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.     

Evaluation of porosity and discontinuities in zinc phosphate coating by means of voltametric anodic dissolution (VAD)

In this study, the porosity and the presence of discontinuities, such as cracks, in zinc phosphate coatings were evaluated by the voltametric anodic dissolution (VAD) method. Zinc phosphate (PZn), zinc phosphate with niobium (PZn + Nb), and zinc phosphate with ammonium niobium oxalate (Ox) and benzotriazole (PZn + Ox + BTAH) coatings deposited on SAE 1010 carbon steel were investigated. Coating porosity was evaluated by estimating the charge densities associated with the substrate passivation process for samples with a phosphate layer and comparing the results to the charge densities for passivation of the same substrate without a coating phosphate layer. Weight loss measurements, induced coupled plasma optical emission spectroscopy (ICP-OES), and scanning electron microscopy (SEM) were also used to investigate the solubility of the phosphate layers tested. The electrolytes used were four buffer solutions with pHs of 7.0, 8.0, 10, and 12. Scanning rates of 30, 50, 100, and 150 mV s^− 1 were used in the VAD tests. The porosities of the PZn, PZn + Nb, and PZn + Ox + BTAH layers were estimated by VAD to be 4.35, 1.96, and 1.37%, respectively. The lower porosities of the PZn + Nb and PZn + Ox + BTAH layers are related to their morphologies, which promote better surface coverage compared to the PZn layer.     

Effects of Niobium Ammonium Oxalate and Benzotriazole on the Corrosion Resistance of Zinc Phosphate Layer

In this study, the viability of nickel replacement in zinc phosphate baths by niobium ammonium oxalate (Ox) and benzotriazole (BTAH) was studied. Samples of carbon steel (SAE 1010) were phosphated in two zinc phosphate baths, one containing nickel (PZn+Ni) and another containing Ox and BTAH (PZn+Ox+BTAH). The corrosion resistance of the phosphated samples was analyzed in a NaCl 0.5 mol/L solution using open circuit potential measurements, anodic and cathodic polarization curves, and electrochemical impedance spectroscopy. Surface analysis was carried out by scanning electron microscopy and energy dispersive spectroscopy. The results showed that the PZn+Ni layer presented a needle-like morphology and that the crystals of PZn+Ox+BTAH showed granular morphology. The electrochemical results showed that the highest corrosion resistance was associated to the PZn+Ox+BTAH layer.     

The effects of nano-SiO2 additive on the zinc phosphating of carbon steel

In this paper, nano-SiO₂ was used as an accelerator for improving the microstructure and the corrosion resistance of phosphate coating on carbon steel. The chemical composition and microstructure of the coatings were analyzed by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The effects of nano-SiO₂ on weight, roughness and corrosion resistance of the phosphate coatings were also investigated. Results show that the compositions of phosphate coating were Zn₃(PO₄)₂·4H₂O (hopeite), and Zn₂Fe(PO₄)₂·4H₂O (phosphophylite). The phosphate coatings became denser due to the addition of nano-SiO₂ which reduced the size of the crystal clusters. The average weight of phosphate coatings approximately linearly increased with the nano-SiO₂ content in the bath from 0 to 4 g/L, and the phosphate coatings formed in bath containing 2 g/L nano-SiO₂ showed the highest corrosion resistance in 5 wt.% sodium chloride solution at ambient temperature. Nano-SiO₂ would be widely utilized as a phosphating additive to replace the traditional nitrite, due to its less pollutant and its better quality of the coating.     

The corrosion of two NiNb alloys under 1 atm O2 at 600–800 °C

The oxidation of two NiNb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in pure oxygen under 1 atm O₂ at 600–800 °C. The scales formed on both alloys under all conditions show an external scale, generally duplex, containing an outermost layer of nearly pure NiO and an innermost region of NiO mixed with the double NiNb oxide NiNb₂O₆. Moreover, the samples corroded at all temperatures also show a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both alloy phases Ni₈Nb and Ni₃Nb. No important depletion of niobium was observed in the alloy close to the interface with the zone of internal oxidation, while the depth of this region is generally much higher than measured for the corrosion of the same alloys under low oxygen pressures at the same temperatures. The corrosion mechanism of these alloys is examined with special reference to the effects of the low solubility of niobium in nickel.     

A study of chromium-free pickling process before electroless Ni-P plating on magnesium alloys

A chromium-free pickling process of magnesium alloys in H₃PO₄ + Na₂MoO₄ solution for electroless Ni-P plating was described. The dosage of Na₂MoO₄ was established by detecting adhesion and corrosion resistance of chemical nickel coatings. Electrochemical behaviors of pickling solution of H₃PO₄ + Na₂MoO₄ and NH₄HF₂ activation solution were investigated with the open circuit potential curves and the polarization curves. The results show that the Na₂MoO₄ has strong inhibition ability. Na₂MoO₄ in H₃PO₄ solution can reduce active sites of microcathodic and microanodic zones in the corrosion cells of the substrate surface of magnesium alloys and plays an inhibition role. The activation film with some oxides and fluorides can prevent the substrate magnesium from the fierce displacement and corrosion reaction of electroless plating bath. The chemical Ni-P coating with good adhesion and corrosion resistance was obtained by the pretreatment of 200 cm³ dm⁻³ 85% H₃PO₄ + 5 g dm⁻³ Na₂MoO₄ pickling bath and activation in 200 g dm⁻³ NH₄HF₂ solution. This procedure of surface pretreatment before electroless nickel plating can replace the existing acid pickling containing chromium and HF activation.     

Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc-calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn-Ca phosphate conversion coatings are mainly composed of hopeite (Zn₃(PO₄)₂·4H₂O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF₂, Zn or ZnO and hopeite. Both of the zinc and zinc-calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn-Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn-Ca coating.     

Microstructural characterization and surface properties of zinc phosphated medium carbon low alloy steel

The present study concerns an in-depth investigation of the influence of process parameters on zinc phosphating of medium carbon steel to have an understanding of nucleation and growth mechanism and the microstructures developed thereafter. Furthermore, the wear and corrosion behavior of phosphated steel were evaluated in details. Phosphating was carried out in zinc phosphate bath (6–8% ZnPO₄) using 0.43 wt.% Fe as catalyst at 90 °C with variable time ranging from 60 s to 1800 s followed by water rinsing and oil sealing. The microstructural evolution with time was recorded to understand the nucleation and growth phenomena. The morphology of coating was both plate and needle shaped. The microhardness of the coated layer is significantly lower than that of as-received steel. Fretting wear behavior against hardened steel ball showed that phosphating reduced the wear resistance which was possibly attributed to the presence of phosphate needles causing an increased coefficient of friction and the reduced hardness of the phosphate layer. Corrosion potential of the phosphated steel shifted towards noble direction as compared to the as-received one. The study of the corrosion rate also shows that the rate is considerably reduced by phosphating. Pitting corrosion resistance was also found to be increased (in terms of increased critical potential for pit formation) due to phosphating as compared to as-received steel.     

Characterization and corrosion behavior of ceramic coating on magnesium by micro-arc oxidation

In this study, the commercial pure magnesium was coated in different aqueous solutions of Na₂SiO₃ and Na₃PO₄ by the micro-arc oxidation method (MAO). Coating thickness, phase composition, surface and cross sectional morphology and corrosion resistance of coatings were analyzed by eddy current method, X-ray diffraction (XRD), scanning electron microscope (SEM) and tafel extrapolation method, respectively. The average thickness of the coatings ranged from 52 to 74 μm for sodium silicate solution and from 64 to 88 μm for sodium phosphate solution. The dominant phases on the coatings were detected as spinal Mg₂SiO₄ (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg₃(PO₄)₂ (Farringtonite) and MgO (Periclase) for sodium phosphate solution. SEM images reveal that the coating is composed of two layers as of a porous outer layer and a dense inner layer. The corrosion results show the coating consisting Mg₂SiO₄ is more resistant to corrosion than that containing Mg₃(PO₄)₂.     

Characteristics of a fast low-temperature zinc phosphating coating accelerated by an ECO-friendly hydroxylamine sulfate

A fast low-temperature phosphating processing accelerated by an ECO-friendly hydroxylamine sulfate (HAS) is developed. The zinc phosphate coating was fast formed on high-carbon steel in a low-temperature phosphating bath. Growth stages and characteristics of the phosphate coating were investigated by open circuit potential (OCP), SEM, EDS and XRD techniques. The phosphating process can be divided into three stages, namely amorphous precipitation, anodic depolarization and growth of phosphate coating. The phosphate coating consists of Zn₃(PO₄)₂ · 4H₂O and Zn₂Fe(PO₄)₂ · 4H₂O phases. The addition of HAS makes the three stages' time shorten to 53%, 31% and 50%, respectively, while markedly reduces the size of phosphate crystals from 100 µm to about 50 µm, and increases the Zn₂Fe(PO₄)₂ · 4H₂O content from 30% to 44% in the coating. HAS would be widely used as a low-temperature phosphating accelerator to replace the traditional nitrite, due to its less pollutant, higher phosphating rate and better quality of the coating.     

Surface characteristics, nano-indentation and corrosion behavior of Nb implanted NiTi alloy

NiTi shape memory alloy has been modified by Nb implantation with different implantation parameters including incident dose and current. The surface morphology and chemical components are determined by atomic force microscopy (AFM), Auger Electron Spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The results show that Nb implantation leads to the formation of compact Nb₂O₅/TiO₂ thin film about 30 nm in thickness on the surface of the NiTi alloy and decreases the surface concentration of Ni. A larger incident dose or incident current causes a higher surface roughness and a higher Nb content in the implantation layer of NiTi alloy. The nano-indentation measurements indicate the obvious reduction of both nano-hardness and Young's modulus of the Nb implanted NiTi alloy in the implantation layer and even in deeper NiTi matrix. The results of potentiodynamic polarization test show that the corrosion resistance of NiTi alloy in Hanks's solution has been evidently improved by Nb implantation. The NiTi alloy with a moderate implantation parameter of 1.5 × 10¹⁷ ions/cm² and 2 mA exhibits the best corrosion resistance ability.     

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.     

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.     

Enhanced electrochemical performances of multi-walled carbon nanotubes modified Li3V2(PO4)3/C cathode material for lithium-ion batteries

The multi-walled carbon nanotubes (MWCNTs) modified Li₃V₂(PO₄)₃/C composite is synthesized by polyvinyl alcohol (PVA) based carbon-thermal reduction method using MWCNTs as a highly conductive agent. PVA mainly supplies a reductive atmosphere to reduce V⁵⁺ and provides a network of carbon to inhibit the aggregation of Li₃V₂(PO₄)₃ particles. The amorphous carbon coating and MWCNTs co-modified composite shows excellent high-rate lithium intercalation/deintercalation property and cycling performance between 3.0 and 4.3 V. The discharge capacities of 131.7 and 122.9 mAh g⁻¹ are obtained at rates of 1 C and 10 C, respectively, for the Li₃V₂(PO₄)₃/(C + MWCNTs). These improvements are attributed to the valid conducting networks of C + MWCNTs and the reduced Li₃V₂(PO₄)₃ particle size by the network carbon from the pyrolysis of PVA.     

Influence of pre-oxidation on the hot corrosion of Ti3SiC2 in the mixture of Na2SO4-NaCl melts

Polycrystalline Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO₂ would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti₃SiC₂ corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.     

Influence of fluoride ions on the amorphous phosphating of aluminium alloys

The influence of the F⁻ ions containing compounds: NaF, HBF₄, H₂SiF₆, NH₄F·HF and their concentrations on formation of phosphate coatings on Al in МоО₄²⁻ ions containing solutions, as well as protective properties of the phosphate coatings in a 0.5 M Na₂SO₄ solution have been studied. The studies of film composition by the XPS method have shown, that the phosphate coating consists of the metal phosphates from aluminium alloy and the Mo (IV, V) phosphates, which are formed during reduction of МоО₄²⁻ ions. The mass of phosphate layer and that of etched metal depend on the nature of fluoride ions, which can be arranged in the following order according to the decrease in strength of their influence: H₂SiF₆ > NH₄F·HF > HBF₄ > NaF. Polarization measurements in a 0.5 M Na₂SO₄ solution and the calculated electrochemical parameters testify that phosphated Al samples exhibit a lower corrosion rate and a higher corrosion resistance as compared to non-phosphated Al substrate.     

A SERS investigation of the electrodeposition of Au in a phosphate solution

The spectroelectrochemical behaviour of cyanide and phosphate ions adsorbed on a Au electrode during electrodeposition and corrosion in a KAu(CN)₂ phosphate bath was studied by in situ SERS (Surface Enhanced Raman Spectroscopy) and cyclic voltammetry. SERS spectra exhibiting features corresponding to CN⁻ and PO₄⁻ vibrations were recorded under potentiostatic control in an interval from − 1600 to + 1500 mV vs. Ag/AgCl. Surface CN⁻ species are the dominating ones under both cathodic and anodic polarisations. As far as the phosphate vibrations are concerned, bands related to PO₄⁻ are visible in both cathodic and anodic conditions. A fluorescence background was evident in the potential range − 1300 ÷ + 1300 mV. At cathodic potentials phosphate ion adsorption on electrode surface prevails over cyanide for potentials more cathodic than − 1000 mV. At − 700 mV a discontinuity is noticed in the Stark tuning of CN⁻, probably due to a change of Au(CN)₂⁻ adsorption mechanism. At anodic potentials bands due to cyanide oxidation could be observed.     

Effect of underpotential deposition of lead on polarization behavior of nickel in acidic perchlorate solutions at room temperature

The effect of Pb²⁺ on polarization behavior of nickel has been investigated in 0.1 M NaClO₄ + 10⁻² M HClO₄ + x M PbO solutions (x = 0, 10⁻⁵, 10⁻⁴, 10⁻³) at room temperature. The cyclic voltammogram has suggested that Pb²⁺ degrades the stability of the passive film on Ni. The corrosion potential of Ni shifted to the more noble direction and the anodic current peak of Ni dissolution decreased with increasing Pb²⁺ concentration in solution, indicating that Pb²⁺ suppresses significantly the anodic dissolution. The underpotential deposition (UPD) of lead on Ni in the potential range more noble than −0.215 V (SHE) corresponding to the equilibrium potential of the Pb²⁺ (10⁻³ M)/Pb electrode was confirmed by XPS and GDOES analyses. The anodic Tafel slope, b⁺, of Ni dissolution changed from b⁺ = 40 mV decade⁻¹ in the absence of Pb²⁺ to b⁺ = 17 mV decade⁻¹ in the presence of 10⁻⁴ or 10⁻³ M Pb²⁺, which was ascribed to the increase in active sites of Ni surface emerged as a result of electrodesorption of Pb adatoms. The roles of Pb adatoms in active dissolution and active/passive transition of Ni were discussed from the above results.     

Phosphate-permanganate conversion coatings on the AZ81 magnesium alloy: SEM, EIS and XPS studies

Conversion coatings on the magnesium alloy AZ81 were prepared using the phosphate-permanganate baths differing in composition. The corrosion behavior of the coated and uncoated alloys has been investigated by electrochemical impedance spectroscopy (EIS) and linear polarization methods. The choice of proper electric equivalent circuit (EEC) is discussed. The effect of temperature, bath composition and time of conversion as well as etching in acids before application on the corrosion resistance of the coated alloy has been evaluated. The best corrosion resistance was obtained for the samples coated in the bath containing 25 g KMnO₄, 150 g Na₂HPO₄ and 50 ml H₃PO₄ in 1 dcm³, applied at 80 °C. Differences in the morphology and composition of coated surfaces were investigated by the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques and correlated with the corrosion resistance of the samples.     

The corrosion behavior of plasma sprayed Fe2Al5 coating in molten Zn

Aluminum coating was plasma sprayed on Fe-0.14-0.22 wt.% C steel substrate, and heat diffusion treatment at 923 °C for 4 h was preformed to the aluminum coating to form Fe₂Al₅ inter-metallic compound coating. The corrosion mechanism of the Fe₂Al₅ coating in molten zinc was investigated. SEM and EDS analysis results show that the corrosion process of the Fe₂Al₅ layer in molten zinc is as follows: Fe₂Al₅ → Fe₂Al₅Zn_x (η) → η + L(liquid phase) → L + η + δ(FeZn₇) → L + δ → L. The η phase and the eutectic structure (η + δ) prevent the diffusion of zinc atoms efficiently. Therefore the Fe₂Al₅ coating delays the reaction between the substrate and molten zinc, promoting the corrosion resistance of the substrate.