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ALCOAT PROJECT

Expected Outcome

Two families of aluminium coatings will be developed. Coatings for flat sheets for the automotive, building, and home appliance industries will be applied by continuous hot dipping. They are expected to contain less iron to keep the product formable. Coatings for large structures exposed in the marine environment, such as towers of wind power plants, will be applied by thermal spraying.

The expected advantages of the new coating compositions are the following.

Reduction of the coating mass. The main component of the coatings developed in ALCOAT for steel protection will be aluminium. Compared to the currently dominant zinc, aluminium has a 2.6-time lower density. Thus, the replacement of zinc coatings with Al-Fe-based coatings will have a strong impact in view of the reduction of metal usage. Even if the novel coatings are applied at identical coating thicknesses, the coating weight reduction will reach about 60 %. In the case of thermal and cold spray coatings, a further reduction in the coating weight can be achieved by lowering the coating thickness because aluminium is more resistant to corrosion than zinc in most conditions. Thus, to obtain the same level of corrosion protection, a thinner coating can be used. For spray coatings, the weight reduction is therefore estimated to be at least 70 %.

Cost reduction. Primary aluminium is about 20 % cheaper than primary zinc. Furthermore, since aluminium coatings will be produced from iron contaminated aluminium scrap sold with a sharp discount of 30 to 85%, new Al-Fe-X (-Y) coatings can be produced significantly cheaper than Zn-based coatings that dominate the market today. Combining the lower density, thickness (in case of thermal and cold sprayed coatings), metal cost, and discount for Fe-contaminated aluminium scrap, the ALCOAT coatings are expected to have the material production cost reduced by at least 75 %. It represents a strong potential for helping to foster necessary investments and rapid acceptance of the Al-Fe-X-(Y) coatings by industry.

Wider application of lighter high-strength steels, reducing the mass of components and inducing fuel economy. High-strength steel (HSS) grades with ultimate tensile strength from 1000 MPa provide a clear advantage compared to traditional carbon steel grades with two to five times lower strength. Consequently, thinner and lighter HSS products can be used. The automotive industry is the first adopter of HSSs because they not only reduce material costs, but also improve safety and reduce fuel consumption. A 10% reduction in vehicle weight has been shown to lead to a 6 to 8% decrease in fuel consumption and a 5% reduction in emissions. Such savings are obviously advantageous for both car producers and end-users. Similarly, important material and installation savings can be achieved for construction materials and many other fields of human activity. However, it is well documented that the risk of HE and the decrease in mechanical properties are directly proportional to steel strength. In parallel, the incorporation of hydrogen into the steel in defects is enhanced in the presence of zinc coatings galvanically protecting steel. The potential risk of failure of HSS under service conditions characterised by elevated corrosivity thus limits the further growth of HSS applications. Any application in which traditional low-strength steel is used instead of HSS due to the risk of HE can be seen as a wasted opportunity to save steel and, in the particular case of transportation, also fuel and limit emissions. The novel Al-Fe-X-(Y) coatings with tailored corrosion potential will reduce the risk of HE in corrosive environments and thus enlarge the application area for HSS.

New market for iron-contaminated aluminium scrap. The price of aluminium scrap decreases with increasing iron content because there is no economically viable method to remove iron from aluminium today. Consequently, a part of aluminium scrap with high iron content is landfilled, whereas the rest is sold at a large discount of up to 85 %. If the new families of Al-Fe-X-(Y) coatings are successfully accepted on the market and applied for steel protection, demand for polluted aluminium scrap will increase, creating new incentives for aluminium recycling.

Longer life and thus lower total cost of ownership (TCO) of the structures protected with the new coatings. The zinc corrosion rate in most parts of Europe is up to 1 µm/year in the first year of exposure. Non-activated aluminium corrodes at a much lower rate below 0.1 µm/year. To protect steel galvanically in defects, the ALCOAT Al-Fe-X(-Y) coatings need to corrode at a somewhat higher rate of 0.2–0.5 µm/year. This means that in addition to the impacts listed above, the new coatings will also be able to protect steel structures for a longer time. Such an increase in lifetime will obviously reduce the TCO, bringing an additional economic benefit to end-users.

Reduction of environmental impact. Primary energy demand for zinc production accounts for 37.5 GJ/ton and a climate change impact of 2.6 tons CO2-eq./ton. In the hypothetical situation that all zinc coatings are replaced by Al-Fe-X(-Y), the project has the potential to save some 1.7·1017 J of primary energy and 11.7 million tons of CO2-eq. Furthermore, it is estimated that aluminium recycling saves approximately 95% of the energy required for primary aluminium production and 97% of greenhouse gas emissions generated in the primary production process.

Improved safety of steel structures due to reduced risk of premature failures due to hydrogen embrittlement. Zinc coatings provide efficient corrosion protection to steel structures, but they may polarise steel in defects to negative values increasing thus hydrogen entry in corrosive environments. Application of the ALCOAT’s chemically-tailored potential difference concept will allow for fine-tuning the potential difference between Al-Fe-X(-Y) and steel in order to ensure adequate galvanic protection of the steel and reduce maximally the hydrogen entry. If the concept proves viable, the application of HSS grades will expand to additional domains that are currently considered risky in view of potential HE failures.

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