Light Metals Zinc for Corrosion Protection
Worldwide, almost 14 million tonnes of zinc were produced in 2015, and a further 4 million tonnes were obtained from recycled materials. About half of this amount is used as corrosion protection for steel by galvanizing. What are the most common methods of galvanizing?
Steel is an important construction material - for example, for the construction and maintenance of infrastructure, in construction and automotive engineering, and in the production and transmission of renewable energies. For steel to perform its function safely and with minimum maintenance over many years, it must be reliably protected against corrosion. Over 50 % of the world's zinc production is used to protect steel from corrosion by galvanizing. Classical processes for construction, infrastructure, and energy are hot-dip galvanizing (batch galvanizing) and thermal spraying with zinc. In the automotive industry, strip galvanized steel is processed, smaller parts such as wheel nuts are electro galvanized. New alloys are currently expanding the range of applications for galvanizing in the automotive industry, among others.
Active and Passive Corrosion Protection
Corrosion is the oxidation of metals by environmental influences. Important factors influencing the course of corrosion are moisture and the type of metals involved. Corrosion processes are manifold and often complex. When steel oxidizes, a reddish corrosion product is formed, which is commonly called "rust." Corrosion can impair the function of steel components.
Zinc protects steel from corrosion in two ways. Firstly, a zinc or zinc-containing separating layer creates a physical separation between steel and the corrosive environment. Zinc has the advantage that it forms a so-called patina on its surface, which significantly slows down the corrosion of the zinc itself. Besides this passive corrosion protection, zinc also has an active effect. In humid environments, zinc and iron form a so-called local element. Chemically speaking, zinc is less noble than iron, gives off electrons, and dissolves. While the nobler element - steel - remains. This is chemically called a redox reaction and metaphorically speaking, zinc "sacrifices" itself for steel.
Depending on the environment and function in which steel is used, different processes are used as corrosion protection. What these processes have in common is the use of zinc.
Why is Corrosion Protection Important when Using Steel?
There are many reasons for protecting steel against corrosion for as long as possible:
- Cost savings: A long and maintenance-free service life of steel structures and machinery saves the costs of repairs and new purchases.
- Resource efficiency: If steel lasts longer and does not need to be replaced, less steel has to be extracted from ore bodies. Less energy is also needed to produce steel.
- Climate protection: Every tonne of steel that does not need to be replaced by new steel before its expiry, does not have to be produced. As a result, less CO2 is produced during steel production.
- Closing material cycles: The recycling of zinc and steel from galvanized steel is not only possible - it has also been practiced successfully for many years. In Europe, strict legislation promotes the recycling of valuable contents in residual materials, which among other things, leads to a high recycling rate of zinc-containing steel mill dust of around 98 percent.
When to Use Which Type of Corrosion Protection?
Corrosion influences the effectiveness, performance, and safety of plants, machines, and materials - and it shows very different characteristics. A cable car in the Alps is exposed to corrosion in a different way than offshore wind turbines or lock gates, which are subject to continuous exposure to fresh or saltwater and also wastewater for decades. A fender on a car has to withstand different stresses than an exhaust silencer. Even on a steel bridge, the loads differ in the spray salt area and in the upper part of the pylons. The processes that have proven their worth as corrosion protection for steel are tailored to different areas of application:
- Hot-dip galvanizing (piecewise galvanizing)
- Strip galvanizing
- Thermal spraying with zinc
- Thin layer galvanizing
- Zinc dust paints