Zinc metal has a number of characteristics that make it well suited for use as a coating for protecting iron and steel products from corrosion. Its 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. The excellent field performance of zinc coatings results from their ability to form dense, adherent corrosion product films and a rate of corrosion considerably below that of ferrous materials, some 10 to 100 times slower, depending upon the environment. While a fresh zinc surface is quite reactive when exposed to the atmosphere, a thin film of corrosion products develops rapidly, greatly reducing the rate of further corrosion. Click the link above to view a chart depicting the expected service life to first maintenance (5% red rust) of iron and steel based on the zinc coating thickness and the environment.
A number of different types of methods of applying zinc coatings to steel are commercially available, each of which has unique characteristics. The products produced by each of these processes have different uses depending on their applicability, relative economics and expected service life. To find out more about the various zinc coatings click one of the topics below.
High Temperature Galvanizing
High-temperature galvanizing is very similar to batch hot-dip galvanizing at a conventional temperature (830ºF). The steel is run through a batch process beginning with multiple chemical pretreatment tanks used to clean residues from the steel. The pretreatment of the steel begins with a degreasing to remove dirt and oils from the steel. This stage is then followed by an immersion in an acid pickling tank that removes oxides and mill scale from the surface of the steel. After the steel has been adequately cleaned, it is submerged in a tank containing a flux solution, which provides protection against oxidation prior to entering the galvanizing kettle.
The difference between this process and the normal hot-dip galvanizing process exists in the galvanizing kettle. Not only is it run at higher temperatures, typically 1020 to 1040ºF (550 to 560ºC), but also the vessel holding the zinc is usually much smaller than the conventional process. Due to the high temperatures that the holding vessel is operated at, it must also be constructed of ceramics. Due to the poor heat conductivity of the ceramic, the vessel is normally top-heated using fossil fuel burners, electric radiant elements, induction heating, or immersion burners.
The high temperature galvanizing process is commonly used for small hardware items such as nuts and bolts. The coatings produced are extremely uniform and allow for easy assembly without zinc buildup on the threads. Another advantage of the high temperature process is the ability to control coating thickness on silicon-rich or reactive steels. The coatings produced on these steels are thinner and more tightly adherent than coatings produced in the conventional galvanizing process. Size limitations on steel that can be galvanized at high temperatures exist due to the smaller size of the galvanizing kettle.
The coating produced by high temperature galvanizing is very similar to that of the conventional hot-dip galvanizing process, but with the absence of the Eta layer. The coatings produced are typically duller than conventional galvanized coatings because the reaction between iron and zinc goes to completion, mostly forming a coating of intermetallic iron-zinc alloy. The thickness of the coating is highly dependent on the temperature. Coatings produced at temperatures of approximately 1030ºF (555ºC) are typically 4 mils thick, depending on the immersion time in the galvanizing bath. The corrosion performance of this coating is similar to the coatings produced by conventional galvanizing and is strictly dependent on the coating thickness. To view a service-life chart on the performance of zinc coatings, click here.
In-line, Continuous (Sheet) Galvanizing
The continuous sheet galvanizing process is also a hot-dip process. Steel sheet, strip or wire is cleaned, pickled, and de-oxidized on a processing line 500 feet in length, running at speeds of over 300 feet per minute. In the coating of sheet or strip, the zinc bath contains larger amounts of aluminum than used in conventional hot-dip galvanizing (0.15 to 0.25%). The aluminum (Al) suppresses the formation of the zinc-iron alloys, resulting in a coating that is mostly pure zinc. In-line heat treatment can be used to produce a fully alloyed (Fe-Zn) coating, called galvannealed steel.
Sheet products continuously coated with zinc-aluminum alloys are also commercially available. Two alloy compositions currently in use are 55% Al-43.6% Zn-1.4% Si and a 95% Zn-5% Al-trace mischmetal (cerium, lanthanum). These coatings are used to enhance the product life for certain applications.
After galvanizing, the continuous zinc coating is physically wiped using air knives to produce a uniform coating across the width of the strip. A variety of coating weights and types is available, ranging up to just under 2 mils (50 µm) per side. One of the most common coatings is Class G90, which has 0.9 oz./ft2 of sheet (total both sides) or about 0.75 mils (18 µm) thickness per side.
Continuously galvanized sheet steels are used to make cars, appliances, corrugated roofing and siding, and culvert pipe. The coated product can be suitably treated for painting for aesthetics or to increase service life. Because of the thin coating, this product normally is used for interior applications or where exposure to corrosive elements is mild.
Electrogalvanized coatings are applied to steel sheet and strip by electro-deposition. Electrogalvanizing is a continuous operation where the steel sheet is fed through suitable entry equipment, followed by a series of washes and rinses, into the zinc plating bath.
The most common zinc electrolyte-anode arrangement uses lead-silver, or other insoluble anodes, and electrolytes of zinc sulfates. Soluble anodes of pure zinc are also used. In this process, the steel sheet is the cathode. The coating is developed as zinc ions in the solution are electrically reduced to zinc metal and deposited at the cathode. Grain refiners may be added to help produce a smooth, tight-knit surface on the steel.
The electrodeposited zinc coating consists of pure zinc tightly adherent to the steel substrate. The coating is highly ductile and the coating remains intact even on severe deformation. The coating is produced on strip and sheet materials to coating weights up to 0.2 oz/ft2 (60 g/m2), or thickness of up to 0.14 mils (3.6 µm) per side. On wire, coating weights may range up to 3 oz/ft2 (915 g/m2). Heat-treated and electro-coated wire can be cold drawn to about 95% reduction in area, depending on the chemical composition of the wire, heat treatment, and diameter.
The electrogalvanized coating is paintable with suitable treatment, and the sheet product is used in automobile and appliance bodies. Due to the extremely thin zinc coating on the sheet, painting or other topcoating is recommended to improve the service life.
Zinc plating is identical to electrogalvanizing in principle in that both are electrodeposition processes. Zinc plating is used for coatings deposited on small parts such as fasteners, crank handles, springs, and other hardware items. The zinc is supplied as an expendable electrode in a cyanide, alkaline non-cyanide, or acid chloride salt solution. Cyanide baths are the most operationally efficient, but they potentially create a pollution and hazardous material problem.
After alkaline or electrolytic cleaning, pickling to remove surface oxides, and rinsing, the parts are loaded into a barrel, rack, or drum and immersed in the plating solution. Various brightening agents may be added to the solution to add luster, but careful control of the bath and brightener is needed to ensure a quality product. Post-plating treatments may be used to passivate the zinc surface and at the same time impart various translucent colors to the coating. These post-plating treatments may be used to provide a desired color or to extend the life of the plated coating.
The normal zinc-plated coating is dull gray in color with a matte finish, although whiter, more lustrous coatings can be produced, depending on the process or agents added to the plating bath or through post-treatments. The coating is thin, ranging up to 1 mil (25 µm), restricting zinc-plated parts to very mild (indoor) exposures. ASTM Specification B 633 lists four classes of zinc plating: Fe/Zn 5, Fe/Zn 8, Fe/Zn 12 and Fe/Zn 25. The number indicates the coating thickness in microns. The coating finds application in screws and other light fasteners, light switch plates and other small parts. Materials for use in moderate or severe applications must be chromate conversion coated.
The coating is entirely pure zinc, which has a hardness about one-third to one-half that of most steels.
The sprayed zinc coating is rough and slightly porous, with a specific gravity of 6.4, compared to zinc metal at 7.1. Zinc corrosion products tend to fill the pores as the zinc corrodes in the atmosphere. The coating adherence mechanism is mostly mechanical, depending on the kinetic energy of the sprayed particles of zinc. No zinc-iron alloy layers are present.
The coating can be applied in the shop or field; it gives good coverage of welds, seams, ends and rivets, and can be used to produce coatings in excess of 10 mils (250 µm). Coating consistency is dependent on operator experience and coating variation is always a possibility. Coatings may be thinner on corners or edges and the process is not suitable for coating recesses and cavities.
Small iron and steel parts may be coated by drum tumbling with a mixture of proprietary promoter chemicals, zinc powder and glass beads. After cleaning, the parts, which are usually limited in size to about 8-9 inches (200-300 mm), and weighing less than one pound (0.5 kg), are flash copper coated and loaded into a plating barrel. The barrel is then filled with chemicals, glass beads and zinc powder, then tumbled. The tumbling action causes the beads to peen the zinc powder onto the part. Thickness is regulated by the amount of zinc charged to the plating barrel and the duration of tumbling time. After coating, the parts are dried and packaged, or post-treated with a passivating film, then dried and packaged.
Materials mechanically plated must be simple in design. Complex designs with recesses or blind holes may not be thoroughly coated because of inaccessibility to the peening action of the glass beads. The media used as the compaction agent is also important: it must be large enough to avoid being lodged in any cavities, recesses or small threads in the parts.
The mechanically plated coating consists of a flash coating of copper followed by the zinc coating. Coating thickness requirements contained in ASTM Specification B 695 range from 0.2-4.3 mils (5 to 110 µm). While thicker coatings are possible, the common thickness on commercial fasteners is 2 mils (50 µm). The coating has a density of about 0.45 oz./ft2/mil compared to the hot-dip galvanized coating density of about 0.6 oz./ft2/mil. The hot-dip coating has over 30% more zinc per unit volume than the mechanical coating.
The coating, upon micro cross-section, appears to consist of flattened particles of zinc loosely bonded together. The bond between zinc and steel, and zinc-to-zinc, being mechanical in this process, is weaker than the metallurgical bond found in hot-dip galvanizing. Edge, corner and thread coating thicknesses are usually lower at these sharp radii areas due to minimal peening action at these locations.
Zinc-rich paints contain 65-94% metallic zinc in the film of the paint after it dries. The paints are usually applied by brushing or spraying onto steel that has been cleaned by sandblasting. While white metal blasting (NACE No. 1) is preferred, near white (SSPC-SP 10) or commercial blast cleaning (SSPC-SP 6) are acceptable.
When the zinc dust is supplied as a separate component, it must be mixed with a polymeric-containing vehicle to provide a homogenous mixture prior to application. Application is usually by air spray, although airless spray also can be used. The paint must be constantly agitated and the feed line kept as short as possible to prevent settling of the zinc dust. Uneven film coats may develop if applied by brush or roller, and cracking may occur if too thick a paint coating is applied.
Zinc-rich paints are classified as organic or inorganic, depending on the binder, and must be applied over clean steel.
Organic or inorganic zinc-rich paints usually are applied to a dry film thickness of 2.5 to 3.5 mils (64-90 µm). Organic zinc paints consist of epoxies, chlorinated hydrocarbons and other polymers. Inorganic zinc paints are based largely on organic alkyl silicates. The zinc dust must be at a concentration high enough to provide for electrical continuity in the dry film. Otherwise, cathodic protection will not occur. Even so, there is some question as to whether cathodic protection is possible at all due to the encapsulation of the zinc particles in the binder.
Adhesion bond strengths of zinc-rich paints are in the order of a few hundred pounds per square inch (psi), while galvanized coatings measure in the several thousand psi range. Zinc-rich painting is similar to metallizing in that large articles can be coated in either the shop or field. Limitations include cost, difficulty in applying, lack of coating uniformity (particularly at corners and edges), and the requirement for a clean steel surface. Zinc-rich paints should be topcoated in severe environments.
Inorganic zinc-rich paints that adhere by mild chemical reactivity with the substrate have good solvent resistance and can withstand temperatures up to about 375ºC (700ºF). Cleanup is easier than with organics, and they do not chalk, peel, or blister readily, and are easy to weld through.
Zinc contents of inorganic zinc-rich paints range up to about 0.35 oz. zinc/ft2/mil or about one-half less zinc per mil than hot-dip galvanized coatings.
The properties of organic zinc-rich paints depend on the solvent system. Multiple coats may be applied within 24 hours without cracking. Zinc-rich paints are often used to touch up galvanized steel that has been damaged by welding or severe mechanical impact.
Organic zinc-rich paints do not have the temperature resistance of inorganic zincs, being limited to 200 to 300ºF, are subject to ultraviolet (sunlight) degradation, and are not as effective as inorganics in length of corrosion prevention.
Zinc dust/zinc oxide paints (MZP) are classified under Federal Specification TT-P-641G as Type I, Type II, or Type III, depending on the vehicle. The vehicles used are linseed, alkyd resin, and phenolic resin, respectively. These paints are widely used as either a primer or topcoat and show good adhesion to galvanized steel, making them the logical choices for painting that substrate. Type I is good for outdoor applications, Type II for heat-resistant applications and Type III for water immersion or severe moisture conditions. Because of their lower metallic zinc content, zinc dust/zinc oxide paints (MZP) cannot provide sacrificial protection to the base steel. When used as a coating over galvanized steel, the service life of the galvanized coating is extended because of the increased barrier protection of the paint. The service life of the paint is extended in the event of a scratch or cut through the paint, since the volume of the zinc corrosion product, being considerably less than that of rust, reduces the incidence of lifting and separation of the paint film. MZPs can be top-coated with a variety of paint types if colors other than gray, green or tan (from pigmented additives) are required.
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.
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.
|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.|
|SSPC-PS Guide 12.00, 22.00
SSPC-PS Paint 20
|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.
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.
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.
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.
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. Visit www.galvanizeit.org/galvanizingcost to run your own analysis.