Geraint Williams

Chromate Inhibition of Corrosion-Driven Organic Coating Delamination Studied Using a Scanning Kelvin Probe Technique

The scanning Kelvin probe is used to study the influence of trivalent cerium cations (Ce 3+ ) on the kinetics and mechanism of corrosion-driven delamination processes affecting polyvinyl butyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanized steel. Placing aerated aqueous sodium chloride onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O 2 reduction at the delamination front is coupled to anodic zinc dissolution at the coating defect by a thin (2.5-5 μm) layer of electrolyte which ingresses beneath the delaminated PVB film. Soluble Ce(III) salts in the external electrolyte reduce coating delamination rates by <25% because Ce 3+ (aq) cations migrating beneath the coating immediately encounter a countercurrent migration of cathodically produced OH anions. Consequently, Ce(OH) 3 precipitation occurs at the coating/ defect boundary, with little or no Ce 3+ (aq) reaching the active sites of cathodic O 2 reduction at the delamination front. In contrast. dispersions of silica and bentonite-based Ce 3+ cation-exchange pigments in the PVB coatings enable Ce 3+ (aq) to exchange directly into the under-film electrolyte and significantly retard coating delamination. Delamination is not halted completely because under-film Ce 3+ (aq) exchange continually lags behind the advancing delamination front so that precipitated Ce(OH) 3 increases under-film resistivity by displacing electrolyte but does not form a coherent cathodic film capable of stifling O 2 reduction.

Latent Fingerprint Detection Using a Scanning Kelvin Microprobe

Electrochemical interactions between latent human fingerprints and metal surfaces in ambient air are investigated using a scanning Kelvin microprobe. Inorganic salts present in sweat deposited at fingerprint ridges locally depassivate the metal surface producing a Volta potential decrease of greater than 200 mV. Volta potential patterns may persist for months and prints may be visualized by potential mapping, even when overcoated with a polymer layer. Because the Volta potential differences are produced by involatile salts they persist when the organic components of the fingerprint residue have been volatilized by heating to 600 degrees C.

Anion-exchange inhibition of filiform corrosion on organic coated AA2024-T3 aluminum alloy by hydrotalcite-like pigments

Novel anion-exchange anticorrosion pigments consisting of nitrate, carbonate, and chromate exchanged hydrotalcite (HT) are shown to effectively inhibit the propagation of filiform corrosion (FFC) on organic coated AA2024-T3 aluminum alloy. The HT-pigments are dispersed in polyvinyl butyral (PVB) coatings applied to the AA2024-T3 surface. Following FFC initiation by application of aqueous HCl to a penetrative coating defect, the time-dependent extent of coating delamination is determined both optically and by means of repeated in situ scanning using a scanning Kelvin probe apparatus. All HT-pigments are shown to profoundly reduce rates of coating delamination by comparison to unpigmented samples. However, inhibitor efficiency is dependent upon the exchangeable anion and increases in the order

Inhibition of Filiform Corrosion on Polymer Coated AA2024-T3 by Hydrotalcite-Like Pigments Incorporating Organic Anions

Benzotriazolate, ethyl xanthate, and oxalate anion-exchanged hydrotalcite (HT) pigments are evaluated with respect to their ability to inhibit filiform corrosion (FFC) on organic coated AA2024-T3 aluminum alloy. FollowingFFC initiation by application of aqueous HCl to a penetrative coating defect, the time-dependent extent of coating delamination was determined both optically and by repeated in situ measurements performed using a scanning Kelvin probe (SKP). Inhibitor efficiency is shown to increase in the order ethyl xanthate « oxalate < benzotriazolate. Benzotriazolate is also shown to interact specifically with the alloy surface, depressing Volta potential values by up to 0.4 V.

Selectivity studies on tin oxide-based semiconductor gas sensors

Abstract Two sensors previously developed at Swansea have been greatly improved. The resistance in clean air of the CO-selective sensor can be reduced by a factor of up to 200 by introducing small amounts of Sb 2 O 3 (0.2–2%); production of the methane-selective device can be improved by introducing a presintering stage. Further study of the tin oxide-bismuth oxide system has led to the production of a third selective sensor, which is sensitive to the presence of hydrogen but shows no response to the presence of either carbon monoxide or methane.

Polyaniline Inhibition of Corrosion-Driven Organic Coating Cathodic Delamination on Iron

The polyaniline emeraldine salt of paratoluenesulfonic acid (PAni-pTS) is dispersed in polyvinylbutyral coatings adherent to an iron surface. Such dispersions are shown to effectively inhibit corrosion-driven coating delamination (cathodic disbondment) when 5% w/v (0.86 M) aqueous chloride electrolyte contacts a penetrative coating defect. A scanning Kelvin probe is used to quantify the influence of PAni-pTS volume fraction (Φ p a ) on delamination rate and the potential of the intact (undelaminated) coated surface (E i n t a c t ). Secondary ion mass spectroscopy and atomic force microscopy are used to determine the time-dependent thickness of an oxide layer developing at the coating-iron interface. At 20°C, high relative humidity (93%), and Φ p a = 0.3, the iron oxide layer grows at a constant rate of 5.6 X 10 - 3 nm s - 1 . E i n t a c t increases monotonically from 0.2 to 0.56 V vs. SHE, and coating delamination rates decrease by ca. 95% as Φ p a is increased from 0 to 0.25. Addition of 0.01 M Na-pTS to the experimental electrolyte has no effect. An inhibition mechanism is proposed in which through-coating cathodic O 2 reduction is suppressed by the ennoblement of substrate potentials and the OH - product of O 2 reduction is absorbed through PAni-pTS mediated pH buffering.

Gas sensing properties of nanocrystalline metal oxide powders produced by a laser evaporation technique

Nanosized powders of Al2O3 , ZrO2 and SnO2 have been produced by laser ablation of ceramic samples followed by condensation in a controlled gaseous atmosphere. X-Ray powder diffraction, transmission and scanning electron microscopy have been used to investigate the morphology and structure of the materials. A study of the gas sensing characteristics of thick films prepared from these materials revealed that zirconia based sensors respond to H2 at a temperature of 100 °C, while thick films of alumina display marked changes in both resistance and capacitance as a function of relative humidity at room temperature. Nanocrystalline SnO2 sensors respond markedly to mixtures of H2 , CO and CH4 in the 100–600 °C temperature range and display sensitivity values which are higher than those obtained using SnO2 powder samples prepared via a conventional wet chemistry route. Optimisation experiments showed that sensor characteristics were influenced both by pre-treatment temperature and film thickness.

Inhibition of Filiform Corrosion on Organic-Coated Aluminum Alloy by Hydrotalcite-Like Anion-Exchange Pigments

Abstract Novel anion-exchange anticorrosion pigments consisting of nitrate-, carbonate-, and chromate-exchanged hydrotalcite (HT) are shown to inhibit the propagation of filiform corrosion (FFC) ef...

NOx response of tin dioxide based gas sensors

Abstract Three types of SnO 2 based sensors developed at Swansea have been tested in NO x containing environments. Polycrystalline Bi 2 O 3 doped devices exhibit a reverse sensitivity to NO x both in air and in nitrogen at temperatures of 300 °C or less. In this case sensor resistance falls markedly upon exposure to the oxidising gas. In contrast sensors prepared from undoped SnO 2 which had been precalcined at 1500 °C behave in a conventional manner, where marked resistance increases are observed in the presence of NO x . Gas sensitive thin films fabricated via the oxidation of a metallic Sn layer display high sensitivity to NO x while remaining unresponsive to common interfering gases such as carbon monoxide or methane.

Dopant Effects in Polyaniline Inhibition of Corrosion-Driven Organic Coating Cathodic Delamination on Iron

Coatings comprising various volume fractions of particulate polyaniline dispersed in a polyvinylbutyral binder are applied to iron substrates. A scanning Kelvin probe is used to measure substrate potentials in humid air and follow corrosion-driven coating delamination (cathodic disbondment) when 5% w/v (0.86 M) aqueous NaCl contacts a coating defect. Emeraldine base has no effect on substrate potential or delamination kinetics. Emeraldine salts (ES) doped using p-toluenesulfonic (HpTS), camphorsulfonic (HCS), phosphoric (H 3 PO 4 ), and phenylphosphonic (H 2 PP) acids increase substrate potentials by up to 0.36 V and inhibit delamination with efficiency order of HpTS < HCS < H 2 PO 4 << H 2 PP. Dopant salts added to the corrosive electrolyte do not inhibit delamination. It is proposed that inhibition arises primarily from cathodic O 2 reduction becoming relocated from the ennobled substrate onto the ES coating. However, Fe 3 (PO 4 ) 2 and FePP salt films formed at the ES-substrate interface also contribute by hindering interfacial electron transfer.