Cracking mechanisms in high temperature hot-dip galvanized coatings
Introduction
Hot-dip galvanizing is one of the major methods used to apply zinc-based coatings on steel in order to provide sacrificial protection against corrosion. Difficulties arise during galvanizing of Si-containing steels; the enhanced reactivity of steel in the presence of Si under typical galvanizing conditions results in very thick and brittle coatings with unacceptable appearance [1], [2]. High temperature galvanizing has been put forward as a solution for this problem; by galvanizing at 560°C instead of the typical temperature of 460°C the reactivity of steel is reduced and thin, uniform coatings with acceptable appearance are produced [1], [3], [4].
While the microstructure and the growth kinetics of high temperature galvanized coatings have been studied in detail [1], [5], no effort has been made on the study of adhesion and mechanical behavior of these coatings. The mechanical response to loading affects significantly the protection efficiency of the coatings and the formability of coated strips; cracking and delamination of the coatings during forming and during service decrease corrosion resistance by reducing coating weight and exposing the substrate (steel) to the corrosive environment.
This work investigates the cracking behavior of high temperature hot-dip galvanized coatings when tested in 3-point bending under displacement control. Although the specific subject has not received a proper attention, a plethora of information on the cracking of coatings attached to substrates is available. A thorough review on cracking of layered materials has been presented by Hutchinson [6]; the effects of residual stresses on the fracture and failure of coatings have been studied extensively [7], [8], [9], [10]; mechanisms of cracking and decohesion of thin coatings have been presented [11], [12], [13], [14]; fracture and failure of bi-material interfaces have been described in detail [15], [16], [17], [18]; and finally, strain-induced crack spacing in coatings are discussed in [19], [20], [21].
Coatings produced by hot-dip galvanizing are complex multi-layer systems consisting of phases with different thermomechanical properties, making the analysis of the mechanical behavior of the coated system (substrate and coating) difficult. The analysis of the mechanical behavior of galvanized systems becomes more troublesome considering the lack of information on the thermomechanical properties of individual phases that constitute the coating, as well as the interfacial properties. The intent of the present work is to critically use all available information in order to assess cracking mechanisms and failure of the studied coatings. This work evaluates the influence of processing history, as mirrored through residual stress and microstructure, and quantifies the effects of bending angle (related with the applied strain) and immersion time (related directly with the coating thickness) on the induced damage. Based on the results of this work, the cracking mechanisms that lead to the failure of high temperature hot-dip galvanized coatings are described.
Section snippets
Procedure
Low carbon cold-rolled steel, kindly provided by Hellenic Steel Co., Thessaloniki, Greece, was used as the substrate for the high temperature hot-dip galvanized coatings. Its chemical composition is shown in Table 1. Strip samples 8.0 cm long, 1.4 cm wide and 0.5 mm thick, were galvanized in a bath prepared by melting high purity zinc pellets (99.99%) in a laboratory electric furnace using a graphite crucible.
Prior to galvanizing, the steel samples were initially degreased by immersion to an
Microstructure
Metallographic and X-ray diffraction analysis of the galvanized coatings revealed a sequence of layers in accordance with the published literature: a coherent layer of the δ1 phase was present next to the substrate, with its outer region fragmented. This layer was followed by a two-phase layer consisting of a mixture of δ1 crystals dispersed into the η phase (Zn solid solution). An outer layer of pure η phase was frequently present. The ζ phase was not detected. A typical microstructure is
Effects of residual stress
It has been shown that significant residual stresses develop in galvanized coatings during the formation of the coating and during subsequent cooling [1]. Residual stresses (either compressive or tensile) may arise during the formation of individual layers, such as the δ1 and Γ phases due to differences in molar volume as it varies with iron content [28], as well as during phase transformations during cooling from the dipping temperature. More importantly, significant residual stresses arise
Conclusions
This analysis has elucidated the cracking behavior of high temperature hot dip galvanized coatings. Tensile residual stresses developed in the δ1 phase during processing are responsible for the formation of a microcrack network within the δ1 phase. These microcracks initiate at discontinuities created by hot corrosion attack of the δ1 phase by liquid zinc during galvanizing that act as stress concentration points when residual stresses develop. Upon subsequent mechanical loading, a fraction of
Acknowledgements
Steel samples were provided by Hellenic Steel Co., Thessaloniki, Greece. The first author acknowledges the contribution of Ms Binu L. Jacob, Drexel University, Philadelphia, PA, USA, for her assistance with ANOVA, and of Dr Vassilis Stamos (European Commission, Joint Research Centre, Institute for Advanced Materials) for his stimulating discussions, valuable comments, and thorough reading of the manuscript.
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