Galvanization (also spelled galvanisation)[1] is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are coated by submerging them in a bath of hot, molten zinc.[2]
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The zinc coating, when intact, prevents corrosive substances from reaching the underlying iron.[3] It's main function is to act as a sacrificial anode to prevent the iron from rusting by cathodic protection. Zinc is more reactive than iron, so the zinc coating preferentially oxidizes to zinc carbonate, preventing the iron from corroding, even if there are gaps in the zinc coating. Additional electroplating such as a chromate conversion coating may be applied to provide further surface passivation to the substrate material.[4]
The process is named after the Italian physician, physicist, biologist and philosopher Luigi Galvani (9 September – 4 December ). The earliest known example of galvanized iron was discovered on 17th-century Indian armour in the Royal Armouries Museum collection in the United Kingdom.[5]
The term "galvanized" can also be used metaphorically of any stimulus which results in activity by a person or group of people.[6]
In modern usage, the term "galvanizing" has largely come to be associated with zinc coatings, to the exclusion of other metals. Galvanic paint, a precursor to hot-dip galvanizing, was patented by Stanislas Sorel, of Paris, on June 10, , as an adoption of a term from a highly fashionable field of contemporary science, despite having no evident relation to it.[7]
Hot-dip galvanizing deposits a thick, robust layer of zinc iron alloys on the surface of a steel item. In the case of automobile bodies, where additional decorative coatings of paint will be applied, a thinner form of galvanizing is applied by electrogalvanizing. The hot-dip process generally does not reduce strength to a measurable degree, with the exception of high-strength steels where hydrogen embrittlement can become a problem.[8]
Thermal diffusion galvanizing, or Sherardizing, provides a zinc diffusion coating on iron- or copper-based materials.[9][10]
Galvanized steel can last for many decades if other supplementary measures are maintained, such as paint coatings and additional sacrificial anodes. Corrosion in non-salty environments is caused mainly by levels of sulfur dioxide in the air.[11]
This is the most common use for galvanized metal; hundreds of thousands of tons of steel products are galvanized annually worldwide. In developed countries, most larger cities have several galvanizing factories, and many items of steel manufacture are galvanized for protection. Typically these include street furniture, building frameworks, balconies, verandahs, staircases, ladders, walkways, and more. Hot dip galvanized steel is also used for making steel frames as a basic construction material for steel frame buildings.[12]
In the early 20th century, galvanized piping swiftly took the place of previously used cast iron and lead in cold-water plumbing. Galvanized piping rusts from the inside out, building up layers of plaque on the inside of the piping, causing both water pressure problems and eventual pipe failure. These plaques can flake off, leading to visible impurities in water and a slight metallic taste. The life expectancy of galvanized piping is about 40–50 years,[13] but it may vary on how well the pipes were built and installed. Pipe longevity also depends on the thickness of zinc in the original galvanizing, which ranges on a scale from G01 to G360.[14]
Galvanized steel is a steel that has a protective zinc coating on its outer surface. The zinc is used to increase the steel's corrosion resistance and is applied most commonly by hot dipping the steel into molten zinc. The zinc can also be annealed after the hot dipping process or electroplated to the steel. Galvanized steel can be used in a variety of applications, the only common factor being the applications needed for corrosion resistance. Galvanized steel is used in construction, aerospace, automotive, oil & gas, and many other industries.
This article will discuss what galvanized steel is, how it is made, its properties, and its uses.
Galvanized steel is a form of standard steel in which the steel is coated in zinc to ensure enhanced corrosion resistance. This galvanized coating protects the steel substrate from damage typically caused by moisture or temperature changes. The steel is dipped into molten zinc, which then bonds itself to the steel and becomes part of its chemical composition.
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The manufacturing process of how to galvanize steel is relatively straightforward to understand. Normally, the manufacturer will dip steel into a bath of molten zinc at a temperature of approximately 450 ºC, which in turn binds itself to the steel, as is the case with any alloy. The zinc becomes part of the steel’s chemical composition, therefore, affording it the properties of both as a gradient mixture of the two metals. The most common form of galvanization is hot-dipped galvanization. It is possible to spray zinc onto the steel, however, a weaker layer of zinc is created by this method. The galvanized steel coating then acts as a barrier to protect the metal underneath from environmental conditions and enables it to be more resistant to corrosion.
There are four ways in which steel can be galvanized, each affecting the type of layer formed and the final application. The four methods are described below:
Galvannealing is the combination of hot-dip galvanizing and annealing processes to produce a specialized coating on steel sheets. The finished product is a zinc-iron alloy product, with a dull matte surface finish that is ideally suited for paint adhesion. This process works by hot-dipping the base metal, and then heating it in an annealing oven at a temperature of roughly 565 ºC to induce alloying between the steel and molten zinc coating. Examples of galvannealing include: welding operations, marine applications, and any high-tensile stress applications.
Hot-dip galvanizing is a form of zinc coating applied to steel sheets (or manufactured steel items). This occurs by passing the steel objects through a bath of molten zinc and then cooling them. The zinc bonds to the iron within the steel. Hot-dip galvanizing works by forming a protective layer on either side of the metal to protect the finished product from corrosion. Galvanized steel does not require any secondary or additional coating or painting. Examples of hot-dip galvanizing include: industrial flooring, motorway guardrails, brackets, and fixings.
Electrogalvanization is a process in which steel is galvanized with a zinc coating using electroplating. Steel is submerged in a zinc salt solution and an electrical current is applied to create electrogalvanized steel. The current creates a reaction between the zinc salt and the surface of the steel, leaving behind a thin layer of zinc. This zinc laver is much thinner than the hot dipping method. Electrogalvanized steel is used in: acoustic ceiling tiles, door frames, and switchboards.
The pre-galvanizing process, also referred to as mill galvanization, is performed at the very initial stages of production on metals which are likely to have a limited number of processes after pre-galvanization. This is because carrying out any process on galvanized steel will result in the exposure of bare metal to the atmosphere increasing the risk of oxidation. Components that are pre-galvanized are usually steel plates and bars. This method enables galvanizing to be done quickly and efficiently on a larger scale than is possible with traditional hot-dipping. This process works by rolling the sheet metal (or other shaped steel stock) in a cleaning agent to prepare the material to be passed through the molten zinc, which is then rapidly recoiled. Pre-galvanizing is primarily used for materials that already have a specific shape before being cut.
Table 1 below shows the chemical properties of galvanized steel:
Due to the additional zinc layer of protection against many different environmental conditions, galvanized steel has a range of advantages, such as:
Despite the many advantages of galvanized steel, there are still some disadvantages to this type of steel, which means galvanizing is sometimes not an option to protect steel due to:
The main difference between galvanized steel and regular steel is the protective barrier formed by the addition of zinc to the surface of galvanized steel. This zinc makes galvanized steel much more durable, robust, and rust-resistant. Whereas galvanized steel has been coated in zinc, regular steel is predominantly made of iron which will corrode when exposed to moisture.
To learn more, see our article on Galvanized Steel vs Aluminum.
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