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Changing the structure and properties of hot-dip zinc coatings using diffusion annealing

Zinc coatings are widely used to protect steel goods from corrosion. The physics of methods of zinc applying to steel determines the coating morphology. Hot-dip zinc and thermodiffusion coatings are produced on steel by a diffusion mechanism and contain intermetallic phases according to the Fe-Zn diagram; however, they have different phase morphology, corrosion resistance, and consequently, different operating corrosive media. Thermodiffusion coatings have the highest corrosion resistance. However, their applying technique imposes a restraint on the dimensions of goods due to the small size of a chamber, and it requires much more time (several hours) compared to hot-dip zinc coatings applying for several minutes. In this respect, the authors suggest using the diffusion annealing of hot-dip galvanized goods to produce the entire intermetallic structure of the coating. The goal of the paper was to study the influence of the diffusion annealing modes on the microstructure and properties of hot-dip zinc coatings. The study showed the changes in the microstructure and elemental composition of the zinc coating phases as the result of soaking at the temperatures of 500 and 600 °С for 5 and 10 minutes. The authors researched the influence of annealing modes on the porosity of a coating and its microhardness. The coatings were quickly tested for corrosion resistance in the initial state and after thermal treatment. As a result of diffusion annealing, zinc coating becomes completely intermetallic, more homogeneous; its chemical composition becomes uniform. The study identified that to obtain the balance of physical-mechanical and technological properties, it is recommended to use the diffusion annealing mode with a temperature of 500 °С and soaking in a furnace for 5 minutes.