Chromate Conversion Research Articles

Surface oxide in replicated microcellular aluminium and its influence on the plasticity size effect

The oxide film present along the interface between metal and pores in microcellular aluminium produced by replication depends on the leaching medium that is used to dissolve the NaCl preform: leaching in water produces a thick and irregular multilayered hydroxide layer, whereas leaching in a chromate conversion solution produces a smooth layer of oxide, ≈10 nm thick. The pore-size-dependent flow stress of replicated microcellular aluminium exhibits a marked dependence on the leaching process used to produce the foam when the pore size is below roughly 100 μm. This dependence on the leaching medium is a result of the influence exerted by the outer surface oxide layer on dislocational glide in micron-sized metal struts making the foam.

O.043 The need of a programmable gravitational valve for hydrocephalus treatment

Electrochemical Impedance Spectroscopic technique (EIS) and Local Electrochemical Impedance Spectroscopic technique (LEIS) have been used to investigate the corrosion behavior of zinc–nickel + chromate conversion coating on carbon steel. The measurements were conducted in chloride media, 0.6 M and 10 mM NaCl solutions with coating with and without artificial defect. An electrical equivalent circuit was proposed to analyze the EIS spectra in 0.6 M NaCl solution. LEIS was also performed in the vicinity of a scratch exposing the carbon steel to the corrosive solution. It was shown that the local impedance measurement was very useful to characterize in situ the electrochemical reactivity around a defect.

Cerium–permanganate conversion coatings for a Zn–Co alloy

The present work aims at investigation of the corrosion resistance of cerium–permanganate conversion coatings (CPCC) electrolessly deposited on a Zn–Co (0.6%) alloy. For comparison, Zn–Co (0.6%) alloy with a commercial chromate treatment was also examined. CPCCs on a Zn–Co (0.6%) alloy have been studied by potentiodynamic polarization and the immersion test. The experimental results of potentiodynamic polarization and the immersion test showed that the corrosion resistance of CPCC on a Zn–Co (0.6%) alloy is close to the corrosion resistance of chromate conversion coating on Zn–Co (0.6%). The microstructure of conversion coatings was analysed by scanning electron microscopy (SEM) and the composition of conversion coatings was analysed by X-ray photoelectron spectroscopy (XPS). It has been found that the cerium–permanganate conversion coating is homogeneous as compared with the chromate coating. XPS has determined that the CPCC contains the following compounds: CeO 2(IV), Ce 2O 3(III) and Mn oxides and hydroxides.

An organic chromium-free conversion coating on AZ91D magnesium alloy

Traditional conversion coatings on magnesium alloys are usually immersed in a solution containing hexavalent chromium compounds. However, the replacement treatments have been proposed by the present environmental driving to eliminate hexavalent chromium. In this work, a tannic acid based conversion coating on AZ91D magnesium alloy was obtained by treatment in a solution containing tannic acid and ammonium metavanadate. SEM, XPS and IR were used to determine the morphology and structure of the conversion coatings. Continuous and uniform conversion coating was deposited on AZ91D alloy and the main components of the coatings were Al 2O 3, MgF 2 and penta-hydroxy benzamide–magnesium complex. The formation mechanism of the coating was discussed. Polarization measurement and salt spray test showed that the corrosion resistance of the conversion coating was much higher than that of traditional chromate conversion coating.

Selective iron removal from spent passivation baths by ion exchange

AbstractBACKGROUND: Post‐treatment of zinc plated surfaces is required to provide corrosion protection; for this purpose chromium passivation is usually used. In the EU, the older hexavalent chromium passivation baths have been replaced by trivalent chromium conversion coatings. However, the life of Cr(III) passivation baths is reduced due to iron and zinc impurities generated in the process. Thus, the objective of this work was to investigate the regeneration of spent passivation baths from the plating industry by ion exchange using a new chelating resin (Purolite S‐957).RESULTS: Three genuine passivation baths with different iron and zinc concentrations were investigated; the effect of loading flow rate, temperature and resin regeneration conditions on the uptake of iron ions was also studied. The removal performance of iron was increased at higher temperature (55 °C) and lower flow rates. The best regeneration conditions were found to be 6 bed volumes (BV) of 30 wt% HCl.CONCLUSION: This work has shown that iron can be selectively removed from trivalent chromium passivation baths using a new chelating resin (Purolite S‐957) and no addition of chromium (III) is required. Low feed flow rate and high bath temperature (up to 55 °C) are recommended for high iron uptake. Copyright © 2008 Society of Chemical Industry

Corrosion protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na 2SO 4 electrolyte, and accelerated atmospheric corrosion tests (humid, SO 2 polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce–Zr–Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce–Zr–Nb-treated samples, with or without an additional layer of epoxy–polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO 2, Ce 2O 3, ZrO 2, Nb 2O 5, MgO, and MgF 2 as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na 2SO 4. This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.

Using ToF-SIMS and EIS to evaluate green pretreatment reagent: Corrosion protection of aluminum alloy by silica/zirconium/cerium hybrid coating

Increasing environmental concern has led to the restrictive use of chromate conversion coatings to protect Al-alloys from corrosion. Our research is under way to find environmentally compliant substitute coating such as Si/Zr/Ce hybrid coating. The corrosion protection effect of green pretreatment reagent consisted of Si-containing base solution, Ce- and Zr-containing sealing solutions on the corrosion protection of Al-alloys was studied with a 3.5% NaCl aqueous testing solution. The correlation between the corrosion resistance measured by electrochemical impedance spectroscopy (EIS) and surface chemical composition of the hybrid coating measured by time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was studied. The proposed green pretreatment reagent was found improve the corrosion protection of Al-alloys, presumably due to the formation of protective oxide film acting as an oxygen barrier.

Characterization of Hexavalent and Trivalent Chromium Conversion Coatings on Electrogalvanized Steel

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Corrosion Protection of Alternative Coatings on High-Strength 4130 Steel

The electrochemical behavior of high-strength 4130 steel with alternative coating systems as well as current cadmium plating and chromate conversion coatings have been investigated using anodic and cathodic polarization, electrochemical impedance and ion chromatography techniques. During anodic polarization, Cd-chromate conversion coated, aluminum-chrome, SermeTel T, and Alseal basecoat exhibited sacrificial coating protection ability while ion vapor deposited-aluminum, SermeTel W, SermeTel W basecoat and SermeTel T basecoat exhibited barrier coating protection. The cathodic polarization data shows that most of the coatings tested exhibit a cathodic corrosion resistance except for the Alseal basecoat. Results of the electrochemical impedance measurements indicate that SermeTel W exhibited barrier protection properties while the remaining coatings exhibited conductive properties. Visual inspection of the IVD-Al and SermeTel W coatings indicated they were intact after 6 days immersion in 5 wt% NaCl. The remaining coatings exhibited surface damage.

Corrosion-Resistant Zeolite Coatings

Abstract As a promising, environmentally friendly alternative for the toxic and carcinogenic chromate conversion surface finishing, zeolite coatings on various metal alloys have shown excellent corrosion resistance, strong adhesion to substrates, and extraordinary thermal and mechanical properties. This review summarizes recent technological advances of zeolite corrosion-resistant coatings from the conventional hydrothermal in situ synthesis method to the newly developed ambient pressure deposition process. The topic of zeolites as smart inhibitor pigments is also discussed. Future perspectives of zeolite corrosion-resistant coatings are envisaged at the end of this review.

Hexavalent Chromium-Free Pre-Treatment for Galvanized Steel

The corrosion protection provided by an organic inhibitor containing an alkanolamine salt of a polycarboxilic acid that is suggested as non-toxic alternative to chromate and phosphate conversion coatings on galvanized steel was studied. Open circuit corrosion potential evolution, linear polarization, potentiodynamic polarization, electrochemical impedance spectroscopy, x-ray diffraction, salt spray (fog) tests and immersion - weight loss tests were used to evaluate the provided protection in a 0.5 M aerated NaCl solution. All tests gave concordant results, indicating that the chromate-free 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, and except for the phosphated samples, the corrosion rate diminishes due to the appearance and growth of passivating corrosion products layers, mainly con stituted by zinc hydroxychloride (Zn5(OH)8Cl2•H2O) and two varieties of zinc hydroxide.

A benchmarking test for surface treatment products for zinc die cast

AbstractCorrosion protection of zinc surfaces was successfully done in the past by chromate conversion layers. To reduce the amount of Cr(VI) products in the environment substitution processes have been developed. For the zinc industry it was a question whether surface treatment products are on the market which can substitute the conventional chromating process. A benchmarking test should be performed to show how good the Cr(VI) free products are in comparison to the state of the art process. Several corrosion tests have been performed to show the corrosion behaviour of treated zinc die cast products in comparison to Cr(VI) treated and plain surfaces. Additionally electrochemical measurements have been performed to discriminate further between the products. Especially CoulCount measurements show clearly the protection capability of the various surface treatments. No one of the tested products shows the same good behaviour like Cr(VI) treated products, but the performance of several products looks not bad and has to be proofed further under real service conditions.

Surface treatment and corrosion behaviour of Fe–32Mn–6Si shape memory alloy

Abstract Fe–Mn–Si-based shape memory alloys are characterized by a good shape-memory effect, but with a modest corrosion resistance. The goal of this study is to try out a suitable surface treatment in order to improve the corrosion resistance of the Fe–32Mn–6Si alloy without impairing its shape-memory properties. The implemented solution is a zinc-electrodeposited coating achieved by chromium conversion treatment. The results show, thanks to electrochemical and thermomechanical measurements, and corrosion tests performed in salt spray chamber, that zinc-electrodeposited coating followed by chromium conversion layer can be considered as a good anticorrosion solution of the Fe–32Mn–6Si alloy.

Preparation and Electrochemical Properties of Trivalent Chromium Coating on 6063 Aluminium Alloy

Chromium (Cr3+) conversion coating was prepared on 6063 aluminium alloy using chromic potassium sulfate and phosphoric acid. Tafel polarization curves and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical properties of the coating formed under different conditions. The optimum conditions were obtained as following: temperature was 30-40 ℃, deposition time 9 min, the pH was adjusted to 2.0-3.0, the KCr(SO4)2 and H3PO4 concentrations were controlled within 15-25 g·L-1 and 10-20 g·L-1, respectively. The polarization result showed that the coating had more positive corrosion potential (Ecorr) and pitting corrosion potential (Epit), and lower corrosion current density (icorr), indicating that the coating formed under the optimum conditions had better corrosion resistance. To explain the electrochemical properties of the coating, a simple equivalent circuit was designed. Electrochemical parameters of EIS, such as the resistance (Rcoat) and capacitance (Ccoat), the charge transfer resistance (Rct) and double layer capacitance (Cdl), were obtained by fitting the EIS plots. The results of EIS were in good agreement with those of Tafel polarization curves.

Influence of Conversion Coating on Magnesium and Aluminum Alloys by Adhesion Method

Magnesium alloys are lightweight materials with high specific strength and have electromagnetic shielding characteristics. On the other hand, chromate is known as carcinogen and it leads a limited use of the conversion coating on magnesium. In recent years, adhesion technology is essential and widely used in producing electronic equipment and sporting goods from magnesium alloys. Adhesives can easily bind different materials. Adhesives may even inhibit the contact corrosion of magnesium, and we may expect adhesion to be used to effectively join different metals.This paper describes and compared with the influence of various non-chromate conversion coatings on the bonding strength of magnesium and aluminum alloys.In this study, specimens of AZ31B and A2017, A6022 were used. The adhesive cements used were epoxy and acrylic resin. The results of a pull-off test and tensile lap-shear strength test ware evaluated.The Mg(OH)2 coating on AZ31B and Boehmite on A6022 exhibited a higher tensile shear strength (16 MPa) with epoxy resin. In the acrylic resin, Mg(OH)2 coating on AZ31B and Boehmite on A6022 had a tensile shear strength of 14 MPa.It is obvious that these conversion coatings can be alternatives to chromate conversion coatings as surface treatment for adhesion of magnesium alloys and aluminum alloys.

クロム化成皮膜中クロム（ＶＩ）の簡易高感度分析法

The chromate conversion coatings treatment of aluminum was treated with 90 cm3 of boiling water for 5 min. The resultant solution was poured into solid phase extraction column (IBC Advanced Technologies Inc., Cr-02 type) and eluted with 2 cm3 of 3 mol/dm3 hydrochloric acid solution. The resultant solution was measured with Digital pack test for Cr (VI) (Kyoritsu Chemical-Check. Lab., Corp). The required time for total analytical procedure was faster than 10 min. The solid phase extraction column could be used repeatedly about 300 times or more. The minimum limit of determination of chromium (VI) was 0.004 mg/dm3.

Microstructure of Trivalent Chromium Conversion Coating on Electrogalvanized Steel Plate

The microstructure of trivalent chromium conversion coating on electrogalvanized steel plate has been characterized. The as-deposited coating consisted of three layers with a thin inner layer, a porous intermediate layer, and a thin outer layer. Baking at 90°C for 30 min reduced the population density and size of cracks in the coating. Meanwhile, the outer layer became denser, but the immediate layer became highly porous. During baking at elevated temperatures, the chemical species and voids within the coating redistribute, which, in turn, suppresses the formation of microcracks that are commonly seen in the hexavalent chromium counterpart.

Inside Cover: Ionothermal Synthesis of Oriented Zeolite AEL Films and Their Application as Corrosion-Resistant Coatings (Angew. Chem. Int. Ed. 3/2008)

Zeolite coatings present a safe and viable alternative to the commonly used but toxic chromate conversion coating for aluminum alloys and other metals, but they normally require high-pressure synthesis. In the Communication on page 525 ff., Y. S. Yan and co-workers present the first ionothermal method by which an oriented zeolite coating can be synthesized on Al alloy at ambient pressure by using microwave radiation. This coating, when sealed with silane, shows exceptional corrosion resistance.

Ionothermal Synthesis of Oriented Zeolite AEL Films and Their Application as Corrosion‐Resistant Coatings

Does this coat your airplane? Zeolite coatings on aluminum alloys and other metals are a safe alternative to the commonly used, toxic chromate conversion coating. However, zeolite coatings are usually synthesized in aqueous medium under high pressure. Using an ionothermal method, an oriented SAPO-11 coating on an Al alloy was prepared at ambient pressure using microwave radiation. This coating shows exceptional corrosion resistance.

Tin–Manganese Alloy Electrodeposits: II. Corrosion Performance Studies

The corrosion performance of tin–manganese alloy electrodeposits either immersed in a quiescent sodium chloride solution or exposed to neutral salt spray (NSS) was studied. The results obtained were then compared with those for conventional pure zinc and zinc–nickel alloy electrodeposits with or without chromate passivation treatment. It was found that tin–manganese alloy electrodeposits provided much longer sacrificial protection against the corrosion of mild steel substrates than zinc–nickel alloy coatings, although they were inferior to pure zinc coatings. The prolonged sacrificial property was attributed to the high polarization resistance of tin–manganese alloy electrodeposits as revealed by linear polarization resistance techniques, which would only allow the corrosion and the consequent ennoblement of these coatings to proceed at a slow rate. The result of NSS tests indicated that tin–manganese alloy electrodeposits, with a time to red rust of about , were as good as chromated zinc–nickel alloy coatings but far outperformed pure zinc coatings and nonchromated zinc–nickel coatings. The high corrosion resistance of tin–manganese alloy electrodeposits without any passivation treatment suggests that they may be an environmentally friendly replacement to zinc alloy coatings, which need toxic chromate conversion coatings for corrosion-resistance enhancement.