Jing Xu

Date of Graduation

2007

Document Type

Thesis

Abstract

This research investigates the mechanisms of corrosion attack and dross build-up in molten Zn-Al systems. In hot-dip coating processes, molten Zn-Al baths corrode the submerged hardware, thus requiring frequent line stoppages for maintenance, repair, and replacement of parts. Similarly, containment of liquid Al/Al alloy during melting and recycling processes leads to corrosion of refractory walls, contamination of melt, and subsequent product quality deterioration as well as energy loss. The goals of the research are to (1) explore the corrosion mechanism in molten Zn/Zn-Al baths by studying corrosion rates and interfacial metallography; (2) determine the kinetics of dross adhesion to the bath hardware under both static and dynamic conditions, for example, investigate the affect of roll rotation speed on dross nucleation-growth-transformation; (3) clarify the reactive wetting behaviors of both metallic and composite materials in contact with molten Zn/Al alloy; (4) develop and apply electrochemical methods as effective research tools for the study of in-situ behavior of molten metal instant corrosion rates, and interfacial performance. The research achievements, based on these objectives, will significantly contribute to energy efficiency and productivity in the manufacturing of steel, aluminum, and refractory materials as well as in hot-dip coating operations such as galvanize, galvaneal, galvafan, galvalume, and aluminize. Moreover, other industries involved with the containment, handling, and application of molten metal could also benefit from this research. A quantitative model of dross formation is developed to help understand dross buildup on pot hardware. The dross particle size in a lab-scale galvanizing bath exhibited a normal distribution with an average value of 33 microns, which compares well with results from industrial baths (35 micron). Agreement of laboratory results with industrial measurements helps confirm subsequent experimental conclusions drawn for corrosion resistance and dross formation mechanisms. The static dipping tests of various types of steel in molten Zn baths focus on investigating the general corrosion rate, based on the weight change of samples. The instantaneous corrosion rate was investigated through electrochemical means and discussed in a following paragraph. Dynamic effects of the rotation of the hardware were found to accelerate degradation of the tested materials, namely 316L stainless steel, WC-Co thermal spray, and MSA2020 weld overlay. After evaluating corrosion and dross buildup resistance, it is found that MSA2020 displays the best performance, followed by the WC-Co thermal spray. Stainless steel 316L displayed severe dross attachment on the surface. Moreover, both metallurgical composition and lattice structure play important roles in molten metal corrosion behavior. The reactive wetting behavior in molten aluminum (Al) and Al alloy for three types of alumina-silicon carbide composite refractory materials was investigated using an optimized sessile drop method at 900°C in a purified Ar-4% H2 atmosphere. The time dependent behavior of the contact angle and drop geometry was monitored and the wetting kinetics were determined. The initial contact angle between the liquid Al/Al alloy and the three refractory substrates was found to be greater than 90° and to remain greater than 90° although gradually decreasing with time during the first hour of the experiment. Among the three refractory substrates tested, namely TC, TQ and MC, it was found that TC showed higher contact angle values than TQ and MC, which indicated that TC has better non-wetting performance with molten Al/Al alloy. The difference in wetting properties among the three types of refractories is attributed to variations in their microstructure and composition. Magnesium in the molten Al alloy drop accelerates the reactive wetting processes. Electrochemical investigations were developed and carried out as an effective tool for research on molten Zn-Al systems. The in-situ behavior of molten metal corrosion and its corresponding instant corrosion rate, as well as in-situ interfacial performance was analyzed with electrochemical techniques. The role of electrochemical reactions in corrosion of coated steel is investigated at room temperature (∼25°C). It is found that dross phases display a noble potential so as not to be dissolved by a polarization in a NaCl solution. High temperature (>450°C) electrochemical tests including electromotive force (EMF) tests were carried out to study in-situ interface characteristics by calculating charge transfer numbers in a dominant reaction. In-situ corrosion and dross nucleation-transformation behaviors are also studied from the EMF plots and AC impedance plots. Numeric models are proposed to quantitatively analyze the kinetics of corrosion mechanisms. The EIS spectrum simulation allows a study of the Faradaic system through the equivalent circuit element and, therefore, predicts the behavior of the system with regard to variation of the experimental conditions. Three time constants are found when the electrochemical data are interpretated at different frequency domains, indicating the resistance, capacitance, and inductance properties at the interface. The main findings and contributions of this research are (1) experimental identification of dross size distribution under GI operating conditions; (2) a ranking of metallic hardware materials based on resistance to corrosion and dross build-up in GI hot-dip processes, as well as a ranking of refractory materials based on the resistance to reactive wetting; (3) development and application of high temperature electrochemical research tools for in-situ determination of nucleation and transformation characteristics of dross phases in GI operations.

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