Paul M. Natishan

Chloride Ingress into Aluminum Prior to Pitting Corrosion An Investigation by XANES and XPS

Two distinct chloride (Cl - ) species were detected on and/or in the passive oxides of polycrystalline Al samples, which were anodically polarized below the stable pitting potential in Cl - -containing solutions. Chloride was found to be present as an adsorbed specie at the surface of the Al oxide, as well as an incorporated specie within the passive oxide. The two species of Cl - were recorded by X-ray absorption near edge structure (XANES), using both an electron yield detector and an X-ray fluorescence detector, and by X-ray photoelectron spectroscopy (XPS). Electron yield XANES and XPS results indicate that adsorbed Cl - migrates from the solution/Al oxide interface into the passive Al oxide film, prior to stable pit initiation. Cl - migration occurs once a critical anodic potential or critical adsorbed Cl - concentration is reached. The migration of Cl - is followed by a loss of oxidized Al from the passivating film, as determined by XPS, and can be attributed to (i) metastable pitting events or (ii) oxide dissolution. The ingress of Cl - into the oxide appears to be a key factor for the onset of metastable pitting or passive film dissolution.

Fabrication of a high surface area boron-doped diamond coated metal mesh for electrochemical applications

An electrode for electrochemical uses is made of a conductive metal mesh coated with boron-doped diamond. The electrode may be used in electrochemical reactions either as a cathode or as an anode, or can be used with an alternating current.

Salt fog corrosion behavior of high-velocity oxygen-fuel thermal spray coatings compared to electrodeposited hard chromium

Abstract The corrosion behavior of several coating/substrate combinations was determined using the ASTM B117 Salt Fog Test. The coatings were electrodeposited hard chromium (EHC) and two high-velocity oxygen-fuel (HVOF) thermal-sprayed coatings, tungsten-carbide/cobalt (WC/Co) and Tribaloy 400 (T400). The substrates were 4340 steel, 7075 aluminum alloy, and PH13-8 stainless steel. On the 7075 Al alloy, a sulfamate nickel layer was deposited prior to the deposition of hard chromium. The results indicated that on the 4340 steel none of the coatings provided significant protection, with equivalent performance between the EHC and WC/Co coatings and slightly poorer performance for the T400. On the 7075 Al alloy, the EHC with sulfamate nickel exhibited excellent performance as no pits or blisters were noted on the faces or edges of the samples. The WC/Co showed no pitting or blistering on the faces but had a significant amount of pitting along the edges. The EHC and WC/Co coatings performed well on the 13-8 stainless steel as no pits or blisters were noted on the faces or edges. The T400 coatings had rust stains on the faces and edges but defects could not be seen with the unaided eye or at a 7× magnification.

Metastable pitting behavior of aluminum single crystals

Abstract The metastable pitting behavior of aluminum single crystals (99.999% pure) with orientations of (100), (110) and (111) was followed as a function of potential at potentials below the pitting potential in a deoxygenated 0.6 M NaCl solution. The (111) crystal face exhibited the highest number of metastable events at a given potential. The (100) face showed the second highest number of events, while the (110) had the least number of events. The peak pit currents for the three surfaces were not statistically different in the potential region −0.785 to −0.745 V vs. SCE indicating that the amount of electrochemical charge passed in a breakdown event is independent of orientation. These results are discussed in relation to the nature of the oxide film and the atomic planar densities of the three orientations.

Carburization-induced passivity of 316 L austenitic stainless steel

A low-temperature (450-500°C) gas-phase process for introducing substantial amounts of carbon, without carbide formation, into 316L austenitic stainless steel has been developed. This process, termed low-temperature colossal supersaturation (LTCSS), provides surface carbon concentration as high as 14 atom % and dramatically improves the localized corrosion resistance of 316L austenitic stainless steel in ambient temperature seawater. In particular, the LTCSS-treated steel increases the seawater breakdown potential by more than 600 mV. This result is remarkable, as traditional carburization methods have historically decreased the corrosion resistance of stainless steels.

Ion beam assisted deposited tantalum oxide coatings on aluminum

Abstract Ion beam assisted deposition (IBAD) and sputter deposition were used to produce tantalum oxide surfaces on aluminum substrates. The corrosion behavior of the sputter deposited coatings and IBAD modified regions was studied by anodic polarization in deaerated, 0. 1 M NaCl solutions. The electrochemical results showed that the tantalum oxide surfaces significantly improved the pitting corrosion resistance of the substrate metal. The best results were obtained for samples with 1.8 μm thick tantalum oxide IBAD modified surfaces. These samples have an average pitting potential value that is 0.850 V higher than that of pure aluminum. X-ray photo-electron spectroscopy showed that the surface of the IBAD samples was composed of Ta 2 O 5 as well as tantalum carbide and tantalum suboxide species.

Chloride Uptake by Oxide Covered Aluminum as Determined by X‐Ray Photoelectron and X‐Ray Absorption Spectroscopy

The uptake of chloride by aluminum polarized at potentials below (less positive than) the pitting potential in 0.1 M NaCl solutions was studied using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The XPS chloride spectra showed that two distinct sets of doublets are present. One doublet is related to chloride on the surface and the second is related to chloride incorporated in the oxide film. In the case of XAS, deconvolution of the spectrum obtained for samples polarized below the pitting potential also showed the presence of chloride in the near surface region and in the oxide film. The important point of this work is that the observed chloride was present in two different chemical environments as determined with both XAS and XPS.

Diamond tubes and fibers

Abstract Diamond-coated fibers, diamond tubes, and diamond fibers were produced using a hot-filament-assisted chemical vapor deposition process. Diamond coatings were deposited on prepared fiber substrates of silicon carbide, copper, tungsten, and copper-coated graphite. Diamond tubes were produced in situ as a result of the removal of the substrate by atomic hydrogen or after deposition by dissolving the substrate using a chemical etch. Diamond fibers in the shape of a half-cylinder were produced by masking a portion of the substrate fiber and removing the diamond after deposition. The diamond tubes and fibers that have been produced are typically 2–3 cm in length and are self-supporting structures.

Carburization-Enhanced Passivity of PH13-8 Mo: A Precipitation-Hardened Martensitic Stainless Steel

Interstitial hardening of the martensitic stainless steel PH13-8 Mo has been achieved by low temperature gas-phase carburization. After treatment, hardness is increased to a depth of =50 μm, with a surface hardness that is twice the core hardness and a corresponding improvement in pin-on-disk wear resistance. Pitting potential is increased by ≈ 0.5 V in 0.6 M NaCl. Elemental analysis and X-ray diffraction suggest the formation of a thin (= 2 μm) carbidic surface layer that is both wear and corrosion resistant.

Chloride Interactions with the Passive Films on Stainless Steel

The uptake of chloride (Cl - ) by the passive oxide film on 316L austenitic stainless steel polarized at potentials below (less positive than) the pitting potential in 0.6 M NaCl solutions was studied using X-ray photoelectron spectroscopy (XPS) and compared to our previous work on aluminum. The XPS spectra for the stainless steel showed that chloride was not adsorbed or present in the passive oxide film. This is quite different from the case of aluminum, where XPS data for the presence of chloride on the surface and incorporated into the oxide film are unambiguous. These differences in the adsorption and incorporation of chloride for these two metals will be discussed in the context of the mechanisms of passive film breakdown.