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.     

Studies on the Surface Treatment of Aluminium and Zinc

Comparative corrosion and adhesion tests have demonstrated the value of chemical surface treatments for improving the bare corrosion resistance and adhesion of paint to zinc and aluminium. Recent developments have enabled architectural green coatings to be produced on aluminium with consistent colour and chromate-fluoride coatings to be applied on high speed strip lines. It is now possible to correlate the colour of Chromate coatings on zinc with the ratio of the hexavalent chromium to the other anion present; best results are obtained when this anion is sulphate. Yellow Chromate films of very good protective power can be produced from comparatively dilute solutions. Colourless Chromate films can also be obtained from these solutions by adding ammonium sulphate to the processing solution.     

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.     

Chromium electrodeposition from Cr(III) aqueous solutions

The aim of this work is to study an alternative plating process to obtain chromium coatings through electrodeposition from baths containing trivalent chromium, as aqueous solutions of Cr (III) are significantly less dangerous, in terms of human health and environmental impact, as compared to the traditional Cr (VI) baths employed for this purpose. In order to overcome problems regarding the reduction of Cr (III) in aqueous solution, two approaches were followed: i) the possibility of co-depositing chromium along with a second metal, which could help the process of discharge of Cr³⁺ on the substrate; ii) the use of a specific ligand for the Cr³⁺ ion, which can generate easily reducible complexes at the metal-solution interphase. Both approaches led to interesting results: in particular, the co-deposition enabled us to obtain NiCr alloy with a high percentage of chromium, and the deposition using specific complexing agents allowed optimal bath compositions to be developed both for decorative and hard chromium plating.     

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.     

Corrosion effects of plasma immersion ion implantation-enhanced Cr deposition on SAE 1070 carbon steel

Chromium electroplated coatings are the most common coatings used for several decades in the general industry as a protective layer against corrosion and wear. The waste of Cr plating is an environmental problem because of the toxic and carcinogenic Cr⁶⁺ compounds resulting from the Cr baths. In this study, we suggest an alternative method for thin Cr films deposition: plasma immersion ion implantation-enhanced deposition (PIIIeD) that aims at achieving highly adherent films to carbon steel surfaces with very high resistance to corrosion attacks. Chromium films formed by PIIIeD were studied with respect to corrosion resistance, surface morphology and elements profile. Potentiodynamic scans showed a decrease in the corrosion current density, promoting higher corrosion resistance, and a hydrophobic character of the PIIIeD Cr films, turning the samples less susceptible to saline corrosion. GDOES depth profiles showed the increase of film and mixing layer thicknesses with the increase of GD power.     

Corrosion Behaviour of Zn and Zn Alloy Coatings in Alkaline Media

The corrosion behaviour of electrodeposited Zn, Zn-Co, Zn-Fe and Zn-Ni coatings without and with Chromate films was studied in alkaline solutions and mortar probes. DC polarization measurements were used for electrochemical characterization of the investigated samples, while AES XPS techniques were applied for the Chromate film analysis. The stability of the anodically formed oxide films on unchromated coatings appeared to be dependent on their composition. The oxide layer stability in Cl free NaOH solution was higher for Zn-Ni and Zn-Co alloys, while introduction of Cl ions caused lowering of stability especially for Zn-Ni coating. The detected corrosion current values implied that unchromated Zn alloys did not possess higher corrosion resistance in alkaline media containing Cl ions. Meanwhile, chromated Zn-Co coatings exhibited the beneficial effect of alloying for corrosion in alkaline solutions and concrete. A higher concentration of Cr(Vl) compounds in the passive Chromate layer of the alloyed sample may be the reason for its superior corrosion resistance.     

The corrosion-protective traits of electroplated multilayer zinc-iron-chromium deposits

A novel kind of three-layer zinciron-cbromium electroplated coating was tested and proven to possess good corrosion protective characteristic as well as acceptable decorative properties. A citric-chloride bath containing Fe(III)-complexes is suggested for electrodeposition of iron intermediate layer on the zinc deposit surface. Due to the low acidity (pH 5) and medium operating temperature (20-30°C), the electrolyte involved has no a strong etching effect and can be used for direct iron deposition on zinc. A finish chromium layer is deposited from formate-sulfate trivalent bath. The zinc layer corrodes through the pores of Fe and Cr layers and protects the steel substrate against corrosion. The iron and chromium layers decelerate the corrosion rate of zinc. The finish chromium layer provides with good decorative properties, and the intermediate iron layer ensures adhesion of chromium to the zinc.     

Studies on the Influence of Hexadecylamine on Chromate Phosphate Coating on Aluminium

The effect of hexadecylamine (HDA) on a Chromate phosphate coating on aluminium was studied using an optimized Chromate phosphate bath, The addition of HDA was found to decrease the coating weight, but to enhance the coating quality and corrosion resistance. The inhibitory effect of HDA helps in regulating the excessive attack on the metal and its ability to reduce Cr⁶⁺ to Cr³⁺ compensates the possible time delay for the initiation of coating deposition due to the inhibition.     

Molybdate/Al(III) composite films on steel and zinc-plated steel by chemical conversion

A molybdate(VI)-Al(III) chemical conversion process was developed as an alternative to the chromate processes. Steel and zinc-plated steel specimens were treated in the solutions of 0.16 mol l⁻¹ ammonium alum (AlNH₄(SO₄)₂ · 12H₂O) with small amounts of ammonium molybdate(VI) (0.002-0.016 mol l⁻¹ (NH₄)₆Mo₇O₂₄ · 4H₂O) at 60 °C for 10-30 min in an ultrasonic rinsing apparatus. The formed films were composed of oxyhydroxides containing Mo(V,VI), Al(III), Fe(II,III), and sulfate ions (and Zn(II) ions in the case of zinc-plated steel), and showed good corrosion resistance in an aerated 0.5 mol l⁻¹ NaCl-0.15 mol l⁻¹ H₃BO₃ solution (pH=7). The films macerated during the corrosion test, but they did not detach and functioned as a protective layer. This process may be useful in forming undercoats for paints and polymer coatings on steel and zinc-plated steel.     

The effect of complexing on the zinc dissolution in dichromic acid with HF additions

Two concentration ranges of activating hydrofluoric acid additions were distinguished when dissolving zinc in a 2.5 M CrO₃ solution. Zinc dissolves slowly in a range of 0.05 to 0.3 and quickly from 0.4 to 1 mol/l HF. The effect is explained by the involvement of F^- ions in the formation of Cr(III) fluoride complexes during the reduction of dichromic acid anions and ZnF⁺ cations during zinc ionization. Ionic equilibrium was calculated after long-term holding zinc in the CrO₃ solutions with various HF contents. It was shown that complexing hinders the growth of passive oxide and salt films based on ZnF₂ or basic Cr(III) chromate on zinc. Original Russian Text ? A.M. Shcherbakov, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 3, pp. 300–305.     

Electrodeposition of Tin For Refrigerating Purposes

Results are presented of experiments on Chromate films on zinc, produced by dipping in sodium dichromate solutions of the type used in the Cronak process and variations thereon. The work covered the effect of solution composition, time of immersion, etc., on the films produced and their characteristics, in particular the protective value under humid corrosion conditions. Preferred conditions for carrying out the filming process are described.     

Corrosion performance of a hot-dip galvanized steel treated by different kinds of conversion coatings

The anticorrosive performance of various kinds of conversion coatings including Cr(VI) and Cr(III) were studied on hot galvanized steel (HDG). The effect of Co(II) and Ni(II) ions was also examined on the Cr(III) coating performance. DC and electrochemical impedance spectroscopy (EIS) were conducted when the specimens were immersed in 3.5 wt.% sodium chloride solution. The coating morphology was observed using SEM. Micro-cracks were observed on both Cr(III) and Cr(VI) treated samples. The results indicated a decrease in the corrosion rate of HDG when the Cr(III) or Cr(VI) conversion coatings were found on the surface. The greater anticorrosion resistance of a Cr(VI) treated sample (compared with a Cr(III) one) was obtained at low immersion times. However, a significant decrease in self healing behavior and therefore corrosion resistance of the Cr(VI) treated sample was observed at longer immersion times. The anticorrosion resistance of Cr(III) treated samples was significantly improved using Co(II) or Ni(II) conversion coatings on Cr(III) treated samples. On the other hand, a lower decrease in corrosion performance of samples treated by Cr(III) and Co(II) or Ni(II) was obtained after a long immersion time in corrosive environment. It was found that, the passive and barrier layer which the Co(II) or Ni(II) conversion coating can produce on the Cr(III) coating can superiorly improve its anticorrosion resistance.     

Optimization of Cr/Mo molar ratio in FeCoCrMoCBY alloys for high corrosion resistance

The corrosion behavior of bulk metallic glasses (BMGs) (Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂)_100–xCr_x (x=0, 4, 8, 12, molar fraction, %) was investigated in 1 mol/L HCl aqueous solution with electrochemical tests. The electrochemical measurements demonstrate that the passive current density of Fe-based amorphous alloy is reduced by about one order of magnitude, and meanwhile, the stability of passive film can be guaranteed by the Cr/Mo molar ratio. The Mott–Schottky (M–S) curves show that the passive film is the densest when the molar ratio of Cr/Mo is between 1.37 and 1.69. X-ray photoelectron spectroscopy (XPS) analysis was performed to clarify chemical states of elements in the passive films. The results show that the corrosion resistance of the alloy is related to the molar ratio of Cr/Mo. The stability of passive film is determined by the synergistic action of Cr and Mo elements. The main component of the passive film is Cr³⁺ oxide. When the potential is greater than 0.5 V (vs SCE), Mo⁶⁺ ions play an important role in keeping the stability of the passive film. The appropriate molar ratio of Cr/Mo can reduce the dissolution rate of the passive film.     

电镀锌三价铬彩色钝化膜封闭剂的制备

目的制备性能良好的封闭剂,以提高电镀锌三价铬彩色钝化膜的耐蚀性。方法以氟化钠、硅酸钠、OP为主要原料制备封闭剂。采用5%醋酸铅点滴法测定封闭处理后的电镀锌三价铬彩色钝化膜的耐蚀性能,探讨了封闭剂的配方,分析了封闭时间、烘干温度、烘干时间等工艺条件对耐蚀性的影响,并通过正交试验对封闭工艺进行了优化,确定了以氟化钠和硅酸钠为主盐的封闭剂的钝化处理工艺规范。结果试验得到的封闭剂组成为:氟化钠0.2 mol/L,硅酸钠0.05 mol/L,OP 1 m L/L。采用该封闭剂,在浸渍时间20min、烘干温度80℃的条件下,醋酸铅点滴实验最高达到近900 s。结论封闭后膜层呈彩虹色,色泽明亮,钝化膜耐蚀性提高。     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

SECM Study of Effect of Chromium Content on the Localized Corrosion Behavior of Low-Alloy Steels in Chloride Environment

This paper investigates the effect of chromium (Cr) content (0, 1, 3 and 5% Cr) in epoxy-coated alloy steel against corrosion using in situ electrochemical techniques such as EIS and SECM in a 3% NaCl solution. The EIS results revealed that the epoxy-coated Cr steel exhibited higher impedance values than carbon steel, which is attributed to the greater resistance of Cr steel toward corrosion. Based on the cyclic voltammogram results, the tip potentials were set at ?0.7, 0.04 and 0.60 V for determining the concentration of dissolved oxygen at cathodic region, and oxidation of Cr²⁺ and Fe²⁺ at anodic region, respectively. The SECM measurements showed that, the tip current in the anodic region has decreased with increase in Cr content of the sample, which indicates that the oxidation of Fe²⁺ and Cr²⁺ decreases (corrosion is reduced) with the increase in Cr content of the steel. Besides, 5% Cr steel can maintain the highest corrosion resistance, and 1 and 3% Cr steels have higher corrosion resistance than the 0% Cr steel. This higher corrosion resistance of Cr steel samples could be due to the formation of Cr-rich hydro-oxide layers [Cr(OH)₃ as a corrosion product] on the surface of the samples. Thus, the epoxy-coated Cr alloy steel has greater corrosion resistance in a chloride-containing environment than the carbon steel. Hence, epoxy-coated Cr alloy steel can be successfully used as a construction material in structures.     

Zinc modified polypyrrole coating on mild steel

Polypyrrole (PPy) films (∼ 1.7 μm thick) have been electrodeposited on mild steel (MS) substrates from 0.1 M pyrrole containing aqueous oxalic acid solution, by using cyclic voltammetry technique. Then, the polymer coatings were modified with deposition of zinc particles (∼ 1 mg/cm²), at a constant potential value of − 1.20 V in 0.2 M ZnSO₄ solution. The corrosion performance of zinc modified PPy coating has been investigated in 3.5% NaCl solution, by using electrochemical impedance spectroscopy and anodic polarisation curves. Also, the corrosion behaviours of zinc modified PPy coated platinum and single PPy coated MS samples have been investigated, for comparison. It was shown that zinc modified coating exhibited very low permeability and provided important cathodic protection to MS for considerably long immersion period. The voluminous zinc corrosion products are formed during exposure time in aggressive solution, giving rise to a blocking effect on the porous structure and led to effective barrier behaviour of zinc modified PPy coating, even after 96 h of exposure time to corrosive solution.     

Evolution of corrosion protection for sol-gel coatings doped with inorganic inhibitors

One the way to improve the self-repair properties of sol-gel films on aluminium alloy is the addition of corrosion inhibitors which could be released from the coating, minimising the corrosion of the unprotected area. Environmentally friendly corrosion inhibitors are studied (chromium III, molybdate, permanganate and cerium III) and compared to the standard corrosion inhibitor, i.e. hexavalent chromium.Corrosion performances are studied by polarisation resistance (Rp) in chloride medium. Evolution of composition of sol-gel coatings, during the corrosion test, is examined by GDOES (glow discharge optical emission spectroscopy).The results show that the morphology of sol-gel and the solubility of the additive play a role in the effectiveness of corrosion protection for a long term. Additives such as molybdate and permanganate ions decrease the sol-gel network stability and are too soluble (they are rapidly lost from the sol-gel films, in an aggressive medium), decreasing the power to prevent corrosion. Incorporation of Ce III is not efficient for a long time due to its high solubility. Sol-gel films containing Cr VI and Cr III provide adequate corrosion protection, due to the sol-gel stability and their low solubility.     

Corrosion properties of Fe-24Cr stainless alloy modified by plasma immersion ion implantation in 0.5 M sulfuric acid solution

Ruthenium (Ru) and mixed Ru-Ti ions were implanted respectively into ferritic Fe-24Cr alloy using the plasma immersion ion implantation (PI³) technique. Potentiodynamic and potential-time response measurements were used to characterize the corrosion behavior of Fe-24Cr alloy exposed to a deaerated sulfuric acid solution. The results showed that the ferritic alloy does not passivate in dilute sulfuric acid. However, plasma-implanted Ru ions can induce spontaneous passivation of the alloy exposed to a sulfuric acid solution. X-ray photoelectron spectroscopy (XPS) analysis showed that Ru is incorporated as Ru⁴⁺ species in the hydrated chromium oxyhydroxide passive film formed on the Ru-implanted Fe-24Cr alloy.