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The corrosion of Zn-4.8%Al sacrificial coatings used for the protection of steel / CALLUM GALLAGHER

Swansea University Author: CALLUM GALLAGHER

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DOI (Published version): 10.23889/SUthesis.59489

Abstract

This work set out to elucidate the microstructural corrosion mechanisms of sacrificial corrosion coating, Galvalloy® (Zn-4.8wt.%Al), which is used extensively in the strip steel industry. Corrosion of Galvalloy® occurs at the surface, where only Galvalloy® is exposed, and the cut edge, where Galvall...

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Published: Swansea 2022
Institution: Swansea University
Degree level: Doctoral
Degree name: EngD
Supervisor: Sullivan, James H.
URI: https://cronfa.swan.ac.uk/Record/cronfa59489
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Abstract: This work set out to elucidate the microstructural corrosion mechanisms of sacrificial corrosion coating, Galvalloy® (Zn-4.8wt.%Al), which is used extensively in the strip steel industry. Corrosion of Galvalloy® occurs at the surface, where only Galvalloy® is exposed, and the cut edge, where Galvalloy® and steel are coupled, which increases the corrosion rate of Galvalloy®. Chapter 3 demonstrates the time-lapse microscopy (TLM) technique being used to analyse, in-situ, the microstructural mechanism of surface and cut-edge corrosion of Galvalloy® immersed in pH 7 1 wt.% NaCl. Rotating disk electrode (RDE) and potentiodynamic polarisation (PD) tests were performed on the individual phases of Galvalloy® to identify their anodic and cathodic activity in 1 wt.% NaCl. TLM showed that surface corrosion initiates and propagates through the binary eutectic Zn-Al phase, whereas cut-edge corrosion initiates within the primary zinc dendrite phase and proceeding through the entire microstructure. The electrochemical data validated this as the RDE showed that the Al containing phases could not support cathodic activity as well as the primary zinc phases and PD showed that the zinc phases are more susceptible to anodic dissolution when polarised. Chapter 4 investigated, using TLM and PD, the corrosion rate and mechanism of the Galvalloy® surface across pH 3, 7, 10 and 13 in 1 wt. % NaCl. At pH 3 and 13, D showed a maxima of corrosion rate was seen and TLM illustrated no precipitation of corrosion product. PD showed pH 7 having the lowest icorr, however, the precipitated corrosion product formed at a smaller radius relative to active anodes during TLM experiments of pH 10 compared to pH 7. Chapter 5 utilized ZRA and TLM to investigate the rate and mechanism of the corrosion of Galvalloy® next to a steel ‘cut-edge’ across pH 3, 7, 10 and 13 in 1 wt. % NaCl. The corrosion rate of Galvalloy® was greater compared to the surface corrosion, due to the polarisation imposed by the connection to the steel substrate and the same corrosion rate to pH trend in Chapter 4 was see. At pH 7, 10 and 13, corrosion initiation occurs in the zinc dendrites, whereas at pH 3 the corrosion is generalised. Chapter 6 investigated the effect of increasing steel to Galvalloy® on the corrosion rate of Galvalloy® at pH 7 in 1 wt.% NaCl using ZRA and TLM. ZRA demonstrated a linear trend, whereas TLM showed a non-linear trend which is suggested to be due to the increased ease of precipitation in the experimental set-up. Chapter 7 is an example of a real-world corrosion problem involving organically coated Galvalloy®. 2 µL of HCl, FeCl2, NaCl and Acetic acid (CH3OOH) were administered to a scribed region of PVB coated Galvalloy® and exposed to a high relative-humidity environment for a month to induce under-film corrosion in order to compare the results to deduce which salt was responsible for the real-world corrosion. NaCl was the salt that posed the greatest similarity and cathodic delamination is the postulated corrosion mechanism.
Item Description: A selection of third party content is redacted or is partially redacted from this thesis due to copyright restrictions.ORCiD identifier: https://orcid.org/0000-0001-6005-2775
Keywords: Corrosion, coatings, galvanised, microscopy, electrochemistry, zinc, aluminium, strip steel
College: Faculty of Science and Engineering