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Corrosion

Selection of Zinc Coatings






After the decision is made to use a zinc coating for corrosion protection, some factors must be considered to ensure that the proper coating is selected for the intended application and service environment. Obviously, zinc coating processes that are limited to small parts such as fasteners or other small hardware, or operations limited to continuous lines in steel mills, such as continuous galvanizing and electrogalvanizing, cannot be considered for the protective coating of structural steel members.

Coating Thickness vs. Coating Weight

The usual criterion for determining the expected service life of zinc coatings is thickness: the thicker the coating, the longer the service life. This is an acceptable criterion when comparing zinc coatings produced by the same process.

When comparing zinc coatings produced by different processes, the thickness criterion cannot be used without considering the amount of available zinc per unit volume. It is also important to keep in mind various ASTM or other specifications as they relate to coating weight or thickness, and reduce the coating requirements to a common denominator prior to making a comparison of different zinc coatings.

It is also important to remember that for all continuous galvanized sheet materials, including electrogalvanized, the coating weight is given in weight per unit area of sheet. To obtain the amount of zinc per unit area of surface, the weight given must be divided by two, assuming equal distribution on both sides. For example, an ASTM A 653 Class G90 sheet contains 0.90 oz. zinc/ft2 of sheet or about 0.45 oz./ft2 on a surface. A G210 (2.10oz/ft2) sheet would have to be specified to obtain about 1 oz/ft2 on each side of the sheet.

While the coating densities for some of the different types of zinc coatings are nearly identical, others differ considerably. The coating densities, in terms of thickness required to equal 1 oz. of zinc per square foot of surface, are:



Hot-dip galvanizing (batch or continuous), electrogalvanizing, zinc plating 1.7 mils (43 µm)
Zinc Spraying (Metallizing) 1.9 mils (48 µm)
Mechanical plating 2.2 mils (55 µm)
Zinc-rich paint 3-6 mils (75 – 150 µm)

Each of these thicknesses, representing the same weight per unit area of zinc, would be expected to provide equivalent service life; i.e. 1.7 mils of hot-dip galvanized would give about the same service life as 2.2 mils of mechanical plating or 3-6 mils (depending on the paint formulation) of zinc-rich paint, assuming bond strength and edge protection are not factors.

The figure below lists the different types of zinc coatings and representative applications for each. While a coating is not limited to those uses listed, the applications represent the most common types of products coated by the process.

Zinc Coatings Method Process Specification Coating Thickness Applications
Electrogalvanizing Electrolysis ASTM A 879 Up to 0.28 mils1 Interior. Appliance panels, studs, acoustical ceiling members.
Zinc Plating Electrolysis ASTM B 633 0.2 to 1.0 mils2 Interior or Exterior. Fasteners and hardware items
Mechanical Plating Peening ASTM B 695 0.2 to 4.3 mils2 Interior or Exterior. Fasteners and hardware items.
Zinc Spraying (Metallizing) Hot Zinc Spray AWS C2.2 3.3 to 8.3 mils Interior or Exterior. Items that cannot be galvanized because of size or because on-site coating application is needed.
Continuous Sheet Galvanizing Hot-Dip ASTM A 653 Up to 4.0 mils1 Interior or Exterior. Roofing, gutters, culverts, automobile bodies.
Batch Hot-Dip Galvanizing Hot-Dip ASTM A 123
ASTM A 153
ASTM A 767
CSA G 164
A minimum of 1.4 to 3.9 mils3 Interior or Exterior. Nearly all shapes and sizes ranging from nails, nuts and bolts to large structural assemblies, including rebar.
Zinc Painting Spray
Roller
Brush
SSPC-PS Guide 12.00, 22.00
SSPC-PS Paint 20
SSPC-PS 12.01
0.6 to 5.0 mils/coat Interior or Exterior. Items that cannot be galvanized because of size or because on-site coating application is needed. Large structural assemblies. Aesthetic requirements.

1 Total for both sides of sheet
2 Range based on ASTM minimum thicknesses for all grades, classes, etc., encompassed by the specifications.
3 Range based on ASTM and CSA minimum thicknesses for all grades, classes, etc., encompassed by the specifications.

Economic Considerations

Selection from the wide range of coatings available for steel will normally depend on the suitability of the coating for the intended use and the economics of the protective system. Factors that affect the economics for a particular application include:

  • Initial cost of the coating
  • Coating life to first maintenance
  • Cost of maintenance
  • Hidden costs, such as accessibility of the site, production loss due to maintenance re-coating, and rising wages for labor-intensive coatings, such as metal spraying and painting

The choice of the most economical system should include both an initial and life-cycle cost analysis. The American Galvanizers Association has developed an online calculator, which taking data from a paint industry survey conducted by KTA Tator, Inc. and a galvanized industry survey conducted by AGA, will provide an initial and life-cycle cost comparison of hot-dip galvanizing to a number of paint systems.  


Related References:
1. About Zinc
2. About Hot-Dip Galvanizing
3. HDG Hot-Dip Galvanizing Last Time
4. Cost of Galvanized Steel
5. Selection of Zinc Coatings
6. Zinc Coatings-Galvanized|Electrogalvanized|Galvanneal|Galfan
7. Physical Properties of HDG Hot-Dip Galvanized
8. HDG Hot-Dip Galvanized Abrasion Resistance Resistance to Mechanical Damage
9. Hot-Dip Galvanized Corrosion Protection and the Zinc Patina
10. HDG Hot-Dip Galvanized High Temperature Exposure
11. HDG Hot-Dip Galvanized Surface Reflectivity
12. HDG Hot Dip Galvanized Coating Structure
13. HDG Hot Dip Galvanized Bond Strength
14. HDG Hot Dip Galvanized Coating Uniformity
15. HDG Hot Dip Galvanized Coating Thickness
16. Powder Coating Hot Dipped Galvanized Steel
17. Painting Hot-Dippped Galvanized Steel
18. Painting Hot-Dipped Galvanized Steel Surface Preparation
19. Surface Coatings for Corrosion
20. Hot-Dip Galvanizing Surface Preparation
21. Hot-Dip Galvanizing Galvanizing
22. Hot-Dip Galvanizing Inspection
23. Characteristics of Zinc
24. Hot-Dip Galvanizing Performance in Atmosphere
25. Hot-Dip Galvanizing in Atmosphere Time to First Maintenance
26. Hot-Dip Galvanizing Performance in Soil
27. Soil Corrosion Data for Corrugated Steel Pipe
28. Hot-Dip Galvanizing Performance in Water
29. Cause of Zinc Corrosion
30. Corrosion of Zinc Coated Steel in Selected Natural Fresh Water
31. Corrosion of Zinc and Zinc Coated Steel in Sea Water
32. Corrosion of Zinc Coating in Industrial and Domestic Water
33. Concrete Corrosion of Hot Dip Galvanizing
34. Concrete corrosion resistance of hot dip galvanized reinforcing
35. Removal of Forms Concrete Corrosion
36. Zinc Reaction in Concrete Corrosion
37. Concrete Corrosion References
38. Hot-Dip Galvanizing Performance in Chemical Solutions
39.Hot-Dip Galvanizing Performance in Contact with Other Metals
40. Hot-Dip Galvanizing Performance in contact with Treated Wood
41. Hot-Dip Galvanizing Performance in contact with Food
42. Hot-Dip Galvanizing Performance in Extreme Temperature

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