Chromate Conversion Research Articles

Effect of Trivalent Chromium Based Treatment on the Protective Properties of Steel Coated with Polymeric Film

Three main requirements are mandatory in order to achieve optimal quality in canned food: adequate processing of the product, definition of the preserving method, and a correct alloy for the deposit. Lacquer coatings are applied on food cans to enhance corrosion protection and also with decorative purposes. In this investigation, steel coated with a Cr3+ based treatment layer and polyethylene teraphtalate polymeric film (PET), applied by co- extrusion, was characterized by electrochemical impedance spectroscopy (EIS) measurements, in NaCl 0.5 M solution, to evaluate the coating corrosion resistance in this environment. The experimental results showed that this new material (tin free) has good corrosion resistance regardless of the porosity found in the polymeric film, indicating that it can be a non-polluting alternative to replace tinplate and hexavalent chromium conversion layer in the food industry, being compatible with the packed product.

The hazardous hexavalent chromium formed on trivalent chromium conversion coating: The origin, influence factors and control measures

In this paper, the effects of processing parameters and constituents of treating-agent on the presence of hazardous hexavalent chromium on trivalent chromium conversion coating were studied. Results showed that shorter immersion time, lower bath pH value as well as lower working and baking temperatures retarded the presence of hexavalent chromium. In addition, the concentration of hexavalent chromium on conversion coatings prepared by the oxalic acid treating-agent was far greater than those on conversion coatings prepared by formic acid and acetic acid treating-agents. Results also indicated that the concentration of hexavalent chromium on conversion coatings was enhanced due to the addition of bivalent cobalt and nitrate anion in treating-agent, especially for oxalic acid conversion coating. However, the addition of hydroxyl compound d-gluconic acid in treating-agent could reduce the concentration of hexavalent chromium effectively. Moreover, a possible formation mechanism of hexavalent chromium on trivalent conversion coating was proposed. Findings of this study provide a better understanding of the formation of hexavalent chromium on trivalent chromium conversion coating and can facilitate the management of trivalent chromium treating-agents and trivalent chromium fasteners.

On conversion coating treatments to replace chromating for Al alloys: Recent developments and possible future directions

Anticorrosive protection of Al alloys still depends heavily on the use of chromates, which are widely and universally employed as chromate conversion coating and chromic acid anodising pretreatments. The replacement of chromate based treatments with more environmentally compliant processes and materials has been identified as a high priority. The aim of this paper is to review the most recent developments in the application of common inorganic protection layers based on conversion coatings for Al alloys. The review lists and discusses the majority of conversion coatings, including those formed through anodisation, on Al alloys as potential replacements to the most widely used treatments based on chromate chemistry.

The Formation Process of Black Trivalent Chromium Conversion Coating on Zn Electrodeposit

A black trivalent chromium conversion coating (CCC) was obtained on Zn deposit in chromium trichloride solution at 55 °C. The formation process of trivalent CCC on Zn deposit were thoroughly investigated by using X-ray fluorescence spectrometer and X-ray photoelectron spectroscopy in combination with atomic force microscopy surface analysis. The trivalent CCC on Zn deposit is consisted of Zn, Cr, O, Ni, Co, P and N elements, with visible micro-cracks on the film’s surface. The growth rate, the surface color and the morphology of this conversion coating vary with the dipping time.

Characterization of trivalent chromium process coating on AA2024-T3

Trivalent Chromium Process (TCP) coatings, which have been used recently as an alternative to chromate conversion coatings (CCCs), have been characterized. The TCP coating was found to be a dense layer consisting of particles, hundreds of nm in size on the coating surface, which is similar to CCCs. The TCP coating has a two layered structure, with zirconium-chromium mixed oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating interface. No Cr(VI) was found, which supports its use as an environmentally friendly replacement for CCC. The TCP thickness was in the range of 40–120nm, considerably thicker than the zirconium based coating without chromium species. The high vacuum condition in the SEM dehydrates the coating quickly resulting in the formation of mud-crack artifacts. The TCP coating provides corrosion protection to the AA2024-T3 through suppressing the oxygen reduction reaction on aluminum alloy surfaces by acting as a protective barrier layer. Two pretreatments were investigated but found to result in little difference in the resulting TCP formed on AA2024-T3.

ZrO2 sol–gel pre-treatments doped with cerium nitrate for the corrosion protection of AA6060

Sol–gel coatings represent an alternative corrosion protection method to the chromate based systems which must be replaced. Recently, it was shown that ZrO2 based sol–gel coatings deposited on AA6060 can provide a good corrosion protection to AA6060. However, ZrO2 based sol–gel systems are not able to provide the self-healing effect which is instead the peculiar property of chromate conversion coatings. The structure of the ZrO2 based sol–gel systems does not contain species able to restore the barrier properties when defects or damages impair the coating protection.In this work, ZrO2 based sol–gel coatings containing cerium nitrate were deposited on AA6060 aluminum alloy in order to evaluate the corrosion inhibition provided by the incorporated cerium ions. The cerium nitrate was added to the starting sol based on zirconium alkoxide precursors. Three types of samples were produced: a non inhibited type consisting of 3 layers of ZrO2, an inhibited system consisting of two layers of ZrO2 with an intermediate layer doped with cerium nitrate and an inhibited system consisting of two layers containing cerium nitrate with a top layer of ZrO2.

The determination of Cr(vi) in corrosion protection coatings by speciated isotope dilution ICP-MS

Chromium conversion coatings are used as decorative finishes and to improve the corrosion protection and strengthen the wear resistance of metallic surfaces. Chromium electroplating frequently involves the use of hexavalent chromium (Cr). To reduce environmental impacts, several EU directives restricted its use to threshold values of 0.1% Cr(VI) by weight per homogeneous material in vehicles, and 1000 mg kg−1 of Cr(VI) in electronic and electrical equipment. There are few analytical procedures reported that comply with legislative demands, therefore, in our work a selective, quantitative and sensitive analytical procedure for determination of Cr(VI) in the corrosion protection coatings was developed. The investigation was performed on metallic plates homogeneously treated by chromium conversion or hard chrome coatings on copper or zinc electroplated steel surfaces. An alkaline solution (pH 12) was used for ultrasonic extraction of Cr(VI). Speciation analysis was performed by anion-exchange HPLC-ICP-MS. Species interconversions during the analytical procedure were followed using enriched isotopic solutions of 50Cr(VI) and 53Cr(III). To prevent Cr(III) oxidation, Tris, EDTA or MgCl2 was added along with a double isotopically enriched spike and extractions were performed over different time periods. Under optimal conditions that prevent any species interconversion (30 min ultrasonic extraction at 70 °C using 2% NaOH + 3% Na2CO3 + MgCl2 as an extraction agent), six consecutive extractions were necessary to quantitatively extract Cr(VI) from the protective layers; its content was determined by speciated isotope dilution ICP-MS. Concentrations in the samples investigated ranged from 2 to 7 ng of Cr(VI) per mm2. The accuracy of the HPLC-ICP-MS determinations was checked by analysis of the certified reference material CRM 545, Cr(VI) in welding dust, using both external calibration and speciated ID-ICP-MS. Good agreement was obtained between the determined and the reported certified values (±0.7% for external calibration and ±0.2% for speciated ID-ICP-MS). The high sensitivity of the procedure developed (LOQ 0.0107 ng Cr(VI) per mm2 on a surface of 250 mm2) and the possibility to use external calibration for quantification of separated Cr(VI) instead of ID-ICP-MS allows its application to be extended to the routine laboratory use.

Transient Formation of Chromate in Trivalent Chromium Process (TCP) Coatings on AA2024 as Probed by Raman Spectroscopy

We report evidence for the transient formation of Cr(VI) oxide species in the Trivalent Chrome Process (TCP) coating on AA2024-T3 alloys. TCP is the leading replacement coating for the environmentally-unfriendly chromate conversion coating (CCC) that is commonly used to pretreat aerospace aluminum alloys. Transient formation of hexavalent chrome in these coatings is important to learn about as the presence of this species will affect the active level of corrosion protection as well as the environmental impact of the coating. Specifically, we tested the hypothesis that dissolved O2 is reduced to H2O2 at the Cu-rich intermetallic sites and that H2O2 diffuses to nearby sites in the coating to oxidize native Cr(III) oxide (Cr(OH)3 or Cr2O3) to Cr(VI) oxide (CrO42−). Raman microprobe spectroscopy was used to spatially distinguish between the two species based on the different vibrational modes of Cr(III)-O (520–580 cm−1) and Cr(VI)-O (840–904 cm−1). Results are presented that support a mechanism whereby the formation of Cr(VI) oxide species is linked to the production of H2O2 locally within the coating. The H2O2 is presumed to arise from the 2e− reduction of dissolved O2 at Cu-rich intermetallic sites. A variety of Cr(VI) oxides and mixed oxides are formed in the coating during full immersion testing in air-saturated electrolyte solutions or aging in air.

Determination of chromium(III), chromium(VI) and total chromium in chromate and trivalent chromium conversion coatings by electrospray ionization mass spectrometry

Developed were the determination methods of CrVI and CrIII by electrospray ionization mass spectrometry, where HCrO4− (m/z 117) and [CrIII(cydta)]− (m/z 394) were measured, respectively. Moreover, total Cr was also determined by measuring [CrIII(cydta)]− after reduction of CrVI with ascorbic acid. Here, cydta denotes trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid. The detection limits were ca. 13nmol/dm3 for CrVI and 56nmol/dm3 for CrIII and total Cr. The proposed methods were applied to analyze chromate and trivalent chromium conversion coatings and gave consistent results with those obtained by a diphenylcarbazide spectrophotometric method for CrVI and ICP-AES for total Cr.

The formation of neodymium conversion coating and the influence of post-treatment

In this paper, neodymium-based conversion coating is used as a substitute for toxic chromate conversion coating on AZ91D magnesium alloys. Its formation and growth were observed via SEM, EDS, XPS, electrochemical tests and weighting experiment. The influence of post-treatment on neodymium conversion coating was measured by FTIR and electrochemical experiments in terms of morphology, component, surface functional group and corrosion resistance. The dissolution of matrix and the deposition of neodymium/magnesium oxides compete with each other in initial time. Then the deposition of neodymium oxides dominates the process. Compact coating is obtained after 20min immersion and it is mainly made of neodymium oxides and a small amount of magnesium oxides/hydroxides. The coating post-treated is rich in OH− and PO43+. The post-treatment can improve the corrosion resistance of the neodymium conversion coating effectually examined by EIS.

Evaluation of Corrosion Behavior of Galvanized Steel Treated with Conventional Conversion Coatings and a Chromate-Free Organic Inhibitor

Conventional weight loss tests and both DC and AC electrochemical techniques were used to study if an organic inhibitor containing an alkanolamine salt of a polycarboxylic acid can substitute toxic coatings as chromating and certain phosphating procedures in the protection of galvanized steel. The electrolyte used was a 0.5 M aerated NaCl solution. All tests gave concordant results, indicating that the chromate-free organic inhibitor does protect galvanized steel in this environment, even though the provided protection was less than that of the chromate conversion coating. It was observed that, after a moderate initial attack, the corrosion rate diminishes due to the appearance and growth of passivating corrosion products layers, mainly constituted by zinc hydroxychloride and two varieties of zinc hydroxide, among other crystalline compounds.

Synergistic effect of potassium metal siliconate on silicate conversion coating for corrosion protection of galvanized steel

Silicate conversion coating is considered as an alternative to hexavalent chromium conversion coating for corrosion protection of galvanized steels. However, the coating must be treated with hot air afterward in order to obtain a water resistant silicate layer and to increase corrosion resistance of the coating. Moreover, it is difficult to apply the uniform thermal treatments on the large irregular shaped metal parts. Furthermore, the heat treatment is an energy consuming process. In this work, the effects of adding potassium methyl siliconate (PMS) into the silicate conversion coating on the anti-corrosion properties were studied. The silicate coating layers were formed by the simple immersion into the silicate solutions and the subsequent ambient drying at room temperature. It was verified that the addition of PMS increased the temporary corrosion protection ability without the post-thermal treatments. Coating thickness was around 100nm and K ion was distributed preferentially at zinc-silicate interface. It was proved that the more hydrophobic surface was formed mainly due to the substantial decrease of the concentration of K ion at the coating surface by the addition of PMS. It was a methyl group of PMS that changed the hydrophilic silicate layers into hydrophobic ones. Subsequently, the enhanced water barrier property of the hydrophobic silicate layers increased the corrosion resistance of the silicate-coated galvanized steel.

Sol-Gel Films Prepared with Pure Alumina Sol by Re-Dissolution on Aluminum Alloy

The development of non-hexavalent chromium conversion coating on aluminum alloy is imminent, hexavalent chromium Cr6+ compounds have serious threat to the environment. The appearances, viscosity, particle size of aluminum nitrate sol prepared by re-dissolving were tested by orthogonal experiment. The results show that a better condition for preparing sol by re-dissolving the aluminum nitrate sol is: [Al3+] of 0.3mol/L, sol at 55~85°C or 0.6mol/L, at 55°C. The pH value ranges 7~9 before re-dissolving. The salt spraying resistance for the sol-gel film prepared by sol of which viscosity ranges 4.8 ~ 6 mPa/s is better than five grade. The dry adhesion for paint film is qualified when viscosity ≤ 5.6 mPa/s and the average size of particles <920 nm for sol. In terms of sol appearance, dry adhesion and salt spray properties of the sol-gel film made by blue and white or gray sol are good. [Al3+] concentration and sol temperature for preparing sol-gel film with better corrosion resistance are equally important, secondly is pH before re-dissolved. Dry adhesion between paint film and aluminum alloy is qualified when [Al3+] ranges 0.3 ~0.6mol/L. Corrosion resistance for alumina sol-gel film of [Al3+] concentration of 0.3 and 0.77mol/L is worse than that of 0.6mol/L. The times of spray coating has no influence on corrosion resistance, while the increased frequency of coating times, and film adhesion decrease. In short, if you want that painting film adhesion and corrosion resistance are both good, Al3+ concentration should be 0.6mol/L for pure aluminum sol. The preferable prepared conditions are that aging at 55 °C, pH of sol re-dissolved with nitric acid is 4 after adding ammonia until pH reaches 8, coated one times, two or more times, the adhesion of the coating will reduce.

Molybdate and molybdate/permanganate conversion coatings on Mg–8.5Li alloy

A novel environment-friendly conversion coating for Mg–8.5Li alloy was obtained by immersing in a solution of molybdate. The concentration of ammonium molybdate and the addition of potassium permanganate were discussed in this experiment. The surface morphology of the conversion coatings was observed by scanning electron microscopy (SEM), and the chemical composition was investigated by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The corrosion resistance of Mg–8.5Li alloy and conversion coatings were investigated by means of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and weight loss measurement. The results showed that the coatings with cracked morphology were homogeneous and uniform. The conversion coatings were mainly composed of metal-oxide as detected by XPS. The results of electrochemical measurement and weight loss measurement revealed that the molybdate conversion coating had better corrosion resistance than bare alloy and chromate conversion coating, and the molybdate/permanganate conversion coating had lower corrosion current density and higher coating resistance than the molybdate conversion coating.

Simple preparation routes for corrosion protection hybrid sol‐gel coatings on AA 2024

AbstractIn recent years, many hybrid inorganic‐organic systems have been proposed in order to replace the traditional conversion coatings on metals like aluminum, and some results have been promising. However, many proposed solutions are based on complicated processes which are not easy to be adapted to industrial scale. The aim of this study was to establish a simple process leading to the production of highly efficient corrosion protective hybrid sol‐gel coating systems for the aluminum alloys as replacement for the highly hazardous conventional chromate conversion coatings. Hybrid coatings have been realized by means of the sol‐gel process. CeO2 and ZnO have been introduced as dispersions of nanoparticles in the system and used as corrosion inhibitors. The aim of this work was to obtain pore‐free coatings with increased barrier properties using nanoparticles that possess the double function of pore fillers and corrosion inhibitors. The proposed processes led to coating materials with good adherence to the aluminum substrate and an extremely long life in the accelerated neutral salt spray test according to DIN ISO 9227. Electrochemical impedance spectroscopy approves these results by high impedance values in the low‐frequency region of the Bode plot. Copyright © 2011 John Wiley & Sons, Ltd.

Effect of Phase on the Electrochemical and Morphological Properties of Praseodymium-Based Coatings

The corrosion protection of epoxy-polyamide primers containing either hexagonal Pr2O3 or cubic Pr6O11 inhibitors deposited on chromate conversion coated Al 2024-T3 substrates was studied. Electrochemical evaluations, scanning electron microscopy with energy dispersive spectroscopy, and x-ray diffraction were used to characterize primers. The electrochemical response of coated panels prior to and following ASTM B117 salt spray exposure was also studied. The addition of PrOx species to the primer matrix improved the corrosion protection of artificial defects by decreasing icorr values from ~10-7 to ~10-9 A/cm2, increasing the passivation range (> 300 mV), and shifting Ecorr values to more negative potentials. After salt spray testing, Pr-hydroxide/hydroxycarbonate species were present on the surfaces of the primers. The addition of Pr2O3 afforded enhanced corrosion protection compared to the addition of Pr6O11 because Pr2O3 is a more reactive phase that provided more Pr3+ species for transport and reaction than Pr6O11.

Self healing ability of silicate conversion coatings on hot dip galvanized steels

The self-healing ability of a coating is an important aspect for evaluating its corrosion resistance. Chromate conversion coatings show excellent anti-corrosion properties because of their compact microstructure and their strong self-healing ability. Silicate conversion coatings, which are potential replacement candidates for chromate conversion coatings, have good corrosion resistance, but their self-healing abilities have not been determined. In this paper, silicate conversion coatings are prepared by immersing hot dip galvanized steel sheets in sodium silicate solutions with SiO2:Na2O molar ratio of 1.00 and 3.50. The coatings are scratched with a blunt knife edge and corroded in a neutral salt spray chamber for certain lengths of time. The corrosion products in and near the scratched area are investigated by scanning electron microscopy and energy-dispersive spectroscopy. The self-healing ability of silicate conversion coatings is discussed. The results show that silicate conversion coatings have self-healing abilities under the experimental conditions. Higher SiO2:Na2O molar ratios lead to improved self-healing abilities. During the corrosion process, the silicate anions in the coating migrate to the scratched area, where a new conversion coating composed of Zn, O, and Si is formed. This delays corrosion in the scratched area.

Nanostructured surface pre-treatment based on self-assembled molecules for corrosion protection of Alclad 7475-T761 aluminum alloy

Coatings based on self-assembled molecules (SAMs) for corrosion protection of aircraft aluminum alloys have been studied to evaluate their potentialities as replacements to yellow chromate conversion coatings (CCC), due to the toxicity of these leading to environmental problems. In this work, the influence of alkane diphosphonates self-assembling molecules on the corrosion resistance of the AA7475-T761 cladded with AA7072 was investigated by electrochemical impedance spectroscopy and cathodic and anodic polarization curves, in naturally aerated 0.5 mol/L Na2SO4 aqueous solution, with pH adjusted to 4. Corrosion accelerated experiments (salt spray tests) were also carried out to examine the resistance of the SAM treated samples against corrosion. The results suggested that the development of boehmite (aluminum oxide) layer with incorporation of SAM was beneficial to the corrosion resistance of the tested aluminum alloy. Samples surface treated with SAM or aluminum oxide/SAM (without and with subsequent polyester layer) showed better corrosion resistance results than samples with CCC, indicating that this last type of coating containing hexavalent chromium could be replaced by the environmentally friendly pre-treatment corresponding to boehmite growth followed by incorporation of SAM.

Development and industrial scale-up of ZrO 2 coatings and hybrid organic–inorganic coatings used as pre-treatments before painting aluminium alloys

Development of Cr-free coatings is an important issue for replacement of chromate conversion coatings in aerospace and automotive industry. This aspect was targeted in the framework of MULTIPROTECT IP project funded by the European Community. This paper is an overview of research activities carried out by our groups in the framework of the MULTIPROTECT IP project.Our research approach in the project considered the development of thin ZrO 2 coatings (100–200 nm) by means of sol–gel technology for AA2024 aluminium alloy. ZrO 2 films were deposited employing solutions of Zr precursors. Coating application was carried out by dipping and spraying technology at a laboratory scale. In addition, substrates pre-treated with ZrO 2 films were painted with an organic primer and top coat in order to evaluate paint adhesion and electrochemical behaviour of painted systems. Corrosion inhibitors were introduced in the ZrO 2 coatings in order to improve the corrosion behaviour of the sol–gel films. In parallel, hybrid organic–inorganic methacrylate silane-based films containing SiO 2 nano-particles doped with cerium nitrate were developed in the framework of the project. At a later stage of the project, coating deposition was carried out with a robot-controlled air pressure gun in spraying cabins in order to evaluate industrial production of the ZrO 2 films and hybrid organic–inorganic films. The results obtained in the development phase clearly indicate that it is possible to deposit ZrO 2 coatings and hybrid organic–inorganic films on the substrate by dipping technique. These thin pre-treatments provide good barrier properties to the substrate and promote adhesion of an organic primer and top-coat. Cerium nitrate introduced in hybrid organic–inorganic films evidences an inhibition effect on corrosion processes, which can be attributed to cerium compounds. The up-scaling phase proves that it is possible to produce water-based ZrO 2 pre-treatments and hybrid organic–inorganic films by spraying technology using industrial equipment.

An On-Site Colorimetric Technique for Routine Determination of Chromium, Iron and Copper in Bath Solutions for Chromium(III) Conversion Coating

An on-site colorimetric technique was developed for the routine determination of chromium, iron and copper in bath solutions for the chromium(III) conversion coating. A portable colorimeter with a red-green-blue light emitting diode was used for the absorbance measurements. Iron and copper were determined as Fe(III)-thiocyanate and Cu(I)-bathocuproindisulfonate, respectively. Chromium(III) was determined simultaneously with iron or copper using green and blue light. A correction method of the matrix effect was proposed and its applicability was demonstrated. Analytical errors were within 500, 5 and 0.3 mg L(-1) for chromium(III), iron and copper, respectively.