High-throughput investigation of zinc corrosion under simulated aerosols and droplets of various sizes and chemistries

Abstract

Atmospheric corrosion is a complex multistage phenomenon involving chemical reactions, equilibria and transport phenomenon in the gaseous, aqueous and solid phase. Where, small changes in atmospheric chemistry will exert a large impact on the corrosion of Zn and to date a detailed understanding of how droplet chemistry, composition, and size interact to lead to corrosion has yet to be reported. The complexity of atmospheric environments makes the understanding and quantification of different parameters that effect atmospheric corrosion difficult especially using traditional field exposure and electrochemical techniques. Therefore, this dissertation endeavours to explore atmospheric corrosion of Zn through an in-depth review of the literature, development of a novel high-throughput testing methodology, and a thorough investigation of a number of the surface oxides that form during corrosion. To carry out this work a detailed study of different surface preparation techniques was conducted to ensure appropriate test surfaces for subsequent droplet experimentation. A high-throughput investigation of corrosion under droplets designed to mimic the aggressive atmospheric environments and of various sizes was conducted using an optimized cleaning procedure and damage quantification in terms of volume loss as measured through optical profilometry. Measurements were sensitive to variations in electrolyte chemistry, allowed relatively rapid and reproducible discrimination between the effects of different atmospheric parameters, and offered a high degree of experimental control not possible using other experimental techniques. This high-throughput approach of experimentation enabled the screening of more specific interactions occurring at droplet/metal interface. Quantification through volume loss measurements were combined with surface and cross-sectional analysis using Raman spectroscopy, XRD-GADDS and FIB-SEM-EDS to highlight important droplet/metal interactions which includes the role of droplet size, concentration of aggressive ions, pH and the type of acidifying agent used to attain highly acidic pH values. Additionally, in-situ monitoring of droplet pH, volume loss measurements, identification and distribution of crystalline and amorphous phases from corrosion under different droplets were used to understand the role of acidification for coarse size (diameter ~ 600–1400 μm) droplets. Results for various droplet chemistries are discussed in terms of initiation mechanism, phase distribution and surface morphology in conjunction with chemical equilibrium calculations. Further, for the first time corrosion damage from fine size droplets (diameter ~ 0.1–5 μm) of various chemistries, deposited using an unmodified inkjet printer, was quantified in terms of volume loss as determined through optical profilometry. SEM-EDS and FIB milling were used to characterise corrosion products and perform cross-sectional analysis of surface oxides. Results show synergistic interactions between chloride concentration and the types of acids used for acidification. Corrosion under fine size droplets was found to be dependent on the initial volume of aerosols, oxygen diffusion, surface area to volume ratio and likely the microstructural features of the underlying metal.