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Impact of galvanizing of couplings of tubings in the oil industry on the structure and properties of coatings
Engineering Education # 04, April 2013
DOI: 10.7463/0413.0559838
Application of protective coatings is one of the promising areas to increase durability of the "pipe-coupling" threaded junction of tubing pipes for oil production. The authors investigated the structure, mechanical and corrosion properties of electroplating, thermal diffusion and hot zinc coatings on steel pipes 45, 30G2 and 37G2F 37G2S with a varying strength group. Thickness of the structure, phase composition, microhardness and pitting resistance of coatings were determined. Zinc coatings of different application techniques differ significantly in thickness, hardness, corrosion resistance and structure, but values ​​of these parameters within a single technology are virtually independent of the base metal. Hot Zn-coating of 100 microns is composed of several layers: T, T1-, δ1-, ζ - and η-phases with a  different content of zinc in accordance with the phase diagram of Fe-Zn. Thermal diffusion coating (40 m) is composed of phases D and T1 with a complex cubic lattice, and the δ1-phase with a hexagonal lattice. The composition of plating (20 microns) are η-phase - solid solution of iron in zinc, Zn and zinc oxide ZnO on the surface. GC and GVC coatings have significantly lower hardness as opposed to thermal diffusion, which is caused by the presence of the solid phase δ1. Pitting resistance of uncoated steel is lower than with coatings. Coating has a maximum resistance of MCC, which has zinc oxide on the surface, and a minimum - GC coating with a two-phase η + ζ-structure. The most resistant to pitting corrosion coating on steel was 37G2S doped by1% silicon, and the most unstable was on steel 45. It is shown that chemical composition and heat treatment of the investigated pipe steels have practically no influence on thickness of coatings, but steel after rolling provides a more uniform plating without obvious lamination.
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