May 27, 2020
By Philip G. Rahrig
Hot-dip galvanizing has been applied to reinforcing steel in concrete used for precast panels, barriers, and bridge decks since the early 1970s. While new technologies have surfaced in decades past, none have matched the performance or economic savings that galvanized reinforcing steel has delivered. To comprehend how and why, it is important to understand the intrinsic properties of zinc in concrete, the corrosion mechanisms that are at play in chloride-rich exposure conditions (proximity to salt water/mist, road salts used for de-icing), what the sacrificial (anodic) properties of zinc mean for precast forms made of dissimilar (non-zinc) metals, and the flexibility in fabrication, handling, and placement of hot-dip galvanized (HDG) rebar.
Hot-dip galvanizing process and coating metallurgy
Unlike paint coatings that form an adhesive bond with the underlying steel, galvanized coatings develop a metallurgical bond with the steel. This occurs because when steel is placed in a molten zinc bath heated to approximately 454 C (850 F), the iron diffuses out of the steel and forms a series of iron-zinc alloy layers. The iron and zinc become metallurgically bonded together at about 24,821 kPa (3600 psi). However, for these intermetallic layers to form properly, the steel must be prepared and processed in a specific sequence. The galvanizing process consists of three basic steps (Figure 1):
Each of these steps is important in obtaining high-quality HDG coatings because if the steel is not white (i.e. free of iron oxide [rust], and organic material such as oil, dirt, and grease) the iron and molten zinc will not react with each other.
It is essential the steel surface is clean and uncontaminated for a uniform, adherent coating. Surface preparation is usually performed in sequence by caustic (alkaline) cleaning, water rinsing, acid pickling, and water rinsing. The caustic solution is used to clean the steel of organic contaminants such as dirt, paint markings, grease, and oil, which are not readily removed by acid pickling. Scale and rust are normally removed by pickling in hot sulfuric acid at 65 C (150 F) or hydrochloric acid at room temperature. The rinsing in a water bath after each step simply prevents contamination of the liquid in the next step of the process.
The final steel cleaning is performed by a flux solution of zinc-ammonium chloride. Flux is required at this step to remove any oxidation that may take place after the steel is taken out from the acid cleaning bath, and to prevent further oxidation of the clean steel between the time it takes for the product to dry and its immersion in the molten zinc. Steel must be completely dry prior to immersion in the molten zinc. If not, zinc will violently splash out of the kettle. The method of applying the flux to the steel depends on whether the ‘wet’ or ‘dry’ galvanizing process is used. Dry galvanizing requires the steel to be dipped in an aqueous zinc ammonium-chloride solution prior to immersion in the molten zinc bath and then thoroughly dried. This ‘pre-flux,’ as it is called, prevents oxides from forming on the material surface before galvanizing. Wet galvanizing uses a flux layer of zinc ammonium-chloride floating on top of the more viscous molten zinc. The steel is passed slowly through the flux and any oxides are removed.
The steel to be coated is immersed in a molten zinc bath maintained at a temperature of 435 to 460 C (815 to 860 F). Typical bath chemistry used in hot-dip galvanizing contains a minimum of 98 per cent zinc with a variety of trace elements or alloy additions. These additions, which could include lead (up to 1.2 per cent), aluminum (up to 0.005 per cent), tin (about 0.05 per cent), nickel (up to 0.1 per cent), and bismuth (about 0.1 per cent), can be mixed into the zinc to enhance the appearance of the final product or to improve the drainage of the molten zinc, as the material is withdrawn from the bath.
Three different galvanizing processes are used today to produce HDG rebar, but it is important to note all three produce a zinc coating durable for decades by isolating the bar from corrosive chlorides in the concrete. Production to exacting ASTM specifications ensures consistent performance from galvanizer to galvanizer.
The traditional method of galvanizing is performed to ASTM A767, Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement, Class I or Class II. The resultant coating is often comprised of alloy layers only and is at least 135 μm (5 mils) and 85 µm (4 mils), respectively.
The modified traditional method is also performed to ASTM A767 Class II, but the coating is a minimum 85 μm thick and is made up of approximately 50 μm (2 mils) of pure zinc eta (the outermost layer) (Figure 2).
The third is a continuous method produced to ASTM A1094, Standard Specification for Continuous Hot-Dip Galvanized Steel Bars for Concrete Reinforcement, and applies to nearly 100 per cent eta layer (i.e. pure zinc to a minimum coating thickness of 50 μm).
The differences in the coatings produced has implications for fabrication and service life, and will be discussed later in this article.
Galvanized coating structure
During the traditional galvanizing process, a series of alloy layers form as a result of the metallurgical (diffusion) reaction between the molten zinc and the steel. Figure 3 shows the cross-section of a coating developed on steel with a low silicon (less than 0.03 per cent). The coating consists of a thin gamma layer next to the steel substrate, a blocky delta layer, and a columnar growth of zeta crystals. The various alloy coatings contain different amounts of iron, with the highest content in the layers closest to the steel. The iron-zinc intermetallic layers are covered by a surface of pure zinc eta layer that is formed when the product is withdrawn from the molten zinc bath. This outer layer gives the galvanized product its distinctive shine and spangled appearance.
Measuring galvanized coating performance (corrosion rate) in concrete
By the nature of reinforcing steel being imbedded in concrete, it is difficult to assess the actual performance of the galvanized coating. The bar cannot be visually inspected without drilling core samples, and it is not as simple as measuring the electrical potential (measurements indicating the cathodic protection taking place [i.e. the zinc is giving up electrons to protect the steel]) to see how much corrosion is occurring. Zinc in concrete has different corrosion mechanisms than bare reinforcing steel. Regardless of how much engineers and scientists want to measure corrosion rates of galvanized rebar by accelerating one variable like chloride, doing so in accelerated tests such as salt fog or salt spray do not reflect what is taking place in a precast panel or barrier in the real world where it may take several decades before the chloride levels in the concrete are high enough to begin the zinc corrosion process. Parameters can be accelerated but the actual lifetime data in the test environment cannot be correlated to the field performance. Accelerated tests of chloride threshold for reinforcing bars have found zinc coatings have higher thresholds than bare steel by a factor of between five and 10 to one. This higher threshold, defined as the time when active corrosion of the zinc coating begins, is in part due to the formation of calcium hydroxyzincate film on the surface of the zinc coating during the concrete curing process, and indicates galvanized reinforcing steel should have a longer lifetime than bare steel. In addition to the higher threshold for chloride attack, the zinc corrosion products migrate away from the bar and fill the small capillaries and voids, thus preventing chloride ions from penetrating the concrete and coming into contact with the reinforcing steel. Black/bare rebar corrosion products such as iron-oxide are more voluminous than steel and cause spalling pressure, which eventually causes cracks at the concrete surface.
Based on coring of two bridge decks in different exposures, as indicated in Figure 1 (page 29), the chloride threshold for the galvanized rebar is not reached until year 78 and 102 for a bridge in a northern climate (Tioga bridge, Pennsylvania) where road salts are used and for a bridge in a southern climate (Boca Chica bridge, Florida) with marine exposure, respectively. Conversely, the chloride threshold for black/bare bar would be reached sometime around 10 to 15 years after the concrete pour (Figure 4).
It is important to note after the chloride threshold is reached, the galvanized coating still protects the substrate bar for additional years, as a function of the zinc coating thickness. Depending on whether bar was produced by the traditional method to ASTM A767 Type I, the modified traditional method to ASTM A767 Type II, or via the continuous method to ASTM A1094, protection may be extended for several decades beyond the chloride threshold point (Figure 5). Longer protection means little or no maintenance costs for a bridge deck. The galvanized rebar will usually last much longer than the life of the concrete it is giving strength to.
What precasters should know
Significant tonnage of galvanized rebar is used by precasters to give their products a longer life, but certain performance characteristics of galvanized rebar should be known in advance to maximize
Forming and fabrication of galvanized rebar
Welding, cutting, and drilling of the steel should be done prior to galvanizing to minimize the exposure of unprotected edges and to take advantage of the protection afforded by the zinc coating. Good fabricating practices state, before galvanizing, steel should be bent with a minimum six to 10 times the bar diameter, per Table 2 of ASTM A767. Heat-treating before bending is required for bends tighter than that. This bend radius minimizes possible cold working of the steel structure that could lead to strain-age embrittlement after galvanizing. Strain-age embrittlement sometimes occurs when bending stresses are induced in the steel fabrication process and then released by the heat in the galvanizing process. The galvanized steel becomes frail.
There are, however, situations where the galvanized products need to be assembled, cut, and/or fabricated in the field. For these instances, it is important to use rebar produced by either a proprietary method or the continuous process. Both yield a thinner coating comprised mostly of the pure zinc eta layer, which is ductile and stretches during forming.
If overlapping of galvanized rebar is required, it is important to know the same requirements for black/bare bar apply. In studies conducted by the University of California Berkeley, the bond strength of galvanized bars is equal to or greater than black/bare bar (Figure 3).
Abrasion and impact resistance of galvanized coatings
The zeta and delta alloy layers are harder than many base steels (Figure 6). These alloy layers offer excellent abrasion resistance during transport, placement, and severe service conditions, meaning special handling procedures are not required.
Corner and edge protection
Since galvanizing is a total immersion process, all areas of the product are coated, including hidden or hard to reach places. Galvanized coatings on edges and corners are at least as thick, and sometimes thicker, as on other parts of the product. Unlike epoxies or spray-applied coatings, the coating does not thin out on edges and corners due to the formation of an alloy layer. These areas are where protection is typically needed the most (Figure 7).
Welding of galvanized steel
Welding can be accomplished by either grinding away the zinc coating and directly welding the base metal, or by welding through the galvanized coating. Materials that have been galvanized may be welded easily by all common welding techniques. Generally, anything that can be welded before galvanizing can be welded too; but some minor changes to the technique need to be incorporated to insure full weld penetration. These changes are intended to allow the galvanized coating to burn off at the front of the weld pool.
Dissimilar metal forms in contact with galvanized rebar
Zinc is anodic to most metals commonly used in construction (i.e. it sacrificially corrodes to protect the metal it is in contact with). Metal forms should be electrically isolated from the galvanized rebar by non-conductive spacers to prevent dissimilar metal reactions during the concrete curing. If metal forms are not isolated from the galvanized rebar, then zinc ions can be released from the galvanized coating to try to protect the metal form, resulting in a change in the concrete appearance near the galvanized bar placement.
Since types of cement with naturally low-occurring levels of chromates may react with zinc, it is important to ensure forms and supports are not removed before the concrete has developed the required strength to support itself. Normal form removal practices may be utilized if the cement contains at least 100 ppm of chromates in the final concrete mix or if the HDG bars are chromate-passivated according to ASTM A767. Too little chromate may allow the zinc to react with hydrogen in the water of the concrete mix, thus causing bubbling, which makes its way to the surface and results in poor concrete quality almost immediately.
Design considerations of reinforcing steel durability in precast concrete for specific exposure conditions, lowest life-cycle cost, and demonstrated performance are paramount in the decision-making process used by architects and specifiers. HDG reinforcing steel offers an objective option, and with some research into existing projects, architects and specifiers may just have found the best solution for
For 26 years, Philip G. Rahrig has been the executive director of the American Galvanizers Association, working in all facets of market/product promotion and positioning, as well as in technical support. Additionally, he is responsible for worldwide co-ordination of objectives with international organizations with common industry interests. His prior experiences include 10 years with the U.S. Steel Distribution Division, Thyssen Industrial Automation, and IBM. He is a graduate of Xavier University, Cincinnati, Ohio, with a BSBA – Management and a minor in physics. He can be reached at firstname.lastname@example.org.
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