Chromate Conversion Research Articles

Carbon nanotube field emission cathodes fabricated with trivalent chromium conversion coated substrates

The properties of carbon nanotube (CNT) field emission cathodes fabricated by a dip coating method with trivalent chromium conversion coated substrates are studied. Two kinds of substrates with different morphologies, one with a rough crackled surface and the other with a smooth surface, were used for making the CNT cathodes, and their I– V curves and emission patterns were evaluated. The results show that, as compared to the smooth substrate surface, the rough surface with self-assembled sub-micro-cracks on the substrate can dramatically enhance the uniformity of the emission pattern and the emission efficiency. The cathode fabricated with the crackled substrate shows good field emission properties such as high brightness, good uniformity, a low turn-on field (0.86 V/μm) and a high current density of 10 mA/cm 2 at 2.5 V/μm.

Corrosion protection behavior of AZ31 magnesium alloy with cathodic electrophoretic coating pretreated by silane

As an alternative process to phosphate and chromate conversion coatings, silane pretreatment was used to improve the performance of cathodic electrophoretic coating (E-coat) on AZ31 Mg alloy in this study. The galvanic corrosion behavior of AZ31 Mg alloy with E-coat coupled with Q235 steel was investigated. Compared to bare Mg alloy and Mg alloy with conventional painting, the corrosion properties of the AZ31 Mg alloy pretreated with silane and subsequently E-coated were studied during salt solution immersion and salt spray testing. The surface morphologies of the Mg alloy were examined in detail after immersion in NaCl solution for different times using digital photography and scanning electron microscopy (SEM). The corrosion current density of the specimens was characterized by DC polarization tests. It was found that silane pretreatment of AZ31 Mg alloy followed by subsequent E-coat led to much better corrosion protection than that without silane treatment. The silane pretreatment and E-coat delayed the galvanic corrosion of Mg alloy coupled with 235 steel bolts.

Effect of refining grain size on the corrosion behavior of Cr(III) conversion layers on zinc coatings

Cr(III) conversion layers were synthesized on the nanocrystalline zinc with two grain sizes, 22 nm and 40 nm. Their corrosion behaviors were examined by potentiodynamic polarization and electrochemical impedance spectroscopy. The results shows that a more uniform and none-micro-crack conversion layer is formed on sample with grain size 22 nm compared with that formed on the sample with grain size 40 nm. Refining the grain size can markedly enhance the corrosion resistance of trivalent chromium conversion layer. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Rare earth conversion coating on Mg–8.5Li alloys

The conversion coating formed by immersion in a solution containing rare earth salt on Mg–8.5Li alloy was studied and the corrosion resistance was evaluated as well. The surface morphology was observed by scanning electron microscopy (SEM), and the chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). The corrosion behaviors of Mg–8.5Li alloy and conversion coating were assessed by means of potentiodynamic polarization curves, electrochemical impedance spectra (EIS) and immersion tests. The experimental results indicated that the coating with cracked morphology was homogeneous. It was mainly composed of La 2O 3, CeO 2, Mn 2O 3 and MnO 2 as detected by XPS. The results of electrochemical measurements and immersion tests revealed that the rare earth conversion coating possessed better corrosion resistance than bare alloy and chromate conversion coating.

A new method of potassium chromate production from chromite and KOH‐KNO3‐H2O binary submolten salt system

AbstractA new method of chromate production by applying a new reaction system of KOH‐KNO3‐H2O (binary submolten salt system) is proposed and proved feasible. Under conditions of temperature 350°C, KOH‐to‐chromite ore ratio 2:1, stirring speed 700 rpm, KNO3‐to‐chromite ore ratio 0.8:1, oxygen partial pressure 50%, and gas flow 1 L/min, chromium conversion ratio obtained is >98% with reaction time around 300 min. The decomposition of chromite ore in the system is a typical process of solid– liquid–gas reaction, which is coordinately controlled by mass diffusion in product layer and interface reaction. Apparent activation energy of decomposition in the temperature range from 280 to 370°C is 55.63 kJ/mol. During reaction, oxygen dissolves into KOH‐KNO3‐H2O melt system first and some cluster, e.g. O, is formed and the mass diffusion coefficient of the cluster was calculated. The system can be considered as both a media of oxygen transportation and reactant donator. Potassium nitrate plays a role of catalyst in the oxidation decomposition reaction of chromite ore and potassium hydroxide. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Trivalent chromium for enhanced corrosion protection on aluminum surfaces

Summary The trivalent chromium pretreatment formulated and invented by NAVAIR performs very well for replacing hexavalent chromium on aluminum. Its performance varies depending on the type of detergent and its concentration, temperature, and cycle time. Also important is the surface activation and the type of acid and/or deoxidizer that is used for the application. However, the most important factor is the type of aluminum alloy undergoing chromate conversion coating. It was concluded from the study that 2024 is the most difficult alloy due to its metallurgical content; if satisfactory results can be obtained for 2024, then it would not be so difficult to achieve successful results for other aluminum alloys. The EPA, when added to a standard trivalent chromium pretreatment (NAVAIR) bath, exhibited outstanding performance in NSS' testing. This additive is designed to enhance corrosion resistance of an aluminum alloy for trivalent chromium pretreatment performance. An EPA offers consistent corrosion resistance performance for difficult alloys, such as 2024. It is an additive that improves standard NAVAIR trivalent chromium application on aluminum for robust performance. It is sufficient to say that the most important aspect of using this newly developed, patented additive for trivalent chromium pretreatment its effective and consistent corrosion-resistance performance.

Characterization of cerium-based conversion coatings for corrosion protection of AISI-1010 commercial carbon steel

The role of hydrogen peroxide in the formation of cerium conversion coatings by immersing AISI 1010 commercial carbon steel substrates into solutions containing various concentrations of CeCl3 (0.1, 1, and 10 g L−1) has been investigated as an alternative method for their protection against corrosion. The deposits prepared from the solutions with H2O2 consist of yellow thin and non-uniform coatings with agglomerates of small CeO2 and Ce2O3 crystallites whose sizes increased over the metallic surface as the cerium concentration was increased. Cerium pre-treatments in the presence of H2O2 displayed layers that were rougher than those synthesized without H2O2. A comparison with the chromate conversion pre-treatment is also simultaneously carried out with the discussion of the possible reactions involved in the different stages of process. The coating obtained from the solution containing 0.1 g in 1,000 mL produced better corrosion resistance on the substrate than that observed for its counterparts due to the fact that the surface was more uniformly covered by the conversion coating. The addition of H2O2 to the cerate baths improves visible roughness, corrosion resistance of the conversion coatings and bond strength because hydrogen peroxide acts as an oxygen source during the formation of the coatings.

Chromate conversion coating on Al–0.2 wt.% Fe alloy

SEM and TEM investigations revealed that the chromate coating developed rapidly over the macroscopic alloy surface. The coating thickness increased over the immersion period employed in this study and was about 50 nm and 200 nm for coatings formed after 30 s and 120 s respectively. The coating is composed of chromium compounds with aluminium compounds probably concentrated at the alloy/coating interface.

Synthesis and Characterization of Chromate Conversion Coatings on GALVALUME and Galvanized Steel Substrates

The morphology, composition, and corrosion performance of chromate conversion coatings (CCCs) formed on GALVALUME (Fe-Al-Zn) and galvanized steel (Fe-Zn) samples have been studied, and different immersion times (0, 10, 30, and 60 seconds) have been compared. The coated surfaces were analyzed using light microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical measurements in a NaCl solution (3 wt pct). The electrochemical measurements were carried out using the polarization resistance, Tafel, and ac impedance methods. A nonuniform growth of the CCCs having a porous morphology and cracks that appear extended to the base metal was observed. The XRD patterns show that the coatings mainly consist of CrO3, Cr2O3, and traces of Cr2O 7 −2 . The electrochemical results show that GALVALUME presents a better behavior than that of the galvanized steel alloys at each dipping time. The SEM micrographs show that the galvanized steel treatments resulted in the formation of a more uniform film, but their protection barrier broke down faster than that of the GALVALUME samples in contact with the aggressive media. The samples that underwent the lowest degree of dissolution were those with a dipping time of 30 seconds. The difference in the corrosion protection given by the two substrate types could be attributed to the structural properties, grain size, composition, and roughness, which affect oxygen diffusion.

Barrier and self‐healing coating with fluoro‐organic compound for zinc

AbstractThe barrier and self‐healing abilities of a corrosion‐protective coating with a fluoro‐organic compound for zinc was investigated. Several types of fluoro‐organic compounds, having different terminal groups and various numbers of carbon atoms, were coated on a pure zinc plate by dip coating. A fluoro‐organic compound having the terminal group of COF showed excellent barrier resistance, which increased as the number of carbon atoms in the compound increased. The optimal conditions — concentration of fluoro‐organic compound in the solution and the pH of the solution — were determined for coating, based on those that demonstrated more ability than chromate conversion coating. Monitoring the corrosion resistance of a scratched specimen in corrosive solution and observing the surface appearance of the specimen after the corrosion test confirmed the self‐healing ability of the coating with the fluoro‐organic compound. The fluoro‐organic coating had excellent self‐healing ability — equivalent to that of a chromate conversion coating. The self‐healing effect of the coating appeared to be due to the release of the fluoro‐organic compound as a result of the pH increase caused by cathodic reaction in the corrosion process, and the formation of a film on the defect.

Trivalent chromium for enhanced corrosion protection on aluminum surfaces

The trivalent chromium pretreatment formulated and invented by NAVAIR performs very well for replacing hexavalent chromium on aluminum. Its performance varies depending on the type of detergent and its concentration, temperature, and cycle time. Also important is the surface activation and the type of acid and/or deoxidizer that is used for the application. However, the most important factor is the type of aluminum alloy undergoing Chromate conversion coating. It was concluded from the study that 2024 is the most difficult alloy due to its metallurgical content; if satisfactory results can be obtained for 2024, then it would not be so difficult to achieve successful results for other aluminum alloys. The EPA, when added to a standard trivalent chromium pretreatment (NAVAIR) bath, exhibited outstanding performance in NSS-testing. This additive is designed to enhance corrosion resistance of an aluminum alloy for trivalent chromium pretreatment performance. An EPA offers consistent corrosion resistance performance for difficult alloys, such as 2024. It is an additive that improves standard NAVAIR trivalent chromium application on aluminum for robust performance. It is sufficient to say that the most important aspect of using this newly developed, patented additive for trivalent chromium pretreatment its effective and consistent corrosionresistance performance.

Characterization of chromate conversion film on tinplate substrate by XPS and electrochemistry methods

AbstractThe chromate conversion coating formed on commercial tinplate via a cathode electrolytic dichromate treatment has been studied by X‐ray photoelectron spectroscopy (XPS) and electrochemistry methods. Through the analysis of the XPS, it was shown that there existed Cr, O, and Sn in the chromate coating and the chromate film consisted mainly of Cr(OH)3, Cr2O3, Sn, and SnOx. The current density decreased with increasing of the electric charge. The corrosion resistance for tinplate is relative with the content of chromium in the passivation film. Copyright © 2009 John Wiley & Sons, Ltd.

The protection of galvanised steel using a chromate‐free organic inhibitor

PurposeThe purpose of this paper is to investigate corrosion protection provided by a water‐soluble corrosion inhibitor as a non‐toxic alternative to the chromate and phosphate conversion coatings on galvanised steel.Design/methodology/approachUntreated galvanised steel samples were assessed simultaneously with galvanised steel samples treated with the chromate‐free organic inhibitor, a conventional chromate conversion coating or a zinc phosphate modified with nickel, by means of immersion – weigh loss and electrochemical tests, using a naturally aerated 0.5 M NaCl aqueous solution as corrosive medium. In addition, selected superficial conditions of galvanised steel were submitted to a salt fog test. The electrochemical tests used were: open circuit corrosion potential logging, linear polarization, potentiodynamic polarization and electrochemical impedance spectroscopy. Corrosion products formed on samples withdrawn from the solution at different intervals were characterized using X‐ray diffraction.FindingsAll tests gave concordant results, indicating that the chromate‐free inhibitor showed moderate protective properties in this electrolyte. Among the considered superficial conditions, the phosphate coating showed the most deficient performance. In all cases, it was observed that after moderately intense initial attack, the corrosion rate diminishes due to the formation and growth of insoluble corrosion product layers, which exhibit a passivating action.Research limitations/implicationsThe chromate‐free organic corrosion inhibitor protected galvanised steel in this environment but the degree of protection was less than that provided by the chromate conversion coating.Originality/valueThe paper presents an alternative to the toxic treatments with chromates, since the inhibitor works as an additional coating, sealing pores and other discontinuities found in the zinc coating.

Corrosion resistance of cerium conversion film electrodeposited on Mg-Gd-Y-Zr magnesium alloy

The cerium conversion film was applied to improving the corrosion resistance of Mg-Gd-Y-Zr magnesium alloy. The film was electrodeposited on the surface of the Mg-RE alloy in cerium nitrate solution. The compositions and morphologies were analyzed by X-ray diffraction(XRD), scanning election microscopy (SEM). The corrosion behaviors of the film were investigated electrochemical impedance spectroscopy (EIS), potentiodynamic polarization tests and immersion tests. The results show that the optimum parameters for electrochemical deposition are as follows: pH 10.0, time 30 min, 50 mmol/L Na2CO3 and temperature 25 °C by the designed experiments according to the orthogonal table L(9, 34). The corrosion protection efficiency is dependent on the deposition parameters. The cerium conversion film shows better corrosion protection behavior than chromate conversion film on Mg-Gd-Y-Zr magnesium alloy.

Application of a factorial design to study a chromate-conversion process

Chromate conversion coatings are commonly deposited on plated zinc to improve the corrosion resistance of the substrates, and to increase the adherence of painted films. The coatings’ properties depend on the composition and the structure of the conversion films as well as on the process parameters. A fractional factorial design was implemented to optimize the experimental conditions of the chromating process (bath composition, rinses composition, draining, treatment time). Surface analytical techniques such as scanning electron microscopy, glow discharge optical spectroscopy, and atomic absorption spectrometry were used to characterize the chromate conversion coatings.

Analysis of Valence for Chromate Conversion Coatings Using Laboratory XAFS Spectrometer and a Comparison with X-ray Photoelectron Spectroscopy

We performed a nondestructive and in laboratory evaluation technique of hexavalent chromium (Cr(VI)) in chromate conversion coatings using a laboratory X-ray absorption fine structure (XAFS) spectrometer equipped with W anode X-ray tube. The recorded XANES spectrum of a chromate conversion coating sample with fluorescence mode possessed the Cr–K X-ray absorption edge. We have obtained calibration curves of the ratio of Cr(VI)/Cr(III) by the height of pre-edge peak and the Cr–K X-ray absorption edge energy of the reference samples prepared by Cr2O3 (Cr(III)) and CrO3 (Cr(VI)) particle reagents. The calibration curves showed the high correlation coefficients such as 1.00 for the height of pre-edge peak and 0.99 for the value of the energy on the Cr–K X-ray absorption edge. The ratio of Cr(VI)/Cr(III) in the chromate conversion coating has also been evaluated by the X-ray photoelectron spectroscopy (XPS) and 1,5-diphenyl-carbazide absorption spectrometry (DPC) methods. The ratio of Cr(VI)/Cr(III) evaluated by the laboratory XAFS spectrometer was almost identical to that by XPS method.

Electrochemical behaviour of ceramic yttria stabilized zirconia on carbon steel synthesized via sol–gel process

Chromate conversion coatings have been widely applied for the corrosion of different metallic substrates. However, the waste containing Cr 6+ has many limitations due to the environmental consideration and health hazards. An interesting alternative seems to be the deposition on metallic surface of thin layers of yttria or zirconia or both by the sol–gel process. In this study, Ytttria and Yttria stabilized zirconia (YSZ, 8% Y 2O 3) thin films were used for coating commercial carbon steel substrates by sol–gel method and the dip-coating process. The evolution of organic compounds up to crystallization process as a function of heat treatments was study by FT-IR spectroscopy. The structure and morphology of the coatings were analysed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The anticorrosion performance of the coatings has been evaluated by using electrochemical techniques in an aggressive media (3.5 wt.% NaCl). The corrosion behaviour of sol–gel method was compared with traditional chromate conversion coatings. Differences in the electrochemical behaviour of YSZ coatings are related to the development of microcracks during the sintering process and to the presence of organic compounds during growth film. Electrochemical results showed that sol–gel YSZ and Y 2O 3 coatings can act as protective barriers against wet corrosion; however yttria films displayed low adhesion to substrate. The corrosion parameters provide an explanation of the role of each film and show a considerable increase in the corrosion resistance for coated samples in comparison to the bare steel samples.

The effect of glow discharge sputtering on the analysis of metal oxide films

The potential of radiofrequency glow discharge optical emission spectrometry (rf-GD-OES) for the quantification and the solid-state speciation of metal oxide films has been investigated in this work. Two types of oxide coatings, an iron oxide film deposited on silicon and a chromate conversion coating (CCC), were studied at 700 Pa of pressure and 30 W of forward power. The metal to oxygen ratios in the quantitative depth profiles (Fe/O and Cr/O, respectively) were used to evaluate the oxidation states of iron and chromium in the oxide films, demonstrating the capability of GD-OES technique for depth-resolved solid-state speciation. Furthermore, the effect of glow discharge sputtering on the samples surface in terms of modifications in the surface morphology and species transformations, were investigated by using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The iron and chromium oxidation states were carefully studied by XPS at the original samples surface and at the bottom of GD craters, and a systematic reduction of metal elements was observed after rf-GD-OES analysis. In the case of thin oxide films, preferential sputtering can be considered as a critical factor since oxygen atoms can be preferentially sputtered, leaving a metal-enriched surface and, therefore, promoting the reduction of metal elements. In the present study preferential sputtering was found to be sample dependent, changing the proportion of the metal reduction in the oxide film with its composition. Additionally, alternative sputter-depth-profiling techniques such as secondary ion mass spectrometry (SIMS), femtosecond laser ablation (fs-LA), and XPS ion gun were used for the analysis of the CCC in order to evaluate the reduction of Cr 6+ to Cr 3+ depending on the sputtering mechanism.

Morphology and Corrosion Performance of Chromate Conversion Coatings on Different Substrates

The aqueous corrosion resistances of galvalume (Fe- Al+Zn) and steel galvanized (Fe-Zn) alloys, passivated by immersion in chromating solution with different immersion times (0, 10, 30 y 60 s) are compared. The electrochemical behaviour of various chro- mated and non-chromated coatings was investigated in 3 wt-% NaCl solution using polarization techniques. For comparison, AISI 1010 carbon steel substrates were also analyzed. The characterization mea- surements and polarization curves revealed that textural properties of Fe alloys influence the growth of the films and both the cathodic and anodic reactions. An optimum anticorrosive protection was obtained at 30 s. CCCs treatments inhibited the corrosion of zinc to a significant degree, but less effectively than coatings formed with alu- minium-zinc alloys. The difference in the corrosion protection given by the two substrates types was attributed to the structural properties, thickness coating, grain size and roughness observed, which affect oxygen diffusion.

Predicting the Long-term Performance of Coating Systems from Short-term EIS Testing

Barrier property measurements of organic coatings can take long times to assess, can vary significantly in magnitude as a function of sample position, and can change unexpectedly over time. This study uses short-term EIS testing to re-examines the role of the coating-substrate interface in governing long-term coating performance through the use of short-term EIS testing. 32 coating systems comprised of various combinations of chromate and non-chromate conversion coatings, chromate and non-chromated primers, and topcoats on AA2024-T3 substrates were exposed to 0.1M (NH4)2SO4 + 0.005M NaCl for 8 days. A through film defect was made with a diamond-tipped scribe on the 4th day of immersion. Another group panels with these 32 coating systems was exposed to ASTM B117 salt spray for 3000 hours. In a blind comparison, it was determined that the EIS data acquired after the introduction of the defect was essential in the prediction of 3000 h salt spray results. These results show the importance of interfacial information in the prediction of field performance, and provide a promising short-term method for the prediction of long-term performance.