Effect of Ni content on the corrosion behavior of Cu-Ni alloys in neutral chloride solutions

The effect of systematic increase of Ni content on the electrochemical behavior of the Cu-Ni alloys in neutral chloride solutions was investigated. The pitting corrosion behavior of Cu-Ni alloys with different Ni contents, namely, 5, 10, 30 and 65 mass% Ni, in a stagnant 0.6 mol dm⁻³ NaCl solution of pH 7.0 was studied. The effect of chloride ion concentration on the electrochemical behavior of these alloys was also investigated. The results show that the increase in nickel content decreases the corrosion rate of the alloys in the neutral chloride solution. The increase of chloride concentration up to 0.3 mol dm⁻³ increases the corrosion rate. At higher concentrations ([Cl⁻] > 0.3 mol dm⁻³) the corrosion rate decreases due to the hydrolysis of Cu(I) chloride to form the passive Cu(I) oxide film. The breakdown potential depends on the chloride ion concentration and the nickel content of the alloy. For these investigations conventional electrochemical techniques and electrochemical impedance spectroscopy (EIS) were used. The impedance measurements have shown that the increase of the Ni content and the immersion time of the alloys in the chloride solution increase the corrosion resistance of the alloys. The experimental impedance data were fitted to theoretical values according to a proposed equivalent circuit model.     

Environmentally safe corrosion inhibition of Mg–Al–Zn alloy in chloride free neutral solutions by amino acids

The corrosion inhibition of Mg–Al–Zn alloy was investigated in stagnant naturally aerated chloride free neutral solutions using amino acids as environmentally safe corrosion inhibitors. The corrosion rate was calculated in the absence and presence of the corrosion inhibitor using the polarization technique and electrochemical impedance spectroscopy. The experimental impedance data were fitted to theoretical data according to a proposed electronic circuit model to explain the behavior of the alloy/electrolyte interface under different conditions. The corrosion inhibition process was found to depend on the adsorption of the amino acid molecules on the metal surface. Phenyl alanine has shown remarkably high corrosion inhibition efficiency up to 93% at a concentration of 2 × 10⁻³ mol dm⁻³. The corrosion inhibition efficiency was found to depend on the concentration of the amino acid and its structure. The mechanism of the corrosion inhibition process was discussed and different adsorption isotherms were investigated. The free energy of the adsorption process was calculated for the adsorption of different amino acids on the Mg–Al–Zn alloy and the obtained values reveal a physical adsorption of the inhibitor molecules on the alloy surface.     

Effect of nickel content on the electrochemical behavior of Cu-Al-Ni alloys in chloride free neutral solutions

The electrochemical behavior of Cu-Al-Ni alloys in chloride free neutral solutions was investigated. The effect of Ni content on the corrosion resistance of the alloys was examined and evaluated. Conventional electrochemical techniques and electrochemical impedance spectroscopy, EIS, have been used. Potentiodynamic measurements revealed that the increase in the Ni content decreases the stability of the Cu-Al-Ni alloys. The polarization measurements were confirmed by EIS experiments. The morphology of the alloy surface was investigated by scanning electron microscopy, SEM, and surface analysis was made by energy dispersive X-ray technique. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model representing the electrode/electrolyte interface. The results of these experiments are discussed in reference to the potential-pH (Pourbaix) diagrams of the alloying elements.     

Electrochemical behavior of Mg and some Mg alloys in aqueous solutions of different pH

The electrochemical behavior of Mg, Mg-Al-Zn and Mg-Al-Zn-Mn alloys were investigated in aqueous acidic, neutral and basic solutions. Conventional electrochemical techniques such as open-circuit potential measurements, polarization methods and electrochemical impedance spectroscopy (EIS) were used. The results have shown that the rate of corrosion in acidic solution is relatively high compared to that in neutral or basic solutions. The presence of Al, Zn and Mn as alloying elements decreases the rate of corrosion of the alloy. The activation energy of the corrosion process occurring at the surface of Mg or Mg alloys in aqueous solutions is less than 40 kJ mol⁻¹. This value indicates a one electron transfer electrode as a rate controlling process. The impedance data were fitted to equivalent circuit models that explain the different electrochemical processes occurring at the electrode/electrolyte interface.     

Corrosion inhibition of mild steel in sulfamic acid solution by S-containing amino acids

R_p, potentiodynamic polarization curves and EIS techniques were applied to study the effect of five S-containing amino acids on the corrosion of mild steel in 5% sulfamic acid solution at 40 °C. The compounds are effective inhibitors and the inhibition efficiency follow the order: N-acetylcysteine (ACC) > cysteine (RSH) > S-benzylcysteine (BzC) > cystine (RSSR) ≅ methionine (CH₃SR). The inhibitors affect the anodic dissolution of steel by blocking the anodic sites of the surface. EIS measurements indicated that charge transfer is the rate determining step in the absence and presence of the inhibitors and the steel/solution interface can be represented by the equivalent circuit R_s(R_ctQ_dl). Adsorption of RSH, CH₃SR and RSSR follows the Langmuir model while the Temkin isotherm describes the adsorption of ACC and BzC. From the application of the Flory–Huggins isotherm, the number of water molecules displaced by the adsorbing inhibitor molecules was estimated. The potential of zero charge pzc of mild steel without and with the inhibitors is calculated and the mechanism of corrosion inhibition is discussed in the light of the molecular structure.     

Electrochemical studies with a Cu-5wt.%Ni alloy in 0.5 M H2SO4

The electrochemical behavior of the annealed Cu-5wt.%Ni alloy in 0.5 M H₂SO₄ was studied by means of open-circuit potential (E_OCP) measurements, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and quasi-stationary linear potential sweep. The hydrodynamics of the system was also studied. This material is constituted by a single α₁ phase. The anodic behavior of a Cu-Ni alloy in H₂SO₄ consists fundamentally on the electrodissolution of Cu, its main component, and the formation of a sulfur-containing passive layer. The presence of Ni decreases the rate of Cu oxidation, mostly at high positive potentials. The impedance spectra, obtained for the unrotating electrode, can be interpreted in terms of a simple charge-transfer reaction across a surface layer. When the electrode is rotated, the occurrence of an inductive loop evidenced the existence of an adsorbed layer. All the resistance estimated from the proposed equivalent circuits diminished with the electrode rotation rate, emphasizing the influence of ion transport in the overall electrode process. The system presented two anodic Tafel slopes: 40 mV dec⁻¹ for E<255 mV and 67 mV dec⁻¹ for E>275 mV. A Tafel slope of 40 mV dec⁻¹ evidences that copper dissolution can be interpreted in terms of the mechanism proposed by Mattsson and Bockris. The second Tafel suggests that at potentials more positive than 275 mV, copper dissolves according to a mechanism that considers the disproportionation of adsorbed Cu(I) species.     

Corrosion of tin in aqueous solutions of mono-, Di- and trichloroacetic acids

The corrosion behaviour of tin in stagnant mono-, di- and trichloroacetic acids solutions in the pH range 1–6 and at concentrations 4.0 to 5 × 10^?4 M was investigated. The results indicate behaviour that is generally the same but there is some dependence on the acid concentration and the pH value. In 4.0 to 10^?2 M solutions, the corrosion rate (W) increased with increasing acid concentration and decreasing pH value from 1 to 4 as follows: log W=a+b log C, where b=0.70, 0.42 and 0.35 for tri-, di- and mono-chloroacetic acids respectively. At high concentrations 4.0 to 10^?2 M and in the pH range 1–6, the steady state corrosion potential shifted in the negative direction with increase of acid concentration accompanied by an increase in the corrosion rate, indicating that the corrosion process becomes anodically controlled by the complexing of Sn²⁺ ions with organic acid anions and that the order of aggressiveness is mono-<di-<trichloroacetic acids. In dilute solutions (10^?2 to 5.10^?4 M) in the pH range 1–6 the steady state potential shifted in the noble direction with increase of acid concentration (accompanied by a remarkable decrease in the corrosion rate). Corrosion inhibition in dilute solutions was attributed to film formation on the surface of tin which may result from the hydrolysis of tin species.     

Corrosion resistance of ternary NiP based alloys in sulfuric acid solutions

NiP based alloy films were prepared by autocatalytic deposition and their structure, chemistry and corrosion behaviors in sulfuric acid solutions were studied as a function of their composition. The as-prepared Ni-based alloys are nanocrystalline, and their grain size decreases with increasing P content. Addition of a third element (W or Mo) influences the observed grain size. At low anodic overpotential NiP based alloys present a lower exchange current and lower reactivity than Ni, both improving with increasing P content. Contrary to Ni however, the NiP based alloys do not passivate at higher anodic overpotentials. Addition of W to NiP alloys can improve their corrosion resistance, while addition of Mo has little or no beneficial effects on corrosion properties.     

Pyrrole-based silane primer for corrosion protection of commercial Al alloys. Part II. Corrosion performance in neutral NaCl solution

A chromate-free, direct-to-metal treatment using pyrrole-based silane (PySi) was developed for protection against corrosion of as-received commercial Al alloys, following the typical procedure for silane deposition. The protection performance of composite PPySi films, containing polysiloxane linkages and polypyrrole units, was evaluated in near neutral NaCl solution by simple corrosion tests such as single-cycle anodic polarization, corrosion potential monitoring and long-term immersion experiments. Control coatings of polymethylsiloxane (PMeSi) and electrochemically synthesized polypyrrole (Ppy) were also studied. The superior performance of PPySi with respect to PMeSi and Ppy was attributed to highly crosslinked, well-packed and adherent composite films of thickness of the order of microns, manifesting both barrier action and active protection. The use of pyrrole-based silane for corrosion protection of Al alloys constitutes a promising approach for effective replacement of chromium-based treatments in practical applications. Further investigation from the fundamental point of view is deserved.     

Effect of sulfur-containing amino acids on the corrosion of mild steel in sulfide-polluted sulfuric acid solutions

The influence of cysteine (RSH) and cystine (RSSR) on the corrosion behavior of mild steel in sulfide-polluted H₂SO₄ solutions was studied by potentiodynamic polarization methods and AC impedance technique. The results show that S²⁻ accelerates the corrosion process markedly, especially the anodic dissolution of iron. Tafel polarization curves show that RSH and RSSR act mainly as anodic-type inhibitors without affecting the mechanism of the hydrogen evolution reaction or iron dissolution. Adsorption of RSH and RSSR in most sulfide-polluted H₂SO₄ solutions obeys Temkin’s isotherm. Impedance studies indicate that in the inhibited and uninhibited solutions, charge transfer controls the corrosion process either at E_corr or at 30 mV vs E_corr. Potentiodynamic anodic polarization curves show that RSSR effectively inhibits the steel dissolution both in the active and passive states and greatly reduces the current oscillations observed in the passive region.     

Corrosion of dissimilar alloys: Electrochemical noise

A new approach for interpreting the electrochemical noise generated by freely corroding electrodes is presented. For a single electrode, with well-defined anodic and cathodic regions, it is proposed that the instantaneous values of the corrosion current and open circuit potential are functions of the time-invariant potential difference between the anodic and cathodic reactions and the time-dependent impedances associated with the anodic and cathodic reactions. The fluctuations in the values of anodic and cathodic impedances are determined by the interaction between the aggressive environment and the oxide, hydroxide or adsorbed surface layer covering the anodic and cathodic regions; the instantaneous values of impedances modulate the corrosion current. The mathematical implications of these assumptions are discussed and the results are compared with the traditional approach, based on time-dependent current sources acting on constant impedances. Subsequently, the findings for a single electrode are transferred to two dissimilar electrodes and an experimental technique that enables the estimation of the time evolution of anodic and cathodic resistances for each of the dissimilar electrodes is presented.     

Electrochemical corrosion properties of metal alloys used in orthopaedic implants

This study concerns an investigation of the corrosion behavior of 316 stainless steel, CoCrMo and Ti6Al4V alloys in simulated body conditions (ringer lactate) at 37 °C by the use of Tafel plots, mixed potential and electrochemical impedance spectroscopy (EIS). Ti6Al4V alloy has the highest corrosion resistance followed by CoCr alloy. Ti6Al4V–CoCrMO was the best couple for galvanic corrosion with the minimum galvanic potential and current values according to mixed potential theory and Tafel method. It was concluded that Ti6Al4V was the most suitable material for implant applications in the human body.     

Influence of rare earths addition on the corrosion behaviour of Zn-5%Al (Galfan) alloy in neutral aerated sodium sulphate solution

The effect of rare earth metal (Ce, Er, Y) additions on the corrosion behaviour of Zn-5Al Galfan alloy has been investigated. The corrosion resistance of Zn-5Al-1Ce, Zn-5Al-1Er and Zn-5Al-1Y alloys has been assessed by various electrochemical tests, such as corrosion potential measurements, polarization curves and electrochemical impedance spectroscopy. They have been performed in a 0.1 M Na₂SO₄ solution, at approximately neutral pH, without stirring and in contact with the air. For comparison, the electrochemical tests have also been carried out on the Zn-5Al alloy. Moreover, the surface morphology and nature of the corrosion products have been investigated. The results indicated that rare earths’ addition improves the corrosion behaviour of Galfan, the effect being more pronounced for the Er- and Y-containing alloys. The mechanism by which the corrosion resistance of Zn-5Al alloy is enhanced in presence of the rare earths is discussed.     

Effect of Al content on the corrosion behavior of Mg-Al alloys in aqueous solutions of different pH

The effect of systematic increase of Al content on the electrochemical behavior of the Mg-Al alloys in aqueous solutions of different pH was investigated. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy, EIS, were used to investigate the electrochemical behavior of the alloys in aqueous solutions. The results have shown that Mg-5Al is easily corroded due to the microgalvanic effect between α-phase and β-phase, its corrosion rate is even higher than that of Mg itself. The increase of Al content increases the corrosion resistance of the alloy due to the formation of the β-phase (Mg₁₇Al₁₂) together with the Mg α-phase. The ranking of the corrosion rate of these alloys was Mg-5Al > Mg > Mg-10Al ≅ Mg-15Al. The corrosion rates of the alloys in acidic solutions are pronouncedly high compared to those measured in neutral or basic solutions. The impedance measurements are in consistence with the polarization techniques and the impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface.     

Corrosion of zinc in simulated carbonated concrete pore solutions

The electrochemical behaviour of pure Zn and galvanized steel in solutions simulating the pore solution of carbonated concrete has been studied by means of potentiodynamic polarization tests and polarization resistance measurements. Pure Zn was chosen because it simulates well the behaviour of galvanized steel, yielding more reproducible results. The effect of different degrees of carbonation and the presence of different chloride contents in the simulated pore solutions was investigated. Results show that at a given pH (about 9.5) the corrosion susceptibility of Zn depends on anions concentration (carbonate and bicarbonate). The results obtained in simulated carbonated concrete pore solutions show that with low anion concentration Zn does not passivate while in presence of high levels of carbonate and bicarbonate the corrosion resistance is improved. Besides, the presence of chloride increases the corrosion susceptibility.     

Complexities of corrosion behaviour of copper-nickel alloys under liquid impingement conditions in saline water

Flow-assisted electrochemical corrosion of Cu-Ni disc electrodes, subjected to a submerged impinging jet in saline water, was evaluated and quantified using the anodic current densities (j_a), corrosion potentials (E_corr), interfacial capacitances (C) and polarization resistances (R_p). The Reynolds numbers in the range of applied impinging velocities suggested that the flow between the solution and the electrode is turbulent in the systems investigated.The role of Ni content in the resistance of spontaneously formed barrier oxide film and diethyldithiocarbamate inhibitor film on Cu-Ni alloys (10, 20, 30 and 40 at.% Ni) was examined and evaluated using electrochemical impedance spectroscopy in dependence of the impinging velocity and the immersion time.The results are discussed in terms of the corrosion mechanisms and their relevance to the use of Cu-Ni alloys in desalination plants and in the marine engineering field because fluid hydrodynamics plays an important role in Cu-Ni alloys application. Based on the analysis presented in the paper, it is anticipated that the Cu-10Ni alloy is the best choice for application in the marine environment.     

Electrochemical behavior of magnesium alloys AZ91D,AZCe2, and AZLa1 in chloride and sulfate solutions

The influence of Cl⁻ and SO₄²⁻ on the electrochemical behavior of AZ91D, AZCe2, and AZLa1 was studied. For all alloys, there was a current plateau in the anodic polarization curves in Na₂SO₄ solutions. In 0.5% NaCl solution, there was a small current plateau, whereas there was none in the 3.5% and 5% NaCl solutions. This indicated that SO₄²⁻ is less aggressive than Cl⁻. The range of the current plateau decreased with increasing SO₄²⁻ concentration. For all alloys, the high frequency capacitive loop in the Nyquist plots decreased with increasing concentration consistent with the decrease in corrosion resistance with increasing Cl⁻ and SO₄²⁻ concentration.     

Corrosion study of surface-modified vanadium-free titanium alloys

The present work is part of an investigation aimed to improve the corrosion resistance of three vanadium-free titanium alloys of biomedical interest, Ti-6Al-7Nb, Ti-13Nb-13Zr and Ti-15Zr-4Nb, by growing on their surfaces an oxide protective layer. For this goal, different samples were oxidized in air at 750 °C for times ranging from 6 to 48 h. Thickness, morphology and composition of the oxide scales for different oxidation times were investigated by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). After equal oxidation time, the Ti-6Al-7Nb alloy exhibited a thinner, more compact and dense oxide layer than the TiNbZr alloys, indicating a slower oxidation rate. Several oxidation times were used in order to obtain oxide scales with similar properties for the three alloys. Different electrochemical techniques were then applied to evaluate the corrosion behavior of the different samples. The oxidized Ti-6Al-7Nb alloy showed the lowest corrosion current densities as well as the best pitting corrosion behavior, and is thereby considered as the best of these materials for biomedical applications.     

Environmentally Friendly Nonionic Surfactants Derived from Jatropha Oil Fatty Acids as Inhibitors for Carbon Steel Corrosion in Acidic Medium

The inhibition effect of four novel environmentally friendly inhibitors (derived from vanillin and a fatty acid mixture obtained from the hydrolysis of Jatropha oil) on carbon steel corrosion in 0.5 M H₂SO₄ was investigated by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results show high efficiency of the inhibitors. The inhibition efficiency increases by increasing the polyethylene glycol chain length and the inhibitor concentration. From polarization curves, the inhibitors act as cathodic inhibitors. EIS spectra display a large capacitive loop at high frequencies followed by a large inductive loop at low frequencies. Environmental study shows inhibitors tendency towards biodegradation by the action of microorganisms after 28 days.     

The corrosion behaviour of rare-earth containing magnesium alloys in borate buffer solution

In this work, the corrosion behaviour of magnesium alloys ZK31, EZ33 and WE54 was studied in sodium borate buffer solution at pH 9.2.The electrochemical processes were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The composition and morphology of the alloys and corrosion products formed were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).The experimental findings highlighted the differences in the corrosion mechanisms of the different alloys tested. The results showed that the presence of rare-earth elements (RE) only increases the corrosion resistance when present in solid solution, as is the case of the WE54 alloy. At pH 9.2, an amorphous yttrium oxide/hydroxide thick film was formed, which possesses greater stability when compared to magnesium oxide/hydroxide. The role of RE in the corrosion mechanism was discussed.