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Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage / REBECCA WALDRAM

Swansea University Author: REBECCA WALDRAM

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DOI (Published version): 10.23889/SUthesis.58625

Abstract

Nickel plating has been used as a barrier coating to protect more reactive metals against corrosion, to improve their wear resistance or as a decorative feature since the early 1800’s. Since then there have been many changes to the bath composition to develop different deposit characteristics. The m...

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Published: Swansea 2021
Institution: Swansea University
Degree level: Doctoral
Degree name: EngD
Supervisor: Williams, Geraint
URI: https://cronfa.swan.ac.uk/Record/cronfa58625
first_indexed 2021-11-11T16:58:17Z
last_indexed 2021-11-12T04:26:10Z
id cronfa58625
recordtype RisThesis
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Circulation coinage is made from a diverse range of metals and alloys, which varies widely across the world. Until recently, these tokens were produced solely from solid metal sections; however rising metal prices for those commonly used such as nickel, copper and zinc meant alternative, cheaper production methods required investigation. Nickel plating was selected to reduce raw metal costs, by only using a thin layer of this more expensive metal over a cheaper substrate, such as steel. Nickel has excellent corrosion resistance in a wide variety of environments, good wear resistance and an attractive lustre. Part of this thesis examines the use of nickel-plated coin sections in conjunction with other metallic sections to discover their compatibility for circulation, in terms of the risk of a galvanic corrosion cell forming. Comparisons between such sections were made using in-situ SVET and SKP analysis to obtain current density distribution maps to show the locations of anodic and cathodic activity, as well as classic electrochemical techniques including open circuit potential to form a galvanic series and zero resistance ammetry to measure the current generated when sections were immersed in the same electrolyte. The results from these experiments indicated that where bi-metallic construction is used there is a risk of galvanic corrosion to varying extents under both immersion and atmospheric conditions. Different countries use coins of varying compositions, and results from four bimetallic coins showed some compositions are more prone to galvanic corrosion than others. Nickel-plated coatings have previously been found to suffer from porosity, and therefore can be susceptible to pitting corrosion when immersed in aggressive chloride-based environments. To monitor this in atmospheric conditions, defects were made in the nickel-plated deposits and electrolyte droplets containing chlorides were introduced. A variety of hole sizes and chloride concentrations were used, and the spreading of these droplets was monitored using time-lapse photography and a Scanning Kelvin Probe. Overall, increased chloride concentration up to 2 M led to an increase in the observed spreading, although above this a decrease was seen. Defect size was shown to have little effect on the spreading distance. As a possible alternative to standard electrolytic nickel plating, electroless nickel plating solutions were investigated. This technology plates via a chemical process and requires no applied current, which could potentially reduce manufacturing costs. Three commercially available plating solutions were compared to a basic in-house bath, where it was found changing the bath pH and temperature could significantly alter the properties of the final deposit. Increasing plating temperature led to an increase in the phosphorous content, plating rate, corrosion resistance and Vickers hardness of the deposit, whilst increasing bath pH led to a decrease in the phosphorous content and corrosion resistance, and an increase in the plating rate and Vickers hardness of the deposit. Additions of organic compounds taurine and thiamine hydrochloride were made to the in-house bath in varying quantities to effect changes in the deposits and improve the properties. These compounds were selected as they are both amino acids, similar in structure to some additives previously used in electroless nickel baths, are non-toxic, low cost and contain sulphur, previously shown to have a brightening effect on nickel deposits. Taurine additions led to similar phosphorous contents and plating rates and lowered the corrosion resistance of the deposits. 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spelling 2021-11-11T17:16:50.2100097 v2 58625 2021-11-11 Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage 4ae9531141c12cb308f5ddd5b793e185 REBECCA WALDRAM REBECCA WALDRAM true false 2021-11-11 Nickel plating has been used as a barrier coating to protect more reactive metals against corrosion, to improve their wear resistance or as a decorative feature since the early 1800’s. Since then there have been many changes to the bath composition to develop different deposit characteristics. The most widely used bath for nickel electrodeposition was developed by Professor O.P. Watts in 1916, which, with a few minor alterations is still commonly used today. Circulation coinage is made from a diverse range of metals and alloys, which varies widely across the world. Until recently, these tokens were produced solely from solid metal sections; however rising metal prices for those commonly used such as nickel, copper and zinc meant alternative, cheaper production methods required investigation. Nickel plating was selected to reduce raw metal costs, by only using a thin layer of this more expensive metal over a cheaper substrate, such as steel. Nickel has excellent corrosion resistance in a wide variety of environments, good wear resistance and an attractive lustre. Part of this thesis examines the use of nickel-plated coin sections in conjunction with other metallic sections to discover their compatibility for circulation, in terms of the risk of a galvanic corrosion cell forming. Comparisons between such sections were made using in-situ SVET and SKP analysis to obtain current density distribution maps to show the locations of anodic and cathodic activity, as well as classic electrochemical techniques including open circuit potential to form a galvanic series and zero resistance ammetry to measure the current generated when sections were immersed in the same electrolyte. The results from these experiments indicated that where bi-metallic construction is used there is a risk of galvanic corrosion to varying extents under both immersion and atmospheric conditions. Different countries use coins of varying compositions, and results from four bimetallic coins showed some compositions are more prone to galvanic corrosion than others. Nickel-plated coatings have previously been found to suffer from porosity, and therefore can be susceptible to pitting corrosion when immersed in aggressive chloride-based environments. To monitor this in atmospheric conditions, defects were made in the nickel-plated deposits and electrolyte droplets containing chlorides were introduced. A variety of hole sizes and chloride concentrations were used, and the spreading of these droplets was monitored using time-lapse photography and a Scanning Kelvin Probe. Overall, increased chloride concentration up to 2 M led to an increase in the observed spreading, although above this a decrease was seen. Defect size was shown to have little effect on the spreading distance. As a possible alternative to standard electrolytic nickel plating, electroless nickel plating solutions were investigated. This technology plates via a chemical process and requires no applied current, which could potentially reduce manufacturing costs. Three commercially available plating solutions were compared to a basic in-house bath, where it was found changing the bath pH and temperature could significantly alter the properties of the final deposit. Increasing plating temperature led to an increase in the phosphorous content, plating rate, corrosion resistance and Vickers hardness of the deposit, whilst increasing bath pH led to a decrease in the phosphorous content and corrosion resistance, and an increase in the plating rate and Vickers hardness of the deposit. Additions of organic compounds taurine and thiamine hydrochloride were made to the in-house bath in varying quantities to effect changes in the deposits and improve the properties. These compounds were selected as they are both amino acids, similar in structure to some additives previously used in electroless nickel baths, are non-toxic, low cost and contain sulphur, previously shown to have a brightening effect on nickel deposits. Taurine additions led to similar phosphorous contents and plating rates and lowered the corrosion resistance of the deposits. Thiamine hydrochloride lowered the phosphorous content of the alloy, increased the plating rate and lowered the corrosion resistance of the samples produced. E-Thesis Swansea Nickel plating, Electroless nickel plating, Corrosion, Galvanic corrosion, Atmospheric corrosion, Coins, SVET, SKP 11 11 2021 2021-11-11 10.23889/SUthesis.58625 COLLEGE NANME COLLEGE CODE Swansea University Williams, Geraint Doctoral EngD Sponsor: The Royal Mint; Funders: COATED2 EPSRC Centre for Doctoral Training 2021-11-11T17:16:50.2100097 2021-11-11T16:55:29.9039366 Faculty of Science and Engineering School of Engineering and Applied Sciences - Uncategorised REBECCA WALDRAM 1 58625__21501__565e800c28924101bef4fddc6453ba4f.pdf Waldram_Rebecca_EngD_Thesis_Final_Cronfa.pdf 2021-11-11T17:12:42.7525005 Output 8012981 application/pdf E-Thesis – open access true Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage © 2020 by Rebecca K. Waldram is licensed under a CC BY license. true eng https://creativecommons.org/licenses/by/4.0/
title Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
spellingShingle Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
REBECCA WALDRAM
title_short Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
title_full Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
title_fullStr Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
title_full_unstemmed Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
title_sort Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
author_id_str_mv 4ae9531141c12cb308f5ddd5b793e185
author_id_fullname_str_mv 4ae9531141c12cb308f5ddd5b793e185_***_REBECCA WALDRAM
author REBECCA WALDRAM
author2 REBECCA WALDRAM
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institution Swansea University
doi_str_mv 10.23889/SUthesis.58625
college_str Faculty of Science and Engineering
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hierarchy_top_id facultyofscienceandengineering
hierarchy_top_title Faculty of Science and Engineering
hierarchy_parent_id facultyofscienceandengineering
hierarchy_parent_title Faculty of Science and Engineering
department_str School of Engineering and Applied Sciences - Uncategorised{{{_:::_}}}Faculty of Science and Engineering{{{_:::_}}}School of Engineering and Applied Sciences - Uncategorised
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description Nickel plating has been used as a barrier coating to protect more reactive metals against corrosion, to improve their wear resistance or as a decorative feature since the early 1800’s. Since then there have been many changes to the bath composition to develop different deposit characteristics. The most widely used bath for nickel electrodeposition was developed by Professor O.P. Watts in 1916, which, with a few minor alterations is still commonly used today. Circulation coinage is made from a diverse range of metals and alloys, which varies widely across the world. Until recently, these tokens were produced solely from solid metal sections; however rising metal prices for those commonly used such as nickel, copper and zinc meant alternative, cheaper production methods required investigation. Nickel plating was selected to reduce raw metal costs, by only using a thin layer of this more expensive metal over a cheaper substrate, such as steel. Nickel has excellent corrosion resistance in a wide variety of environments, good wear resistance and an attractive lustre. Part of this thesis examines the use of nickel-plated coin sections in conjunction with other metallic sections to discover their compatibility for circulation, in terms of the risk of a galvanic corrosion cell forming. Comparisons between such sections were made using in-situ SVET and SKP analysis to obtain current density distribution maps to show the locations of anodic and cathodic activity, as well as classic electrochemical techniques including open circuit potential to form a galvanic series and zero resistance ammetry to measure the current generated when sections were immersed in the same electrolyte. The results from these experiments indicated that where bi-metallic construction is used there is a risk of galvanic corrosion to varying extents under both immersion and atmospheric conditions. Different countries use coins of varying compositions, and results from four bimetallic coins showed some compositions are more prone to galvanic corrosion than others. Nickel-plated coatings have previously been found to suffer from porosity, and therefore can be susceptible to pitting corrosion when immersed in aggressive chloride-based environments. To monitor this in atmospheric conditions, defects were made in the nickel-plated deposits and electrolyte droplets containing chlorides were introduced. A variety of hole sizes and chloride concentrations were used, and the spreading of these droplets was monitored using time-lapse photography and a Scanning Kelvin Probe. Overall, increased chloride concentration up to 2 M led to an increase in the observed spreading, although above this a decrease was seen. Defect size was shown to have little effect on the spreading distance. As a possible alternative to standard electrolytic nickel plating, electroless nickel plating solutions were investigated. This technology plates via a chemical process and requires no applied current, which could potentially reduce manufacturing costs. Three commercially available plating solutions were compared to a basic in-house bath, where it was found changing the bath pH and temperature could significantly alter the properties of the final deposit. Increasing plating temperature led to an increase in the phosphorous content, plating rate, corrosion resistance and Vickers hardness of the deposit, whilst increasing bath pH led to a decrease in the phosphorous content and corrosion resistance, and an increase in the plating rate and Vickers hardness of the deposit. Additions of organic compounds taurine and thiamine hydrochloride were made to the in-house bath in varying quantities to effect changes in the deposits and improve the properties. These compounds were selected as they are both amino acids, similar in structure to some additives previously used in electroless nickel baths, are non-toxic, low cost and contain sulphur, previously shown to have a brightening effect on nickel deposits. Taurine additions led to similar phosphorous contents and plating rates and lowered the corrosion resistance of the deposits. Thiamine hydrochloride lowered the phosphorous content of the alloy, increased the plating rate and lowered the corrosion resistance of the samples produced.
published_date 2021-11-11T04:58:16Z
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score 11.109323

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