Microstructural study of galvanized coatings formed in pure as well as commercial grade zinc baths

Abstract

This investigation is an effort to have a better understanding of the growth kinetics and morphology of the coating formed during the galvanizing process in pure as well as commercial grade zinc baths. The protective coating that is formed during hot dip galvanizing, normally between 450 and 480 o C, consists of a series of Fe-Zn intermetallic layers, which have been identified as gamma (Γ), delta (δ), zeta (ξ) and an outer eta (η) layer, highly rich in zinc. There is apparently no delay in the formation of ξ or δ phases in both pure as well as the commercial grade zinc baths. The gamma (Γ) phase is formed after an incubation time of about 30 s at a bath temperature of 470 o C in the pure zinc bath. Its formation is further delayed in the commercial grade zinc bath. The last morphological feature is the formation of a second ξ layer at the ξ/δ interface in the pure zinc bath. In the commercial grade zinc bath two different morphologies of ξ phase are seen starting from the lowest dipping time, and also the overall coating is considerably thicker due to formation of several iron-zinc intermetallics which degrade its ductility and outward appearance. Commercial grade zinc also enhances the dross formation in the bath and deteriorates the quality of the coating. Presence of transverse cracks as well as entrapment of dross particles in the coating is attributed to the less compact coating that is formed in the commercial grade zinc bath.

Efecto de la composición química en el comportamiento mecánico de recubrimientos galvanizados por inmersión en caliente: una revisión

“…Demostrando que en los recubrimientos fabricados con baños comerciales, se retarda la aparición de la fase Γ (Fe 3 Zn 10 ) y se aprecian dos morfologías diferentes de fase ζ (FeZn 13 ), las cuales se observan con menores tiempos de inmersión, y también el recubrimiento total es considerablemente más grueso debido a la formación de varios compuestos intermetálicos Zn-Fe que degradan su ductilidad y apariencia externa [9].…”

Influencia del tamaño del cristal de cinc de láminas de acero galvanizadas en ambientes que contienen cloruros y sulfatos

“…En la Fig. 3a se observa la formación de islas asociadas a cristales pseudo-hexagonales que se ubican en la superficie del galvanizado [7]; dichos cristales, inicialmente, se asociaban a productos de corrosión comunes, como la simonkolleita (Zn 5 (OH) 8 En los procesos de galvanizado por inmersión en caliente se adicionan algunos elementos al baño del recubrimiento de cinc, como plomo, estaño, cobre, níquel y aluminio, siendo este último uno de los elementos más importantes en el proceso, ya que no solo mejora la resistencia a la corrosión y aumenta el brillo del revestimiento, sino que asegura una buena adherencia del recubrimiento en el sustrato y le proporciona ductilidad [10].…”

Evolution of microstructures of galvanised and galvannealed coatings formed in 0·2 wt- aluminium–zinc bath

“…The use of zinc coated steels for automotive, construction and appliance applications has increased continually at a greater rate than the overall growth in steel industry during the past decade 1 1,2. In recent practices, galvanised baths containing aluminium in the range of 0·2 wt- are used to inhibit formation of Fe–Zn intermetallics by forming an extremely thin interfacial barrier layer of Fe 2 Al (5–x) Zn x (0< x <1) at the substrate/coating interface 1 – 8.…”

“…The use of zinc coated steels for automotive, construction and appliance applications has increased continually at a greater rate than the overall growth in steel industry during the past decade. 1,2 In recent practices, galvanised baths containing aluminium in the range of 0?2 wt-% are used to inhibit formation of Fe-Zn intermetallics by forming an extremely thin interfacial barrier layer of Fe 2 Al (5-x) Zn x (0,x,1) at the substrate/coating interface. [1][2][3][4][5][6][7][8] It is now well established that during galvanising, the coating adherence and quality improve due to the formation of a fully developed Fe 2 Al (5-x) Zn x (0,x,1) inhibition layer at the interface.…”