Protecting iron and steel from corrosion, commonly known as rust, has long been a challenge for builders and engineers. The longevity of infrastructure and manufactured goods depends heavily on effective methods to shield these metals from the elements. Over centuries, various techniques have been explored, leading to the development of galvanization, a highly effective process that forms the backbone of modern construction and industry.
Defining Galvanization
Galvanization applies a protective coating of zinc to iron or steel to prevent rusting. The most common modern technique is hot-dip galvanizing, which involves submerging the cleaned metal part into a bath of molten zinc heated to approximately 450°C (850°F). This method creates a metallurgical bond, forming a series of zinc-iron alloy layers on the steel surface.
The protection works in two ways: as a physical barrier and through cathodic protection. Zinc is more electrochemically active than iron, meaning it preferentially corrodes when the coating is scratched or damaged. The zinc acts as a sacrificial anode, corroding away slowly to protect the underlying steel from rust.
Early Precursors to Zinc Protection
The idea of using zinc to protect iron predates the formal invention by nearly a century. In 1742, French chemist Paul Jacques Malouin presented a paper describing a method to coat iron by dipping it in molten zinc. This technique was initially applied on a small scale to protect household utensils from corrosion.
Further scientific understanding emerged in the early 19th century through the study of electricity. In 1824, Sir Humphry Davy demonstrated that connecting two dissimilar metals immersed in water caused one to corrode while protecting the other. Michael Faraday later noted the sacrificial action of zinc in protecting iron when they were in contact in a saline solution. These observations established the theoretical basis for the zinc-iron relationship.
The Official Invention and Patent
The formalization of the galvanizing process occurred in France in the early 19th century. The process that defined modern galvanizing was patented by the French civil engineer Stanislas Tranquille Modeste Sorel. Sorel filed his patent in 1837 for a method of protecting iron from rust by coating it with molten zinc.
Sorel’s innovation included a necessary pre-treatment step, not simply the dipping itself. His patented process involved first cleaning the iron with an acid solution, such as diluted sulfuric acid, and then treating it with a flux like ammonium chloride. This preparation ensured a clean surface, allowing the molten zinc to bond effectively with the iron. Sorel also named the process “galvanizing,” taking inspiration from the field of “galvanic” electricity, though the coating is not applied electrically.
Evolution of Galvanizing Methods
Following Sorel’s patent, the hot-dip process quickly became industrialized, with the British industry using significant amounts of zinc for iron protection by 1850. While dipping steel in molten zinc remains the primary method, application techniques have evolved considerably.
The 20th century introduced alternative and more specialized galvanizing methods. Electrogalvanizing uses an electrical current to deposit a pure zinc coating, resulting in a thinner, smoother coating often used in the automotive industry. Sherardizing involves heating the steel with zinc powder, creating a zinc-iron alloy coating through vapor diffusion. Continuous strip galvanizing, developed in the early 1900s, revolutionized the process by allowing for the rapid, continuous coating of steel sheets.