Operando style, non-contact infrared thermography has been used to study the change in surface metal temperature between the zinc bath compared to just above the gas jet knives at an industrial, continuous galvanising line (CGL). Measuring photons in the wavelength range 7-12 mu m at 30 frames per second (fps), the change in photon count was 4608. Using an emissivity of 0.069, corresponding to zinc, this correlates to a minimum temperature drop of 14 degrees C. Using higher emissivities, linked with oxidized surfaces, suggests an even higher temperature drop (up to 19 degrees C). These data are key in understanding the influence of coating weight processing parameters on continuously galvanised sheet steel with implications for surface finish, microstructural morphology and resultant corrosion resistance of the material. The infrared data is validated using static measurements of molten zinc and zinc dross between 430 degrees C and 470 degrees C in a hot dip galvaniser simulation pot containing 40 kg of molten GI (Zn 0.2 wt. %Al).

Galvanising pot roll bearings are subjected to severe deterioration due to the corrosion of the bearing materials in liquid Zn, resulting in maintenance stops that can cost thousands of pounds per hour in downtime. Dynamic wear testing in molten Zn-Al and Zn-Al-Mg was conducted to assess the corrosion and wear resistance of three material pairs using a bespoke testing rig. The materials investigated in this study were Wallex6TM coated with WC-Co, stainless steel 316L coated with Al2O3, and as-received Wallex6TM and Wallex4TM alloys. It was found that only the Al2O3 coating remained unreactive in Zn alloy, whereas the materials containing Co were corroded, as evidenced by the formation of intermetallic compounds containing Al-Co-Zn-Fe. The results also highlighted that the dissolution of the Co matrix and diffusion of Zn and Al from the bath occurred in Wallex6TM and Wallex4TM. However, the diffusion of Zn into the WallexTM alloys was reduced by approximately 60% in the Zn-Al-Mg bath compared to Zn-Al. The wear scars were analysed to determine the wear coefficient of the worn specimens. Out of the three material couplings investigated in this study, minimal wear damage in both Zn-Al and Zn-Al-Mg was only obtained by pairing Wallex6TM with Al2O3 coatings.

The production and quality of automotive-grade galvanised steel are affected by the limited service life of the pot roll bearings used in continuous galvanising lines. The journal bearings are subjected to severe degradation as they react with the molten Zn bath, and coatings can provide corrosion protection to the bearing materials. This research investigates the performance of Al2O3 coatings applied via the HVOF thermal spray process to stainless steel 316L substrates. Immersion tests were conducted in baths of different compositions, namely GI (Zn-0.3 wt.% Al) and ZMA (Zn-1.5 wt.% Al-1.5 wt.% Mg). Material characterisation after testing showed evidence of coating degradation after 1 week, as the coating tended to crack and detach from the substrate, allowing the molten Zn to attack the underlying steel. The coefficient of thermal expansion of Al2O3 and steel was measured, and a difference of 13 × 10−6 K−1 was found, leading to the development of cracks in the coatings. Zn penetration through cracks was determined to be the main failure mechanism of the Al2O3 coatings, which otherwise remained inert to Zn-Al. Conversely, the coatings immersed in Zn-Al-Mg reacted with the Mg in the molten metal bath, showing that changing bath composition affected the performance of the coatings in molten Zn alloy.

In this investigation, 0.19–1.8 wt.% of Ge was introduced into a ternary Zn-Mg-Al alloy. The introduction of Ge had a significant impact on the microstructure, leading to the formation of Mg2Ge. The area fraction of the eutectic phase diminished with increasing Ge additions. Small-scale test techniques were utilised to evaluate the mechanical properties due to the changes in microstructure. Zn-Mg-Al alloys were found to be inherently harder compared to standard hot-dip Zn-containing 0.2 wt.% Al. The hardness and strength of the Zn-Mg-Al alloys decreased with the increase in Ge additions.

Time-lapse Microscopy, scanning vibrating electrode technique and potentiodynamic methods were used to study the influence of increasing coating weight (80–310 gm–2) on microstructure, cut-edge and surface corrosion of Zn-Mg-Al coatings in 0.17 M NaCl. Cut-edge corrosion was similar for all coatings due to the oxygen reduction reaction becoming diffusion-limited. A 64% reduction in surface corrosion was observed for high coating weights through increases in eutectic volume fraction. Spatial and temporal corrosion mechanisms were controlled by microstructural morphological differences as coating weight varied. 80 g.m–2 coatings demonstrated lateral anodic spreading potentially reducing coating penetration rates despite their higher surface corrosion rate.

Systematic investigations into the individual effects of increasing alloy additions of Mg and Al on the corrosion mechanisms of ZMA coating have demonstrated that microstructural phases and their surface distribution determine the anodic and cathodic behaviour of the coating. Increasing Al and Mg from 1 to 3 wt.% leads to an improved corrosion performance of the coating yet the improvement was greater for Al compared to Mg. Simultaneously increasing both Al and Mg to 3 wt.% again increased the corrosion performance. However, it was greater than 3 wt. % Mg but was lower the 3 wt.% Al.

Microscopy, electrochemical techniques and mechanical testing are used to investigate the effect of varying antimony additions (0.45–1.8 wt%) on the microstructure and corrosion properties of zinc-magnesium-aluminium coating alloys. Samples were produced by splat casting to produce high cooling rates similar to those seen in a continuous galvanising line. X-Ray Microscopy reveals that the Sb additions produce disk-shaped Mg3Sb2 intermetallics, subsequently reducing or eliminating the MgZn2 eutectic. Electrochemical testing in 1 wt% NaCl shows that the Mg3Sb2 phase is cathodic with respect to the bulk alloy with slower oxygen reduction kinetics. The decrease in eutectic content leads to less intense anodic activity. The combined effect is anodic and cathodic deactivation, which leads to a 43% reduction in corrosion rate as measured through LPR compared to the base alloy. This work shows that quaternary additions to ZMA coating alloys can be a potential route to improved corrosion resistance for galvanic protection.

Environmental targets to increase the whole-life efficiency of many products have led to an increased focus on improvements in metallic coating performance, with view to increase durability and minimise consumption of raw materials. Zinc Magnesium Aluminium (ZMA) alloy galvanised coatings have been the focus of numerous research studies over the past two decades. Research has predominantly been based around the improved corrosion resistance of these coatings for automotive and construction products exposed to external environments. A variety of compositions exist, each forming complex microstructures containing primary zinc within eutectics rich in magnesium and aluminium. These compositions generally include, but are not limited to, alloy additions that range between 1 and 2 wt% to zinc. The alloying additions enhance the corrosion resistance and pressing performance. It has been previously identified that limited systematic research has been undertaken to date to understand what influences adverse mechanical behaviours of these microstructures. Studies have shown that eutectic phases are susceptible to cracking on forming. More recent research has attributed the genesis of the cracks, specifically to the twinning locations within the eutectic phase, MgZn2. This study investigates four Zinc Alloy coatings containing: 1Mg1Al; 3Mg3Al; 1Mg3Al and 3Mg1Al wt% additions. These compositions formed different morphologies, with varying volumes of primary and eutectic phases. All four specimen coatings are approximately 30 microns thick and have been created using identical heat treatment processing parameters and cycle. The galvanic alloy coatings have been mounted upon 0.7mm gauge Interstitial Free (IF) steel substrate. This substrate has been selected as typically used for car body panels in the automotive industry, and cold formed light gauge structural sections in the construction industry. IF steels offer good tensile strength, as well as good formability capability. A Hot Dip Processing Simulator was used to apply the IF steels with the four separate specimen coatings, to isolate variables, such as: substrate grade, substrate thickness, coating thickness, heat treatment cycles and post coating cooling. Uniaxial tensile tests are governed by the structural substrate mechanical properties. Typically, with elongation limits of approximately 20%. Traditional soft-tempered galvanic coatings can achieve as much as 65% elongation, which is desirable when forming pre-galvanised sheet material. However, bending material presents a stress distribution of both tension and compression. Elongations as much as 100% (theoretical 0T bend) can be induced on the coated material surface, without total structural failure of its substrate. BS EN 13523-7 provides a Standard for testing and specifying a minimum bend radius for coated sheet material. As bend radius and material thickness govern bending tensile strain, bends are categorised relative to material thickness. A systematic study was undertaken that incrementally induced relatively high levels of strain to the material surface, by bending the pre-coated steel sheet material. The cracked microstructures of the four zinc alloy coating specimens were quantified before accelerated corrosion tests were undertaken. All specimen samples were immersed in 1wt% NaCl electrolyte solution and test data recorded over a 24-hour period, at room temperature. A GAMRY Potentiostat recorded Open Circuit (OCP) and Linear Polarisation Resistance (LPR) data of both strained and unstrained samples and results were compared. The results showed that for a comparable strain, each of the cracked coating specimens were cathodically polarised. The depths of coating cracks were observed using a scanning electron microscope, during a fracture analysis exercise of cross-sectional images. The cracks were observed to be the full depth of the coating full thickness, exposing the surface of the steel substrate. The characteristics of the coating fractures correspond with the electrochemical test results. The corrosion resistance of these zinc alloy coating specimens was compromised when the coatings cracked on the tension surface at small radius bends, when compared to flat unstrained samples.

Powder recycling refers to the reuse of unused powder feedstock in the laser powder bed fusion (PBF-LB/M) process. This approach is crucial for the economic viability and sustainability of PBF-LB/M, as powder accounts for a large proportion of the total production cost. However, through powder recycling, the physical and chemical properties of powder are liable to change. This variation in powder properties can subsequently lead to knock-on effects on the mechanical properties of a fully built component.This research has investigated the changes that occur to stainless steel 316L (SS316L) powder as a result of recycling. This includes changes to powder size distribution (PSD), flowability, chemistry and phase composition. Likewise, the impact that these changes have will also be assessed in PBF-LB/M SS316L components manufactured from powders after different levels of recycling and subjected to alternative post processing routes such as hot isostatic pressing (HIP). This comprehensive investigation involves a thorough examination of both macro- and microstructures, encompassing detailed analyses of chemical composition, microstructural features, and defects. The study aims to elucidate differences in mechanical behaviour through a series of experiments, including uniaxial tensile tests, Charpy impact assessments, and low cycle fatigue (LCF) experiments. Additionally, the investigation will be complemented by pitting potential tests, providing a holistic understanding of the material's performance and characteristics.Although moderate changes to powder were observed for both PSD and chemistry, this was found to be negligible and not enough to result in any adverse changes to part performance. In addition, the microstructure of SS316L remained stable across differing levels of powder recycling. Whereas the porosity content increased marginally as the fine particle content of powder was reduced, this was not found to be sufficient to affect the LCF performance of the material. After powder recycling, increases in ductility and Young’s modulus were attributed to a reduction in oxides present in the microstructure, which were sources of localised damage and deformation.

A B S T R A C T Additions of 1 wt.% Ca were made to Zn (Zn-1Ca) to generate favourable chemical conditions during corrosion for enhanced performance of phosphate inhibitors. Zn-1Ca displayed a reduced corrosion rate in 0.17 M NaCl measured using the Scanning Vibrating Electrode Technique and polarisation. In-situ timelapse microscopy and SEM-EDS demonstrated that intermetallic CaZn 13 preferentially corroded releasing Ca 2 + that increased corrosion product precipitation, cathodically deactivating the system. 1 × 10 (cid:0) 3 mol dm (cid:0) 3 phosphate additions to 0.17 M NaCl decreased the corrosion rate of Zn and Zn-1Ca. Inhibition was greater for Zn-1Ca where uniform precipitation of mixed metal phosphates produced anodic and cathodic inhibition.

Ceramics are considered to be candidate materials for galvanising pot bearing materials due to their excellent corrosion resistance in many molten metals. Galvanising pot roll bearings must have excellent wear resistance, and, therefore, it is important to understand the wear behaviour of prospective bearing materials. This study investigates the friction- and wear-resistant properties of select ceramics, namely, pure hBN, BN M26, AlN-BN, Macor, 3YSZ, Al2O3 and Si3N4. The ceramics were tested at different sliding speeds using a pin-on-disc device equipped with SiC pins. The lowest coefficient of friction (COF) achieved was below 0.1, and it was measured for pure hBN at a 3.14 m/min sliding speed. However, a wear scar analysis showed that the BN grades suffered from severe wear. The highest wear rate was obtained for BN M26 at a 9.42 m/min sliding speed and was equal to 17.1 × 10−6 mm3 N−1 m−1. It was shown that the wear coefficient of the tested ceramics varied exponentially with hardness. The lowest wear was observed on the 3YSZ, Al2O3 and Si3N4 ceramics, which showed no volume loss, and, for this reason, they can be potentially used as bearing materials in continuous galvanising lines.

A major influence on performance and lifetime of steel-based structures is corrosion. Some such structures include ground support equipment (GSE). GSE plays a vital role in the safety and efficiency of aircrafts and airports worldwide. GSE encompasses all support equipment at airports, necessary for the function and logistical operation of airports, air bases and other aviation facilities. According to the IMPACT study undertaken by the National Association of Corrosion Engineers (NACE) in 2013, the global cost of corrosion was estimated to be US$2.5 trillion, equivalent to 3.4% of the global GDP at the time. The corrosion of GSE can be attributed to a number of factors present including, but not limited to, moisture in air, salt particles in coastal winds and industrial pollutants from fuel. For these reasons, corrosion prevention through means of protective coatings is of utmost importance; as well as a strict programme of corrosion inspection and repair. Traditional means of protection against corrosion centre around Hot-Dipped Galvanising (HDG). This is the process of applying a protective zinc layer to steel in order to prevent corrosion of a component or structure. One of its major issues is cost. This is primarily due to the need to outsource galvanising to external contractors, especially for manufacturers that do not have the capacity or capability to undertake the process in-house. HDG also happens at high temperatures of ~450°C. This requires a great amount of energy and would be impossible to implement in-house for manufacturers. The repair of HDG sections is also a concern. This often sees the protective Zn coating removed and the section welded to repair. In this instance, the steel substrate is left unprotected and susceptible to corrosive attack. The common solution here is Cold Galvanising Coatings (CGCs) or Zn Rich Paint (ZRP). ZRPs are, however, laden with issues; poor adhesion, UV degradation and poor galvanic contact with the substrate. These shortcomings mean the service life of ZRPs and CGCs are limited. The current work focuses on developing a lower-temperature alternative centred around fusible alloys. The aim is to provide a corrosion-resistant coating that can bond with the substrate at lower temperatures, be applied on-site and offer galvanic protection. The fusible alloy is mixed with zinc powder to create a novel corrosion resistant coating. The coating may also be used for repair of galvanised GSE, which often sees welded areas left susceptible to corrosive attack. Microstructural analysis was carried out as a function of Zn loading weight as well as curing time and temperature. A systematic study on curing time and temperature identified the optimal processing parameters for the novel coating. Heating of below 250°C was used to melt the fusible alloy to envelop the zinc. This can be achieved using a conventional oven, while Near Infrared (NIR) heating was explored as a rapid heating option. The resulting optical and scanning electron microscope (SEM) images show that the fusible alloy acts as a matrix, while “islands” of Zn are dispersed therein to offer galvanic protection. The use of a metal matrix in the microstructure allows for superior electrical contact between the sacrificial Zn and the structural steel substrate, unlike that given by most CGCs and ZRPs. Electrochemical studies, such as Zero Resistance Amplitude (ZRA) and Open Circuit Potential (OCP), and accelerated corrosion tests, implementing the Scanning Vibrating Electrode Technique (SVET), have shown that the coating offers excellent galvanic protection to steel in a 1% NaCl electrolyte. ZRA galvanic corrosion tests demonstrated that current flow between components was negligible. This suggests no obvious self-corrosion in the coating, despite the nobility of fusible alloy components. Current OCP results show that adding Zn to the fusible alloy shifts the OCP from -425mV to -970mV in a 24hr test in 1%NaCl. This is comparable to that of Zn which is stable at -1V and shows promise when compared to bare steel at -700mV, meaning the coating offers galvanic protection to the steel. This is reinforced by SVET maps of coated steel samples, confirming that steel remains cathodic to the coating throughout tests, showing that performance improves as a function of Zn content. Adhesion tests, using the 0t method, show the peak performance comes at a known Zn wt% with cracks appearing and propagating as Zn wt% is increased. Overall; peak microstructure, corrosion protection and adhesion are achieved and demonstrated. The resulting product is a robust, protective, and dynamic coating; a step forward in the fight against corrosion of steel and protection of HDG.

Galvanizing is the process of applying a protective zinc or zinc alloy coating to steel or iron which acts as a sacrificial anode to protect the underlying steel or iron. These coatings are mainly used in the automotive and construction industry where corrosive environments will be encountered. There has been a drive to improve the performance of galvanised coatings through alloy additions, initially Al (5 – 55 wt%) and more recently through the development of Zn - Al - Mg coatings. Here, Al and Mg are added as minority additions (typically 1 - 3 wt %) to improve corrosion resistance allowing thinner coatings to be used thus saving material, weight and cost. The assessment of the corrosion performance of such coatings is often evaluated through standardised tests. An industry standard test known as Prohesion or ASTM G85 Annex A5 is carried out on metallic coatings as it is thought to better represent long-term natural exposure than a regular salt spray test. The Prohesion test is a cyclic accelerated corrosion test using an electrolyte of 0.4% Ammonia sulphate and 0.05% sodium chloride (NaCl). This investigation aims to determine the effects on the corrosion performance of zinc-aluminium alloys and zinc-aluminium-magnesium alloys of varying compositions immersed in Prohesion solution. Previous work on this topic suggests that magnesium containing alloys were particularly susceptible to accelerated corrosion in Prohesion solution. Here, several zinc based alloys were immersed in Prohesion solution as well as a control 0.45% sodium chloride solution and subjected to electrochemical testing. The electrochemical techniques used to test said substrates are linear polarisation resistance (LPR), scanning vibrating electrode technique (SVET) and time-lapse optical microscopy (TLM) all for 24 hours. The alloys that underwent testing are shown in Table 1. Table 1: Table showing the composition of substrates used. Sample number Composition 1 Zn-<0.2wt%Al 2 Zn-5wt%Al 3 Zn-55wt%Al 4 Zn-1.2wt%Al-Zn1.2wt%Mg 5 Zn-3.5wt%Al-3wt%Mg 6 Zn-6wt%Al-3wt%Mg The results from the TLM showed differences in the corrosion mechanism that is dependent on the alloy composition and electrolyte. Generally, all substrates immersed in the control 0.45% NaCl showed localised corrosion with distinct anodes developing surrounded by a ring of corrosion product and white rust is observed on the surface of the substrate. The initial site of anodic attack in the microstructure was dependent on the alloy composition. However, when immersed in the Prohesion solution significant differences were observed dependent on the alloy composition. For Zn-Al-Mg coatings rapid attack of zinc-magnesium phases was observed especially in the eutectic regions followed by aggressive attack of the entire exposed surface of the alloy within 6 hours. Additionally, no corrosion product rings formed. For Zn-Al alloy coatings, the zinc phases were preferentially attacked with the aluminium containing phases appearing to be resistant to corrosion during the duration of the tests. No white rust was seen on the surface of the substrates and instead a dark surface/corrosion product film was observed. The corrosion rate from the LPR data measured at hourly intervals over 24 hours increases when substrates are immersed in Prohesion solution compared to NaCl, with the most noticeable increase seen when there are zinc-magnesium phases present within the alloy. The largest increase in corrosion rate mm per year (mmpy) was seen in sample 5 with it increasing from 0.15 mmpy after 24 hours in the control solution to 0.55 mmpy in the Prohesion solution after 24 hours. The same is observed with the SVET results showing that the ammonium ions are having a significant damaging effect on the Zn-Al-Mg metallic coatings. The proposed corrosion mechanism for Zn-Al-Mg alloy coatings in Prohesion solution is preferential dissolution of MgZn2 in eutectic phases due to reactivity of Mg producing Mg(OH)2 which is a highly basic hydroxide. Due to the basic nature of Mg(OH)2, Ammonium (NH4 +) cations react with Mg(OH)2 producing Mg2+ and Ammonia (NH3). Mg(OH)2+2NH4 +=Mg2++2NH3+2H2O Zn(OH)2 does not undergo this reaction to the same extent as it hydrolyses in water (producing H+) therefore is not basic enough to support much NH3 production. Neutral NH3 reacts with metallic Zn or Zn2+ compounds to produce highly soluble zinc (II) tetra-ammine complex cations. Therefore, the protective Zn and Mg compounds forming at the metal surface will dissolve and the surface will become activated. This is demonstrated by the lack of white rust shown from samples immersed in the Prohesion solution. Al phases are resistant to NH3 hence the better performance of Zn-Al coatings and the preferential attack of the Zn in the Zn-Al timelapse microscopy.

This work compares the mechanical and corrosion properties of 316L steel manufactured by Laser Powder Bed Fusion (LPBF) and post treated by Hot Isostatic Pressing (HIP) to wrought 316L. HIP is often used by default on LPBF components to reduce porosity and obtain the best mechanical properties, however, if the HIP temperatures are too high, there is a risk of reducing mechanical strength and corrosion resistance. The purpose of this work was to investigate the HIP parameters and understand the trade-off in properties. By choosing various HIP temperatures (700 ?, 1125 ?, 1200 ?), pressures (100 MPa, 137 MPa and 200 MPa) and hold times, optimal cycles were investigated based on the most favourable mechanical properties (density, hardness, tensile and low-cycle fatigue), and pitting corrosion resistance. Microstructural features associated with LPBF such as melt pools, melt pool boundaries and sub granular cells were observed. These features were found to disappear with longer and higher temperature treatments, accompanied by increased grain sizes. Low and mid temperature point HIP treatments resulted in higher ultimate tensile strength but lower fracture elongation. The decreasing hardness and tensile strength trends were consistent with decreased grain boundary strengthening and decreased dislocation strengthening (with dis-appearing sub grain boundary and granular cells). Only one HIP condition, consisting of a low temperature and medium pressure, produced samples that achieved runout under low cycle fatigue testing for both the lower and higher stresses. Despite this, most higher temperature HIP cycles reduced the fatigue resistance. This was again attributed to the coarsening of the microstructure at the higher temperature treatments. The spread of the pitting potentials of HIP treated samples was reduced by 52.46 % compared to the as-built material, although none were better overall compared to the wrought material. Of all the properties, porosity appears to play the most influential role on pitting corrosion, and to this extent, despite having a larger variation in results, some of the treated parts demonstrated improved pitting resistance and some demonstrated improved repassivation potentials compared to wrought 316L.

The microstructural corrosion mechanism of a zinc-magnesium-aluminium (ZMA) alloy coated steel (Zn- 2wt%- Al- 2wt% Mg) immersed in 0.17 M NaCl was observed using in-situ time-lapse optical microscopy. The effect of pH on the corrosion mechanisms was assessed by using 0.17 M NaCl adjusted to pH 3, 7, 10 and 13 respectively. The pH of the electrolyte had an influence on the corrosion mechanism of the ZMA alloy. At pH 7 and 10, preferential corrosion of the MgZn2 lamellae within the eutectic phases was observed followed by subsequent dissolution of Zn-rich phases. A reduction in corrosion was observed using time-lapse microscopy at pH 10, most likely due to the magnesium hydroxide corrosion products being more stable in alkaline conditions, thereby retarding the kinetics of cathodic oxygen reduction and overall rate of corrosion. At pH 3 and 13, the corrosion mechanism appeared to change from localised corrosion, seen at pH 7 and 10, to more generalised corrosion. At pH 3, the MgO corrosion products that seemingly impart benefit to the substrate are dissolved in such acidic conditions. Gravimetric analysis and polarisation experiments were also conducted at the varying pH levels, to compliment the results from time-lapse microscopy.

Ductile cast iron (DCI) is a high carbon iron alloy with a composition of up to 3.7% C, 2.3% Si, 0.35% Mn and various smaller constitutes. Upon casting of DCI, the high carbon content results in solidification in a secondary phase in the form of graphite nodules. These graphite nodules form around a matrix of ferrite (α-Fe) and pearlite (α-Fe + Fe3C) and impart desirable properties such as high strength properties and corrosion resistance due to graphite’s cathodic potential. DCI is commonly used for water distribution pipework and contribute to 40% of all drinking water distribution pipes in Wales. Several different corrosion inhibitors are used to prevent corrosion of DCI in drinking water distribution applications, such as sodium silicate (Na2SiO3) and ortho-polyphosphates. However, the use of deoxyribonucleic acid (DNA) could provide a novel alternative to conventional corrosion inhibitors currently in service. The DNA molecule is formed by a double helix polynucleotide strand of phosphate and sugar groups via covalent bonds and base pairs bonded by hydrogen bonds to the molecule. The main advantage of DNA as a corrosion inhibitor is the surplus of phosphate molecules available to be chemisorbed to the surface of DCI. Alongside the potential of aromatic heterocyclic compounds found in base pairs, which could also provide corrosion inhibition via physicochemical adsorption. This work aims to assess the use of deoxyribonucleic acid (DNA) on the corrosion of ductile cast iron (DCI) in a corrosive electrolyte. DNA is considered a green, non-toxic inhibitor that shows potential when used in immersion corrosion conditions. Literature has demonstrated the use of DNA as a corrosion inhibitor for other materials such as Magnesium alloys, Copper and X80 steel. However, there is little published work on the use of DNA as a corrosion inhibitor on DCI. The DNA used in this study was extracted from salmon sperm (Sigma-Aldrich) and dissolved into a 0.17 mol dm-3 sodium chloride (NaCl) solution at the concentrations of 50, 100 and 500 mg/L adjusted to pH 7 via Sodium Hydroxide (NaOH). Electrochemical analysis and surface characterisation was carried out over a range of time periods. The corrosion inhibition afforded by DNA was determined through using a variety of electrochemical techniques such as: open circuit potential (OCP), anodic and cathodic polarisation and linear polarisation resistance (LPR). In-situ microscopy was used to study the corrosion mechanisms that occur on the surface microstructure of DCI. Post corrosion, the DCI surface was characterised using scanning electron microscopy (SEM) and FTIR (Fourier transmission infrared spectroscopy). Electrochemical results, performed over a 24-hour period, showed that as concentration of DNA increased an increase in voltage potential and decrease in corrosion rate (mpy) was observed. A decrease of 60% in corrosion rate after 24 hours is provided by 500 mg/L DNA compared to the control of 1% NaCl. However, after 30 minute’s immersion followed by cathodic polarisation studies showed an increased concentration of DNA can behave as a cathodic inhibitor. A theory for this inhibitive effect is that a passivating film forms on the DCI surface and that it is most effective after 30 minutes exposure due to the high efficiency observed during the longer-term studies. This could suggest that a film formed on the surface is most efficient after 30 minutes adsorption due to the high efficiency seen during the longer-term studies. After 72 hours of immersion there is a change in the surface of DCI, with a more robust scale appearing at higher concentrations. This change in coating surface may provide the increase in corrosion inhibition as the pathway for further corrosion attack is blocked by a protective scale. The protective scale is characterised using FTIR to examine the changes in chemical structure of the corrosion layer. Whereby PO-2 and base pairs are identified on the corrosion scale surface after 24-hours immersion in DNA. The corrosion inhibition observed using DNA on DCI in neutral immersion conditions is afforded to the adsorption of inhibitive species onto the DCI surface, providing an insoluble layer.

In-situ scanning vibrating electrode technique and time-lapse microscopy are used to investigate the influence of germanium additions (0.19-1.8 wt.%) on the corrosion performance of zinc-aluminium-magnesium model alloys immersed in 0.17 mol.dm(-3) NaCl. The addition of Ge results in the formation of Mg2Ge and a decrease in the fractional area of eutectic phase. A 58 % decrease in SVET derived mass loss is achieved at 1.8 wt.% Ge. It is proposed that Mg2Ge crystals are anodically attacked and behave as reservoirs of Mg2+ ions. Mg(OH)(2) is precipitated and local electrolyte pH stabilises to values at which the zinc surface is passive.

A combination of in-situ Scanning Vibrating Electrode Technique (SVET) and time-lapse immersion optical microscopy (TLM) is used to investigate the effect of microstructural refinement on patterns of localized corrosion affecting zinc-aluminum-magnesium (ZAM) galvanized coatings on steel. Model ZAM coatings comprising Zn-2.7 wt% Al-1.5 wt% Mg are produced on 0.7 mm mild steel sheet by hot dipping, and the resulting coating microstructure is systematically refined by increasing the cooling (solidification) rate from 5 degrees C.sec(-1) to 1000 degrees C.sec(-1). The intact ZAM coated surface is immersed in 0.17 M aqueous NaCl, and SVET and TLM are used to follow the resulting localized corrosive attack. TLM shows that corrosion initiates preferentially within MgZn2 and spreads laterally over the ZAM surface by preferentially following MgZn2 rich phases. In coarse microstructures, large primary zinc grains tend to deflect and constrain lateral spreading whereas in fine microstructures the smaller primary zinc grains do not. Consequently, lateral spreading rate increases with microstructural refinement. SVET shows that global corrosion rates are similar for all the ZAM coatings but that increased lateral spreading results in lower rates of through-coating penetration for the refined microstructures. These findings are explained in terms of the lateral diffusion of aggressive anolyte species. (C) The Author(s) 2019. Published by ECS.

The scanning vibrating electrode technique is used to study the localized corrosion of unpolarized zinc in near-neutral aqueous sodium chloride electrolyte of varying concentration [NaCl]. As [NaCl] is reduced from 1% (w/v) to 0.0005% (w/v) the morphology of attack changes from large, irregular, areas of anodic zinc dissolution and cathodic oxygen reduction, to a regular array of anodic pits set in an otherwise cathodic surface. In dilute electrolyte ([NaCl] < 0.01%) the total (area averaged) corrosion current density, J, (A . m(-2)), obtained from a numerical area integral of SVET anodic current density data, is shown to vary with approximately the square root of [NaCl]. The number density of pits, n (m(-2)) is shown to decrease with increasing [NaCl]. The mean individual pit current, <(i(pit))over bar>, (A = J/n) varies with approximately the square root of [NaCl]. A simple geometric analysis, based on a calculation of the radial dependence of electrical resistance exhibited by a hemispherical shell of electrolyte concentric with the pit, is used to explain the relationship between ohmic potential drop in the external solution, solution conductivity, and the radial distance away from an existing active pit. (C) The Author(s) 2019. Published by ECS.