Chromate Film Research Articles

Chemical composition and corrosion resistance of passive chromate films formed on stainless steels 316 L and 1.4301

The chemical composition and corrosion behavior of the natural and formed by chemical treatment in chromium-containing solution passive films on 316 L and 1.4301 stainless steel surfaces have been investigated by means of X-ray photoelectron spectroscopy and electrochemical in situ method of anodic polarization curves. It have been established that the oxide films formed by the chemical treatment have different chemical composition (Cr-enriched), color and reduced corrosion resistance compared to the natural passive films on both steels. The results have shown that the lower part of the oxide layer represents a uniform modified passive film and the upper one is a porous Cr 3+-enriched film with an island-like structure. The latter permits a direct contact of the solution with the modified passive film which controls the corrosion resistance by the dissolution of the Fe 3+ oxides present in the lower thinner layer.

Investigation of the chromate conversion coating on Alclad 2024 aluminium alloy: effect of the pH of the chromate bath

The parameters of the chromate bath, like temperature, pH, and fluoride content, strongly affect the morphology and chemical composition of the chromate conversion coating and as a consequence have a large influence on its corrosion performance. In this paper, electrochemical impedance spectroscopy (EIS) was used in combination with other techniques to investigate the role played by the pH of the chromate bath on the properties of the chromate film formed on Alclad 2024 aluminium alloy. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE) have shown the formation of a thicker and less dense chromate layer when the pH of the chromate bath is changed from 2.4 to 1.2. The analysis of the EIS spectra have highlighted that this change in pH leads to the formation of more protective and more resistant chromate corrosion products (CCP) inside the defects of the chromate film. When a thin, dense and protective layer of CCP is formed in the defects, the corrosion behaviour of the chromate conversion coating improves for two main reason: (a) further attack of the defects is avoided or delayed; (b) the change in volume caused by the formation of the CCP is limited resulting in a low level of stress in the film, which as a consequence is not detached from the aluminium substrate.

Influence of surface preparation on performance of chromate conversion coatings on Alclad 2024 aluminium alloy: Part II: EIS investigation

Chromate conversion layers, formed on Alclad 2024-T3 aluminium alloy after different surface preparation procedures, were investigated by means of EIS measurements performed in aggressive and middle aggressive electrolytes. The morphology of the chromate layer varies depending on the type of solution and on the time of duration due to deposition of corrosion products inside the defects and deterioration of the layer. Therefore, three slightly different equivalent circuits have been used to model the chromate film in the two electrolytes and for different times of exposure. The proposed equivalent circuits enable the study of the effect of variation in the aluminium surface microstructure, caused by different surface pretreatment, on the performance of the deposited chromate conversion coating. The removal of galvanic couples, mainly caused by precipitates, from the aluminium surface appears to be a necessary condition for the formation of a chromate layer with good corrosion resistance. Besides, the properties of the surface of the aluminium substrate largely influence the morphology and composition not only of the chromate film but also of the corrosion products formed at weak locations, which seem to play an important role in the protection provided by the chromate conversion coating.

Influence of surface preparation on performance of chromate conversion coatings on Alclad 2024 aluminium alloy: Part I: Nucleation and growth

The microstructure and morphology of the surface of aluminium alloys strongly affect the nucleation, growth, and protective properties of chromate conversion coatings. Samples of AA 2024-T3 cladded with AA1230 were pretreated with two different procedures before they underwent a chromating process, which consisted of dipping in a commercial Alodine 1200 solution. In one procedure, an extra step, consisting of immersion in nitric–hydrofluoric acid solution for 30 s (desmutting bath), was carried out after immersion in sulfuric –phosphoric acid solution (acid pickling bath). Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and electrochemical investigations highlighted the fact that this extra step in the pretreatment route gave rise to a more etched, but less reactive surface, due to the removal of a large amount of cathodic sites, mainly related to AlFeSi precipitates present in the clad layer. A similar effect, but on a smaller scale, was caused also by the increase of the time of immersion in sulfuric –phosphoric acid solution. The decrease of galvanic coupling gives rise to a more homogeneous nucleation of the chromate conversion coating on the surface of the Alclad 2024-T3. Moreover, the chromate film tends to be denser and with fewer defects.

Corrosion behaviour of chromated Zn and Zn alloy electrodeposits

The influence of a chromate film on the corrosion behaviour of Zn, Zn–Co and ZnFe coatings was investigated. XPS, AES techniques were applied for characterisation of the iridescent chromate films, while the XRD method was used for the corrosion product analysis. Atmospheric field (4 years data), accelerated (neutral salt spray and SO 2 cabinet) tests and electrochemical measurements were applied for the corrosion resistance studies. The beneficial influence of alloying for chromated Zn electrodeposits was observed under the conditions, where Cl − ions were the dominant pollutant (marine test site, Cl − cabinet, Cl − aqueous solution). The degree of protection from the chromate film, in spite of their lower thickness, was detected to be higher for the alloyed samples. A higher concentration of Cr(VI) compounds in the passive chromate layers of alloyed samples may be the reason of their superior corrosion resistance.

Corrosion Behaviour of Zn and Zn Alloy Coatings in Alkaline Media

SUMMARYThe 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.

Thermal Conductivity Measurement of Submicron-Thick Films Deposited on Substrates by Modified ac Calorimetry (Laser-Heating Ångstrom Method)

Technological development, especially in microelectronics, necessitates the development of new and improved methods for measuring the thermal properties of materials, especially in the form of ultrathin films. Previously, modified ac calorimetry (laser-heating Angstrom method) using a scanning laser as the energy source was developed and shown to provide accurate values of thermal diffusivity and derived thermal conductivity for a broad range of materials in the form of free-standing thin sheets or films, wires including fiber bundles, and some films on substrates. This paper describes further applications of the modified ac-calorimetry technique for measurements of the thermal conductivity of thin films deposited on substrates. It was used to measure successfully the thermal conductivities of 1000- to 3000-A-thick aluminum nitride films, aluminum oxide films, etc., which were deposited on a glass substrate. It was also shown to be suitable for developmental measurements on submicron-thick chromatic films deposited on a PET substrate, which are photothermal recording layers, used in the media of CD-R drives of computer systems.

プラズマ溶射法により作成した固定酸化物型燃料電池用Ca<sup>2+</sup>置換ランタンクロマイト膜セパレーターのキャラクタリゼーション

プラズマ溶射法により作成した固定酸化物型燃料電池用Ca<sup>2+</sup>置換ランタンクロマイト膜セパレーターのキャラクタリゼーション

樹脂クロメートの皮膜構造

樹脂クロメートの皮膜構造

Structure and Function of Ferricyanide in the Formation of Chromate Conversion Coatings on Aluminum Aircraft Alloy

Raman and infrared spectroscopy were used to determine the structure of and its reaction products in chromate conversion coatings (CCCs) on AA 2024-T3 aluminum aircraft alloy. In addition, Raman spectroscopy was used to monitor CCC growth rates and their dependence on coating bath composition. The IR and Raman spectra of the air‐dried CCC corresponded to those of Berlin green, a polymer, and physisorbed on . No other cyano‐containing products were observed. When was excluded from the coating bath, CCC formation rate greatly decreased. In addition, it was observed that could rapidly oxidize AA 2024-T3, and rapidly reduced Cr(VI) in bath conditions. These results indicate a redox mediation action for , which greatly increases the reduction of Cr(VI) to Cr(III) by the alloy. This process is normally quite slow, and redox mediation by is critical to CCC formation. could substitute for to produce a chromate film with properties very similar to a conventional CCC. The results establish redox mediation as the mechanism of acceleration of CCC formation, but provide no evidence for any additional role of in corrosion protection. © 1999 The Electrochemical Society. All rights reserved.

Formability of chromate conversion coating on zinc electroplated steel sheet

A conversion coating is a coating produced by chemical or electrochemical treatment of a metallic surface that gives a superficial layer. This layer which is actually a compound of the metal gives corrosion protection and guarantee paint adhesion. A chromating liquid for forming the chromate film was prepared. The solution is a mixture of Cr+6 and Cr+3 at a ratio of 1.5 to 2.0, 1-20g/l of H2S04, 1-20g/1 of HNO3, 1-20 ml NaOH and 1-10% Si oil. The solution was applied on a surface of zinc-based plated steel sheet of approximately 0.25 mm thickness and then dried. The chromate film formed on the sheet surface was approximately 0.5-1.0 I¼m in thickness. The steel sheet with chromate film was then analysed and mechanically tested. It was found that the chromate solution has better stability over wide range of pH and the formability of its film is improved whence providing a better corrosion resistivity.

Zn and Zn-Sn alloy coatings with and without chromate layers. Part I: Corrosion resistance and structural analysis

The aqueous corrosion resistances of Zn and Zn–Sn (∼ 20 wt% Sn) electrodeposits, passivated by immersion in chromating solution with different ratios of Cr(vi) to activating ions, are compared. The electrochemical behaviour of various chromated and nonchromated coatings were investigated in deaerated 0.5 mol dm−3 Na2SO4/pH 5 solution using a.c. impedance and d.c. polarization techniques. The polarization curves revealed that the chromate layers influence both the cathodic and anodic reactions. The corrosion rate of each specimen decreases with time due to the accumulation of corrosion products. The dark yellow (DY) chromate film on the Zn–Sn alloy and the iridiscent yellow (IY) on Zn yields the best protective ability in agreement with the assessment of prolonged salt spray chamber tests. These chromate layers resembling cracked mud become permeable to the electrolyte after immersion and, as a consequence of the transformation and the leaching of certain Cr compounds, a very porous agglomerate of corrosion products forms. The morphology and structure of dark yellow chromated Zn–Sn alloy was also investigated by transmission electromicroscopy (TEM) and scanning electronmicroscopy with microprobe (SEM/EDS) analyses before and after corrosion. The depth profile of the corroded surface chemical composition was determined by X-ray photoelectron spectroscopy (XPS).

Corrosion Protection of Untreated AA‐2024‐T3 in Chloride Solution by a Chromate Conversion Coating Monitored with Raman Spectroscopy

The behavior of chromate conversion coatings (CCCs) on the aluminum aircraft alloy AA 2024‐T3 was examined by several types of experiments, using Raman spectroscopy as a primary technique. First, Raman spectra of the CCC film made from a commercial process revealed a Raman feature characteristic of Cr(VI) which was distinct from Raman bands of pure or . Second, Raman spectroscopy was used to monitor migration of chromate species from a CCC film to an initially untreated alloy sample. The release of chromate from a CCC was demonstrated, as was redeposition of a chromate film on the fresh alloy surface. Formation of a Raman‐observable Cr(VI)‐containing deposit was more rapid in or near pits in the untreated alloy sample, and the deposit was spectroscopically very similar to the original CCC film. The initially untreated alloy became much less active toward corrosion after migration of chromate from the nearby CCC film, with the polarization resistance increasing by at least two orders of magnitude and the pitting potential increasing by 60 mV. The results clarify the mechanism of self‐healing exhibited by CCC films, in which chromate species released from the CCC migrate to an actively corroding region and stop aluminum dissolution. The migrating chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/chromate precipitate or by adsorption by previously formed corrosion products.

State analysis of electrolytic chromate film by XPS and SXS

The chemical state of Cr in the interior and on the surface of silica-free and silica-added electrolytic chromate and that at the interface between the chromate films and electrogalvanized (EG) steel sheet were quantitatively investigated by SXS and XPS. Specimens for the interface analysis were prepared by dissolving the Zn coating in mercury after adhering stainless steel sheets to the films. The Cr 3+ ( Cr 3++ Cr 6+) ratio (Cr 3+ ratio) of the interface was substantially the same as that of the interior, and that of the surface was 20% lower than that of the interior. Addition of silica to the film does not affect the chemical state of Cr. The Cr 3+ ratio of the interface as well as the surface increased with the increase in the insoluble Cr 3+ ratio obtained by an immersion test in boiling water, indicating that Cr 3+ is insoluble in water. The surface of the chromate film was Cr 6+-rich and the interior of the film is Cr 6+-poor, which is attributed to the fact that Cr 6+ is easily reduced to Cr 3+ during chromate formation at the interface between the chromate bath and EG surface.

5527641 Coated metal sheet for dry cell positive electrode can and positive electrode can formed of such metal sheet

A coated metal sheet for alkaline dry cell positive electrode cans, comprising a metal sheet covered via a chromate film, preferably a chromate film containing carbon black, with a coating film mainly composed of at least one of an epoxy resin, an epoxy-acrylic resin and a polyester resin, containing at least one of graphite powder having a purity of not less than 95%, an aspect ratio of not less than 50 and an average particle diameter of not larger than 50 μm and carbon black having an oil absorption of not less than 250 ml/100 g as measured using DBP, or a coating film further incorporated with a cross-linking agent. This coated metal sheet may be only worked by drawing to produce the positive electrode cans.

Formation of black chromate conversion coatings on pure and zinc alloy electrolytic deposits: role of the main constituents

Preparation of electrolytic zinc coatings covered by a chromate conversion coating is of great interest for automobile companies. Apart from the protective properties required, the most important factor in this field is the aethetic aspect. Principally, black conversion films are now used. The purpose of this study is to understand and explain the mechanism of formation of chromate conversion coatings and how the black colour is obtained. Electrochemical measures showed parameters that influence the kinetic formation of the chromate layers. The use of surface analysis methods (SEM, ESCA, GDOS) showed the role of anions in a chromate electrolyte and the effect of the transition elements in zinc electrodeposits. The chromate film is made of two layers superimposed: a very thin black oxide layer at the zinc/chromate coating interface and a gel type layer at the surface. The oxide layer containing the transition elements is the essential factor for the formation of the black colour.

Internal stresses in zinc-chromate coatings

Zinc coatings immersed in a chromating solution acquire a mixed potential characteristic of zinc dissolution and the deposition of a chromium hydroxide layer. Zinc deposits are initially in a compressive stress but a zinc-chromate coating has a tensile stress which increases with chromate thickness. The stress in the chromate films after drying increases only slightly compared to a wet chromate film. Scanning electron and scanning probe microscopy reveals microcracks in the chromate coatings due to the tensile stress. As the internal stresses in the chromate film increase, scanning probe studies show an increase in microcrack width and depth.

Ｎｉ表面調整処理によるカラー鋼板の塗膜密着性向上メカニズム

In this study, we investigated a mechanism for improving paint adhesion by using Ni surface conditioning prior to dry-in-place chromate treatment, which is usually applied to hot dip Zn-coated steel sheets as a paint base.The results showed that the scattered deposits of Ni on the Zn surface formed micro cells with the Zn as the anode and the Ni as the cathode.Through these micro cells, insoluble chromate film (Cr(III)) deposited on the Ni through an electrochemical reaction of reducing the Cr(VI) in the dry-in place chromate solution.This mechanism of generating chromate film should contribute greatly to improved paint adhesion.

Novel Diffusion Protective Coatings for Copper/Polyimide MCM.

The effect of two novel diffusion protective coating methods for Cu/polyimide interface, chromate treatment and 1H-tetrazole addition, have been investigated. The protective coating layer of chromate was less than a few nm thick. The Cr structure of the chromate film was assigned as Cr-oxide. 1H-tetrazole which was added into the polyamic acid was effective inhibitor for Cu/polyamic acid reaction. We have confirmed that our protective coatings, the chromate treatment and 1H-tetrazole addition, prevented the Cu diffusion into the polyimide films. Both of these protective coatings are effective and practical from an application point of view.

低スズめっき鋼板の塗料密着性に及ぼすクロメート皮膜の影響

低スズめっき鋼板の塗料密着性に及ぼすクロメート皮膜の影響