Fig . 1 . Microstructure of the hot-dip galvanized coating ( layers of the η-phase of various thickness are clearly visible in the photographs )
» spraying when the energy of flame formed of a mixture of oxygen and combustible gas ( acetylene , more rarely propane or hydrogen ) is used as a heat source and electric-arc spraying using electric arc as a source of energy and zinc wire ( rod ) or zinc powder as a source of metal .
The mechanism of formation of the metallized coating is based on the process of adhesion of molten zinc particles and therefore strength of coating adhesion to the surface of the coated article depends on the degree of surface roughness , the size of the sprayed metal particles , the speed of their flight and the degree of deformation of the particles upon their collision with the substrate . Particles of metal ( zinc ) having conical form are propelled at a high velocity ( 130-300 m / sec ) from the metallizer nozzle . The degree of particle melting decreases from their center to surface . In powder spraying , some of the zinc particles do not have time to melt at all , and when encountering the surface of the product , the zinc particles are flattened , layered on the adjacent particles and solidified forming a generally monolithic solid , albeit porous zinc coating .
Hot-dip zinc coatings are formed when the product is immersed in a bath with molten zinc .
The mechanism of formation of hot-dip galvanized coatings is based on the phenomenon of diffusion . When galvanizing by the liquid-phase method ( in zinc melt ), steel reacts with molten zinc which results in coating formation on the surface of the steel product . The coating consists of iron-zinc compounds ( intermetallic phases ).
The reaction of the interaction of iron ( steel product ) with molten zinc proceeds in the following stages :
1 ) dissolution of iron in liquid zinc ; 2 ) formation of an intermediate phase of intermetallic compounds at the Fe-Zn interface ; 3 ) interdiffusion of iron and zinc atoms through the formed partition of the intermediate Fe-Zn compound ; 4 ) Formation of a zinc coating with its crystallization at the exit of the galvanized product ( e . g . pipe ) from the bath with molten zinc .
The structure of zinc coatings obtained by hot-dip galvanizing is of a multi-phase nature and includes 5 phases ( α- , γ- , δ- , ζ- , η- ). Their sequence is in exact correspondence with the diagram of state of the “ iron-zinc ” system along the line of the zinc coating temperature .
The structure of zinc coatings obtained by hotdip galvanizing is multi-phase and the top phase ( η-phase ) consists of zinc similar to the zinc melt . It is rather ductile and has a low microhardness of not more than 600-900 MPa . It should be noted here that
20 TUBE NEWS June 2018