Introduction
What is the common denominator between the Golden Gate Bridge, automotive chassis parts, and precision fasteners? A very thin layer of zinc that coats them and keeps them safe against the unremitting onslaught of corrosion over decades. This galvanized metallurgical shield is a cost-efficient technique to add longevity to the steel parts in numerous industrial applications.
Knowledge of galvanized steel properties and uses is vital to the engineers, purchasers, and manufacturers who require credible corrosion control without sacrificing the integrity of the structure or busting financial limits. This one-stop reference covers the principles of the galvanizing process, comparisons of the benefits of using hot-dip galvanizing over other methods like Galvanized and aluminum coating, and offers valuable implementation tips to help increase the service life of equipment in harsh environments.
The readers will acquire profound knowledge about galvanized steel production, technical requirements that comply with international standards, the use of the material in specific industries, like the automotive and construction industries, and the important restrictions that have to be taken into consideration when choosing the material to use in long-term projects.
What Is Galvanized Steel (and Why It Matters)
Galvanized steel is carbon steel that is metallurgically treated by applying a protective layer of zinc, and it can be described as the zinc enveloping the carbon steel to avoid oxidation and corrosion, which would otherwise affect the structural integrity. Zinc coating serves as a physical barrier and sacrificial anode, and it corrodes in favor of the underlying steel substrate.
There are three major industrial processes: hot-dip galvanizing produces the thickest and most durable finishes (about 45-150 micrometers) by being dipped in molten zinc baths at 450degC; electrogalvanizing uses thinner, more uniform finishes (5-25 micrometers) through electrochemical deposition; and galvannealing interlayers of hot-dip galvanized with further heat treatment to achieve zinc-iron alloy layers optimized to bond paint and allow welding.
Galvanized steel corrosion protection has an economic payoff that stretches way beyond primary material expenses. NACE International studies showed that corrosion costs the global economy more than 2.5 trillion dollars every year, so practical protection systems such as galvanization are necessary to reduce maintenance and corrosion costs in the industries of construction of precision manufacturing.
How It’s Made: The Galvanizing Process
The hot-dip galvanizing process is the gold standard of heavy-duty corrosion protection with multiple key steps that guarantee metallurgical bonding of zinc and steel substrates. It requires initial degreasing, then acid pickling (usually 15-25 percent hydrochloric acid) to remove mill scale and rust, then using a fluid of zinc chloride solutions to inhibit oxidation as it heats.
Real galvanizing is done when the clean steel parts are submerged in the molten zinc baths heated to 445-465 °C, which is 833-869°F, and form an intermetallic layer of iron-zinc. The formation of various layers of coating takes place in the following order: a gamma layer (Fe3Zn10) is formed together with the steel, then a delta layer and a zeta layer (FeZn7, FeZn13), and finally a pure zinc eta layer is deposited as the ultimate protective layer.
Electrogalvanizing provides the ability to control coating thickness with precision, where a smaller and more consistent coverage is needed. This is accomplished by means of electrolytic cells in which the steel items are used as cathodes in either a zinc sulfate or a zinc chloride solution, deposition of pure zinc coatings atom by atom. Electrogalvanized finishes are finer (5-25 mm vs 45-150 mm in hot-dip), with high uniformity on automotive body panels and precision parts in which dimensional tolerance is highly important.
The protective mechanism works in two ways: barrier protection, keeping the steel surfaces dry and oxygen-free; galvanic protection is to make sure that even in the case of coating damage, zinc disintegrates to save the metal surfaces. Zinc develops a stable zinc carbonate layer of patina that slows down the rate of corrosion even more in environmental conditions over time.
Properties and Benefits
Physical properties of galvanized steel preserve the structural properties of the base steel, and this is accompanied by great corrosion resistance. Normal hot-dip galvanized steel has tensile strengths of 400-550 MPa (equal or somewhat greater than base steel), and coating hardness of 70-250 HV based on the composition of the iron-zinc alloy layers. A density of 7.14 g/cm3, which is added to the zinc coating, adds little weight (usually less than 2 percent) and offers considerable protection.
Hot-dip galvanized steel advantages consist of superior durability with correctly applied coatings giving a maintenance-free service life of 20-50 years in most atmospheric environments. According to the American Galvanizers Association, galvanized finishes corrode at 1-3 micrometers/year in an urban condition, much less than bare metal, which corrodes at 25-100 micrometers/year. This translates to dramatic lifecycle cost savings in the form of no more maintenance painting, less frequent replacement, and less risk of structural failure.
Galvanized steel is also tough and durable, and therefore, suitable for stressful applications. The zinc coating is resistant to substantial mechanical loading, such as moderate forming, drilling, and cutting, with no coating breakdown. ASTM A153 impact resistance test demonstrates that well galvanized parts retain their coating properties even in situations that would corrode painted parts.
Galvanized steel is always at an advantage in the cost efficiency analysis in long-term projects. The initial cost of the material might be 20-40% greater than painted steel, but the removal of the continued maintenance painting (usually once every 5-10 years) and increased service life result in significant total cost of ownership benefits, especially when there is limited or expensive maintenance accessibility, as is the case with infrastructure applications.
Applications Across Industries
The uses of galvanized steel in the construction industry take precedence in structural framework, architectural components, and roofing systems, where durability and lack of maintenance are very important. Structural steel galvanizing is used to protect building frameworks, bridges, and industrial buildings, and the coating is usually specified under ASTM A123 standards of hot-dip galvanized structural steel products. In most climates, galvanized roofing materials such as corrugated sheets and standing seam systems have a service life of 25-40 years and still offer structural integrity and weather protection.
Automotive galvanized steel materials are devoted to body parts, chassis, and fastener components, where the needs of corrosion protection should be weighed against formability and weldability. In automotive applications, galvanised steel sheets are prevalent and offer corrosion resistance of galvanized coatings with increased paint adhesion qualities needed in the automotive finishing process. New high-strength galvanized steels make it possible to pursue lightweighting efforts without compromising crash-related performance or durability specifications.
The pipelines, structural supports, and processing equipment used in oil and gas equipment are made using galvanized steel, which is used in a corrosive environment. Galvanized steel is used in applications in marine and offshore operations because of its performance in salt water, but coating thickness considerations become significantly higher (more than 150 micrometers) to ensure optimum performance at harsh marine exposures.
Precision fastener galvanizing has high stakes in both construction hardware and electronic enclosure fasteners. Hot-dip galvanized bolts, nuts, and washers to ASTM A153 are a dependable corrosion protection with support for threading, and assembly torque standards. Zinc fasteners, electroplated, provide even more accurate dimensional control in those applications where the thickness of the coating to be applied should be minimal.
Galvanized fences, structures of barns, and equipment parts that are in corrosive conditions due to animal waste, fertilizers, and extreme weather are used in agricultural applications. Zinc coatings have self-healing characteristics, and thus galvanized steel is available in agricultural wire products where coating damage due to handling or installation is not uncommon.
Alternatives & Comparisons
Galvannealed vs galvanized steel: A major decision to make when using many materials is galvannealed vs galvanized steel. Hot-dipped galvanized galvannealed steel, the outer layer of zinc is transformed into zinc-iron alloys, which gives it enhanced adhesion to paint and spot welds. Although galvannealed coating can lose a part of its corrosion resistance (typically 10-20% percent loss of service life), it has been noted to perform exceptionally well in the automotive and appliance industry where the coated item needs to be painted or welded.
The galvanneeling process temperatures. On the galvanized steel, the process of reheating is carried out at between 500-565 °C to enable the diffusion of iron in the substrate to form homogenous zinc-iron alloy layers. This removes the pure zinc outer layer that may provoke the adhesion problems with the paint and welding spatter problems that are a reality with standard galvanized finishes.
Galvanized steel vs aluminum to aluminum showcases the strengths of each material to be used in complementary applications. Aluminum has better corrosion resistance in most environments, is lighter (2.7 g/cm3 as compared to steel 7.85 g/cm3), and is highly formable. Nevertheless, galvanized steel offers superior strength-to-cost ratios, improved compatibility with conventional weld gear, and reduced cost of materials to have equivalent structural performance.
In structural applications, where modulus of elasticity is more important (where galvanized steel has a higher modulus of elasticity, 200 GPa, compared to aluminum, 70 GPa), it allows a smaller cross-section to achieve the same level of stiffness, and in many cases, the weight is compensated. Cost analysis normally indicates that galvanized steel provides 30-50% of the overall costs that a similar structural performance requires in most applications.
Stainless steel substitutes are much more resistant to corrosion, but they cost a lot more to substitute (usually 300-500 percent higher than galvanized steel). This choice between galvanized steel and stainless steel is based on the environment, lifecycle needs, and cost-based factors, and features where galvanized steel is selected in moderate environments that require moderate corrosion features, and cost control is paramount.
Limitations & Pitfalls
Environmental considerations. The environmental factors to consider in terms of the galvanizing processes are zinc resources used, energy needed to run the processes at high temperatures, and waste streams. Hot-dip galvanizing normally consumes 200-300 kWh of electricity (ton) processed, and zinc can be used in quantities of 600-1200 kg per ton (depending on coating thickness requirements). Nevertheless, lifecycle analysis reveals net environmental benefits, which are longer service life and less maintenance needs than painted steel options.
Coating defects in galvanized steel. Treating flaws in galvanized steel may have a significant effect on performance and appearance. The typical flaws are bare spots due to poor preparation of the surface, dross inclusions due to zinc bath contamination, and variations in coating thickness due to inappropriate processing parameters. Flux inclusions may form weak points, resulting in premature coating failure, and silicon-based steel may result in overly thick, brittle coating, which may flake.
The problem of brittleness is with the galvanized coatings that undergo severe forming operations following galvanizing. Whereas zinc coating can take moderate deformations, extreme bending, stretching, or hitting may lead to cracking and spalling of the coating. The ASTM A123 standards contain minimum coating ductility requirements, although applications that involve post-galvanizing forming must be evaluated critically in terms of coating integrity.
Temperature limitations: The high temperature of the applications limits galvanized steel in high-temperature environments. Zinc starts oxidizing quickly at temperatures over 200 °C, but the rate of coating degradation increases much higher at temperatures over 300 °C. This restricts the application of galvanized steel in exhaust and furnace components and any other high-temperature application where other protective systems must be used.
When corrosive environments are encountered, the galvanized steel contact with other dissimilar metals, the risks of galvanic corrosion pose risks. Galvanic corrosion may increase the rate of zinc coating consumption with direct contact with copper, stainless steel, or other noble metals, necessitating design considerations and suitable methods of isolating mixed-material assemblies.
Practical Tips for Engineers & Makers
Best practices for galvanizing preparation. The best practices in galvanizing preparation are first in the design consideration, before fabrication. Perform all welding, drilling, and forming operations before galvanizing, where possible, to achieve a uniform coating coverage and prevent the coating damage caused by post-galvanizing processes. Elements that are well drained and ventilated to avoid acid retention during the washing process, and full coverage of zinc coating.
Cleaning of the surface is to be done with care to remove contaminants. Oil, paint, and organic residues are to be completely removed by use of alkaline degreasing, whereas mill scale and rust are to be removed by use of acid pickling or abrasive blasting. Base steel Silicon content must not exceed 0.04 percent or less than 0.15 percent, as it may result in silicon-induced differences in coating thickness, resulting in brittleness and unsightly appearance.
Galvanized steel coating thickness standards Thickness Standards Galvanized steel coating varies according to use and harshness of the environment. ASTM A123 recommends a minimum average thickness of 85 micrometers when using steel that is more than 6mm thick, and a thinner section can have lower requirements. In harsh marine conditions, a coating thickness of 150+ micrometers can be required to perform best.
During transportation and installation, the coating is not damaged due to improper storage and handling. Components made of galvanized steel should not be kept in damp or stuffy places and should have a good spacing to avoid the trapping of water and the formation of white rust. Dry wooden dunnage should be used to allow air to move between galvanized sheets or components to keep the moisture trapped between sheets dry and to avoid galvanic action.
Maintenance strategies for galvanized steel. Galvanized steel maintenance policies aim at maintenance to maintaining the integrity of the coatings instead of replacement. Periodic checking of coating damages, immediate repairing of scratches or corroded areas using zinc-rich paint, and cleaning to get rid of corrosion deposits increase the service life. Lasting cleaning to remove salts or chemicals that may have been deposited on the coating can increase coating life in an industrial setting by two or three times.
The quality control of galvanizing should be performed by measurement of coating thickness with magnetic thickness gauges as per ASTM E376, visual inspection of defects, and adhesion test, where necessary. The distribution of coating across component surfaces must be within the required standards, and the internal surfaces, corners, and complex geometries that are difficult to coat uniformly should be of concern.
Real-World Case Examples
The Verrazano Narrows Bridge shows the long life of galvanized steel in harsh environments at sea. Components of the original galvanized suspension cable still in service after 60+ years of exposure to saltwater in 1964, coating performance remained higher than anticipated. Frequent observation reveals that the quality of the coating wears at rates that are much lower than the estimated ones, which confirms the choice of galvanized steel as long-term infrastructure.
Automotive industry success. Success in galvannealed steel body panels used in the automotive industry has practically completely removed rust-through corrosion in the current vehicles. According to leading manufacturers, galvannealed steel implementation has cut warranty claims due to corrosion damage by more than 90 percent than the past painted steel panels, and allowed thinner gauges to reduce weight without reducing the life expectancy.
Agricultural applications. There are agricultural applications that demonstrate galvanized steel’s performance in adverse environments. Confinement housing with galvanized structural steel and roofing materials has a 2a 5-30 year service life in conditions with a high concentration of ammonia, moisture, and temperature change that would quickly ruin painted steel solutions.
Precision manufacturing applications: Zinc electroplated finishes on small parts and fasteners are used in precision manufacturing processes. Electronic enclosure devices with a zinc coating of 8-12 micrometers offer effective corrosion protection and yet tight dimensional tolerances necessary in high precision assembly processes.
Conclusion
Galvanized steel is a time-tested, cost-efficient corrosion protection in a wide range of applications, such as small manufacturing parts and massive infrastructure projects. Its combination of barrier and sacrificial protection mechanisms provides decades of maintenance-free operation in most atmospheric conditions, and the range of galvanizing processes allows it to be optimized to meet the needs of a particular application. Nonetheless, the implementation should be done with proper attention to constraints such as temperature limits, possible flaws in coating, and environmental forces that influence performance. Depending on the requirements of the service, environmental and lifecycle cost, engineers and procurement professionals are urged to compare galvanized steel to other types of steel, such as galvannealed steel, aluminum, or stainless steel. You can tell us about your experiences using galvanized steel in the comments below, and you can read more of our related guides on galvannealed steel properties, the selection criteria between aluminum and steel, and advanced coating technologies used on special applications.
