Corrosion mechanism and kinetics of Al-Zn coating deposited by arc thermal spraying process in saline solution at prolong exposure periods

Changes in Corrosion Behaviour of Zinc and Aluminium Coatings with Increasing Seawater Acidification

The increase in greenhouse gas emissions has led to seawater acidification, increasing the corrosion rate of metal structures in marine applications. This paper indicates that the spraying of four types of coatings, namely Zn, Al, Zn-Al, and Al-Mg, using the arc-spraying technique on steel substrate S235JR, creates effective protective coatings that interact differently with various pH solutions exposed to varying levels of seawater acidification. The study analyses the structural properties of the coating materials using SEM and XRD techniques. Electrochemical parameters are evaluated in solutions with different pH and salinity levels. The results demonstrate that alloy metallic coatings provide excellent resistance to corrosion in low-pH solutions.

Role of 5 wt.% Mg Alloying in Al on Corrosion Characteristics of Al-Mg Coating Deposited by Plasma Arc Thermal Spray Process

The corrosion of steel structures in coastal areas is a major issue. Therefore, in the present study, the protection against the corrosion of structural steel is carried out by depositing 100 μm thick Al and Al-5 Mg coatings using a plasma arc thermal spray process, immersing them in 3.5 wt.% NaCl solution for 41 days (d). To deposit such metals, one of the best known processes, arc thermal spray, is frequently used, but this process has severe defects and porosity. Thus, to minimize the porosity and defects of arc thermal spray, a plasma arc thermal spray process is developed. In this process, we used normal gas to create plasma instead of argon (Ar) and nitrogen (N₂) with hydrogen (H) and helium (He). Al-5 Mg alloy coating exhibited uniform and dense morphology, where it reduced more than four times the porosity compared to Al, where Mg fills the voids of the coating, resulting in greater bond adhesion and hydrophobicity. The open circuit potential (OCP) of both coatings exhibited electropositive values due to the formation of native oxide in Al, while in the case of Al-5 Mg, the coating is dense and uniform. However, after 1 d of immersion, both coatings showed activation in OCP, owing to the dissolution of splat particles from the corner where the sharp edges are present in the Al coating, while Mg preferentially dissolved in the Al-5 Mg coating and made galvanic cells. Mg is galvanically more active than Al in the Al-5 Mg coating. Due to the capacity of the corrosion products to cover the pores and defects, both coatings stabilized the OCP after 13 d of immersion. The total impedance of the Al-5 Mg coating is gradually increased and is higher than the Al, which can be attributed to the uniform and dense coating morphology where Mg dissolves and agglomerates to form globular corrosion products and deposit over the surface, thereby causing barrier protection. The defect bearing corrosion products on Al coating led to the cause having a higher corrosion rate than the Al-5 Mg coating. A total of 5 wt.% mg in the Al coating improved the corrosion rate by a rate of 1.6 times compared to the pure Al in the 3.5 wt.% NaCl solution after 41 d of immersion.

Flake ZnAl Alloy as an Effective Pigment in Silicate Coatings for the Corrosion Protection of Steel

Spherical zinc is well known as an effective pigment for the corrosion protection of carbon steel. However, a high proportion of spherical Zn in a coating leads to difficulties in fabricating the coating solution and increased cost. In this work, the influence of flake ZnAl alloy in silicate coatings on the corrosion protection properties of steel substrates was investigated. The electrochemical behaviour of coatings containing different flake ZnAl alloy content immersed in NaCl solution (3.5 wt%) was evaluated using an electrochemical impedance spectroscopy (EIS) method. A salt spray test was performed to evaluate corrosion on the coating surface. Pull-off tests of the coatings before and after the salt spray process were performed, and the surface morphology was analysed to determine the degradation of corrosion resistance. The results show that silicate coating containing flake ZnAl alloy (25 wt%) possesses the highest total resistance (1417 Ω) and the longest time to the appearance of white rust (720 h). The surface morphology of the coating containing 25 wt% flake ZnAl alloy was found to include corrosion products with the most compacted surface, which effectively prevents the penetration of electrolytes to the interface between the coating and the steel.

Corrosion Behavior and Sacrificial Properties of Zn and Zn-Al Coatings in Conditions Simulating Deep Geological Disposal of Radioactive Waste at 80 °C

In France, it is planned to manage high-level radioactive waste via deep geological disposal. The carbon steel overpacks containing the waste will face corrosion processes specific to the unusual environment of the disposal. To protect these overpacks against corrosion, the use of metallic sacrificial coatings was considered as a possible solution. Therefore, the corrosion behavior of thermally sprayed Zn and Zn-15wt.%Al coatings (500 µm thick) was studied in a solution simulating the specific environment of the disposal. Galvanic corrosion experiments were performed with a coated steel electrode connected to a bare steel one, which simulated a coating defect. First, it was observed that the Zn coating had lost its sacrificial properties after ~2 months of coupling. XRD and µ-Raman analysis carried out after the experiment demonstrated that the coating was covered with a protective hemimorphite (Zn4Si2O7(OH)2·H2O)-rich layer. In contrast, the Zn-15 wt.%Al coating kept its sacrificial properties all along the 6-month experiment. It proved to be covered with a layer mainly composed of Zn and Zn-Al hydroxysulfates, which did not contain hemimorphite. However, SEM cross-sectional observations showed that the electrolyte had seeped into the coating and had even reached the steel surface. This infiltration led to the oxidation of the splats inside the coating, even close to the steel surface, and induced a large swelling of the coating. Its thickness was increased by 50%.

Effect of Low Chloride and Sulfate Concentrations on Corrosion Behavior of Aluminum and Zinc Arc Thermal Sprayed Coatings

The aim of this study was to determine the suitability of arc sprayed zinc and aluminum coatings as materials for protective coatings of different heating systems. The most aggressive chemical agents occurring in heating water are chloride and sulfate anions. Both ions are responsible for the corrosion of metals due to their high electronegativity and standard electrochemical potential. Water in heating systems should not contain more than 150 mg/L anions, including no more than 50 mg/L of chlorides and 100 mg/L of sulfates. To determine the corrosion resistance of three types of zinc and aluminum coatings, open circuit potential and linear polarization resistance (LPR) tests were conducted in eight alkaline solutions with different sulfate and chloride contents. The SEM/EDS structural properties of sprayed coatings at specific arc process parameters were investigated. Zinc coatings exhibit the most stable corrosion potentials in varying environments but have higher corrosion current density. Aluminum coatings exhibit much higher potential values in a chloride environment than in any other. A chloride environment also causes the lowest corrosion rates for aluminum-coated samples. A small addition of aluminum to the zinc coating (15 wt.%) does not appear to affect the stability of the corrosion potential but does result in a reduction in corrosion rates in chloride solutions.

Surface Engineering Strategies to Enhance the In Situ Performance of Medical Devices Including Atomic Scale Engineering

Decades of intense scientific research investigations clearly suggest that only a subset of a large number of metals, ceramics, polymers, composites, and nanomaterials are suitable as biomaterials for a growing number of biomedical devices and biomedical uses. However, biomaterials are prone to microbial infection due to Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), Staphylococcus epidermidis (S. epidermidis), hepatitis, tuberculosis, human immunodeficiency virus (HIV), and many more. Hence, a range of surface engineering strategies are devised in order to achieve desired biocompatibility and antimicrobial performance in situ. Surface engineering strategies are a group of techniques that alter or modify the surface properties of the material in order to obtain a product with desired functionalities. There are two categories of surface engineering methods: conventional surface engineering methods (such as coating, bioactive coating, plasma spray coating, hydrothermal, lithography, shot peening, and electrophoretic deposition) and emerging surface engineering methods (laser treatment, robot laser treatment, electrospinning, electrospray, additive manufacturing, and radio frequency magnetron sputtering technique). Atomic-scale engineering, such as chemical vapor deposition, atomic layer etching, plasma immersion ion deposition, and atomic layer deposition, is a subsection of emerging technology that has demonstrated improved control and flexibility at finer length scales than compared to the conventional methods. With the advancements in technologies and the demand for even better control of biomaterial surfaces, research efforts in recent years are aimed at the atomic scale and molecular scale while incorporating functional agents in order to elicit optimal in situ performance. The functional agents include synthetic materials (monolithic ZnO, quaternary ammonium salts, silver nano-clusters, titanium dioxide, and graphene) and natural materials (chitosan, totarol, botanical extracts, and nisin). This review highlights the various strategies of surface engineering of biomaterial including their functional mechanism, applications, and shortcomings. Additionally, this review article emphasizes atomic scale engineering of biomaterials for fabricating antimicrobial biomaterials and explores their challenges.

Electromagnetic Shielding Performance of Different Metallic Coatings Deposited by Arc Thermal Spray Process

Advancement in electronic and communication technologies bring us up to date, but it causes electromagnetic interference (EMI) resulting in failure of building and infrastructure, hospital, military base, nuclear plant, and sensitive electronics. Therefore, it is of the utmost importance to prevent the failure of structures and electronic components from EMI using conducting coating. In the present study, Cu, Cu-Zn, and Cu-Ni coating was deposited in different thicknesses and their morphology, composition, conductivity, and EMI shielding effectiveness are assessed. The scanning electron microscopy (SEM) results show that 100 µm coating possesses severe defects and porosity but once the thickness is increased to 500 µm, the porosity and electrical conductivity is gradually decreased and increased, respectively. Cu-Zn coating exhibited lowest in porosity, dense, and compact morphology. As the thickness of coating is increased, the EMI shielding effectiveness is increased. Moreover, 100 µm Cu-Zn coating shows 80 dB EMI shielding effectiveness at 1 GHz but Cu and Cu-Ni are found to be 68 and 12 dB, respectively. EMI shielding effectiveness results reveal that 100 µm Cu-Zn coating satisfy the minimum requirement for EMI shielding while Cu and Cu-Ni required higher thickness.

Sacrificial Thermally Sprayed Aluminium Coatings for Marine Environments: A Review

One of the corrosion mitigation methods that is used for the protection of steel operating in seawater environments involves the application of sacrificial metallic coatings (such as aluminium, zinc, and their alloys). This paper reviews current knowledge about thermally-sprayed (TS) and cold-sprayed (CS) Al coatings for the corrosion protection of steel. It also summarises the key findings of the substantial amount of work that has been devoted to understanding mechanisms and the parameters that control the performance of TS Al coatings, such as the spraying method and its parameters like coating thickness and the application of sealer. The paper includes suggestions for areas of further research that could lead to the development of more resilient and longer-lasting coatings, based on the results from both laboratory and field tests that have been published in the literature. It also highlights the need for conducting simulated laboratory tests at conditions of intended service and the importance of long-term testing.

Electromagnetic Shielding Performance of Carbon Black Mixed Concrete with Zn-Al Metal Thermal Spray Coating

The electromagnetic pulse (EMP) is a destructive phenomenon which harms the building, telecommunication, and IT based infrastructure. Thus, it is required to reduce the effect of EMP using shielding materials. In the present study, we have used different thickness of concrete walls by incorporating 1 and 5 wt% of carbon black, as well as 100 µm thick Zn-Al coating using the arc thermal metal spraying method (ATMSM). The EMP was evaluated using waveguide measurement fixture for shielding performance of the concrete wall in the range of 0.85 to 1 GHz frequency. The results reveal that the maximum value, i.e., 41.60 dB is shown by the 5-300-N specimen before application of Zn-Al coating where the thickness of concrete wall was 300 mm and 5% carbon black. However, once the 100 µm thick Zn-Al coating was applied on concrete specimen, this value was increased up to 89.75 dB. The increase in shielding values around 48 dB after using the Zn-Al coating is attributed to the reflection loss of the metal thermal spray coating. Thus, the Zn-Al coating can be used for EMP application instead of metallic plate.

The corrosion inhibition of stainless steel by ferrocene–polyoxometalate hybrid molecular materials – experimental and first principles studies

The corrosion inhibition mechanism for SS316 coated with FcPMo hybrid material is shown.

Effect of Sodium Phosphate and Calcium Nitrate Sealing Treatment on Microstructure and Corrosion Resistance of Wire Arc Sprayed Aluminum Coatings

Aluminum coating was deposited by arc thermal spraying process onto the steel substrate for the corrosion protection in aggressive environment. However, the arc thermal sprayed coating possesses defects in the coating. Thus, it is important to reduce the defects and enhance the corrosion resistance properties of the deposited coating using post-treatment. In the present study, we have used different concentrations of sodium phosphate mono basic (NaH2PO4) with 0.1 molar (M) calcium nitrate [Ca(NO3)2] as post-treatment solution to fill out the defects of the Al coating. It was observed by scanning electron microscopy (SEM) that 1 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample exhibited 71% reduction in defects compared to as coated samples. X-ray diffraction (XRD) was performed to determine the phases formed on the coating surface after treatments. XRD confirms the formation of sodium aluminum hydrogen phosphate (Na3Al(OH)(HPO4)(PO4)) and brushite (Ca(HPO4)(H2O)2) as composite oxides on the Al coating. Electrochemical results show that 0.5 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample has exhibited the highest charge transfer resistance and the lowest corrosion current density after 89 days of exposure in 3.5 wt.% NaCl solution. The enhancement in corrosion resistance of 0.5 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample attributed to the formation of adherent, sparingly soluble, and stable corrosion products. The volume fraction result of the corrosion products formed on 0.5 M NaH2PO4 with 0.1 M Ca(NO3)2 treated sample after 89 days of exposure in 3.5 wt.% NaCl using XRD confirms the highest amount of Bayerite (α-Al(OH)3) deposition, thus, the corrosion rate of this sample was the lowest.