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  • Open access
  • 63 Reads
Covalent surface modification of 2024 aluminum alloy surface by self-assembly dodecyl phosphate film towards corrosion protection

This work was to prepare a layer of dodecyl phosphate (DDPO4) film on 2024 aluminum alloy substrate for corrosion protection by self-assembling. The prepared DDPO4 self-assembly monolayers (SAMs) properties were characterized by X-ray photoelectron spectroscopy (XPS), water contact angle measurement (CA), and electrochemical impedance spectroscopy (EIS). Experimental results indicated that DDPO4 was successfully assembled on aluminum alloy substrate via covalent bond attachment. The modified surface was hydrophobic due to the DDPO4 attached to the oxide surface and a hydrocarbon tail-up orientation. In order to build a molecular adsorption dynamics model, the impact of temperature and pH values for assembling process has also been evaluated. Accelerated corrosion test showed that the DDPO4 modified 2024 aluminum alloy substrate exhibited excellent corrosion resistance. The charge transfer resistance of the DDPO4 covered aluminum alloy was ∼100 times larger than that of the blank aluminum alloy. The molecular dynamic (MD) simulation for DDPO4 adsorbed on the Al2O3(110) face indicate that the head group of DDPO4 is the active group and bind as tridentate with the O in P=O bond can adsorbed on the Al2O3 surface.

  • Open access
  • 157 Reads
Corrosion of AZ31 with LDH conversion coatings loaded with inorganic inhibitors.

Layered Double Hydroxides (LDHs) coatings were developed for corrosion protection of AZ31 Mg alloy. LDH coatings were fabricated under co-precipitation conditions and applied under hydrothermal conditions. Two different systems Zn-Al LDH and Li-Al LDH were studied. Specimens were post-treated via immersion for 2 h at 45 ºC in Na2WO4·H2O or LiNO3 baths respectively, to produce Zn-Al LDH(W) and Li-Al LDH(Li). The characterization of the coatings was carried out by field-emission scanning electron microscope (FESEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The corrosion process was studied by electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). Surface was also evaluated by water drop contact angle and paint adhesion test by using an epoxy primer. The characterization of the coating revealed two-layered coatings with a denser inner layer and a flaky outer layer. Both coatings improved the corrosion resistance of the AZ31 alloy. Loading with inhibitor further increased the corrosion resistance by one order of magnitude (Bare substrate, Z10mHz ⁓ 102 Ω cm2; LDH, Z10mHz ⁓ 103-4 Ω cm2; LDH-inhibitor, Z10mHz ⁓ 105 Ω cm2)

  • Open access
  • 212 Reads
Corrosion behavior of Fe-based amorphous/nanocrystalline composite coating: correlating the influence of porosity and amorphicity

Recently, Fe-based amorphous coating synthesized by different thermal spraying methods are being investigated as a potential candidate for long-term surface protection of various structures attributed to their outstanding wear and corrosion resistance. Defects like porosity and crystallization are inevitable in the thermal sprayed coatings, which are introduced during the synthesis process. Corrosion behavior of these coatings is adversely affected by the presence of such defects. However, identification of a microstructural feature among amorphous content and porosity to have greater influence on the corrosion resistance of the thermal sprayed Fe-based amorphous/nanocrystalline coating has remained elusive so far. Thus, to address this problem, in-situ amorphous/nanocrystalline composite coatings were synthesized via high velocity oxy-fuel (HVOF) spraying, along with two melt-spun ribbons of different amorphous content (one fully amorphous, FA-Rib and the other having similar level of amorphicity as that of the coatings, PA-Rib). This was done to eliminate the porosity aspect of such composite coatings and investigate the extent of degradation in corrosion resistance caused by amorphicity and porosity individually. Potentiodynamic polarization and electrochemical impedance spectroscopy studies revealed that the corrosion and passive current densities and polarization resistance was greatly influenced by reduced amorphicity compared to porosity. Moreover, extent of corrosion attack increased, whereas fraction of chromium oxide and Cr substituted hematite phase in the Raman spectra of post-polarized samples decreased gradually in the order of FA-Rib < PA-Rib < the coating. Besides, analysis of the passive film by Auger electron spectroscopy exhibited an increment in the thinning of passive film, and lowered Cr/Fe ratio of the passive film in the order FA-Rib PA-Rib the coating. These results established amorphicity as the primary factor that affects the corrosion resistance of Fe-based amorphous/nanocrystalline coating. This study will ultimately help in designing new amorphous composite coatings with improved corrosion resistance.

  • Open access
  • 83 Reads
Cresol red as corrosion inhibitor in chitosan thin layers on zinc

Chitosan (Chit) is a biopolymer which is synthesized by the deacetylation of chitin extracted from the shells of crustaceans. In the past few years, chitosan was widely used in physical and electrochemical research due to its cost- efficiency, low toxicity and eco- friendly nature [1]. The corrosion inhibition potential of chitosan is due to the amino and hydroxyl groups present in the polymer structure. Chitosan can provide a possible temporary coating on several metal layers [2-3]. Cresol red (CR) is a widely used pH indicator, yellow at pH below 7.2 and purple color at pH higher than 8.8 (in the pH range between 7.2 and 8.8 is red) [4]. This study focuses on the influence of cresol red as a possible corrosion inhibitor for chitosan thin layers on zinc substrates. The chitosan-cresol red system was applied on zinc substrates using dip-coating method. The electrochemical measurements were carried out in a three- electrode cell system using a PARSTAT-2273 single channel potentiostat. Electrochemical impedance spectroscopy was used to investigate the protective properties of the impregnated coatings.

[1] R.C. Cheung et al./ Mar. drugs 13 (8) (2015) 5156– 5186

[2] F. Szoke et al. / International Journal of Biological Macromolecules 142 (2020) 423–431

[3] F. Szoke et al./ Carbohydrate Polymers Volume 215 (2019) 63-72

[4] I. Sousa et al./ Macromol. Mater. Eng. 305 (2020) 1900662

  • Open access
  • 189 Reads
Evaluation and prevention of hydrogen embrittlement by NDT methods: A review

This paper comprises of hydrogen embrittlement phenomena in material, factors responsible for the hydrogen embrittlement and Non destructive methods to evaluate the internal defect in machine or component when working in hydrogen atmosphere. Hydrogen embrittlement is responsible for Sub-critical crack growth in materials, fracture and mechanical properties such as ductility, toughness and consequently loss of strength. This hydrogen is induced into the material during electrochemical reactions and in a high pressure hydrogen gas environment. The paper covers the review on the capabilities of non-destructive testing methods with aspects its advantages and disadvantages of it. Sometimes one non-destructive technique is not providing sufficient information about physical integrity and therefore a different combination of methods is required. Ultrasonic testing is very useful to detect internal defects.

  • Open access
  • 88 Reads
Corrosion resistance of aluminium against salt hydrates used for latent heat storage by using different coating materials

Thermal energy storage (TES) is required when using energy sources that are intermittent in order to fill the gap between energy supply and energy demand. Latent heat storage systems are based on phase change materials (PCM) such as salt hydrates, which absorb and release thermal energy with a change in its physical state. However, even though salt hydrates are widely used as PCM, they are potentially corrosive. Since PCM are normally encapsulated in containers, the compatibility with each other has to be assessed in order to create resistant containers. In this work, corrosion resistance of aluminium against two different salt hydrates (SP24E and SP50) in a temperature range of 40 °C to 60 °C was tested. Furthermore, four coatings (anodized, electroless nickel-phosphorous, powder and KTL-cathodic dip) were used to enhance the aluminium corrosion protection. The method used was the immersion corrosion test. Signs of severe localized corrosion were found in uncoated and nickel-phosphorous coated aluminium, while the anodized coated aluminium showed slight uniform corrosion. According to the calculated corrosion rates uncoated aluminium is not recommended to be used for long-term applications when using SP24E as PCM, as well as nickel-phosphorous coated aluminium when using any of the two tested salt hydrates.

  • Open access
  • 175 Reads
Evaluation of chloride-ingress models on concrete bridge exposed to deicing salts

Numerical models for chloride transport in concrete have been improved in the last four decades, however, their application to existing structures is still not at a satisfactory level. While simple models have many limitations that cannot be applied to existing structures in a real environment, more comprehensive models for service life prediction are not suitable for everyday engineering practice. Two chloride ingress models, a more comprehensive 3D chemo-hygro-thermo mechanical (CHTM) model implemented into the MASA software and well-known Life-365, are used for a case study: motorway bridge in the mountain region in Croatia. Both models are capable to predict the chloride content in concrete and match well with measured data on the bridge after 11 and 14 years of exposure to deicing salts. However, calibration with measured results led to higher values of surface chloride content and initial chloride diffusion coefficient for numerical analyses using the Life-365, which assumes that the concrete is uncracked and the surface chloride content is constant. On the other hand, the 3D CHTM model considers more realistic conditions: variable temperature, surface water and chloride contents, wetting and drying cycles, chloride diffusion and convection in cracked and un-cracked concrete. Consequently, input values for chloride diffusivity and surface chloride content do not require calibration for each chloride profile, as is the case for the Life-365 application.

  • Open access
  • 98 Reads
Passive film evolution over 2.5 years of lean-duplex stainless steel reinforcements embedded in mortar containing chlorides

The corrosion behavior, semiconducting properties and chemical composition of the passive film formed on lean-duplex stainless steel LDSS 2001, LDSS 2304 and AISI 304 reinforcements embedded in OPC mortar containing chloride, 0, 0.4, 2 and 4 wt.% Cl; after 2.5 years were studied. The Ceff,film of LDSS 2001 remained constant at 3×10‒5 F/cm2 for all Cl concentrations. Flat-band potential, EFB,A, of LDSS 2001 increased with the Cl concentration, suggesting a Cr2O3 film more prone to accept electrons, reducing the vacancy density. The enrichment in chromium in the passive film of LDSS 2001 after 2.5 years increases from 44% to 61%.

  • Open access
  • 100 Reads
Corrosion of steel in synthetic fly ash pore solution

Herein, the corrosion behavior of steel exposed to fly ash (FA) synthetic pore solution and carbonated FA synthetic pore solution is presented. A better understanding of corrosion phenomena of steel embedded in FA binder will be achieved if the corrosion mechanisms in the pore solution are disentangled. As such a more sustainable cementing material will be suitable for the construction industry. Electrochemical tests such as cyclic potentiodynamic curves (CPP) and electrochemical impedance spectroscopy (EIS) were performed in order to characterize the corrosion behavior of steel in chloride contaminated FA synthetic pore solution as well as the carbonation effect. Results showed that FA synthetic pore solution was able to repassivate the steel even with chloride contents of 0.1M NaCl. Furthermore, the combined effect of chloride and carbonation induced corrosion was observed, showing corrosion rates of 3.547× 10‒5 A/cm2 in FA carbonated pore solution containing 0.6M NaCl.

  • Open access
  • 143 Reads
Unveiling the self-healing effect of cerium ions in PMMA-silica coatings on AA7075: a comparative study of Ce(III) and Ce(IV)

Cerium salts in the form of ammonium cerium(IV) nitrate and cerium(III) nitrate hexahydrate are widely used as corrosion inhibitors due to their ability to provide active protection against corrosion. When incorporated into polymeric, ceramic or hybrid coatings, cerium ions modify their structure and impart beneficial or adverse effects in barrier features, depending on the concentration and type of salt added. In this study, we compare the effect of varying amounts of Ce(III) and Ce(IV) ions on the structure and anti-corrosion properties of poly(methyl methacrylate) (PMMA)-silica hybrid coatings deposited on AA7075 aluminum alloy. The PMMA-silica coatings provided for both additives a long-term protection of AA7075 due to the highly cross-linked structure and a less defective polymeric network [1], however, the self-healing ability as key feature was achieve only by Ce(IV) ions [2]. Electrochemical impedance spectroscopy essays combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy revealed the corrosion inhibition mechanisms occurring in corrosion induced and artificial defects. It was found that intermediate Ce(IV) loadings (500 and 1000 ppm) proved to be more effective in providing a high corrosion resistance with active barrier property, extending the service time up to 720 days in 3.5% NaCl solution. The regenerative action of Ce(IV) can be associated with the faster formation of oxides and hydroxides mainly at intermetallic particle sites of AA7075 at pH ~ 3 compared to those from Ce(III) formed at pH ~ 9 [3]. These results link an optimized hybrid structure provided by cerium ions with their self-healing ability, making PMMA-silica-Ce(IV) hybrids very attractive as low-cost, high-performance and smart chromium-free coatings.