Galvanization as a coating for preventing the damaging effects of corrosion has been popular for more than a century. The galvanization of iron and steel products is normally chosen for its low cost, low maintenance, long life, sustainability and aesthetics. Here we'll take a look at the advantages and limitations of galvanization, the different processes and their applications, along with some of the environmental concerns.
What is Galvanization?
The galvanization process involves applying a zinc coating to the surface of ferrous metals to retard or prevent the corrosion of the substrate metal. A zinc coating produces a physical barrier while also working as a sacrificial anode. When zinc reacts with atmospheric oxygen, zinc oxide is produced, which forms zinc hydroxide in the presence of moisture. The zinc hydroxide further reacts with carbon dioxide in the natural environment, resulting in a thin layer of impermeable zinc carbonate, which binds to the zinc beneath. Zinc carbonate thus protects and preserves the zinc layer from any further corrosive oxidation. This protection is comparable to the passive oxide layer of aluminum that protects an aluminum substrate from further oxidation.
The History of Galvanization
The term galvanization, as it applies to corrosion, was first used to describe the process of applying a coating of zinc to an iron surface when the process was simultaneously developed in France as well as in England in 1837. The method employed then was to manually dip iron sheets into a hot bath of molten zinc. The first patents for galvanized metals were obtained by M. Sorel of France and H. W. Crawford from England. Within two years, galvanized iron sheets became popular for roofing applications in Europe and the U.S.
The Advantages of Galvanizing
Some of the notable advantages of galvanization include:
- It is one of the most effective techniques for protecting the surfaces of iron and steel against corrosion. A ton of coated zinc saves at least around 20 to 30 tons of iron from corrosive destruction.
- The galvanization process provides not only a low initial cost, but also one of the lowest life cycle costs among all of the protective coating techniques. Other corrosion prevention systems, such as polymeric powders and paints, have continuously seen cost increases of high magnitude over the past decade. Moreover, they require frequent inspection and maintenance, which adds to life cycle costs. Both direct maintenance costs for polymeric powders and paints, such as repair costs as well as indirect maintenance costs, add up to several times the initial capital costs. (For information on the suitability of various coatings including zinc, see Anti-Corrosion Coatings for Different Service Exposures.)
- Because of the escalating cost of polymeric coating systems and the astronomical rise in maintenance costs, more and more users have started to consider the life cycle costs when selecting systems, materials and components. Life cycle analysis considers the benefits versus total costs (including inspection, monitoring and maintenance) throughout the expected life of an asset or a system. Because hot dip galvanized steel requires very little maintenance for its life span, which could be as high as 50–70 years, the initial cost alone is the total life cycle cost.
- This technique has the longest history (around 140 years) of providing corrosion protection to iron and steel in most applications.
- A galvanized coating is easy to apply and maintain.
- By-products of the process can be recycled.
- A galvanized surface can withstand UV radiation without suffering any damage.
- A hot dip coated surface will not be damaged by transport and handling.
Different types of galvanizing processes and their pros and cons are explained briefly:
1. Hot Dip (Immersion) Galvanization
Hot dip galvanization involves the immersion of iron or steel products in molten hot zinc after the surface of the base metal has been properly and adequately cleaned. (For a background on each step, read Specifying the Gold Standard for Color and Protection.) This is also suited for the galvanization of large bulky items. It can be completed as a batch process or a continuous galvanizing process.
In batch galvanizing, often called fabrication galvanizing, the iron or steel product to be coated is formed into its required final size and shape, cleaned thoroughly and then immersed for a specific duration in the bath of molten zinc. Any shape of ferrous metal can be galvanized in this batch processing method. Overall, this process involves three major steps:
- Cleansing (caustic cleanser, pickling, rinsing) and flux coating
- Post-treatment such as quenching and grinding
The preparatory cleansing consists of dipping the iron/steel in cleansing baths prior to galvanization. The first bath is a caustic cleanser, which removes grease, oil and dirt. The second bath is a rinsing water bath and the third is a pickling bath containing diluted (10%) sulfuric acid or a hydrochloric acid. After the pickling process, which removes rust and mill scale, the product to be galvanized is again rinsed in water and soaked in a flux solution to keep it ready for galvanizing.
The galvanizing bath consists of a minimum of 98% pure, molten zinc, maintained at around 850°F (455°C) by continuous controlled heating. When the ferrous product reaches the bath temperature, the zinc reacts with the iron on the ferrous metal/steel surface to form a layer of coating that protects the surface. Depending on the thickness of the steel section, it may take around 10 to 15 minutes for the steel to heat up and complete the reaction.
The post-treatment steps can be quenching, if required, as well as the grinding of excess zinc icicles. This is all followed by a final inspection.
- The thick coating formed using the hot immersion galvanization process is suitable for outdoor as well as indoor applications.
- The crystallites in this case are highly visible, and provide an aesthetic element called a spangle. By varying the cooling rate and the chemicals, the shining spangle can be calibrated to be more heterogeneous.
- During the hot dipping process, a structure of hard and brittle iron-zinc alloy is formed in multiple layers between the inner surface of the zinc coating and the iron or steel. Under the bending loads, these brittle layers could flake off. It is also unsuitable for very small components, as the coating thickness cannot be accurately controlled.
2. Electrogalvanizing (Plating)
Electrogalvanizing, or plating, is an electrochemical process involving the immersion of ferrous or steel products in an electrolyte of zinc metal, such as a solution of zinc sulfate. Pure zinc gets plated on the surface of the product to be protected. Electrolytic action deposits a coating of pure zinc on the surface of the product and works as a cathode. The coating produced using this process is very thin.
- Coating thickness can be monitored and controlled very accurately, and provides a bright appearance and aesthetic suitability. It can be adopted for smaller components and can be cheaper compared to a hot dip process. It is often used for the light mechanical parts of bicycles, cars and home appliances.
- Thick coatings similar to those produced by hot-dip galvanizing can't be produced using electro-galvanizing. This is normally suitable only for indoor applications, as otherwise it may get depleted quickly. Additional painting is also required for some electro-galvanized automotive components used in an outdoor environment.
Sherardizing is a thermal diffusion-type of galvanizing, where a zinc coating is formed on ferrous parts by tumbling zinc powder with these parts in a hot sealed drum, resulting in zinc alloy formation on the surface. The drum is heated uniformly, well below the melting temperature of the zinc metal. It is a preferred method for coating small, complex-shaped metals, and for smoothing in rough surfaces on items formed with powder metal.
- Creates a uniform coating of intricate shapes and complex configurations. Coating thickness can be up to 300 microns.
- This process can’t be adopted for bulky items. It's a costly process with a long cycle time.
4. Zinc Spray Galvanizing
The zinc spraying process requires a thorough cleaning of the product surface. Zinc in wire form or powder form is sprayed on the surfaces by using a plasma flame. This process produces a porous coating in the range of 75–200 microns. It is the next feasible option when the hot dip process is ruled out due to the shape and size of the product. It can be used for repairing galvanized coatings.
- The coating is highly malleable and unlikely to crack, peel or rupture on bending.
- The coating is significantly porous and susceptible to early depletion due to moisture.
5. Continuous Galvanizing
Continuous galvanizing and annealing lines are found in large steel plants, and produce coated steels in bulk. The process involves the application of zinc to a strip or a wire when it passes through a molten metal bath. The steel is passed through the bath at very high speeds and is only in the bath for about three to four seconds. After the galvanization, the steel is further processed, as required. This method is feasible only for thin, flexible strips and sheets, as the final shaping has to be completed later on. Steel wires, roof sheets and automobile body panels are products that may use continuous galvanizing.
In this process, the coating thickness is monitored and controlled accurately through an air wiping technique, which is completed as the sheet emerges from the molten bath. The thickness can vary from 7 to 42 microns. The thin zinc-iron alloy layer formed by the coating gives the much-needed flexibility for further processes, such as pressing and forming. In the case of wires, coating thickness may vary from 3 to 43 microns. In the case of the continuous galvanizing of tubes and pipes, only an external coating is feasible and the thickness is typically 12–25 microns. In the case of semi-continuous tube plants the coating thickness is around 65 microns.
New Developments in Continuous Galvanizing
Newly developed precision hot-dip galvanizing and electrolytic coating systems in steel plants use servo-hydraulic adjustment systems, providing a fast reaction time and high-precision coating thickness. This may be used for galvanizing hot-rolled as well as cold-rolled steel coils and steel wires. Newly developed dynamic air knives help high-quality wiping of the steel strip surface as it's leaving the molten zinc bath, thus minimizing zinc coat thickness deviation and ensuring high standards of surface quality. It also provides an aesthetically pleasing decorative super finish to meet the requirements of the appliance, automotive and construction sectors.
Steels that are difficult to galvanize:
- Special high-silicon steels can produce brittle galvanized coatings, depending on the length of immersion time needed for reaction.
- Steels with high manganese content can form a brownish brittle coating, which is quickly lost and damaged in handling and transportation.
- High-sulfur special steels should not undergo galvanization, as they can get eroded during the process.
- Special high-phosphorous steels form a thick coating that easily delaminates and deteriorates.
- Galvanized stainless steels will be affected by liquid metal embrittlement, which can cause failure by fracture during use. Stainless steels need to be galvanized only if they are physically connected to mild steel as part of the assembly in order to avoid galvanic corrosion.
- Cast steel as well as cast iron can be galvanized effectively by ensuring a thorough, abrasive blasting to remove the molding sand and other refractory contaminants before the coating process begins.
- In the case of high-strength steels with over 115,000 psi yield value, pickling must be avoided, as hydrogen embrittlement will reduce its strength.
- Very heavy structural sections may make the molten zinc freeze during the immersion, which would require remolding of zinc and heating of the metal being coated all over again. This may interfere with the cycle time of the process and quality of the coating.
Limitations of Galvanizing in Corrosion Protection
Galvanization of iron and steel items cannot provide absolute protection against severe corrosion from exposure to salt water, reactive acids, alkalis and their compounds in various forms. Cathodic protection and other strategies are needed in such cases, as per the severity of the exposure.
In case of severely loaded applications such as wire ropes, which constantly face cyclical loading and severe bending stress near the drums and pulleys, galvanization can cause loss of yield strength due to hydrogen embrittlement. In such applications, the safe working load has to be reduced, and the system derated properly (safety factors needed to be increased). Ropes will also need frequent inspection, and the use of galvanized ropes for heavy-duty hot metal cranes should be avoided. (For more about loaded applications, see High Pressure Fastener Coating Practices Under Fire: Ian MacMoy Speaks Out.)
Sulfur dioxide in the air, the natural acidity of rain, and other reactive substances may reduce the life of galvanized steels. The high conductivity of seawater also increases the risk of corrosion by converting the coated zinc to water-soluble zinc chloride. Zinc-coated car frames corrode in the winter season as a result of road salt. Additional protection from a sacrificial anode or a paint application needs to be considered in such cases.
Galvanization: Sustainability and Recycling
Unlike the coatings that use volatile organic compounds (VOCs), galvanizing is considered environmentally-friendly and sustainable. A ton of zinc coating saves around 20 to 30 tons of ferrous metals when the coating is properly applied. (The corrosion rate of zinc is around 1/30 to 1/20 of the corrosion rate of unprotected iron.) Gibbs Free Energy (GFE) is one of the measures of ranking the sustainability of metals. It represents the energy used from the mining to the metal stage in MJ/Kg. Zinc has a GFE value of 3.0MJ/Kg, whereas steel has a GFE value of 6.5 MJ/Kg. Zinc is more sustainable than steel, and the zinc requirement is around 5% of the mass of the steel it protects. However, there are opportunities in the recycling of waste products produced in galvanizing, which reduce pollution and ensure adherence to local environmental compliance rules. The pickling, rinsing and cleaning baths of the galvanization process, in particular, provide opportunities for recycling wastes. Acids and other chemicals need to be disposed of as per regulation.
Some quantity of metallic waste is generated in a galvanizing tank as dross as well as skimmings. Skimmings float to the top, and consist of alloys of iron and aluminum, while dross—which sinks to the bottom—is made of iron and zinc. Accumulation of these wastes interferes with galvanization, and hence it is removed and taken off-site. It is possible to recycle most of the metallic waste products. Air pollution that occurs in the heating of a galvanization tank may also be a concern. Safety and health issues related to the hazards of molten hot metal and pickling tank operation may also be issues with this corrosion prevention method.
Inspection of Galvanized Surfaces
When it comes to inspecting galvanized surfaces, automated surface inspection systems are adopted for the online surface quality monitoring of galvanized metals, and for classifying and detecting surface area defects in continuous galvanization processes. In the case of batch production, visual inspection gives fairly accurate results. Zinc does not form a coating on unclean surfaces.
Coating thickness is a critical parameter, as the life of the coating is proportional to coating thickness and its adherence (bonding) to the substrate. Surface finish and the decorative quality are also checked against the specified requirements. Because the molten zinc cannot penetrate small cavities, a coating discontinuity will show up as a bleeding or a blowout. In the case of reactive steels, the flaking of the surface coating may be observed. Contaminants on the surface may also show up as a surface defect. Root-cause analysis of surface defects could lead to appropriate preventive measures.