Formation of Phosphate Coatings on Steel and Corrosion Performance of Phosphated Specimens in Alkaline Solutions

Crystalline and amorphous phosphate coatings were formed on steel samples with the intention of using them for corrosion resistance enhancement of steel reinforcement in concrete. The physical properties of the phosphate coatings were studied using the following methods: the weights of phosphate coating and those of dissolved metal were calculated from gravimetric measurements, surface morphology studies were carried out with an AFM and the phase composition of coatings was analysed by X-ray diffraction. An indicator of film porosity was considered as the maximum difference between potential values of growth completeness of phosphate film and those of metal dissolution. The corrosion behaviour of phosphated steel specimens was studied in 1 × 10^?5 and 0.1 M NaOH solutions (pH=8.5, 13) without/with 0.1 M Cl^? ions. The phosphate coating protection abilities were related to its composition, thickness and porosity. All the crystalline phosphate coatings studied protect steel from corrosion in alkaline solutions in a wide range of pH even when Cl^? ions are present. The best protective properties were exhibited by medium weight crystalline phosphate coatings. Amorphous phosphate coating is not chemically stable in strongly alkaline solutions because of the hydrolysis of iron phosphate present in it.     

Zum Spannungsrißkorrosionsverhalten rekristallisierter Bleche der AlLi-Legierung 8090-T81

On the stress corrosion cracking behaviour of recrystallized 8090-T81 Sheets The stress corrosion cracking behaviour of a recrystallized sheet of the Al-Li-Cu-Mg-Zr alloy 8090-T81 was studied performing accelerated tests under constant deformation, constant load, and slow strain rate conditions. The used electrolytes were an aqueous 3.5% NaCl solution, an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH = 3, and synthetic seawater according to ASTM D1141. Alternately immersed in 3.5% NaCl solution according to ASTM G44 the investigated alloy was found to be susceptible to stress corrosion cracking was not promoted by continuous immersion in aerated 3.5% NaCl solution, 3.5% NaCl solution saturated with carbon dioxide, and in acid chromate inhibited 2% NaCl solution. Using the slow strain rate technique with continuously immersed flat tensile specimens stress corrosion cracking was only observed in synthetic seawater. Under specific environmental conditions hydrogen embrittlement can occur in the investigated material.     

The hydration of barrier oxide films on aluminium and its inhibition by chromate and phosphate ions

Compact oxide films were formed on aluminium in a neutral borate solution (pH 7.4, 20°C) by applying a constant potential of 50 V (SCE), and then immersed in distilled water, a 0.001 mol dm^?3 chromate solution (pH 7.0), or a 0.001 mol dm^?3 phosphate solution (pH 7.0). Changes in the composition profile of the oxide caused by the immersion were examined by X-ray photo-electron spectroscopy (XPS) combined with a film sectioning technique. After a 72 h immersion in distilled water, the OH^? content in the outer part of the film increased to about three times as that before immersion, and the film sustained only 30 V instead of the initial 50 V. As the profile of BO₂^? ions remained unchanged, the deterioration of the film is exclusively due to hydration of the oxide caused by the penetration of water molecules or OH^? ions into the oxide lattice. It was found that the hydration is strongly inhibited by chromate and phosphate. XPS and chemical analysis showed that CrO₄^2? and PO₄^3? ions adsorb on the oxide surface to form a mono- or bi-layer, hindering the penetration of water molecules. The inhibiting behaviour, stabilizing oxide films, is important in explaining the function of these anions as corrosion inhibitors for metals in near neutral solutions.     

Effects of temperature and pH level on the corrosion behavior of amorphous Co69Fe4.5Nb1.5Si10B15 alloy

The corrosion behavior of the amorphous Co₆₉Fe_4.5Nb_1.5Si₁₀B₁₅ (at.%) alloy ribbon in H₂SO₄ solutions (0.001 M or 0.07 M), NaCl solution (0.07 M), and HCl + NaOH solution were examined as functions of solution temperature and pH. The corrosion potential decreased when either the temperature or pH of the solutions increased. The corrosion resistance of the Co₆₉Fe_4.5Nb_1.5Si₁₀B₁₅ alloy in the 0.07 M NaCl solution was higher than the 0.001 M or 0.07 M H₂SO₄ solutions for a given temperature. The corrosion rate increased exponentially with an increase in temperature and was inversely proportional to the pH in the range of 10^?6 A/cm²～10^?4 A/cm².     

Raman microscopy of chromate interactions with corroding aluminum alloy 2024-T3

Raman microspectroscopy was used to observe actively corroding aluminum alloy 2024-T3 directly in a solution containing NaCl and dilute K₂Cr₂O₇. Raman spectra acquired in and near corrosion pits allowed identification of two products of Cr^VI interactions with the corroding alloy. If the alloy potential was fixed negative of the pitting potential, an Al^III-Cr^VI mixed oxide formed in formerly active or metastable pits. This mixed oxide is similar to a covalent compound which can be made synthetically by adding NaOH to solutions containing Al³⁺ and CrO₄²⁻. If the alloy potential was held positive of the pitting potential, the corrosion product was primarily a Cr^III-Cr^VI mixed oxide, formed by partial reduction of Cr^VI to Cr^III by exposed Al metal or by H₂. At the pitting potential, a mixture of Al^III and Cr^III mixed oxides was observed, with the Al^III species located primarily within pits. A model for the formation mechanism of the mixed oxides is proposed, and the consequences of the findings to corrosion inhibition by Cr^VI are considered. In particular, the concentration of Cr^VI in and near pits driven by mixed oxide formation may serve to direct the inhibitor from the solution to sites of corrosion.     

Study of an alternative phosphate sealer for replacement of hexavalent chromium

This study evaluates the possibility of replacing the hexavalent chromium passivation treatment used as a sealer after phosphating of carbon steel (SAE 1010) by a treatment with niobium ammonium oxalate (Ox). Samples of carbon steel (SAE 1010) after being phosphated in a zinc phosphate bath (PZn + Ni) were immersed in solution of niobium ammonium oxalate (250 mg L⁻¹ of Nb) either at pH 3.0 or pH 8.0. A passivation treatment with a solution with CrO₃ (200 mg L⁻¹ of Cr⁶⁺) was also used for reference.The corrosion resistance of the phosphated samples after passivation treatments was analyzed in a NaCl 0.5 mol L⁻¹ solution using electrochemical impedance spectroscopy (EIS) and anodic polarization curves. Salt spray tests were also performed to evaluate their corrosion resistance. The results showed that the highest corrosion resistance was obtained by passivation in a solution with (250 mg L⁻¹ of Nb) at pH 8.0.     

Corrosion of Mg alloys EV31A,WE43B,and ZE41A in chloride- and sulfate-containing solutions saturated with magnesium hydroxide

This study studied corrosion in 0.1 M Na₂SO₄, 0.1 M NaCl, and 0.6 M NaCl, all saturated with Mg(OH)₂, using weight loss, hydrogen evolution, and electrochemical measurements. Corrosion was similar in all cases. Nevertheless, the corrosion rates were alloy-dependent, were somewhat lower in 0.1 M Na₂SO₄ than in 0.1 M NaCl, and increased with NaCl concentration. The corrosion damage morphology was similar for all solutions; the extent correlated with the corrosion rate. The corrosion rates evaluated by the electrochemical methods were lower than those evaluated from hydrogen evolution, consistent with the Mg corrosion mechanism involving the unipositive Mg⁺ ion.     

Nature of the Oxide Film Present on Iron after Inhibition in 0·05 M Chromate Solutions

Abstract

Chemical and microprobe analyses have been used to study the composition of the oxide films formed on iron by 0·05 M potassium chromate, pH 4–8.

Chemical analyses have shown that the air-formed oxide film was thinned and evenly reinforced with a normal iron chromium spinel having a composition in the range Fe²⁺ (Fe³⁺_0·5 Cr³⁺_1·5)O₄ — Fe³⁺ Cr³⁺ O₃

Microprobe analyses indicated enrichment of chromium at scratch lines, but the effect was small compared with the overall thickening of the films.     

Distinctions between the Activating Effects of Sulfuric and Phosphoric Acids on the Process of Zinc Chromating

Activating effect of anions of sulfuric and phosphoric acids (both in the range from 0.025 to 0.200 mol/l) on the process of zinc chromating in acid (pH 1.1) 0.2 M CrO₃ solution is studied by analytical methods. A general balance of oxidized zinc (Zn²⁺) and reduced chromium (Cr³⁺) and their distribution between the solution and chromate film for different solution compositions, as well as the elemental composition throughout the film's depth, are determined. It is found that the zinc oxidation and Cr⁶⁺ reduction reactions do not proceed in the absence of SO^2– ₄ ions (that is, when only PO^3– ₄ ions are present), so that the chromate film cannot form. However, the PO^3– ₄ ions combined with SO^2– ₄ ions increase the Zn²⁺ and Cr³⁺ ions concentration in the solution significantly, while their concentrations in the film correspondingly decrease. SO^2– ₄ ions activate the zinc surface because they form soluble complex compounds with Zn²⁺ and Cr³⁺ ions and increase the part of the surface, on which the cathodic reduction of Cr⁶⁺ to Cr³⁺ occurs. The activating action of phosphoric acid is caused by the increase in the total (analytical) concentration of H⁺ ion in the solution; hence, the deposition of the Cr³⁺ and Zn²⁺ hydroxide compounds onto zinc is retarded, due to the increased near-surface concentration of H⁺ ion.     

Self-healing protective films prepared on zinc by treatments with cerium(III) nitrate and sodium phosphate

Chromate-free, self-healing protective films were prepared on a surface of zinc electrode previously treated in a solution of cerium(III) nitrate Ce(NO₃)₃ by coverage of the surface with a layer of sodium phosphate Na₃PO₄. The self-healing ability of the film was examined by polarization measurements of the electrode after scratching the surface with a knife-edge crosswise and immersed in an aerated 0.5 M NaCl solution for many hours and by observation of pit formation at the scratches. A thin film containing 0.0552 mg/cm² of Na₃PO₄·12H₂O prepared on the electrode previously treated in 1×10⁻³ M Ce(NO₃)₃ at 30 °C for 30 min and dried at 90 °C for 23 h was highly self-healing and protective against corrosion of zinc in 0.5 M NaCl at the scratches. No pit was detected at the scratches on the electrode coated with this film after immersion for 72 h.     

Comparison of corrosion behaviour of zinc in NaCl and in NaOH solutions. Part I: Corrosion layer characterization

The corrosion layer formed on zinc sample in 0.6 M NaCl and 0.5 M NaOH solution under ambient conditions has been investigated. The corrosion layer morphology was analyzed using scanning electron microscopy (SEM). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the corrosion products of zinc. The thickness evolution of the corrosion layer was investigated by glow discharge optical emission spectroscopy (GDEOS). The corrosion layer formed in 0.5 M NaOH solution appeared more compact than that formed in 0.6 M NaCl solution. Zinc hydroxide chloride (Zn₅(OH)₈Cl₂·2H₂O) and zinc hydroxide carbonate (Zn₅(CO₃)₂(OH)₆) were formed on zinc surface in 0.6 M NaCl solution while in 0.5 M NaOH solution, zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂) and zinc hydroxide carbonate (Zn₅(OH)₆(CO₃)₂·H₂O) were detected. Probable mechanisms of zinc corrosion products formation are presented.     

Comparison of accelerated SCC tests performed on the aluminium alloy 2014-T651

The stress corrosion cracking (SCC) behaviour of plates of the Al-Cu-Si-Mn-Mg alloy 2014-T651 was investigated in short transverse direction performing various accelerated tests. Corrosive media used were: aqueous 3.5% NaCl solution, an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH = 3 (according to LN 65666), and substitute ocean water according to ASTM D1141. C-ring and tensile specimens were loaded under constant deformation, constant load and slow strain rate conditions. Alternate immersion tests in 3.5% NaCl solution clearly indicate the low SCC resistance of the alloy 2014-T651 in short transverse direction. Under continuous immersion conditions the acidified 2% NaCl solution containing chromate is an appropriate synthetic environment, while neutral 3.5% NaCl solution does not promote severe stress corrosion cracking. The SCC susceptibility of 2014-T651 is also observed in slow strain rate tests using substitute ocean water as well as acidified 2% NaCl solution inhibited by chromate. In 3.5% NaCl solution the evaluation of slow strain rate data is complicated by pre-exposure effects.     

On the corrosion behaviour of phosphoric irons in simulated concrete pore solution

The corrosion behaviour of three phosphoric irons P₁ (Fe-0.11P-0.028C), P₂ (Fe-0.32P-0.026C) and P₃ (Fe-0.49P-0.022C) has been studied in simulated concrete pore solution (saturated Ca(OH)₂ solution) containing different chloride concentration. This has been compared with that of two commercial concrete reinforcement steels, a low carbon steel TN (Fe-0.148C-0.542Mn-0.128Si) and a microalloyed corrosion resistant steel CS (Fe-0.151C-0.088P-0.197Si-0.149Cr-0.417Cu). The beneficial aspect of phosphoric irons was revealed from potentiodynamic polarization experiments. The pitting potentials and pitting nucleation resistances for phosphoric irons and CS were higher than that for TN. Electrochemical impedance spectroscopy (EIS) studies revealed thickening and growth of passive film as a function of time in case of phosphoric irons and CS in saturated Ca(OH)₂ pore solutions without chloride and in the same solution with 0.05% Cl⁻ and 0.1% Cl⁻. In case of TN, breakdown of passive film resulted in active corrosion in simulated pore solution containing 0.1% Cl⁻. Linear polarization resistance measurements complemented EIS results. Visual observations indicated that phosphoric iron P₃ was immune to corrosion even after 125 days of immersion in saturated Ca(OH)₂ solution containing 5% NaCl. The good corrosion resistance of phosphoric irons in simulated concrete pore solution containing chloride ions has been related to the formation of phosphate, based on ultraviolet spectrophotometric analysis and Pourbaix diagram of phosphorus-water system.     

Influence of Zn on oxide films on Alloy 690 in borated and lithiated high temperature water

To clarify the corrosion control effect of Zn injection into hydrothermal environments, the oxide films on Alloy 690 in the deaerated borated and lithiated water have been investigated using potentiodynamic polarization curves, electrochemical impedance spectra at 300 °C and ex-situ X-ray photoelectron spectroscopy. With Zn injection in the solution, ZnCr₂O₄ and ZnFe₂O₄ were formed in the inner and outer layers of the oxide films on Alloy 690, respectively, through exchange reactions between Zn²⁺ and Fe²⁺/Ni²⁺. A simple model for oxide film structure change and the mechanism of corrosion inhibition by Zn injection is proposed and discussed.     

Effects of Cr addition on the glass-forming ability and the corrosion behaviors of FeCBSiP amorphous alloys

The corrosion behaviors of Fe_83-XC₁B₁₁Si₂P₃Cr_X (X = 0, 1, 2, and 3 at%) amorphous alloys in 0.1 M NaCl solution have been investigated by immersion and electrochemical tests. With the addition of Cr content from 0 to 3 at%, the corrosion rate of amorphous alloys gradually decreases from 2.57 × 10⁻¹ to 1.04 × 10⁻¹ mm·year⁻¹. The minor addition of Cr improves the corrosion resistance through the increase in the E_corr value, which makes it easy to reach a passive state and suppress pitting corrosion. The corroded morphology and products of amorphous alloys have been tested by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The SEM/EDS analysis shows that the high corrosion resistance is due to the formation of dense and stable chromium oxide during immersion in NaCl solutions.     

Scanning Electrochemical Microscopy for the Investigation of Galvanic Corrosion of Iron with Zinc in 0.1 M NaCl Solution

Scanning electrochemical microscopy was used to monitor microscopic aspects of the electrochemical processes at the iron-zinc couple immersed in 0.1 M NaCl aqueous solution. The SECM measured the concentration of chemical species relevant to the corrosion processes. The electrochemical behavior of galvanic Fe/Zn coupling was investigated as a function of time using SECM microelectrode both as Fe/Zn joined together as well as away from each other. SECM amperometric line scan curves were obtained over the Fe/Zn at a constant distance. In the first case, the chemical species participating in the corrosion reactions at the sample are detected at the SECM tip by applying appropriate potential values to the microelectrode. The release of Zn²⁺ ionic species into the solution phase from local anodic sites, as well as the consumption of dissolved oxygen at the corresponding cathodic locations, was successfully monitored. The results revealed that the galvanic couple where Fe/Zn is close to each other will show higher corrosion rate of zinc than that of galvanic couple away from each other. The Fe/Zn couple away from each other showed a decrease in current values with time. This is due to the formation of oxide layer of Zn over the Fe followed by the protection of the corrosion products with further exposure times.     

On the corrosion resistance of porous electroplated zinc coatings in different corrosive media

The corrosion resistance of an electroplated (EP) Zn coating whose surface was chemically etched to produce surface defects (pores) is investigated in this work. Impedance and DC polarisation measurements were employed to study the behaviour of such coating in various corrosive media (NaCl, NaOH and rain water). Four different faradaic relaxation processes were clearly revealed in different NaCl concentrations (from 0.1 to 1 M). In the most concentrated solutions at least three relaxation processes at low frequencies (LF) appeared and were related to zinc deposition and dissolution. At lower concentrations and depending on the pH, only one process was observed. The charge transfer resistance (R_ct) and the corrosion current (I_corr) were practically stable in the pH range 5-10. In deaerated NaCl 0.1 M, the EIS diagrams showed two time constants at very close frequencies. From the EIS diagrams the porous nature of the coating was highlighted and showed that the dissolution mechanisms occurred at the base of the pores.     

Self-healing mechanism of an organosiloxane polymer film containing sodium silicate and cerium(III) nitrate for corrosion of scratched zinc surface in 0.5 M NaCl

A highly protective and self-healing film of 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ polymer containing sodium silicate (water glass) Na₂Si₂O₅ and cerium(III) nitrate Ce(NO₃)₃ was prepared on a zinc electrode previously treated in a Ce(NO₃)₃ solution. The film was examined by polarization measurement of the electrode in an aerated 0.5 M NaCl solution after the electrode was scratched and immersed in the solution for 4-72 h. Self-healing mechanism of the film was investigated by X-ray photoelectron spectroscopy and electron-probe microanalysis for the coated electrode surface after scratched and immersed in the NaCl solution. A passive film composed of Zn(OH)₂, ZnSi₂O₅ and Ce³⁺-Si₂O₅²⁻ salt or complex was formed on the scratched surface and preferential deposition of Si₂O₅²⁻ compounds occurred at a defect of the passive film where Cl⁻ accumulated, resulting in suppression of pitting corrosion at the scratch.     

Anodisches Verhalten und Korrosion von Titan in methanolischen Lösungen

Anodic behaviour and corrosion of titanium in methanolic solutions The anodic behaviour of titanium in methanolic solutions containing halides or water in various concentrations has been investigated and the result were compared with the result obtained by corrosion tests performed on ?U”? bend specimens in similar solutions. Polarization curves can predict the susceptibility to titanium to stress corrosion cracking as in the case the metal surface is not passivable. These conditions can be obtained in the presence of low water content (i.e. 0.1% H₂O) and of activating ions as Cl^? and Br^? even in very small concentration (10^5?, 10^4? M). Presence of higher water contents and/or of F^? or I^? ions can to some extent produce passivation of the metal surface and cracking does not occur. A lower water amount in the solution (i.e. 130 ppm) eliminates any possibility of passivation of the metal surface, so that cracking can occur even in the absence of chlorides or bromides. High Cl^? concentration in the solution. (i.e. 0.1 M NaCl + 0.1% H₂O) can produce intergranular attack even in the absence of applied stress.     

Untersuchung der Eigenschaften von nach einem neuen Verfahren hergestellten Phosphatüberzügen

Investigation into the properties of phosphate layers obtained according layers obtained according to a new process Phosphate coatings were obtained by anodic phosphatizing in sodium phosphate solutions (2.5-5% phosphate, PH 4.2-5.4, current density 2.5-5.0m Amps/cm²). The coatings (coating weight 7.3–20 g/cm₂) consist of ferrous/ferric phosphates, the Fe²⁺: Fe³⁺ ratio depending from bath composition. he hardness of the coatings is lower than the hardness of coatings from Zn and Mn phosphate baths and their absorption capacities for chromic acid, mineral oil and paints are higher because of their fine crystalline structure. With respect to corrosion behaviour they are superior in NaCl and equivalent to Zn and Mn phosphate coatings in moist atmospheres, while the latter seem to be superior in the salt spray seem to be superior in the salt spray test. The minimum coating thickness required for subsequent painting is 8/m² subsequent oil impregnation or cold working require higher weight.