An in situ scanning vibrating electrode technique ͑SVET͒ is used to investigate localized corrosion occurring on unpolarized magnesium ͑Mg͒ samples immersed in 5% w/v aqueous sodium chloride electrolyte. Corrosion is characterized by the appearance of circular, blackened areas which expand radially at a constant rate and evolve hydrogen vigorously. These are shown to consist of a cathodically active center surrounded by a 0.5 mm wide anodic ring. Any localized corrosion currents emerging from the intact ͑uncorroded͒ Mg surface are negligible by comparison. Local anodic current density is shown to be directly proportional to the radius of the local cathode, while corresponding local cathodic current density remains relatively constant with time. Estimates of time-dependent rates of total equivalent Mg loss and H 2 evolution, obtained by numerical integration of SVET-derived normal current density distributions, indicate that corrosion rate is controlled by the area of local cathodic activity. The empirical findings are consistent with a mechanism involving cathodic H 2 evolution on the dark, film-free region which is galvanically coupled with anodic attack of the intact Mg surface. It is proposed that cathodic activation of the film-free, corroded "disk" is caused by a combination of elevated pH and enrichment in noble iron-containing impurity phases.

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A scanning Kelvin probe technique is used to study the influence of chromate false(CrO42−false) 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 aqueous sodium chloride electrolyte onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O2 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 chromates in the external electrolyte reduce delamination rates by less than 25% because CrO42− anions are excluded from the underfilm electrolyte layer by the delamination cell electric field. In contrast, dispersions of particulate SrCrO4 in the PVB coatings allow CrO42− diffusion directly into the underfilm electrolyte layer and profoundly inhibit delamination. It is proposed that replacement of underfilm O2 reduction by a self-limiting CrO42− reduction process is the most significant factor in decreasing delamination rates. © 2001 The Electrochemical Society. All rights reserved.

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 false(ϕnormalpafalse) on delamination rate and the potential of the intact (undelaminated) coated surface false(Enormalintactfalse). 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 ϕnormalpa=0.3, the iron oxide layer grows at a constant rate of 5.6×10−3 nm s−1. Enormalintact increases monotonically from 0.2 to 0.56 V vs. SHE, and coating delamination rates decrease by ca. 95% as ϕnormalpa 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 O2 reduction is suppressed by the ennoblement of substrate potentials and the OH− product of O2 reduction is absorbed through PAni-pTS mediated pH buffering. © 2005 The Electrochemical Society. All rights reserved.

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Self-piercing riveting (SPR) is a cold mechanical joining process used to join two or more sheets of materials by driving a rivet piercing through the top sheet or the top and middle sheets and subsequently lock into the bottom sheet under the guidance of a suitable die. SPR is currently the main joining method for aluminium and mixed-material lightweight automotive structures. SPR was originated half century ago, but it only had significant progress in the last 25 years due to the requirement of joining lightweight materials, such as aluminium alloy structures, aluminium-steel structures and other mixed-material structures, from the automotive industry. Compared with other conventional joining methods, SPR has many advantages including no pre-drilled holes required, no fume, no spark and low noise, no surface treatment required, ability to join multi-layer materials and mixed materials and ability to produce joints with high static and fatigue strengths. In this paper, research investigations that have been conducted on self-piercing riveting will be extensively reviewed. The current state and development of SPR process is reviewed and the influence of the key process parameters on joint quality is discussed. The mechanical properties of SPR joints, the corrosion behaviour of SPR joints, the distortion of SPR joints and the simulation of SPR process and joint performance are reviewed. Developing reliable simulation methods for SPR process and joint performance to reduce the need of physical testing has been identified as one of the main challenges.

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°C.