We have studied the electronic properties and relative stability of the biphenylene sheet composed of alternating eight-, six- and four-carbon rings and its one-dimensional derivatives including ribbons and tubes of different widths and morphologies by means of density functional theory calculations. The two-dimensional sheet presents a metallic character that is also present in the planar strips with zigzag-type edges. Armchair-edged strips develop a band gap that decreases monotonically with the ribbon width. The narrowest armchair strip considered here (0.62 nm wide) presents a large band gap of 1.71 eV, while the 2.14 nm wide armchair strip exhibits a band gap of 0.08 eV. We have also found that tubes made by rolling these ribbons in a seamlessly manner are all metallic, independent of their chirality. However, while the calculated energy landscape suggests that planar strips present a relative stability comparable to that of C(60), in the tubular form, they present a more pronounced metastable nature with a Gibbs free energy of at least 0.2 eV per carbon higher than in C(60).

The formation, structure and corrosion performance of a commercial Zr/Ti-based conversion coating (Bonderite M-NT 5200, Henkel Corp.) on AA2024-T3 was investigated. This coating bath consists of a mixture of fluorozirconic and fluorotitanic acids (ca. 1:3) at a pH of 2-3. The coating was formed on polished, degreased and deoxidized specimens by immersion, and was characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis and Auger electron spectroscopy (depth profiling) for coating morphology and chemical composition. Electrochemical methods were used to assess the corrosion inhibition provided by the coating. The coating consists of a hydrated ZrO 2 /TiO 2 outer layer on the order of 30 nm thick (in vacuo) and an interfacial region of approximately 60-90 nm between the coating and the alloy surface. In the coating, the Ti levels are ca. 2× greater than the Zr levels consistent with the concentration differences in the coating bath. The coating provides no significant corrosion protection to the alloy under the test conditions employed based on similarities in the open circuit potentials, anodic and cathodic polarization curves and polarization resistances for the uncoated and coated specimens measured in a naturally-aerated 0.5 M Na 2 SO 4 . Measurements were also made with two other conversion coatings for comparison. The 5200 provides less corrosion inhibition than does a commercial trivalent chromium process (TCP) coating (Bonderite M-CR T5900, Henkel) or a non-chromium process (NCP) coating (NAVAIR). For example, the polarization resistance, R p , for both of these conversion coatings was 10× greater than that for the 5200. We suppose that the limited corrosion protection of provided by the 5200 coating is due to a thin and porous structure on this alloy. Conversion coatings are used as part of a multi-component system to protect aluminum alloys from corrosion.1-4 In short, they convert the metal surface into a more chemically and electrochemically stable state. Pretreatment conversion coatings prepare the surface for paints and finishes, promote adhesion of these top coats and ideally provide some corrosion protection especially in the event of "break through". The barrier protection of a coating depends on (i) the coating coverage (thickness) and uniformity of that coverage and (ii) its chemical composition. Common surface pretreatments used for aluminum alloys include chromate, 4-8 phosphate 3,9 oxide 4,10-17 and anodized 18,19 coatings. In the aerospace industry, chromate conversion coatings (CCCs) continue to be used as the primary surface pretreatment for aluminum alloys. CCC provides excellent active corrosion protection, so called "self-healing", and promotes adhesion with topcoats. [4][5][6][7][8]20 The active component in this coating, and in numerous primers, is hexavalent chromium in the form of chromate, CrO 4 2− . The carcinogenicity and toxicity of hexavalent chromium as well as the high cost of treating waste contaminated with it are mot...

We report on the physicochemical properties and anti-corrosion performance of a non-chromated Zr/Zn conversion coating (NCP) on AA2024-T3. The immersion coating was formed on polished, degreased and deoxidized specimens. Electrochemical methods were used to assess the corrosion inhibition provided by the coating in laboratory tests. The results were compared with environmental exposure tests to assess the stand-alone corrosion protection. Coated AA6061-T6 and 7075-T6 specimens were also used in the environmental tests. Electrochemical testing in naturally-aerated 0.5 M Na 2 SO 4 + 0.1% NaCl revealed that the NCP coating shifted E corr positive by about 250 mV, suppressed anodic more than cathodic current around E corr by at least a factor of 10x and shifted E pit more noble. The coating functions more as an anodic inhibitor through barrier layer protection. The coating provided excellent corrosion protection to all three alloys during a 14-day full immersion test in 0.5 M Na 2 SO 4 + 0.1% NaCl. However during 14-day neutral salt spray and thin-layer mist tests, NCP failed to provide much stand-alone corrosion protection to the aluminum alloys and the anti-corrosion properties were found to be inferior to TCP conversion coatings of comparable thickness. A 7-day beach exposure revealed the NCP coating also provides little resistance to galvanic corrosion on the aluminum alloys as compared to TCP coatings. The results demonstrate that laboratory evaluation of the anti-corrosion properties of non-chromated conversion coatings does not always reflect coating performance during accelerated degradation or environmental exposure. The inferior anti-corrosion behavior of NCP, as compared to TCP, is due to (i) inherent defect density of the former (i.e., reduced throwing power) and ( AA2024 and AA7075 are high-strength aluminum alloys that derive their properties from their alloying components. They are used on civilian and military aircraft because of their light weight and mechanical strength. 1,2 However, many of the constituent particles, such as the Al 2 CuMg phase (so-called S-phase) in AA2024 3,4 and Mg(ZnCuAl) 2 in AA7075, 5-8 lead to corrosion challenges. More noble intermetallic particles play a critical role in the corrosion susceptibility of aluminum alloys as they can give rise to localized corrosion, such as pitting and exfoliation, because of the formation of galvanic cells with the surrounding aluminum. [9][10][11] The intermetallic phases tend to function as cathodic sites supporting oxygen reduction, which can drive the localized dissolution of nearby aluminum. The shape, size and chemical composition of the intermetallic particles are determined by the processing route (heat-treatment and forming) carried out on the aluminum alloy. [9][10][11] Multilayer coating systems (conversion coating + primer + topcoat) are required to protect aerospace aluminum alloys from corrosion in service. Traditional coating systems contain hexavalent chromium (Cr(VI)) in both the conversion coating and primer, volatile orga...

Carbon fiber reinforced epoxy (CFRE) composites are joined with aluminum alloys in aerospace assets through either adhesive bonding or mechanical fastening. The carbon fibers are noble relative to the alloy so when these materials are electrically connected through a thin electrolyte layer, galvanic current can flow. Oxygen reduction occurs at the exposed carbon fibers leading to the formation of H 2 O 2 that could build up in the gap between the two. Electrochemical methods were used to characterize a standard airframe composite before and after a 7-day (i) neutral salt-spray (ASTM B117) and (ii) moist SO 2 atmospheric (ASTM G87) test when joined with AA2024-T3. The electrochemical properties of the composite were also assessed before and after the imposition of applied cathodic currents from −1 to 1000 μA. Finally, electrochemical methods were used to study the effect of a 14-day H 2 O 2 exposure on the composite properties. Horizontal shear stress testing was performed to determine how the mechanical strength of the composite specimens was impacted by full immersion (14 days) in H 2 O 2 solutions at 25 and 55 • C. The composite is damaged under the cathodic conditions and appears linked to H 2 O 2 . The dominant consequence is epoxy and or fiber sizing degradation, which leads to debonding and cracking.

The anti-corrosion properties of a trivalent chromium process (TCP) conversion coating on Cu-rich aluminum alloy 2024-T3 were studied when galvanically coupled to an uncoated Ti-6Al-4V fastener. Coated and uncoated test specimens were exposed to a 14-day moist SO 2 exposure (40 ± 3 • C, 360 ppm SO 2 , 336 h). The TCP coating (Bonderite T5900, Henkel) provided corrosion resistance to this alloy in this acidic environment both stand-alone and when galvanically coupled. The results revealed that the stand-alone corrosion protection of the coating is excellent during the exposure. Roughness, pit density, pit depth and pit diameter are all significantly reduced as compared to the uncoated control. The coating also provided excellent resistance against galvanic corrosion. Corrosion damage and corrosion product formation were confined to the region under the contacting fastener head. Regions away from the through-hole were largely free of discoloration and pitting. Electrochemical parameters, R p and i corr , were good predictors of the trends observed during the SO 2 testing. Corrosion of the alloy initiates at sites where the conversion coating and or the native oxide does not cover and passivate the surface. Corrosion leads to the formation of a Al x (OH) y (SO 4 ) z product layer that undercuts the TCP coating. The corrosion product eventually passivates the surface. By and large, the TCP coating remains intact and passivating after the 14-day exposure. Aluminum alloys are used in the aerospace industry for structural support because of their light weight and high strength. Strength is imparted to a common material, aluminum alloy 2024-T3, by alloying with copper and magnesium but this comes at the expense of corrosion resistance.1-8 More noble intermetallic particles play a critical role in the corrosion susceptibility of aluminum alloys as they give rise to localized corrosion, such as pitting and exfoliation, because of galvanic cells formed with the surrounding aluminum.4,7,9-11 Many intermetallic compounds tend to function as cathodic sites supporting oxygen reduction, which drives the localized dissolution of the nearby aluminum. 4,10,11 A multi-layer coating system is commonly used to prevent corrosion of this and other Al alloys in service. 12The coating system generally consists of three components: (i) an inorganic conversion coating that provides corrosion protection and adhesion promotion, (ii) a primer layer and (iii) a topcoat used to seal the alloy and restrict contact with the service environment.12,13 The aerospace industry continues to use chromated conversion coatings and primers because of a historical comfort level and their superior performance. These chromated coatings provide excellent corrosion protection to the underlying Al alloy and self-healing capability by releasing Cr +6 species into the solution environment. 13-16 Soluble Cr +6 species in the conversion coating (e.g., CrO 4 −2 ) get reduced at nearby corroding sites on the aluminum to produce a passivating layer of Cr(OH) 3 .13-16 W...