Formation of Mg,Al-hydrotalcite conversion coating on Mg alloy in aqueous HCO3/CO3 and corresponding protection against corrosion by the coating

An environmentally clean method is studied for synthesizing a conversion coating on Mg alloy in aqueous at 50 °C. A precursor layer was firstly formed on AZ91D sample. The precursor layer transformed into a crystallized Mg,Al-hydrotalcite when the sample was continuously immersed in the bath until the bath changed from acidic to alkaline. A rapid conversion treatment was, therefore, developed: it involved maintaining the pH under 6 for precursor layer formation and then increasing it to 11.5 to form crystallized Mg,Al-hydrotalcite coating. No corrosion spot on the Mg,Al-hydrotalcite-coated sample was observed after a 72-hour salt spray test.     

Corrosion protection of steel by bi-layered polypyrrole doped with molybdophosphate and naphthalenedisulfonate anions

Bi-layered polypyrrole (PPy) coating was investigated for corrosion prevention of steels. The PPy coating consisted of the inner layer doped with PMo₁₂ (PMo₁₂) and ions and the outer layer doped with naphthalenedisulfonate (NDS) ions. PMo₁₂ ions doped in the inner layer make the passive oxide film on steel stabilized and the outer layer doped with large organic ions of NDS restricts decomposition and release of PMo₁₂ ions in the inner layer. Corrosion tests were made in 3.5% NaCl aqueous solution for the steels covered by the single PPy-PMo₁₂, single PPy-NDS, and bi-layered PPy-PMo₁₂/PPy-NDS coatings. The single PPy-PMo₁₂ and PPy-NDS coatings maintained the steel in passive state for 34 h and 24 h, respectively. The bi-layered coating kept the steel in passive state and prevented corrosion of the steels for the longer time period.     

Corrosion behaviour of vacuum induction-melted Ni-based alloy in sulphuric acid

A vacuum induction-melted (VIM) Ni-based alloy was immersed in 60% H₂SO₄ solution to investigate its corrosion behaviour and resistance. The results indicate that the microstructure contains a γ-Ni solid solution + Ni₃Si particles, dendrite Ni₃Si, Ni₃B, Cr₇C₃, and CrB. The corrosion started at the zones of the γ-Ni solid solution + Ni₃Si particles and dendrite Ni₃Si. These zones transformed to oxide films and protected the alloy from significant attack. However, the pitting corrosion created paths for acid solution and/or to further attack. Therefore, the corrosion rate decreased and then stabilised at a high value as the immersion time increased.     

The effect of counter anions on corrosion resistance of steel covered by bi-layered polypyrrole film

The bi-layered polypyrrole (PPy) coatings were investigated for corrosion prevention of a carbon steel. The inner layer was doped with the Keggin structure anions of (PMo₁₂) and anions for stabilization of the passive oxide film at the metal-polymer interface, and the outer layer was doped with four organic anions of dihydroxynaphthalenedisulfonate (DHNDS), naphthalenedisulfonate (NDS), anthraquinonedisulfonate (AnqDS) or dodecylsulfate (DoS) for inhibition of the decomposition and release of PMo₁₂. The corrosion tests were performed in 3.5 wt.% NaCl aqueous solution. The corrosion resistance of the steels covered by the bi-layered PPy films was found in the following order: PPy-PMo₁₂/PPy-DHNDS < PPy-PMo₁₂/PPy-NDS < PPy-PMo₁₂/AnqDS < PPy-PMo₁₂/PPy-DoS. The performance of corrosion protection related to the oxidized state of the polymer was discussed.     

Comparison of corrosion-resistance and hydrogen permeation properties of Zn-Ni, Zn-Ni-Cd and Cd coatings on low-carbon steel

Zn-Ni-Cd alloy was electroplated from an alkaline sulfate bath under potentiostatic conditions. The corrosion and hydrogen permeation characteristics of Zn-Ni-Cd alloy coatings electrodeposited from alkaline bath were studied and compared with those of Cd and Zn-Ni coatings obtained using commercial baths. Zn-Ni-Cd alloy was electroplated from an alkaline sulfate bath under potentiostatic conditions. The corrosion potential of this Zn-Ni-Cd coating was −0.62 V vs. SCE, which is still negative potential compared to iron. The corrosion rate of Zn-Ni-Cd coated steel was 0.073 mm y⁻¹, which is estimated in a solution at a pH of 7. This value is much lower than the corrosion rate of Zn-Ni alloy (0.502 mm y⁻¹) and Cd (0.306 mm y⁻¹) coatings deposited from commercial baths. Zn-Ni-Cd alloys are also demonstrated to have superior hydrogen permeation inhibition properties compared to Cd and Zn-Ni coatings. Kinetic parameters of hydrogen permeation such as the transfer coefficient, α, the modified exchange current density, i₀^′, thickness dependent adsorption-absorption rate constant, k^″, recombination rate constant, k₃, surface hydrogen coverage, θ_s, were evaluated by applying a mathematical model to analyze experimental results.     

The application of electrochemical noise resistance to evaluate the corrosion resistance of AISI type 304 SS in nitric acid

The paper presents the application of noise resistance to evaluate the corrosion behaviour of sensitized AISI type 304 SS in nitric acid of varying concentration (4 N, 12 N, 16 N) and temperature (298 K, 323 K, 348 K). Electrochemical noise data was acquired from a three identical electrode configuration in the required conditions at open circuit potential. The noise resistance was evaluated as the ratio of the standard deviation of the potential to that of the current noise after removing the DC component. The inverse relationship between noise resistance and corrosion rate was exploited to qualitatively assess the corrosion behaviour of AISI type 304 SS in nitric acid. Noise resistance decreased with increase in concentration implying an increase in corrosion rate with increase in nitric acid concentration. An increase in temperature from 298 K to 323 K and 348 K decreased the noise resistance in 4 N and 12 N nitric acid implying higher corrosion rates at higher temperatures. The corrosion rates were similar at 323 K and 348 K in these concentrations. The simultaneous measurement of current and potential noise facilitated the evaluation of the frequency dependence of the noise data to determine the spectral noise resistance (R_sn) and the DC limit of the spectral noise resistance . The results from R_sn and also indicated higher corrosion rates at higher concentration and temperature. Also R_n and correlated well in 4 N and 12 N nitric acid at 323 K and 348 K while disparity was observed at room temperature in 4 N and 12 N nitric acid.     

Electrochemical behavior of tin in sodium borate solutions and the effect of halide ions and some inorganic inhibitors

The corrosion of tin electrode in sodium borate (Na₂B₄O₇) solutions was investigated using cyclic voltammetry and potentiostatic current transient techniques. In absence of halide ions, the E/j response exhibits active/passive transition. The active region involves one anodic peak corresponding to the formation of Sn(OH)₂ and/or SnO. Addition of Cl⁻, Br⁻ or I⁻ (C ? 0.01 M) ions inhibits the active dissolution of tin, but higher concentrations enhance the active dissolution and tend to breakdown the passive film and induce pitting attack. The effect of , , and as inorganic inhibitors on the pitting corrosion of tin in (0.1 M Na₂B₄O₇ + 0.1 M NaCl) solution has also been studied. The presence of these anions (except ) inhibits pitting corrosion. Chronoamperometry measurements showed that nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation () increases with increasing halide ions concentration and applied potentials, but decreases with increasing the concentration of the inorganic inhibitors (except ). The inhibition efficiency of these inhibitors decreases in the order:

     

Bis-isoxazolidines: A new class of corrosion inhibitors of mild steel in acidic media

Two new bis-isoxazolidines were synthesized in excellent yields via cycloaddition reaction of 1-pyrroline 1-oxide with 2,7-di(10-undecenyloxy)naphthalene and 1,4-di(10-undecenyloxy)benzene. One of the bis-isoxazolidines, on reaction with two equivalents of 1-bromododecane, afforded a bis-quaternary ammonium salt. All three inhibitor molecules in the presence of 400 ppm at 60 °C achieved inhibition efficiencies (IE) in the ranges 97-98% and 92-96% as determined by gravimetric method for corrosion of mild steel in 1 M HCl and 0.5 M H₂SO₄, respectively. The results obtained by the electrochemical methods using Tafel plots and electrochemical impedance spectroscopy corroborated the findings of the weightloss method. Both bis-isoxazolidines demonstrated remarkable efficiency in the lower range of inhibitor concentrations - in the presence of a meager 1.5 ppm of the inhibitor molecules, IEs of ∼70% in 1 M HCl were achieved. Thermodynamic parameters (, , ) for the adsorption process in the presence of the bis-isoxazolidines were determined. The values of around -90 kJ/mol in 1 M HCl and −50 kJ/mol in 0.5 M H₂SO₄, pointed towards the chemisorption of the inhibitor molecules, especially in HCl media. While the corrosion inhibition by these molecules was predominantly under cathodic control in 1 M HCl, the inhibition in 0.5 M H₂SO₄ was found to be under anodic control. The bis-isoxazolidines were found to provide a suitable inhibition mechanism for the corrosion inhibition in HCl as well as in H₂SO₄ media.     

Semiconducting and passive film properties of nitrogen-containing type 316LN stainless steels

The semiconducting property of passive films of nitrogen-containing type 316LN stainless steels in different electrolytic media (0.5 M NaCl, borate buffer and borate buffer + 0.5 M NaCl) was investigated by electrochemical impedance spectroscopy (EIS). The nitrogen effect on the chemical composition of the passive films was investigated using X-ray photoelectron spectroscopy, (XPS). Based on capacitance results, the semiconducting parameters obtained from the Mott-Schottky plots indicated a decrease in the donor and acceptor density (N_D and N_A) with increase in nitrogen content, and variation in the flat band potential (E_FB), depending on the electrolytic media. Thus indicating that the oxide layers of the passive film are modified by nitrogen addition. The presence of nitrogen and in the passive film was confirmed by the XPS analysis of the passive film. Cyclic polarization for pitting and repassivation corrosion studies indicated a decrease in hysteresis loop with increase in nitrogen content in 0.5 M NaCl solution. In the highest nitrogen-containing alloy (0.556 wt.% N), the hysteresis loop was small and negligible indicating that the pit initiation is minimum in this alloy. Based on the results obtained, an attempt was made to correlate the semiconducting nature of the passive films with pitting corrosion resistance.     

Inhibitive effect of diethylcarbamazine on the corrosion of mild steel in hydrochloric acid

Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), weight loss measurements and atomic force microscopy techniques were used to investigate the inhibitory effect of diethylcarbamazine (DECM) on corrosion of mild steel in HCl solution. The inhibitor showed >90% inhibition efficiency at 5.01 × 10⁻⁴ M. Results obtained revealed that inhibition occurs through adsorption of inhibitor molecules on metal surface without modifying the mechanism of corrosion process. Potentiodynamic polarization studies suggested that it is a mixed type inhibitor, predominantly controls cathodic reaction. Activation parameters (E_a, ΔH and ΔS) and thermodynamic parameters (, and ) were calculated to investigate mechanism of inhibition.     

Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 °C

New Pourbaix diagrams were calculated at 25, 75 and 95 °C for the Nb-H₂O system. The species and were considered. Potentiodynamic polarization and mass loss experiments (14 days) were conducted in concentrated H₂SO₄ (20, 40 and 80 wt%) and HCl (20 and 38 wt%) solutions at 75 and 95 °C. Nb forms a metastable pentoxide (Nb₂O₅) in H₂SO₄ and HCl solutions which dissolves as . Corrosion rates decrease between the 40% and the 80% H₂SO₄ solutions. SEM micrographs show generalized pitting in the 20% and 40% H₂SO₄ solutions. Mass loss corrosion rates did not exceed 306 μm/yr. Corrosion rates estimated by Tafel extrapolation were within two orders of magnitude of those measured by mass loss and it is shown that this finding is consistent with the thickening of the oxide.     

Photoelectron spectroscopy study of the inhibition of mild steel corrosion by molybdate and nitrite anions

Passive films formed on mild steel in aqueous 8.6 mM NaCl solutions (pH 8), containing either or , have been studied with X-ray photoelectron spectroscopy. For either anion these films are ∼5 nm deep, and the primary chemical state of iron is Fe³⁺. Following exposure to , the film consists of a sub-layer (∼4.1 nm) composed largely of ferric oxide/hydroxide, overlaid by Fe₂(MoO₄)₃ (∼0.6 nm). As regards , spectra are consistent with the film being closely related to γ-Fe₂O₃. Furthermore, a reduction product of , potentially N₂, is present, displaying a depth profile comparable to that of molybdate.     

Effects of inorganic additions on the performance of manganese-based conversion treatments

The electrochemical behaviour of permanganate-based no-rinse conversion coatings on AA3003-H14 aluminium alloy shows that they have promise as an alternative to toxic chromate-based coatings. A selection of inorganic additions has been made here to a simple 0.1 M KMnO₄ formulation investigated previously. The performance of the different coatings has been evaluated from potential-time and polarisation measurements in 0.5 wt % (0.085 M) NaCl solution for periods up to 3 days. The addition of fluoride ions produced no benefit, whereas Al(NO₃)₃ shows an improvement when added in small amounts. Tests with the individual ions reveal that it is Al³⁺ cations, rather than anions, which produce the greatest benefit.     

Prevention of zinc corrosion in oxygenated 0.5 M NaCl by treatment in a cerium(III) nitrate solution and modification with sodium hexadecanoate

A self-assembled monolayer (SAM) of hexadecanoate ion (C₁₆A⁻) was prepared on a zinc electrode covered with a layer of hydrated cerium(III) oxide Ce₂O₃. The protection of zinc against corrosion was examined for the electrode coated with the Ce₂O₃ layer and the C₁₆A⁻ SAM in an oxygenated 0.5 M NaCl solution. A more positive open-circuit potential of the coated electrode was maintained during immersion in the solution for 4 h than that of the uncoated one and polarization curves showed marked suppression of the anodic process, implying that the layer modified with the SAM acted as a passive film. The protective efficiency of the modified layer was extremely high, more than 99%. The zinc surface coated with the Ce₂O₃ layer and the C₁₆A⁻ SAM was characterized by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement with a drop of water.