A comparative study of the corrosion protective properties of chromium and chromium free passivation methods

Commercially available passivation methods for white-rust protection of hot-dip galvanized steel have been investigated. The passivations were either based on trivalent chromium or chromium free. A chromate based conversion coating was used for reference. The treated panels were tested with regard to white rust protection and paintability. The surface chemistry of the conversion coatings was monitored with scanning Auger electron spectroscopy and X-ray photoelectron spectroscopy. Coating thicknesses were measured using Auger electron sputter depth profiling.The passivations were applied with a thickness recommended by the supplier and thus showed large variation. The thickness of the chromium free passivation (Cr-free) is approximately 75 nm. The coating contains the active ions; H₃O⁺, Ti⁴⁺, Mn²⁺, Zn²⁺, PO₄³⁻. The passivation based on trivalent chromium (Cr-III) is approximately 30 nm thick and contains the active ions; H₃O⁺ Cr³⁺, PO₄³⁻, F. The chromate based passivation (Cr-VI) is approximately 5 nm thick and contains the active ions Cr⁶⁺/Cr³⁺, F⁻.The Cr-free and the Cr-III passivations showed similar white rust protection in the corrosion tests. The corrosion resistance was good although it did not fully reach the level of the Cr-VI passivation. The results from the tests of the painted panels showed that the powder paint worked well on all three passivations. The solvent born paint worked best on the passivation based on trivalent chromium. The water born paint showed poor resistance to blistering in the Cleveland humidity test for all three passivations. In this test the passivation with hexavalent chromium showed slightly better results than the chromate free passivations.     

An XPS study of passive films on Nickel and alloy 600 in acids

Compositions of surface films formed on nickel and Alloy 600 in I M HCI, 0.5 M H₂SO₄ and I/3 M H₃PO₁ solutions were investigated as a function of polarization potential. The main constituent of surface films formed on Ni in 0.5 M H₂SO₄ or 1/3 M H₃PO₄ solution was hydrated nickel oxyhydroxide, in which the ratio of O₂ to OH⁻ increased when passivation occurred. The surface films formed on Ni and Alloy 600 at lower potentials in 0.5 M H₂SO₄ solution contain S² ions other than SOP_4²⁻ ions, whereas S²⁻ ions were not incorporated in the passive film. Passivation of Alloy 600 took place by the formation of hydrated chromium oxyhydroxide. Pitting led to no substantial change in the average composition of the film.     

Improvement in the self-healing ability of a protective film consisting of hydrated cerium(III) oxide and sodium phosphate layers on zinc

A self-healing protective film prepared on a zinc electrode by previous treatment in a Ce(NO₃)₃ solution and coverage with a Na₃PO₄ layer was improved by the addition of Zn(NO₃)₂. The protective and self-healing abilities of the films against corrosion of zinc in an aerated 0.5 M NaCl solution were examined by polarization measurements and observation of pit-like anodic dissolution feature (plf) and pit formation after the electrode was scratched with a knife-edge crosswise and immersed in the solution for many hours. The film prepared on the pretreated electrode coated with 9.98 or 19.9 μg/cm² of Zn(NO₃)₂ · 6H₂O and 55.2 μg/cm² of Na₃PO₄ · 12H₂O was significantly protective and self-healing against zinc corrosion on the scratched electrode. The protective efficiency of the film was more than 96% in the region of the immersion time between 72 and 360 h. No plf was observed at the scratches after immersion for 120 h. Results of Fourier transform infrared reflection and X-ray photoelectron spectroscopies revealed that the film was composed of Na₃PO₄, Zn₃(PO₄)₂ and Ce₂O₃ layers.     

Removal of SO4, uranium and other heavy metal ions from simulated solution by sulfate reducing bacteria

In the case of in-situ leaching of uranium, the primitive geochemical environment for groundwater is changed since leachant is injected into the water bearing uranium deposit. This increases the concentration of SO4^2-, uranium and other heavy metal ions and results in the groundwater contamination. The effects of pH values of the simulated solution on the reduction of SO4^2- and the removal of uranium and other heavy metal ions by sulfate reducing bacteria（SRB） were studied. The results show that, when the pH value of the simulated solution is about 8, the reduction rate of SO4^2- by SRB and the removal rate of uranium, Mn^2＋, Zn^2＋, Pb〉 and Fe^2＋ will reach their highest values. A bioremediation technique for remediation of groundwater in in-situ leaching uranium mine can be developed.     

Self-healing mechanism of a protective film prepared on a Ce(NO3)3-pretreated zinc electrode by modification with Zn(NO3)2 and Na3PO4

Self-healing mechanism of a protective film against corrosion of zinc at scratches in an aerated 0.5 M NaCl solution was investigated by polarization measurements, X-ray photoelectron spectroscopy (XPS) and electron-probe microanalysis (EPMA). The film was prepared on a zinc electrode by treatment in a Ce(NO₃)₃ solution and addition of aqueous solutions containing 9.98 or 19.9 μg/cm² of Zn(NO₃)₂ · 6H₂O and 55.2 μg/cm² of Na₃PO₄ · 12H₂O. After the coated electrode was scratched with a knife-edge crosswise and immersed in the NaCl solution for many hours, polarization measurements, observation of pit formation at the scratches, XPS and EPMA were carried out. This film was remarkably protective and self-healing against zinc corrosion on the scratched electrode. The cathodic and anodic processes of zinc corrosion were markedly suppressed by coverage of the surface except for scratches with a thin Ce₂O₃ layer containing a small amount of Ce⁴⁺ and the surface of scratches with a layer composed of Zn₃(PO₄)₂ · 4H₂O, Zn(OH)₂ and ZnO mostly.     

Inhibition of steel and galvanised steel corrosion by zinc and calcium ions in the presence of phosphate

Abstract

Mixtures of zinc, calcium and phosphate ions have been found to be effective (>90%) in inhibiting the corrosion of steel and zinc during immersion in acid rain solution and when used as pigment additions to organic coatings. The presence of calcium ions, in addition to zinc ions, appears to be fundamental in the inhibition mechanism since, in the absence of calcium, the inhibition efficiency is 50% or less. Although some anodic inhibition occurs, the predominant mechanism appears to be classical cathodic inhibition caused by the precipitation of a sparingly soluble, thin, but persistent, solid film. Surface analysis, by XPS, confirms that phosphorus is present in the form of phosphate species containing zinc and calcium. Also, the atomic ratios of zinc:calcium:phosphorus present in the inhibitive film strongly suggest that it consists of a mixture of CaZn₂(PO₄)₂ (Scholzite) and Zn₃(PO₄)₂ (Hopeite) in a 1 : 1 ratio.     

Microstructural aspects of the corrosion of Alloy 800

Transmission electron microscopy studies on solution-annealed Alloy 800 revealed small (100–200 nm), spherical-shaped titanium carbide (face centered cubic structure) and large (200 nm–5 μm), faceted titanium nitride (hexagonal structure) particles randomly distributed in the austenite matrix. The volume fraction of former particles was found to be greater than that of the latter. Corrosion studies of the alloy in acidic, chlorides and acidic chloride environments at room temperature indicated that the passivity of Alloy 800 was adversely affected by the addition Cl⁻ ions. X-ray photoelectron spectroscopy revealed that the surface film formed on the alloy at the onset of passivity consisted of Cr³⁺ (as Cr₂O₃), without any Fe³⁺/Fe²⁺ or Ni²⁺. Scanning electron microscopy studies indicated initiation of pitting at large, faceted particles, not at small, spherical-shaped ones.     

The Corrosion Behaviour of Electrochemical Zn Coated with Conversion Chromate and Phosphate Films in Concrete

The corrosion behaviour of electrochemical Zn coatings with and without chromate (VI, III) and phosphate (amorphous and crystalline) films in aerated 0.1 M NaOH + 0.1 M NaCl solution and concrete, immersed into water has been studied. The estimated corrosion rate in 0.1 M NaOH + 0.1 M NaCl solution is minimal cf. Zn chromatised in Cr³⁺ solution. The value of j_cor of Zn chromatised in Cr⁶⁺ solution is 1.5 times as high as that in Cr³⁺. The experiments carried out in concrete immersed into water (pH 12.5) have shown that all the chromate films studied do not extend the operating time of the sample. The best corrosion protection is provided by crystalline phosphate film.     

XPS and EPMA studies on self-healing mechanism of a protective film composed of hydrated cerium(III) oxide and sodium phosphate on zinc

A thin, protective film was prepared on a zinc electrode by coverage with a hydrated Ce₂O₃ layer and 0.0552 mg/cm² of a Na₃PO₄·12H₂O deposit layer and by drying the layers at 90 °C for 23 h, as has been reported in the preceding paper. This film was highly self-healing at the scratched electrode in an aerated 0.5 M NaCl solution for many hours. The present investigation deals with elucidation on self-healing mechanism of this film by X-ray photoelectron spectroscopy and electron-probe microanalysis. The scratched surface was covered with deposits of Zn(OH)₂, ZnO and small amount of Zn₃(PO₄)₂. Both cathodic and anodic processes of zinc corrosion were markedly suppressed by coverage of the scratches with the deposits, resulting in the self-healing activity at the scratched electrode in 0.5 M NaCl.     

Zinc treatment effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 °C were found to lower the corrosion rate of austenitic stainless steel (UNS S30400) by about a factor of five, relative to a non-zinc baseline test [S.E. Ziemniak, M. Hanson, Corros. Sci. 44 (2002) 2209] after 10,000 h. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, revealed that miscibility gaps in two spinel binaries—Fe(Fe_1−mCr_m)₂O₄ and (Fe_1−nZn_n)Fe₂O₄—play a significant role in determining the composition and structure of the corrosion layer(s). Although compositions of the inner and outer corrosion oxide layers represent solvus phases in the Fe₃O₄-FeCr₂O₄ binary, zinc(II) ion incorporation into both phases leads to further phase separation in the outer (ferrite) layer. Recrystallization of the low zinc content ferrite solvus phase is seen to produce an extremely fine grain size (∼20 nm), which is comparable in size to grains in the inner layer and which is known to impart resistance to corrosion. Zinc(II) ion incorporation into the inner layer creates additional corrosion oxide film stabilization by further reducing the unit cell dimension via the substitution reaction

0.2Zn²⁺(aq)+Fe(Fe_0.35Cr_0.65)₂O₄(s)?0.2Fe²⁺(aq)+(Zn_0.2Fe_0.8)(Fe_0.35Cr_0.65)₂O₄(s)     

Kinetic Regularities of Zinc Corrosion in Chromating Baths

Electrochemical modeling of zinc corrosion in chromate solutions showed that its rate (which is controlled by the limiting current density of Cr(VI) reduction to Cr(III)) is determined by the limiting diffusion current of hydrogen ions to the zinc electrode surface partially covered with the chromate coating. The sizes of active and passive surface sites are comparable with the thickness of the diffusion layer of the solution, which means that the zinc surface corroding in a chromate solution is macroscopically heterogeneous. Investigating the kinetics of the electrode reactions showed that, in the absence of a noticeable film formation, the steady-state cathodic polarization curve of zinc in sulfate–chromate solutions can be described by a kinetic equation that takes into account the pH _s dependence on the dissolution rate of zinc.     

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.     

Electrochemical studies of Na2CrO4-Na2So4 melts at 1200 K

The electrochemical response of Na₂SO₄-Na₂CrO₄ solutions at a Pt working electrode was established by cyclic voltammetry and chrono-potentiometry measurements at 1200 K. Experiments were made under conditions of controlled and electrochemically measured values of melt basicity for basic, neutral, and acidic melts. Despite the thermodynamic stability of chromite ions under reducing conditions, a direct conversion of chromate to chromite upon cathodic polarization was not observed. Rather, the cathodic reduction of chromate ions proceeds by a reversible one-electron charge transfer to an electroactive intermediate species formed through an equilibrium between CrO and oxide ions in the solution. An irreversible chemical reaction reduces the chromium solute species further to precipitate a solid. As a solute in a corrosive fused salt film, a chromate to chromite transition would not be expected to stabilize the oxidation state and basicity. However, chromate ions to provide a reduction reaction more favorable than sulfate reduction to sulfide; chromate reduction leads to the precipitation of solid NaCrO₂ or Cr₂O₃ in basic or neutral solutions, respectively. A large shift in melt basicity can only occur for mildly acidic solutions with high chromate solute.     

Subsolidus phase relations in the system ZnO–P2O5–MoO3

The subsolidus phase relations of the ternary system ZnO–P₂O₅–MoO₃ were investigated by means of X-ray diffraction (XRD). Seven binary compounds and eight 3-phase regions were determined, and no ternary compound was found in this system. The phase diagram of pseudo-binary system Zn₃(PO₄)₂–Zn₃Mo₂O₉ was also constructed through XRD and differential thermal analysis (DTA) methods, and the result reveals this system is eutectic system. The eutectic temperature is 904 °C and the corresponding component is 30% Zn₃Mo₂O₉ and 70% Zn₃(PO₄)₂.     

Inhibition of mild steel by strontium chromate in artificial acid rain solution

Abstract

The inhibition of mild steel in an artificial acid rain solution (pH 4·5) in low concentrations of strontium chromate pigment has been evaluated using a range of methods: potentiodynamic polarisation, electrochemical impedance, X-ray photoelectron spectroscopy and solution analysis. The individual effects of strontium and chromate have been evaluated and have been used to estimate inhibitor efficiency and film composition, and to determine the possible inhibition mechanism. The results show that, at low concentration, strontium chromate affects the cathodic reaction, with reduction of Cr⁶⁺ to Cr³⁺, and the surface film was composed of magnetite and hydrated chromium hydroxide. The presence of strontium was not found significantly to affect the inhibitory performance.     

An environmentally friendly molybdate/phosphate black film on Mg-Zn-Y-Zr alloy

An environmentally friendly molybdate/phosphate black film for light absorption application was developed as an alternative to the chromate system on Mg-Zn-Y-Zr alloy. The microstructure, surface morphology, chemical composition, light absorbance and protection property of the black film were investigated. The initiation and growth mechanisms of the black film were also discussed. The black film consists of MoO₂ and a fraction of Mg₃(PO₄)₂. The MoO₂ contributes to the black color and the Mg₃(PO₄)₂ aims to improve the protection property. The experimental results indicate that the black film can provide not only high light absorbance but also corrosion protection to the Mg-Zn-Y-Zr substrate. The formation mechanism of the black film is associated with the electrochemical heterogeneity of the Mg-Zn-Y-Zr substrate. At the cathodic sites, MoO₄²⁻ is reduced to MoO₂ accompanying with the hydrogen evolution reaction. At the anodic sites, the dissolution of Mg²⁺ bonds with PO₄³⁻ to form Mg₃(PO₄)_2.     

Corrosion resistance of the Ti–50Zr at.% alloy after anodization in different acidic electrolytes

The stability of oxide films potentiodynamically (50 mV s⁻¹) grown up to 8.0 V(SCE) on Ti–50Zr at.% in H₂SO₄, HNO₃, CH₃SO₃H, and H₃PO₄ (pH ≈ 1) was assessed in the growth electrolyte itself or in a Ringer solution. For all anodizing electrolytes, oxide films become stabler as their thickness increases. In the Ringer solution, the oxide film stability is affected by the anodizing electrolyte; for oxides grown up to 8.0 V(SCE), films obtained in H₃PO₄ are slightly less stable than the ones obtained in the other acids, whereas films obtained in H₂SO₄ are clearly the stablest ones.     

Zinc treatment effects on corrosion behavior of Alloy 600 in high temperature, hydrogenated water

Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 °C were found to reduce the corrosion rate of Alloy 600 (UNS N06600) by about 40% relative to a non-zinc baseline test [S.E. Ziemniak, M. Hanson, Corros. Sci., in press, doi:10.1016/j.corsci.2005.01.006]. Characterizations of the corrosion oxide layer via SEM/TEM and grazing incidence X-ray diffraction confirmed the presence of a chromite-rich oxide phase and recrystallized nickel. The oxide crystals had an approximate surface density of 3500 μm⁻² and an average size of 11 ± 5 nm. Application of X-ray photoelectron spectroscopy with argon ion milling, followed by target factor analyses, permitted speciated composition versus depth profiles to be obtained. Numerical integration of the profiles revealed that: (1) alloy oxidation occurred non-selectively and (2) zinc(II) ions were incorporated into the chromite-rich spinel: (Zn_0.55Ni_0.3Fe_0.15)(Fe_0.25Cr_0.75)₂O₄. Spinel stoichiometry places the trivalent ion composition in the single phase oxide region, consistent with the absence of the usual outer, ferrite-rich solvus layer. By comparison with compositions of the chromite-rich spinel obtained in the non-zinc baseline test, it is hypothesized that zinc(II) ion incorporation was controlled by the equilibrium for

0.55Zn²⁺(aq)+(Ni_0.7Fe_0.3)(Fe_0.3Cr_0.7)₂O₄(s)?0.40Ni²⁺(aq)+0.15Fe²⁺(aq)+(Zn_0.55Ni_0.3Fe_0.15)(Fe_0.3Cr_0.7)₂O₄(s)     

Inhibiting the corrosion of stainless steel in phosphoric acid

The effect of Cu²⁺ and NO ₃ ⁻ ions on the stability of the passive state of a X18H10T type steel in phosphoric acid is investigated by constructing true curves of the anodic and cathodic processes. In a 3 M H₃PO₄ at 100°C, the steel is found to be in an unstable passive state. Copper(II)- and nitrate-ions transfer it into a steady passive state. Adding copper (II) ions to phosphoric acid accelerates the cathodic process and impedes the anodic dissolution in the active potential range. The steel passivation potential decreases under their action.     

Kinetics of the passivation of molybdenum in salt solutions as inferred from impedance and potential measurements

The oxide film formed on the molybdenum electrode in sodium salt solutions of different anions was found to thicken according to a logarithmic growth law and the electrode potential varied with time according to the equation E_h = α + β log t. The rate of oxide film thickening in the respective solutions was in the order Na₃PO₄ > Na₂SO₄ > NaCl > NaNO₃. The equilibrium potential was found to be independent of anion concentration in the case of Cl^- and NO^-₃ and was slightly dependent on the PO^3-₄ and SO^2-₄ concentrations. Complex plane analysis showed that the double-layer capacitance affects the measured capacitance and an electrical model was proposed to explain the alternating current behaviour of the electrode-electrolyte interface.