Effect of Mg on the Formation of Periodic Layered Structure during Double Batch Hot Dip Process in Zn-Al Bath

Abstract

The article presents the results of studies on the influence of Mg on the formation of the periodic layered structure of the Zn-AlMg coatings. These coatings were produced by the double batch hot dip method in a Zn bath and then in a Zn-Al(Mg) bath with a content of 15, 23, 31 wt.% Al and 3, 6 wt.% Mg. The microstructure of the coatings (OM, SEM) was revealed and the phase composition (XRD) obtained in two-component Zn-Al baths and Zn-AlMg baths were determined. The periodic layered structure was found to consist of alternating FeAl3 phase layers and a bath alloy (Zn + Al + Mg). Moreover, it was observed that the addition of 3 wt.% Mg reduces the thickness of the coating in baths containing 23 and 31 wt.% Al. However, the addition of 6 wt.% Mg causes complete disappearance of periodic layered structure in a bath with 23 wt.% Al. In a bath with a content of 31 wt.% Al the addition of 6 wt.% Mg creates a compact layer consisting of the FeAl3 phase containing the precipitation of the MgZn2 phase and Fe2Al5 phase. Such a structure of the coating transition layer limits the growth of the periodic layered structure in the coating.

Structure and Corrosion Resistance of Coatings Obtained by the Batch Double Hot Dip Method in Eutectoid ZnAl Bath with the Addition of Mg and Si

“…The formation of the coatings in the two-component ZnAl23 bath leads to the formation of a periodic layered structure, which causes the formation of porous coatings of excessive thickness. The addition of Mg to the bath reduces this phenomenon, but does not eliminate it completely [11]. The research shows that the addition of Si to the bath helps to inhibit the formation of a periodic layered structure.…”

“…The growth of the coating under such conditions also causes the formation of a periodical layered structure (PLS), characterizing the alternating structure of the intermetallic phases layers of the FeAl system and the alloy layers of the metalizing bath. A slight decrease in the reactivity of steel is caused by the addition of Mg to the bath [11]. On the other hand, the coatings obtained in ZnAl baths containing Mg show very good corrosion resistance [6][7][8][9].…”

Thermodynamic Simulation Calculations of Phase Transformations in Low-Aluminum Zn-Al-Mg Coatings

“…Previous studies have focused on the MgZn 2 phase within Zn-Al-Mg coatings, considered a crucial factor in enhancing these coatings’ corrosion resistance [ 3 , 12 , 13 ]. However, as research progresses, some studies have also identified the presence of the Mg 2 Zn 11 phase, indicating that the phase composition of these coatings is more complex than expected [ 8 , 14 , 15 , 16 , 17 ].…”