Zinc Coatings
Zinc coatings protect steel by forming a sacrificial, tightly adherent zinc patina that corrodes up to 10–100 times more slowly than steel. In infrastructure applications—such as bridges, power structures, and water facilities—zinc coatings extend service life by shielding steel from moisture and chlorides, reducing maintenance shutdowns. Compared to paint-only systems, some zinc coatings can have a higher initial cost but significantly lower life-cycle cost because they require less maintenance. Within zinc coating options, batch hot-dip galvanizing typically provides the thickest, most durable coating and longest service life, while other zinc processes, such as metallizing, mechanical plating, and zinc-rich paints, are best suited for specific component sizes, aesthetics, or field repairs.
Zinc, a natural, healthy, and abundant element was first used in construction in 79 AD. Zinc metal has a number of characteristics that make it a well-suited corrosion protective coating for iron and steel products. Zincs excellent corrosion resistance in most environments accounts for its successful use as a protective coating on a variety of products and in many exposure conditions.
This proven corrosion resistance is a result of zincs ability to form dense, adherent corrosion byproducts. While a fresh zinc surface is quite reactive when exposed to the atmosphere, zinc corrosion products develop rapidly on the surface as the coating is exposed to natural wet and dry cycles in the atmosphere. These corrosion products, collectively known as the zinc patina, act as an additional barrier between the steel and the environment.
A number of different methods of applying zinc coatings to steel are commercially available, each of which has unique characteristics. Often all of these coatings are incorrectly labeled galvanizing, but it is important to understand the differences. The products produced by each of these processes have different uses depending on their applicability, relative economics and expected service life. Batch hot-dip galvanizing immerses fabricated steel into a bath of molten zinc to create a thick, alloyed zinc-iron coating metallurgically bonded to the steel. Other zinc coatings—such as continuous sheet galvanizing, electroplating, metallizing, mechanical plating, and zinc-rich paint—provide thinner or purely surface-bonded layers that may be better suited for sheet products, small parts, or touch-up work, but generally do not match batch hot-dip galvanizing’s coating thickness and abrasion resistance on structural steel.
Selection of Zinc Coatings
Once the decision is made to use a zinc coating for corrosion protection, a few additional factors must be considered to ensure the proper coating is selected for the application and service environment. Each zinc coating covered in this section provides varying degrees of corrosion protection and it is important to identify the corrosiveness of the exposure environment to determine if the coating selected will provide adequate service life.
Some zinc coatings will be eliminated by the nature alone, (zinc coating processes limited to small parts or sheet steels cannot be considered for the protective coating of structural steel members); others may be ruled out based on cost, appearance, availability, etc.
| Method | Batch Hot-Dip Galvanizing | Continuous Sheet Galvanizing | Zinc Rich Paint | Thermal Spray Zinc Metallizing | Mechanical Plating | Electro-galvanizing | Zinc Plating |
|---|---|---|---|---|---|---|---|
| Application / Venue Conditions | In shop, factory controlled; no special requirement | In shop, factory controlled; no special requirement | In shop or field; conditions are subjective and prone to human error | In shop or field | In shop, factory controlled; no special requirement | In shop, factory controlled; no special requirement | In shop, factory controlled; no special requirement |
| Specification | ASTM A123, A153, A767, CSA G164, ISO 1461 | ASTM A653 (i.e. G60, G90 coating grade designations) | SSPC-PS Guide 12.00, 22.00; SSPC-PS Paint 20; SSPC-PS 12.01 | AWS C2.2 | ASTM B695 | ASTM A879 | ASTM B633 |
| Coating Thickness Minimum / Typical | 1.4-3.9 milsa / 2-8 mils | 0-3.2 milsb / 0-3.2 mils | 0.6-5.0 mils per coat / 4.0-6.0 mils per coat | 3.3 mils / 4.0-6.0 mils | 0.2-4.2c mils / 0.2-4.2 mils | 0.28 milsb / 0-0.28 mils | 0.2-1.0 milsc / 0.2-1.0 mils |
| Size | Small parts and fasteners to 90' beams | Sheet steel 0.010" - 1.7" thick, 72" wide | Unlimited | Unlimited | Small parts 8-9" and under 1 lb | Sheet steel | Small parts |
| Cure Time | < 1 hour | < 1 hour | 24-72 hours | < 24 hoursd | < 1 hour | < 1 hour | < 1 hour |
| Coverage Consistency | 100% flat surfaces, edges, corners, and interior | Unknown thickness, controlled by air knife | Inconsistent based on operator skill, tends to thin at edges and corners, interior surfaces are uncoated | Inconsistent based on operator skill | Inconsistent thinner at edges, corners and recesses | 100% coverage | 100% coverage |
| Bond to Substrate Steel | Metallurgical ~3,600 psi | Metallurgical ~3,600 psi | Mechanical 400-600 psi | Mechanical ~1,500 psi | Mechanical 400-600 psi | Mechanical 300-500 psi | Mechanical 300-500 psi |
| Abrasion Resistance / Hardness | Intermetallic layers 179-250 DPN | ~70 DPN | Soft and not abrasion resistant | ~70 DPN | 75 DPN | ~70 DPN | 75 DPN |
| Finish / Appearance | Varies; matte gray, shiny, spangle or a combination | Controlled; minimum to highly spangled | Smooth finish; color to suit specifier | Matte gray, rough | Matte gray, rough compared to electroplated | Smooth finish; shiny, unless passivated | Smooth finish; dull gray to shiny; controlled by additives |
| Exposure Conditions | Interior / exterior | Interior or mildly corrosive conditions | Interior / exterior | Interior / exterior | Interior / exterior | Interior | Interior |
| a Range based on ASTM, ISO, and CSA minimum thicknesses for all grades, stylees, etc., encompassed by the specifications. b Total for both sides of the sheet c Range based on ASTM minimum thicknesses for all grades, stylees, etc., encompassed by the specifications. d Dependent on sealer top coat |
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Coating Thickness vs. Coating Weight
The service life of zinc coatings is a linear function of the zinc coating thickness. However, zinc coating thickness alone can be deceiving when evaluating zinc applied by different processes. In addition to thickness, the amount of available zinc per unit volume, or density, is also important.
Various ASTM and/or other specifications require different weights or thicknesses, so it is imperative to convert all coatings to a common denominator for comparison. While coating densities for some 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:
| Coating | Thickness to reach 1 oz/ft2 |
|---|---|
| Hot-dip galvanizing (batch or continuous), electroplating, zinc plating | 1.7 mils (43 µm) |
| Metallizing (zinc spraying) | 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.
It is also important to remember for all continuous galvanized sheet materials, including electrogalvanized, the coating weight is given for the total for both sides of the sheet. To obtain the amount of zinc per unit area of surface, the weight given must be divided in two, assuming equal distribution on both sides. For example, an ASTM A653 Class G90 sheet contains 0.90 oz/ft2 of zinc or about 0.45 oz/ft2 per side.
Zinc Coatings vs. Other Corrosion Protection Systems
Zinc coatings differ fundamentally from barrier-only systems (such as most paints) because zinc is anodic to steel and continues to protect even if the coating is damaged. In infrastructure, this can eliminate several repainting cycles over a bridge or transmission structure’s design life.
- Zinc coatings vs. paint-only: longer time to first maintenance, better edge protection, fewer site closures for repainting.
- Zinc coatings vs. uncoated weathering steel (UWS): more reliable performance in de-icing salt, marine, and industrial environments.
- Zinc coatings vs. stainless steel: lower initial cost for large structural members with comparable corrosion resistance in many atmospheres.
Economic Considerations
Initial cost will always be considered when specifying steel corrosion protection. However, in addition to the initial cost, evaluating the performance of the zinc coating in the intended environment also impacts the economics of the protective system. Hidden costs, such as accessibility of the site, production loss due to maintenance recoating, and rising wages for labor-intensive coatings, such as metal spraying and painting, must also be considered.
The choice of the most economical system is not precise, because neither the timing nor the cost of future maintenance can be predicted accurately. In addition, depreciation of capital investment, tax relief for investment, and maintenance cost and the time value of money must be considered and can change. However, to get the most realistic cost of the coating system throughout the projects life, economic models for comparing the life-cycle costs of different coatings have been developed. The American Galvanizers Association has created an online calculator to facilitate the complex process of evaluating life-cycle costs. The online calculator uses the same net present value economic formula recommended in ASTM A1068, Standard Practice for Life-Cycle Cost Analysis of Corrosion Protection Systems on Iron and Steel Products. Performance and cost data are drawn from real-world exposure results and published reports, including NACE Paper No. 8279 (2008). Visit our Life-cycle Cost Calculator (lccc.galvanizeit.org) to run your own analysis.