A kinetic and electrochemical study of the zincate immersion process for aluminium

To plate aluminium, its surface is often first coated with a thin layer of zinc which is formed by immersion in an alkaline zincate solution. This paper describes a kinetic and electrochemical study of the zincate immersion reaction. Using an aluminium sample in the form of a rotating disc, the effects of varying the zinc concentration (0.01–0.5 m), disc rotation speed (66–1380 rpm), temperature (5–72°C), and sodium hydroxide concentration (1.5–9.0 m) on the kinetics were investigated. It was found that the reaction was usually first order. When the zincate concentration was 0.1 m, the reaction was chemically controlled with an activation energy of 35 ± 7 kJ mol^–1. At high zincate concentrations, high disc rotation speeds and low sodium hydroxide concentrations, a thin film of zinc metal was formed on the aluminium surface, blocking the subsequent reaction. It was found that the most compact and adherent zinc films were formed at high zincate concentrations. This finding is consistent with industrial practice. These results are explained using mixed potential measurements and Evans' diagrams.     

醇溶型无机锌车间底漆的电化学行为研究

使用电化学方法对2种不同的醇溶型无机锌车间底漆进行了研究,结合浸泡以及盐雾的实验结果,发现2种车间底漆中锌含量对阴极保护时效有影响,但不是决定车间底漆防锈能力的唯一因素,锌含量与防锈颜料对于车间底漆的防锈性能有协同效果。其中,锌粉在早期提供阴极保护的作用,而防锈颜料在后期对基底提供防锈保护。对于锌粉含量相对较高的车间底漆,锌粉并不是从一开始就全部参加阴极保护作用,而是一部分具有激活潜力的锌粉在另一部分锌粉率先反应之后才由“未激活状态”转变为“激活状态”进而提供阴极保护的作用。     

Effect of impurities on the electrochemical behaviour of aluminium electrodes anodized in phosphoric acid

The electrochemical behaviour of anodic films formed on pure aluminium (99.999% and 1100) and 6061-Al alloy substrates in phosphoric acid has been investigated in an aluminium saturated phosphate solution of pH 6.23. The polarization resistance data obtained after one day immersion in the electrolyte showed a strong correlation with the pit number, assessed from an outdoor atmospheric exposure test after 4 months. The cathodic polarization measurements of anodized aluminium in the electrolyte were also carried out. Electrochemical behaviour, eg corrosion, hydrogen evolution reaction, was interpreted in terms of “active sites” within the anodic film. It is suggested that the active sites were mainly formed in the presence of impurities or alloy constituents in aluminium substrates.     

Corrosion resistance of phosphate coatings obtained by cathodic electrochemical treatment: Role of anode–graphite versus steel

The formation of phosphate coatings by cathodic electrochemical treatment using graphite and steel anodes and evaluation of their corrosion resistance is addressed in this paper. The type of anode used, graphite/steel, has an obvious influence on the composition of the coating, resulting in zinc–zinc phosphate composite coating with graphite anode and zinc–iron alloy–zinc phosphate–zinc–iron phosphate composite coating with steel anode. The corrosion resistance of the coating is found to be a function of the composition of the coating. The deposition of zinc/zinc–iron alloy along with the zinc phosphate/zinc and zinc–iron phosphate using graphite/steel anodes has caused a cathodic shift in the E_corr compared to uncoated mild steel substrates. The i_corr values of these coatings is very high. EIS studies reveal that zinc/zinc–iron alloy dissolution is the predominant reaction during the initial stages of immersion. Subsequently, the formation of zinc and iron corrosion products imparts resistance to the charge transfer process and increases the corrosion resistance with increase in immersion time. The corrosion products formed might consist of oxides and hydroxychlorides of zinc and iron. The study suggests that cathodic electrochemical treatment could be effectively utilized to impart the desirable characteristics of the coating by choosing appropriate anode materials, bath composition and operating conditions.     

The role of iron(iii) and tartrate in the zincate immersion process for plating aluminium

The zincate immersion process is a commercial process used to treat aluminium prior to electroplating. Superior zinc coatings are obtained through modifications to the original process. One such modification involves the addition of ferric chloride, complexed with potassium sodium tartrate, to the plating baths. This paper describes a detailed investigation into the function of these additives. It was found that the addition of tartrate alone did not significantly affect the rate of zinc deposition, the deposit morphology or the position of the aluminium or zinc polarization curves. This suggests that the role of tartrate is solely as a complexing agent to maintain iron (iii) in solution. In contrast to this, the addition of iron(iii)/tartrate had a marked effect on the reaction. It was found that iron(iii) functioned primarily by reducing the size of the zinc crystals as they formed. This produces a thinner and more compact zinc immersion coating.     

Tungstate induced corrosion of some aluminium alloys in acidic chloride solution — An approach for controlling it with morpholine

The corrosion behaviour of some aluminium alloys (1060, 1100 and 3003) in acidic chloride solution (pH=1) have been studied in the presence of various concentrations of tungstate ions. The alloy most vulnerable to the effects of tungstate ions is 1100 followed by 1060 and 3003 alloys. After six hours of immersion the alloys exhibit 6 to 8 times higher corrosion rates compared to that in the blank solution containing only chloride ions. The experiments suggest that the tungstate ion stimulates the corrosion of aluminium by acting as a cathodic depolarizer. Morpholine effectively arrests the dissolution of the alloys in the blank electrolyte as well as in the presence of tungstate ions. Morpholine polarizes local cathodic sites to act as an inhibitor. Tungstate ions are not adsorbed on the surface in the presence of morpholine and a synergistic effect is obtained at higher concentrations of morpholine. A Langmuir adsorption isotherm has been used to explain the data.     

Effect of nickel impurities on zinc electrodeposition

The kinetics of nickel-zinc co-deposition and the effect of nickel on zinc morphology have been studied by means of chemical analysis, X-ray diffraction and SEM observations of cathodic deposits obtained in various experimental conditions, including acid and alkaline baths. It has been shown that nickel ions, even if present in the electrolyte at very low concentrations (1 ppm), do not deposit under mass transport controlled conditions but are dragged by zinc ions whose concentration influences the nickel current. With regard to zinc morphology, it has been observed that nickel reduces the grain size of the zinc crystals formed during the electrodeposition.     

Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests

The formation of zinc phosphate coating by cathodic electrochemical treatment and evaluation of its corrosion resistance is addressed. The corrosion behaviour of cathodically phosphated mild steel substrate in 3.5% sodium chloride solution exhibits the stability of these coatings, which lasts for a week's time with no red rust formation. Salt spray test convincingly proves the white rust formation in the scribed region on the painted substrates and in most part of the surface on unpainted surface. The protective ability of the zinc corrosion product formed on the surface of the coated steel is evidenced by the decrease in the loss in weight due to corrosion of the uncoated mild steel, when it is galvanically coupled with cathodically phosphated mild steel. Potentiodynamic polarization curves reveal that E_corr shifts towards higher cathodic values (in the range of −1000 to −1100 mV versus SCE) compared to that of uncoated mild steel and conventionally phosphated mild steel substrates. The i_corr value is also very high for these coatings. EIS studies reveal that zinc dissolution is the predominant reaction during the initial stages of immersion. Subsequently, the non-metallic nature of the coating is progressively increased due to the formation of zinc corrosion products, which in turn enables an increase in corrosion resistance with increase in immersion time. The zinc corrosion products formed may consist of zinc oxide and zinc hydroxychloride.     

Cathodic polarization characteristics and activation of aluminium in chloride solutions containing indium and zinc ions

This work deals with the study of the cathodic and active behaviour of aluminium in acid chloride solutions containing indium and zinc ions. The cathodic behaviour was studied using potentiostatic and potentiodynamic techniques, complemented by SEM and EDX. During cathodization, preferential adsorption of zinc ions occurs, compared to that of H+ and In3+ ions. Once a critical amount of zinc is deposited, preferred indium deposition begins. The synergistic interaction between zinc and indium ions and aluminium leads to its activation in chloride media. This can be explained by displacement reactions that produce an indium accumulation and preferential zinc dissolution. This situation produces a new interface, quasi-free of Zn but rich in In, which favours Cl– ion adsorption at more negative potentials than aluminium, leading to its activation.     

Study on electrochemical behaviour of zinc in dimethylsulphoxide

The kinetics of cathodic deposition and anodic dissolution of zinc in Zn(BF₄)₂ and Zn(ClO₄)₂ solutions in DMSO has been examined using various experimental techniques. It has been found that magnitudes of kinetic parameters of Zn/Zn²⁺---DMSO systems are very close to the values measured in aqueous solutions of these salts. Cathodic efficiency of Zn attains a value of 95%. However, zinc coat contains impurities mainly as incorporated DMSO molecules, which is a conclusion based on X-ray analysis of Zn deposit.     

Surface treatment for zinc corrosion protection by a new organic chelating reagent

The effect on the corrosion behaviour of zinc of a new organic molecule with chelating groups was investigated. Electrochemical studies of the zinc specimens were performed in aqueous sulfate–chloride solution (0.2 m Na₂SO₄ + 0.2 m NaCl, pH 5.6) using potentiostatic polarization techniques with a rotating disk electrode. Zinc samples, previously treated by immersion in the inhibiting organic solution, presented good corrosion resistance. The influence of the treatment bath pH and temperature on the protection efficiency has been emphasized. The recorded electrochemical data indicated a basic modification of the cathodic corrosion behaviour of the treated zinc resulting in a decrease of the electron transfer rate. Corrosion protection could be explained by a chelation reaction between zinc and organic molecules and the consequent growth of an organometallic layer strongly attached to the metal surface which prevented the formation of porous corrosion products in the chloride-sulfate medium. This protective film was studied using several surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM–EDS) and Fourier transform infrared spectroscopy (FTIR).     

Electrochemical reduction of potassium from potassium chloride in propylene carbonate electrolyte with aluminium anodes

Potassium has been cathodically deposited in a propylene carbonate electrolyte with concurrent dissolution of aluminium. Starting with an AlCl₃-free electrolyte, changes in the composition and conductivity of the electrolyte, as well as those of the electrode potentials, were measured as a function of charge passed. Anodic and cathodic current efficiencies were both close to 100%. The observed behaviour is consistent with the overall reaction stoichiometry This reaction and its analogues for other alkali metals may conceivably form the basis of practical processes at ambient temperatures for the recovery of alkali metals from their chlorides with concurrent production of aluminium chloride.     

Effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte

The effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte has been investigated through thermodynamic analysis and corrosion experiments. The solution chemistry of aluminium, zinc, and iron in aqueous solution in the absence and in the presence of fluoride ions was studied with the construction of the Eh-pH diagrams for the Al–F–H₂O, Zn–F–H₂O and Fe–F–H₂O systems at 25°C. In the presence of fluoride ions, aluminium can form a series of aluminium-fluoride complexes depending on the fluoride concentration and pH whereas zinc and iron can form soluble or insoluble metal-fluoride complex species only at relatively high fluoride concentration and at higher pH values. Experimental results show that in the presence of fluoride ions, the corrosion of pure aluminium in sulphuric acid is due to uniform dissolution and the reaction rate depends on the fluoride concentration. In zinc electrolyte containing fluoride ions, zinc deposits onto the pure aluminium substrate spontaneously and the amount of deposited zinc also depends on the fluoride concentration. On the other hand, the presence of iron in the Al–Fe alloy accelerates the corrosion of aluminium in H₂SO₄ and zinc electrolyte significantly and prevents the deposition of zinc on the aluminium surface. The effect of fluoride ions on zinc adherence to the aluminium is also discussed.     

Corrosion protection of AA2024-T3 using rare earth diphenyl phosphates

Existing corrosion protection technologies for aluminium alloys utilising chromates are environmentally damaging and extremely toxic. This paper presents a preliminary investigation into rare earth diphenyl phosphates as new environmentally benign corrosion inhibitors. Full immersion weight loss experiments, cyclic potentiodynamic polarisation measurements and Raman spectroscopy were used in this study. Results show cerium diphenyl phosphate (Ce(dpp)₃) acts as a cathodic inhibitor, decreasing cathodic current density and E_corr by passivating cathodic intermetallic particles on the alloy surface. Mischmetal diphenyl phosphate (Mm(dpp)₃) acts a mixed inhibitor, shifting E_corr to more noble values, decreasing cathodic current density, increasing the breakdown potential and suppressing pitting.     

Tosyl-hydrazine, 4-nitrobenzoyl-hydrazine and terephthalyl-hydrazine as inhibitors for the corrosion of copper and aluminium in sulphiric acid

The effect of tosyl-hydrazine (THy), 4-nitrobenzoyl-hydrazine (4-NBHy) and terephthalyl-hydrazine (terephthalyl-Hy) on the corrosion of copper and aluminium in sulphuric acid has been investigated. The inhibitive efficiency ranking of these compounds from both weight loss and polarization measurements was found to be terephthalyl-Hy > Thy > 4-NBHy for Cu and THy > 4-NBHy for Al. Both THy and 4-NBHy were found to be much less efficient as inhibitors for the corrosion of aluminium when compared to the corrosion of copper in sulphuric acid. The inhibitors acted cathodically both in the case of copper and aluminium, by merely blocking the cathodic reaction sites in the former and by altering the course of the mechanism of corrosion in the latter. The relative inhibitive efficiency of these compounds has been explained on the basis of structure dependent electron donor properties of the inhibitors and the nature of the metal-inhibitor interaction at the surface.     

Influence of CaF2 and AlF3 on the kinetics and mechanism of the Al electrode reaction in cryolite melts with various alumina contents

Electrochemical techniques were used to study the kinetics and mechanism of the aluminium electrode reaction in two cryolite-based melts containing cryolite with either 11 wt % AlF₃ or 5 wt % CaF₂ additions and variable alumina contents at 1000 °C. A three step electrode process was observed in both melts, comprising a preceding chemical reaction followed by two charge transfer steps. The exchange current density of the cathodic reaction was found to be dependent on the concentration of aluminium fluoride. By a combination of electrochemical impedance spectroscopy (EIS) and galvanostatic relaxation methods (GRM), the exchange current density of the first (slower) charge transfer step, the Warburg diffusion impedance, the double layer capacitance of the aluminium electrode and the rate of the preceding chemical step, were evaluated in the range of 2–8 wt % alumina. The role of the two additives, AlF₃ and CaF₂, was evaluated.     

Acceleration of zinc corrosion in alkaline suspensions containing iron oxides or iron hydroxides

Fast zinc dissolution is of industrial interest in recycling galvanised steel scraps. An acceleration of zinc corrosion in alkaline solutions was observed in the presence of various iron oxides or iron hydroxides. This corrosion was investigated by weight loss, measurements of hydrogen evolution and variation of current in a galvanic cell. The mechanism of this fast zinc corrosion was investigated by electrochemical means and by X-ray diffraction and scanning electron microscopy observations of zinc surface after immersion in alkaline suspensions of iron oxides or iron hydroxides. These insoluble iron compounds were involved in a reduction step leading to iron containing microparticles characterised by a low hydrogen overpotential and which acted as cathodic areas in a galvanic corrosion of zinc.     

Corrosion inhibition of steel by thiourea and cations under incomplete cathodic protection in a 3.5% NaCl solution and seawater

This study investigates corrosion inhibition of steel using thiourea and cations such as aluminium, calcium and magnesium under cathodic control in a 3.5% NaCl solution and in seawater. Steel protection in a 3.5% NaCl solution is normally incomplete under a cathodic potential less electronegative than –1.100 V. However, the protection can be enhanced by nearly 50% by adding either 50 ppm aluminium ion or 75 ppm thiourea in solution, and by almost 90% by the combined use of these additions. This study also analyzes how combining inhibitors and cathodic control may be used to protect steel. Moreover, this investigation monitors pH in the solution, measures zero-resistance current between the graphite-steel couple, as well as analyses cathode reaction products. A mechanism is also proposed to interpret the combined effects of inhibitors and cathodic control on the protection of steel.     

Reduction of cathodic delamination rates of anticorrosive coatings using free radical scavengers

Cathodic delamination is one of the major modes of failure for anticorrosive coatings subjected to a physical damage and immersed in seawater. The cause of cathodic delamination has been reported to be the result of a chemical attack at the coating–steel interface by free radicals and peroxides formed as intermediates in the cathodic reaction during the corrosion process. In this study, antioxidants (i.e., free radical scavengers and peroxide decomposers) have been incorporated into various generic types of coatings to investigate the effect of antioxidants on the rate of cathodic delamination of epoxy coatings on cold rolled steel. The addition of <5 wt% free radical scavengers to epoxy coatings improved the resistance toward cathodic delamination by up to 50% during seawater immersion, while peroxide decomposers had a limited effect. Testing using substrates prepared from stainless steel, copper, aluminum, galvanized steel, and brass also showed a reduction in the rate of cathodic delamination when the coating was modified with a free radical scavenger. The protective mechanism of free radical scavengers investigated for the primers are similar to that of antioxidants used for protection against photochemical degradation by UV-radiation of top coatings. Both substrate corrosion and degradation of a coating exposed to UV-radiation lead to the formation of free radicals as reactive intermediates.     

Influence of methylcellulose-coated paper separators on the corrosion,polarization and impedance characteristics of zinc in concentrated ammonium chloride solution. I. Behaviour in flooded electrolyte

The corrosion characteristics of both pure and amalgamated zinc have been studied in 6.0 M NH₄ Cl using steady-state polarization and a.c. impedance methods, and the influence of methylcellulose-coated and uncoated paper separators have been determined. Irrespective of the type of separator present, the results can be interpreted almost exclusively in terms of charge-transfer effects. In the presence of the base paper the anodic zinc dissolution process is inhibited by 79% whilst the rate of cathodic hydrogen evolution is inhibited by only 43%. Similarly, in the presence of the complete methylcellulose-coated separator formulation the anodic reaction is inhibited by 98–99% whilst the cathodic reaction is inhibited by only 60–74%. In either case it is shown that the anodic inhibition is considerably greater than that expected on the basis of a simple blocking type model, whereas the cathodic inhibition is considerably less. Evidence is presented which suggests that the excessive inhibition of the anodic current is a consequence of the specific influence of the separator on the stability and adsorption behaviour of Zn(I) intermediates in the zinc dissolution reaction. It is also postulated that the deficient inhibition of the cathodic current may result from the small size and high mobility of the proton which allows it to penetrate those regions of the metal-separator interface normally inaccessible to bulkier ions. Overall, the corrosion-inhibiting efficiency of the base paper is 41% whilst that of the complete methylcellulose-coated paper separator formulation is 70–75%. These results reflect a substantial influence of the separator material on the corrosion behaviour of zinc.