Selection of alternatives to hexavalent chromium for inhibitors and inhibiting films for Al and its alloys requires a clear understanding of how hexavalent chromium inhibits corrosion. To this end, the following hypotheses have been proposed: residual chromate in conversion coatings provides self healing; hydrated Cr(III) oxide films are uniquely inert and hydrophobic; oxyanions of Cr(VI) rapidly passivate secondary phases in the highly unstable Cu-rich aluminum alloys; and adsorption of hexavalent chromium on passive aluminum oxide makes it less susceptible to chloride attack. Evidence supporting these hypotheses come from X-ray spectroscopic analysis (XANES) of conversion coatings on aluminum, characterization of Cr(VI) oxyanion adsorption on anodized aluminum via a piezo–electrokinetic (PEK) method, and the electrochemical current vs. voltage response model intermetallic compounds.

Role of hexavalent chromium in the inhibition of corrosion of aluminum alloys

The use of chromium (Cr) as an anticorrosion pretreatment ingredient to protect aluminum (Al) alloys is being restricted because of environmental concerns. Research currently is under way ...

Review: Replacements for Chromium Pretreatments on Aluminum

The formation of chromate conversion coatings (CCCs) on commercially pure Al (AA1050) during immersion in a chromic/ hydrofluoric acid solution has been investigated. A film formation mechanism is proposed based on the combination of open circuit potential measurements and surface analysis techniques, such as Auger electron spectroscopy and atomic force microscopy. It is suggested that the conversion of the Al surface takes place in three distinct stages: activation of the surface, initiation of the film formation, and growth of the conversion layer. In addition to the classical approach, a sol-gel model was considered for the initiation and growth of the film. The effect of the free fluoride and chromic acid concentration was studied using a thermodynamic model whereas the influence of the Al oxide film was investigated by forming an anodic barrier layer prior to the conversion process. It was shown that the rate-determining step in the CCC formation is the activation of the Al surface. Therefore, the morphology and structure of the conversion layer is determined not only by the bath composition but also by the thickness of the Al oxide film.

Chromate Conversion Coating on Aluminum Alloys I. Formation Mechanism

concentrations in the coating continued to increase during this electrochemically quiescent period, suggesting continued CCC evolution. Conversion-coated arrays were subject to anodic potentiodynamic polarization in 0.5 M NaCl until all elements on the array exhibited coating breakdown and substrate pitting. Breakdown potentials were found to increase with coating time up to 120 s, indicating anodic inhibition in CCC corrosion protection. Breakdown was found to be more difficult on electrodes that were net cathodes during coating formation. Results also showed that the NaF and K3Fe~CN!6 in commercial CCC bath formulations strongly contributed to coating corrosion resistance. Without Fv, the Al surface passivated quickly during coating formation, and a nonprotective film formed. Without Fe~CN!3 62 , CCCs exhibited lower breakdown potentials.

/pdf/characterization-of-chromate-conversion-coating-formation-f449if6o3b.pdf

Characterization of Chromate Conversion Coating Formation and Breakdown Using Electrode Arrays

The trivalent chrome process (TCP) is one of the leading non-chromate conversion coatings and has been shown to provide excellent corrosion protection for Al alloy substrates. The goal of this work was to assess and quantify the self-healing or active corrosion inhibition properties of TCP on AA2024-T3 (UNS A92024). Samples exposed in the artificial scratch cell were characterized using electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS) for assessment of various aspects of active corrosion inhibition. Analysis of the electrolyte exposed to the TCP surface was conducted by inductively coupled plasma-optical emission spectroscopy (ICP-OES). TCP treatment was found to greatly improve the corrosion resistance of AA2024-T3 in corrosive environments. After exposure in dilute Harrison's solution, chromium species were detected in the exposed electrolyte, indicating that chromium is released from the TCP coating. Chromium was found on the surface of the uncoated sheets in the ...

Active Corrosion Inhibition of AA2024-T3 by Trivalent Chrome Process Treatment

A study of the composition and structure of chromate conversion coating on aluminum

Liquid-solid based triboelectric nanogenerator (TENG) can be widely utilized for droplet energy harvesting, in which the hydrophobicity of triboelectric layer is crucial for output enhancement. However, poor mechanical properties of hydrophobic triboelectric layers prepared in classic methods greatly limit the TENG’s application. Here, a stretchable hydrophobic triboelectric nanogenerator (SH-TENG) with excellent output, and durability is devised by strongly bonding PTFE micro particles on a flexible substrate even under extreme stretching or abrasion. By synergistic enhancement of the PTFE particles on both contact-separation and charge-transfer between droplet and triboelectric layer, the transfer charge density of SH-TENG is increased to ∼4.74 × 10−3 C/m2·L, along with the open circuit voltage (VOC) and short circuit current (ISC) over 7 times and 6 times higher than TENG without PTFE modified, respectively. Moreover, under a stretching rate of 500%, the SH-TENG shows less 20% decline of its output performance to guarantee applications in flexible electronics. A miniature watch can be sufficiently powered by the SH-TENGs with total area of 0.016 m2 under the simulated raindrop flow rate of 50 mL/min. With such outstanding performance and environmental adaptability, the SH-TENG can be integrated with various objects to collect droplet energy and supply electronic devices. 

Highly stretchable PTFE particle enhanced triboelectric nanogenerator for droplet energy harvestings

Sandwich-structured ion exchange membrane/cotton fabric based flexible high-efficient and constant electricity generator

Application of pH-responsive functionalized hollow mesoporous organosilica nanoparticles for wood preservation

Hydrovoltaic generators (HGs), which harness sustainable energy directly from water flow have garnered tremendous interest worldwide due to their cost-effectiveness, eco-friendliness and potential for powering wearable electronics. However, current HGs generally suffer from poor ion selectivity and resistance matching resulting in low output current, limiting their application scenarios. Herein, a flexible cotton fabric based hydrovoltaic generator (CFHG) fabricated by successive modification with carbon black (CB) and graphene oxide (GO) is proposed, contributing to a simple yet effective performance improvement. Benefiting from the enhanced charge transfer efficiency, ion selectivity and fast capillary flow contributed by CB, GO and fiber network respectively, a considerable open-circuit voltage (Voc) of ∼0.75 V and a short-circuit current (Isc) of ∼8.7 μA are obtained when exposed to water droplets. Experimental results indicate that the power generation performance is influenced by various factors, including the amount of GO, cation size, liquid volume, polarity and pH, as well as temperature and humidity. Moreover, excellent flexibility and porosity of the CF substrate facilitate unconventional serial or parallel connections, providing sufficient power supply for commercial devices and wearable electronics. This research is highly significant for designing versatile power supplies for next-generation intelligent wearable electronics. 

Graphene oxide and carbon black synergistic coated cotton fabric for enhancing energy harvesting from water droplets

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