Corrosion, passivation and breakdown of passivity of Al and Al-Cu alloys in gluconic acid solutions

The pitting corrosion of Al, and two Al-Cu alloys, namely (Al-2.5% Cu) and (Al-7.0% Cu) was investigated in gluconic acid (HG) solution through linear polarization and cyclic voltammetry techniques complemented with ex situ EDX and SEM examinations of the electrode surface. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method of chemical analysis was used to study the effect of alloyed Cu on the rate of the uniform corrosion of Al in these solutions. Results obtained from ICP were compared with those obtained from polarization measurements. For the three Al samples, the anodic responses did not involve active/passive transition due to spontaneous passivation. Addition of HG induced pitting (confirmed from SEM) within the passive oxide film due to the aggressive attack of gluconate (G⁻) anions. Relationships between pitting potential (E_pit), HG concentration, temperature, pH and potential scan rate were established. Alloyed Cu was found to enhance uniform corrosion, while it suppressed pitting attack. Local acidification model is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of G⁻ anions.     

Effect of bromide ions on the corrosion behavior of tantalum in anhydrous ethanol

The electrochemical behaviors of Ta in Et₄NBr ethanol solutions were investigated using potentiodynamic polarization, cyclic voltammetry, potentiostatic current-time transient and impedance techniques. The potentiodynamic anodic polarization curves did not exhibit active dissolution region due to the presence of thin oxide film on the electrode surface, which was followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of Br⁻ anions. The pitting potential (E_b) decreased with the increase of solution temperature and Br⁻ concentration, but increased with increasing potential scan rate and water concentration. The incubation time derived from potentiostatic current-time transients decreased with increasing potentials. The impedance spectra exhibited two time constants for all the potentials and the resistance of passive layer decreased with increasing potential.     

Metastable and stable pitting events on Al induced by chlorate and perchlorate anions—Polarization, XPS and SEM studies

The metastable and stable pitting events of Al were studied in 0.075 M deaerated acidic NaClO₃ and NaClO₄ solutions (pH 3) using potentiodynamic anodic polarization and potentiostatic measurements, complemented with SEM and XPS examinations of the electrode surface. Metastable pits (appeared here as oscillations in current in the nA range) form at potentials close to the pitting potential (E_pit). SEM examinations of the electrode surface showed that the current oscillations resulted in observable pits on sample surface. The repassivated metastable pitting sites are prone to become preferential sites for following metastable pits to nucleate, resulting in accumulated corrosion damages on the surface. Results showed that Cl⁻ ions produced in solution via the reduction of ClO₃⁻ and ClO₄⁻ anions at sufficiently negative cathodic potentials as well as their decomposition at high anodic potentials. Chloride production induced via the reduction of perchlorates is much slower than induced by chlorates. XPS examinations of the electrode surface showed that the amount of ClO₄⁻ and Cl⁻ anions detected on the electrode surface increases with both cathodic and anodic polarizations (even above E_pit). Experimental results revealed that addition of Cl⁻ ions to the ClO₄⁻ solution accelerates pitting corrosion, indicating that these anions cooperate together in passivity breakdown and initiation of pitting. The role of ClO₃⁻ and ClO₄⁻ ions, despite their large size, in pitting process is also discussed here. A point defect model (PDM) is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of Cl⁻ ions.     

Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO₃⁻ concentration and temperature, but increases with increasing potential scan rate. Addition of SO₄²⁻ ions to the nitrate solution accelerates pitting corrosion, while addition of WO₄²⁻ and MoO₄²⁻ ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO₃⁻ concentration, temperature and applied anodic current density. Addition of SO₄²⁻ ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO₄²⁻ or MoO₄²⁻ ions.     

Pitting corrosion of lead in sodium carbonate solutions containing NO3 ions

Pitting corrosion of Pb in Na₂CO₃ solutions (pH=10.8) containing NaNO₃ as a pitting corrosion agent has been studied using potentiodynamic anodic polarization, cyclic voltammetry and chronoamperometry techniques, complemented with scanning electron microscopy (SEM) examinations of the electrode surface. In the absence of NO₃⁻, the anodic voltammetric response exhibits three anodic peaks prior to oxygen evolution. The first anodic peak A₁ corresponds to the formation of PbCO₃ layer and soluble Pb²⁺ species in solution. The second anodic peak A₂ is due to the formation of PbO beneath the carbonate layer. Peak A₂ is followed by a wide passive region which extends up to the appearance of the third anodic peak A₃. The later is related to the formation of PbO₂. Addition of NO₃⁻ to the carbonate solution stimulates the anodic dissolution through peaks A₁ and A₂ and breaks down the dual passive layer prior to peak A₃. The breakdown potential decreases with an increase in nitrate concentration, temperature and electrode rotation rate, but increases with an increase in carbonate concentration and potential scan rate. Successive cycling leads to a progressive increase in breakdown potential. The current/time transients show that the incubation time for passivity breakdown decreases with increasing the applied anodic potential, nitrate concentration and temperature.     

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.     

Electrochemical methods in the study of localized corrosion attack

Electrochemical methods for determining the characteristic pitting potentials of 90Cu–10Ni alloy in slightly alkaline chloride solutions are summarized and the results of measurements carried out using potentiostatic, quasi-potentiostatic, potentiodynamic and galvanostatic techniques, complemented byex situ techniques — SEM and EDXA — are discussed. In borate buffer solution a passive state is established due to the formation of the oxide film with low ionic conductivity. However, in the presence of Cl^– ions, at potentials higher than a certain critical value, breakdown of the anodic passivity occurs, caused by field-stimulated chloride entry into the passive oxide film at singular point defects. The brightening of the pits formed after oxide film breakdown was established to be due to the conversion of the passivating oxide film to one of high ion conductivity Contaminated oxide permits the passage of the metal cation into and through it, finally leaving it at the film/solution interface where pitting can proceed. During localized attack two characteristic potentials have to be distinguished: the potential of pit nucleation,E _n, above which pit nucleation starts, and the breakdown potential,E _b, above which the growth of nucleated pits develops. An attempt is made to compare the values ofE _n andE _b obtained through different methods, and to determine the factors influencing these values in each particular method.Presented at the 11th International Corrosion Congress, Florence, Italy, April 1990     

On the role of NO2 ions in passivity breakdown of Zn in deaerated neutral sodium nitrite solutions and the effect of some inorganic inhibitors: Potentiodynamic polarization, cyclic voltammetry, SEM and EDX studies

As a first step towards studying pitting corrosion of Zn in deaerated neutral sodium nitrite solutions (pH 6.9), we have reported the results of potentiodynamic polarization and cyclic voltammetry measurements on the passivity and passivity breakdown of Zn in these solutions. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) examinations of the electrode surface. The voltammograms involve active/passive transition prior to the initiation of pitting corrosion as a result of breakdown of the passive film by NO₂⁻ ions. The active region displays one anodic peak due to the formation of ZnO passive film on the anode surface. SEM examinations confirmed the existence of pits on the electrode surface. The potential at which pits initiated (E_pit) was determined, together with a pit transition potential (E_ptp) that appeared as an abrupt current discontinuity on the reverse potential scan (hysteresis loop), and a protection potential (E_prot) that appeared at the end of the hysteresis loop. The value of E_pit shifted negatively as either C_nitrite or temperature was increased, while it increased with the increase in potential scan rate. The effect of adding some environmentally acceptable inorganic inhibitors, as tungstates, molybdates and silicates (water glass), on the pitting corrosion behaviour of Zn in nitrite solutions has also been studied. The mechanism of inhibition was discussed.     

Uniform and pitting corrosion events induced by SCN anions on Al alloys surfaces and the effect of UV light

The influence of the alloying elements on the uniform and pitting corrosion processes of Al-6061, Al–4.5%Cu, Al–7.5%Cu, Al–6%Si and Al–12%Si alloys was studied in 0.50 M KSCN solution at 25 °C. Open-circuit potential, Tafel polarization, linear polarization resistance (LPR) and ICP-AES measurements were used to study the uniform corrosion process on the surfaces of the tested alloys. Cyclic polarization, potentiostatic current-time transients and impedance techniques were employed for pitting corrosion studies. Obtained results were compared with pure Al. Passivation kinetics of the tested Al samples were also studied as a function of applied potential, [SCN⁻] and sample composition by means of potentiostatic current transients. The induction time, after which the growth of stable pits occurs, decreased with increasing applied potential and [SCN⁻]. Regarding to uniform corrosion, alloyed Cu was found to enhance the corrosion rate, while alloyed Si suppressed it. Alloying elements of the tested samples diminished pitting attack to an extent depending on the percentage of the alloying element in the sample. Among the investigated materials, Al–Si alloys exhibited the highest corrosion resistance towards uniform and pitting corrosion processes in KSCN solutions. The passive and dissolution behaviour of Al was also studied under the conditions of continuous illumination (300–450 nm) based on cyclic polarization and potentiostatic techniques. The incident photons had a little influence on pit initiation and a marked effect on pit growth. These explained in terms of a photo-induced modification of the passive film formed on the anode surface, which render it more resistant to pitting. The effects of UV photons energy and period of illumination on the morphology of the pitted surfaces were also studied.     

The Influence of Halide Anions on the Anodic Behavior of Nickel in Borate Solutions

The anodic behavior of nickel in Na₂B₄O₇ solutions containing various concentrations of NaCl, NaBr, or NaI as pitting corrosion agents was studied using the potentiodynamic technique. In absence of the halide anions, the E/i curves exhibit active and passive regions prior to oxygen evolution. The passivity is due to the formation of nickel oxides on the electrode surface. The presence of low concentrations of the halide anions has no effect on the mechanism of nickel passivity. High aggressive anion concentrations stimulate the active region and tend to break down the passive film, leading to pitting corrosion. The susceptibility of the nickel anode to pitting corrosion is enhanced with increasing halide anion concentration and is decreased with increasing pH of solution. The aggressiveness of the halide anions towards the stability of the passive film decreases in the order: Cl^– > Br^– > I^–. The addition of increasing concentrations of tungstate, chromate, or molybdate anions causes a shift of the breakdown potential in the noble direction, indicating the inhibitive action of the added anions. The inhibiting tendency of these anions decreases in the order: WO₄^2– > CrO₄^2– > MoO₄^2–.     

The corrosion behavior of sputter-deposited aluminum-tungsten alloys

Thin films on aluminum-tungsten alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source, on glass and sapphire substrates. Completely amorphous films were obtained in the Al₈₀W₂₀-Al₆₇W₃₃ composition range. Passivity and corrosion behavior of amorphous Al-W alloys were investigated in 1 M deaerated hydrochloric acid solution using polarization and impedance spectroscopy measurements and have been correlated with the properties of pure alloy components. Tungsten and sputter-deposited Al-W thin films are inherently passive materials while aluminum undergoes pitting corrosion in hydrochloric acid solution. The passive film formed at the OCP on each alloy possesses excellent electric and dielectric properties comparable to those of the isolating film on tungsten. The absolute impedance increases with increasing tungsten content in the alloy. According to electrochemical polarization measurements, alloying Al with W in solid solution significantly enhances the material's resistance to pitting corrosion by shifting the breakdown potential above 2000 mV (Al₆₇W₃₃) and lowering the corrosion rate at the OCP by more than two orders of magnitude. The most likely mechanism explaining the passivity of amorphous Al-W alloys, the Solute Vacancy Interaction Model (SVIM), involves the formation of complexes between highly oxidized solute atoms (W⁺⁶) and mobile cation vacancies, which restrict the transport of Cl⁻ through the oxide film and inhibit its breakdown in hydrochloric acid solution. The role that film stress relaxation effects and microscopic defects in amorphous Al-W films, of the some composition, and deposited on various substrates play in their corrosion resistance is discussed.     

Formation of anodic films on sputtering-deposited Al-Hf alloys

The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al-Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO₂ and Al₂O₃ distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al-61 at.% Hf alloy. The distributions of Al³⁺ and Hf⁴⁺ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al³⁺-O²⁻ and Hf⁴⁺-O²⁻ bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V⁻¹ respectively.     

Role of Cl in breakdown of Cu-Ag alloys passivity in aqueous carbonate solutions

The electrochemical behaviour of two Cu-Ag alloys was studied in 0.1 M Na₂CO₃ solution containing different concentrations of Cl⁻ ions using linear polarization and current/time transients under the effect of different variables of Cl⁻ ions concentration, scan rate and applied anodic potentials. In Cl⁻ free solutions, the anodic voltammogram consists of two potential regions I and II. The potential region I exhibits three anodic peaks A₁, A₂ and A₃ that correspond to the formation of Cu₂O, Cu(OH)₂ and CuO, respectively. The potential region II exhibited four anodic peaks A₄, A₅, A₆ and A₇ due to the formation of AgO⁻, Ag₂O, Ag₂CO₃ and Ag₂O₂. In the presence of Cl⁻ ions, the anodic voltammograms depends considerable on the concentration of Cl⁻ ions. Increasing the amounts of Cl⁻ ions up the 0.02 M (alloy I) or 0.006 M (alloy II) the heights of all the anodic peaks were decreased and their peak potentials were shifted to less negative values. The existence of pitting was confirmed by SEM micrograph. The pitting potential E_pit was shifted towards more active potential values as the concentration of Cl⁻ ions in the solution was increased. When the scan rate is high, initiation of the pitting can be noticed only at more positive potentials, corresponding to a sufficiently short pit incubation time. The potentiostatic current/time transients show that the incubation time decreases with increasing the applied anodic potential and the Cl⁻ ion concentration and the pitting corrosion can be described in terms of instantaneous three-dimensional growth under diffusion control.     

Electrochemical studies of the pitting corrosion of tin in citric acid solution containing Cl

The electrochemical behavior of a tin electrode in citric acid solutions of different concentrations was studied by electrochemical techniques. The E/I curves showed that the anodic behavior of tin exhibits active/passive transition. Passivation is due to the formation of Sn(OH)₄ and/or SnO₂ film on the electrode surface. Addition of NaCl to citric acid solution, enhances the active dissolution of tin and tends to breakdown the passivity at a certain breakdown potential. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second stages involve pit nucleation and growth and third stage involve repassivation. The impedance spectrum of pure Sn is found to consist of three intersecting capacitive semicircles at the high and medium frequencies with an inductive loop at low frequencies. The capacitive semicircles occurring at the high and medium frequency are due to the dielectric properties of surface oxide film and dissolution of underlying metal, respectively. The inductive loop at low frequencies results from Cl⁻ adsorption at the pitting region. By increasing the potential the pitting corrosion and the fractal dimension of surface due to pitting increase.     

Corrosion and passivation behaviors of some stainless steel alloys in molten alkali carbonates

The corrosion and passivation behaviors of two types of stainless steel alloys (ferritic and austenitic steels) in ternary molten Li₂CO₃-Na₂CO₃-K₂CO₃ mixture at different temperatures (475-550 °C) were studied using galvanostatic polarization and cyclic voltammetry. The galvanostatic polarization curves of the investigated alloys illustrate the passivation and passivity breakdown of the alloys. The passivation potential range for the three investigated steel alloys is about 1.15-1.3 V. During this potential range different oxide and spinels are formed, the nature of which depends on the type of alloy and the anodization potential. At high anodic potentials the decomposition of carbonate takes place, leading to passivity breakdown and oxygen evolution. The values of corrosion parameters (R_p, i_o and i_corr) were calculated. The calculated values indicate that the corrosion resistance of the austenitic stainless steel is higher than that of the ferritic steel. The activation energy of the corrosion process was found to be equal to about 70 kJ mol⁻¹. The results of the cyclic voltammetric investigations indicate that the behavior of the austenitic steels is about the same and differs from that of ferritic steel. The corrosion tests in 0.2 M HCl solutions have shown that the oxide scales formed on the surface of the austenitic stainless steels are multilayered, whereas those formed on the ferritic alloy are uniform.     

Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions

Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na₂SO₄ solutions of different concentrations at 25 ± 0.2 °C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that i_corr and i_c increases with increasing either Cl⁻ or SO₄²⁻ concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl⁻ or SO₄²⁻ concentration.     

Localized corrosion of ultrafine-grained Al-Mg model alloys

The present study investigates passivity and localized corrosion of ultrafine-grained (UFG) binary Al-alloys with 0.5, 1 and 2 wt.% of magnesium subjected to three different equal-channel angular pressing (ECAP) pass numbers. The alloys were investigated by electrochemical techniques and surface analyses in different NaCl solutions and compared with their conventionally grained (CG) counterparts. The results of potentiodynamic polarization experiments indicated that the breakdown potential slightly decreased with increasing the ECAP pass number for all alloying series. Major differences are present in the pit transition potential (E_ptp), in that the potential shifts to more negative values with increasing number of passes in all series. EIS measurements showed no differences in corrosion resistance with increasing the number of ECAP passes at the open circuit state compared to their CG counterparts, indicating that passive layer properties are not affected by volume fraction of grain boundaries and dislocation density. Furthermore, serious pitting occurred for the coarse grained alloys in a kind of laterally spreading crystallographic filiform corrosion. In contrast, the UFG material exhibits solely local crystallographic pitting corrosion propagating in depth. The results indicate that UFG Al-Mg alloys tend with increasing pass number to deep localized pitting corrosion which leads to a hindered repassivation behavior compared to its CG counterpart, reflected in the negative shift in E_ptp.     

Electrochemical characteristics of Al-Mg alloy in seawater for leisure ship: Stress corrosion cracking and hydrogen embrittlement

We investigated the mechanical and electrochemical properties of aluminum alloys. Aluminum alloys do not corrode due to the formation of an anti-corrosive passive film, such as Al₂O₃ or Al₂O₃ · 3H₂O, which resists corrosion in neutral solutions. In seawater, however, Cl⁻ ions destroy this passive film. The current density in the first passivity range during the application of anodic protection had a similar value as that for concentration polarization by dissolved oxygen during the application of cathodic protection. The current density in the first passivity range had the lowest value overall. The lowest current densities in the potentiostatic and galvanostatic tests occurred at potentials of −1.4 to −0.7 V and −0.9 to −0.7 V, respectively.     

Inhibitory effect of tungstate, molybdate and nitrite ions on the carbon steel pitting corrosion in alkaline formation water containing Cl ion

The pitting corrosion of carbon steel in carbonate-formation water solution in the presence of chloride ions and the effect of addition WO₄²⁻, MoO₄²⁻ and NO₂⁻ anions on the pitting corrosion were studied using cyclic voltammetry and potentiostatic current-time measurements and complemented by scan electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) investigations. Cyclic voltammograms of carbon steel in the presence of chloride ions in carbonate-formation water solution show one anodic peak, corresponding to the formation green rust carbonate and the two cathodic peaks. As the addition of Cl⁻ ions concentration increases, the anodic peak current density increases and pitting potential E_pit shifts to more negative potential. It is shown that the rate of pit initiation () decreases and the pitting potential E_pit moves to more positive direction upon the addition of inorganic anions. It was found that pitting inhibition of carbon steel increases in the sequence: (WO₄)²⁻ > (MoO₄)²⁻ > (NO₂)⁻.     

Influence of halides on the dissolution and passivation behavior of AZ91D magnesium alloy in aqueous solutions

The electrochemical behavior of AZ91D in various aqueous sodium halide solutions was investigated using open-circuit potential (E_oc), potentiodynamic polarization and ac impedance (EIS) techniques. Generally, the results reveal that during immersion a protective layer of a salt film is formed on the alloy surface whose passivation performance depends on the halide nature, its concentration and temperature. E_oc shifts positively with time until attaining a steady (E_st) value, which becomes less noble with increasing concentration or temperature of the test solution. At any given conditions, self-passivation was found to be favored in the order F⁻ > I⁻ > Br⁻ > Cl⁻. This sequence is consistent with that for surface film resistance (R_T) and its relative thickness (1/C_T). Nevertheless, in F⁻ medium each of the above parameters increases with [F⁻] up to a critical value of 0.3 M then decreases. Increasing concentration above 0.3 M induces large change in the microstructure of the outermost layer of the fluorinated extremely protective film and depassivation behavior predominates. In Br⁻ and I⁻ solutions, as well as the lower Cl⁻ concentrations (≤0.01 M), AZ91D exhibits pseudo-passive state over the polarization range from the corrosion potential (E_corr) to the knee point (E_pt) in the anodic scan, at which passivity breakdown occurs with rapid increase in the anodic current and hydrogen gas reaction. At Cl⁻ concentrations >0.01 M the negative difference effect (NDE) occurs under cathodic polarization where E_pt lies negative to E_corr. Addition of F⁻ to the Cl⁻ solution can induce large changes in the behavior of AZ91D. Equal concentration addition (1:1) produces the highest propensity of the surface to form passivating layer that can afford better protection.