Steel is the world’s most important metallic construction material due to its advantageous mechanical properties, general availability, and low price. However, it must be protected against corrosion in most environments to provide a sufficiently long service life and meet the safety requirements. Today, zinc coatings are widely used for steel protection in automotive, construction, home appliances, renewable energy, and other industries, with the global market for zinc galvanised steel estimated at 174.6 billion US dollars in 2022. Of approximately 14 million tons of yearly worldwide zinc production, 60 % are used for steel corrosion protection. Zinc coatings applied by continuous hot-dip galvanising of steel sheets dominate the market. They bring strong economic benefits as a result of a longer service life of the final products and improved aesthetic properties. In particular, zinc coatings are unique in their ability to galvanically protect steel, serving as a sacrificial anode in defects. Corroding zinc polarises steel to a more negative potential where it does not corrode.

Although currently indispensable in steel corrosion protection, zinc coatings have several drawbacks, namely:

• Lower corrosion resistance in marine environments, requiring the application of thick coatings or costly additional means of corrosion protection such as organic coatings and cathodic protection for marine structures such as the hulls of ships and the towers of wind power plants.
• Increased risk of hydrogen entry into steel in defects in zinc coatings due to steel polarisation and the consequent risk of hydrogen embrittlement (HE). In an extreme case, it can lead to a fracture of the affected steel part. This is of particular importance for high-strength steels gradually replacing traditional steel grades in automotive, construction, and elsewhere. 
• Low recycling rate of zinc. According to the International Zinc Association, the end-of-life recycling rate for zinc is 45 % worldwide, with even a lower ratio expected for galvanised steel products (exact value not known). Currently, economic reserves are at 250 million tons, covering zinc consumption for another 20 years. In the long term, zinc resources can be depleted.

The only economically viable alternative metallic coating material, aluminium, is cheaper, lighter, widely available, and more corrosion resistant. However, it is unable to provide sufficient protection to steel in defects, leading to red rust formation, and thus it is used only marginally.

To solve these shortcomings, ALCOAT will develop two new families of aluminium alloy coatings for protection of wind towers, ships and other structures exposed to seawater and atmosphere, and steel sheet products for automotive, building, and home appliance industries. The coating composition and microstructure will be designed using advanced computational and molecular modelling. A novel, ground-breaking chemically-tailored potential difference concept will be developed and applied to precisely tailor the potential difference between the coatings and steel substrate in relevant environments. Application of this concept will ensure that the corrosion potential of the coatings is more negative than that of steel, thus guaranteeing sacrificial protection of steel in defects and protection against red rust formation, and is still more noble than that of zinc, which is in a range where the risk of hydrogen embrittlement exists.

The concept is schematically shown below.

The new coatings will be more sustainable than zinc coatings because of the lower corrosion rate, lighter because of the lower specific mass of aluminium, ensure savings of primary raw materials due to the use of iron-contaminated aluminium scrap, and improve the safety of steel constructions because of risk of hydrogen embrittlement.