasion

2) Cathodic Protection

  • Change electrochemistry of corrosion cell
  • Based on the electrochemical series
  • Insure base metal is the cathodic element

3) Hot-dipped galvanizing provides both kinds of protection

  • Strongly resistant to most oxidizing environments
  • Rate of Corrosion is significantly less than steel called “Patina”
  • Life of the Zinc coating depends on zinc thickness and corrosivity of the environment

4) Stability of Galvanized Steel

  • Oxygen, Water, Corrosive ions (Chloride)
  • Thickness
  • Corrosion Rate

Galvanized steel is one of the most often specified materials for the manufacturing of equipment, poles, lattice towers, enclosures and other T&D assets commonly utilized in the electric power utility industry.

Corrosion characteristics of galvanized steel

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.

Hot dip galvanizing has been an attractive and economical means of corrosion protection for construction, utility tubular and lattice structures. Galvanized steel components are protected from corrosion attack due to both barrier effect and also due to galvanic (sacrificial) action of zinc. Zinc does a fine job of protecting a steel components  in moderately corrosive and extremely  corrosive environments. It provides long term protection both above ground and underground portion of structures .

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.

 

 

We have inspected galvanized roofs, galvanized pipes, galvanized lattices in service which date back to early 20th century. Upon inspection we found out that galvanized layer is present even after 100 years of service. The key point in long service life is that the soil in that location provided the protective layer on galvanized surface. However, both  white rust and paint failures have been observed  on galvanized steel components in service after few years in service.

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