In-situ imaging and electrochemical monitoring of damaged thermal spray aluminium coating in synthetic seawater

This paper presents the results of a study that combined electrochemical monitoring with in-situ imaging of Thermal Spray Aluminium (TSA) coating in synthetic seawater at room temperature in quiescent condition. The coatings were obtained by twin-wire arc spraying of 1050 aluminium alloy on S355 carbon steel substrate. TSA-coated steel samples were evaluated by analysing sequential images of the surface: (i) without defect; (ii) with defects machined before immersion (5% and 30% of exposed steel surface); (iii) with a defect machined after 35 d of immersion (10% of exposed steel surface); and (iv) after the removal of calcareous deposits formed on top of the exposed steel surface. Variations in the coating and the defect were captured and correlated with the evolution of Open Circuit Potential (OCP) during 35 days of full immersion. Determination of calcareous deposit formation time on the top of exposed steel was also carried out. The defect created before immersion impacted the cathodic reactions, resulting in a faster formation of corrosion products and calcareous deposits compared to the defect machined after exposure to synthetic seawater. The penetration time of the electrolyte in the coating and the activation of the surface are key in the protection mechanism and the kinetics of corrosion.UTB, MInciencias, Lloy's Register Foundatio

Near-inflection point inflation and production of dark matter during reheating

We study slow roll single field inflationary scenario and the production of non-thermal fermionic dark matter, together with standard model Higgs, during reheating. For the inflationary scenario, we have considered two models of polynomial potential - one is symmetric about the origin and another one is not. We fix the coefficients of the potential from the current Cosmic Microwave Background (CMB) data from Planck/Bicep. Next, we explore the allowed parameter space on the coupling $(y_\chi)$ with inflaton and mass $(m_\chi)$ of dark matter (DM) particles $(\chi)$ produced during reheating and satisfying CMB and several other cosmological constraints.Comment: Prepared for Proceedings of XXV Bled Workshop "What comes beyond the Standard models?

Prediction of thermal spray coatings performance in marine environments by combination of laboratory and field tests

Cost-effective corrosion mitigation of offshore steel structures can be achieved by thermal spray coatings. These coatings, when comprised of Al, Zn and their alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. Due to the complexity of marine environments, laboratory tests should be combined with field work in order to understand the corrosion protection offered by these coatings. The work presented here was carried out with thermal spray coatings of aluminum alloys (AA1050, AA1100, Al-5Mg) and Zn-15Al prepared by Twin Wire Arc Spray onto low carbon steel substrates. The resulting coatings were ~300 μm in thickness, and 5% of surface area defects were artificially machined in order to expose the steel substrate, simulating mechanical damage or erosion of the coating. Electrochemical data collected over a 90 days period showed a good correlation between laboratory and real marine environment results. Aluminum alloys showed better corrosion protection in fully immersed conditions, while zinc alloys performed better in atmospheric and splash zones. Overall, these results aim to improve design of thermal spray coatings to protect carbon steel in marine environments

Comparative study of corrosion performance of HVOF-sprayed coatings produced using conventional and suspension WC-Co feedstock.

Corrosion properties of nanostructured coatings deposited by suspension high-velocity oxy-fuel (S-HVOF) via an aqueous suspension of milled WC-Co powder were compared with conventional HVOF-sprayed coatings. Microstructural evaluations of these coatings included x-ray diffraction and scanning electron microscopy equipped with an energy-dispersive x-ray spectroscopy. The corrosion performance of AISI440C stainless steel substrate and the coatings was evaluated in a 3.5 wt.% NaCl aqueous solution at ~25 °C. The electrochemical properties of the samples were assessed experimentally, employing potentiodynamic polarization and electrochemical impedance spectroscopy. The potentiodynamic polarization results indicated that coatings produced by S-HVOF technique show lower corrosion resistance compared with the coatings produced by HVOF-JK (HVOF Jet Kote) and HVOF-JP (HVOF JP5000) techniques. Results are discussed in terms of corrosion mechanism, Bode and Nyquist plots, as well as equivalent circuit models of the coating–substrate system

Effect of Damage on the Corrosion Performance of Thermal Spray Aluminium (TSA) Coating in Synthetic Seawater

A widely used coating system for corrosion mitigation of offshore steel structures is thermally sprayed aluminium (TSA). Even though these coatings have been used for decades, it is not always clear how they perform in service over long periods, particularly if damaged during installation or in service. To understand the corrosion behaviour of damaged TSA coatings in seawater and their tolerance to levels of damage, TSA coatings (1050 Al) were prepared on carbon steel substrates using wire arc spray and tested in synthetic seawater. Prior to testing, various levels of holidays or damage (~5%, 10%, 15% and 18%) reaching the steel substrate were drilled on the front surface of the coated specimens. Open circuit potential was measured and linear polarization resistance technique was used to calculate the corrosion rate. The work showed that the TSA coatings polarised steel to potentials below −800 mV (Ag/AgCl) at 25 °C, even in the presence of damage or holiday (up to ~18%). The SEM/EDX and XRD data confirmed the presence of brucite and aragonite in the damage region. The presence of damage impacted the short-term corrosion rate at the start, but did not significantly affect the overall corrosion performance of the TSA coatings in 420 days of testing

Special Issue: “Coatings for Harsh Environments”

The operation of numerous safety-critical components in industries around the world relies on protective coatings. These coatings often allow process equipment to be purposeful in environments well beyond the operational limit of the uncoated components. Durability, ease of application, repairability, reliability and long-term performance of such coatings are vital to their application. Therefore, this Special Issue of Coatings, “Coatings for Harsh Environments”, is devoted to research and review articles on the metallic, non-metallic and composite coatings used in aggressive environments

Hydrogen in Aluminium-Coated Steels Exposed to Synthetic Seawater

Thermally sprayed aluminium (TSA) coatings provide protection to offshore steel structures without the use of external cathodic protection (CP) systems. These coatings provide sacrificial protection in the same way as a galvanic anode, and thus hydrogen embrittlement (HE) becomes a major concern with the use of high strength steels. The effect of TSA on the HE of steel seems to remain largely unknown. Further, the location of hydrogen in TSA-coated steel has not been explored. To address the above knowledge gap, API 5L X80 and AISI 4137 steel coupons, with and without TSA, were prepared and the amount of hydrogen present in these steels when cathodically polarised to −1.1 V (Ag/AgCl) for 30 days in synthetic seawater was determined. One set of TSA-coated specimens was left at open circuit potential (OCP). The study indicates that the amount of hydrogen present in TSA-coated steel is ~100 times more than the amount found in uncoated steel, and that the hydrogen seems to be largely localised in the TSA layer

Stiffness of Plasma Sprayed Thermal Barrier Coatings

Thermal spray coatings (TSCs) have complex microstructures and they often operate in demanding environments. Plasma sprayed (PS) thermal barrier coating (TBC) is one such ceramic layer that is applied onto metallic components where a low macroscopic stiffness favors stability by limiting the stresses from differential thermal contraction. In this paper, the Young’s modulus of TBC top coat, measured using different techniques, such as four-point bending, indentation and impulse excitation is reported, along with a brief description of how the techniques probe different length scales. Zirconia-based TBC top coats were found to have a much lower global stiffness than that of dense zirconia. A typical value for the as-sprayed Young’s modulus was ~23 GPa, determined by beam bending. Indentation, probing a local area, gave significantly higher values. The difference between the two stiffness values is thought to explain the wide range of TBC top coat Young’s modulus values reported in the literature. On exposure to high temperature, due to the sintering process, detached top coats exhibit an increase in stiffness. This increase in stiffness caused by the sintering of fine-scale porosity has significant impact on the strain tolerance of the TBC. The paper discusses the different techniques for measuring the Young’s modulus of the TBC top coats and implications of the measured values

Inspection of Coated Hydrogen Transportation Pipelines

The growing need for hydrogen indicates that there is likely to be a demand for transporting hydrogen. Hydrogen pipelines are an economical option, but the issue of hydrogen damage to pipeline steels needs to be studied and investigated. So far, limited research has been dedicated to determining how the choice of inspection method for pipeline integrity management changes depending on the presence of a coating. Thus, this review aims to evaluate the effectiveness of inspection methods specifically for detecting the defects formed uniquely in coated hydrogen pipelines. The discussion will begin with a background of hydrogen pipelines and the common defects seen in these pipelines. This will also include topics such as blended hydrogen-natural gas pipelines. After which, the focus will shift to pipeline integrity management methods and the effectiveness of current inspection methods in the context of standards such as ASME B31.12 and BS 7910. The discussion will conclude with a summary of newly available inspection methods and future research directions

Natural Deposit Coatings on Steel during Cathodic Protection and Hydrogen Ingress

The calcareous coating formed during cathodic protection (CP) in seawater is known to reduce the current demand by hindering the transport of species required to support the cathodic reactions and, thereby, improve the economic performance of CP systems. There is, however, uncertainty as to whether the coating reduces hydrogen uptake or indeed enhances it. To ascertain this, two sets of samples were polarized at −1.1 V (standard calomel electrode, SCE) in 3.5% w/v NaCl and synthetic seawater (ASTM D1141) at 20 °C and the diffusible hydrogen content measured over a period of 530 h. Under such conditions reports suggest a deposit with two distinct layers, comprising an initial brucite layer followed by an aragonite layer. Contrary to other findings, a fine initial layer containing Ca and Mg followed by a brucite layer was deposited with a few specks of Ca-containing zones in synthetic seawater. The hydrogen uptake was found to occur within the initial 100 h of exposure in synthetic seawater whilst it continued without the benefit of a deposit coating, i.e., in 3.5 wt % NaCl solution