- Title
- The influence of the 'steel substrate entry temperature' on wetting and interfacial resistance in hot dip metallic coating
- Creator
- Ebrill, Nicholas John
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2000
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- During immersion of a steel substrate in a metallic coating bath reactions occur between the iron in the substrate and the zinc or aluminium in the bath, forming an interfacial alloy layer. This alloy layer provides the vital meallurgical bond and is therefore critical to the integrity of the coated product. An experimental investigation of the influence of the substrate entry temperature on the initial solid-liquid contacting mechanisms and its impact on the alloy layer formation in hot dip metallic coating is presented. The objective of the study was to link the initial wetting to the interfacial resistances to both heat and mass transfer and to resolve their effect on the subsequent formation of the interfacial alloy layer. An experimental technique, based on the sessile drop deposition method, was developed to study the initial dynamic wetting of metal droplets on steel substrates. Unique to the experimental design was the ability to perform non-isothermal experiments where the substrate temperature was varied from ~250°C to ~800°C. The influence of the substrate temperature on the formation of hot dip coatings was studies using an immersion technique. A thermocouple instrumented substrate was developed to enable millisecond resolution measurements of the thermal response during the transient stages of solid-liquid contact. Interfacial heat fluxes and thermal interfacial resistances were calculated from the temperature responses. Reactive Zn-55Al-1.6Si and Zn baths and non-reactive Pb baths were chosen for study. Under isothermal conditions, reactive Zn-55Al-1.6Si and Zn droplets and non-reactive Pb droplets were found to wet clean metallic substrates within the first millisecond of contact. Under non-isothermal conditions, the substrate preheat temperature had a dramatic influence on the wetting. Wetting was improved as the substrate preheat temperature was increased and approached the bath temperature, beyond which the improvement in wetting was negligible. The transition point for non-wetting to wetting was related to substrate temperatures close to the solidification temperatures of the respective droplets. The maximum heat fluxes reflected the changing wetting conditions at the solid-liquid interface, as well as encapsulating the heat released by the exothermic interfacial reactions in the reactive Zn-55Al-1.6Si and Zn baths. The minimum thermal interfacial resistance decreased by an order of magnitude (1x10-4 to 2x10-5 m2K/W) as the substrate preheat temperature was increased from room to bath temperature. The reduction in the resistance was directly attributed to improved contact through improved wetting. The observed increase in the minimum thermal interfacial resistance for preheat temperatures greater than the bath temperature, in the Zn-55Al-1.6Si and Zn baths, was attributed to the increased heat of reaction. Measurements of the alloy layer thickness, for the Zn-55Al-1.6Si and Zn samples immersed for ~ 1 second, showed increased alloy layer thickness with an increase in the substrate preheat temperature. The number of uncoated areas on the substrates were also reduced as the wetting was improved through an increase in temperature. No coating was withdrawn when the substrates were immersed in the Pb bath as there was no interfacial reaction. Consideration of the phenomena occurring at the interface on the microscopic scale clarified the initial solid-liquid contacting mechanisms. It was proposed that the contact area available to heat and mass transfer consisted of points of intimate contact and points of non-contact and was related to the measured "macroscopic" contact angle. The fractional contact area was estimated using a contact angle function derived in classical nucleation theory. The thermal contact resistance was calculated using the estimated fractional area, and was defined as the resistance to heat transfer, by conduction, through the points of intimate contact. A fundamental relationship between the contact angle and the thermal contact resistance was established. Further, interface conditions at low preheat temperatures resulted in poor wetting and were conducive to solidification and a change from the initial solid-liquid contact to a solid - solid contact. The solid-solid contact was reflected in high interfacial resistances. This mechanism correlated well with the calculated melt interfacial temperatures that showed the average melt temperature could cool close to or even below the solidification temperature of the bath at low preheat temperatures. At higher temperatures the wetting was good and conditions were more conducive to interfacial reactions and increased mass transfer at the interface, evinced by low interfacial resistances and increased alloy layer thickness.
- Subject
- metallic coating; steel substrate; interfacial resistance; hot dip
- Identifier
- http://hdl.handle.net/1959.13/1312479
- Identifier
- uon:22403
- Rights
- Copyright 2000 Nicholas John Ebrill
- Language
- eng
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