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The role that applied potential has in controlling the properties of porous silicon formed on highly conductive p-type silicon in diluted HF has been investigated by studying the photoluminescence characteristics along the current–voltage curve and using high resolution transmission electron microscopy (HRTEM) evidence to support the conclusions drawn. A dramatic decrease in the average nanocrystal size was found to take place after the etching current density switched from an exponential dependence on the applied potential to a linear relationship. Importantly this event occurred prior to reaching the Ups potential (usually consider the onset of electropolishing). This rapid decrease in particle sizes has been explained in terms of the partial formation of an oxide film. The presence of oxygen terminated porous silicon allows a trapped exciton states model to be invoked, which removes the quantum confinement restrictions on the minimum particle size. Support for the presence of a partial oxide prior to Ups comes from both FTIR measurements and previous literature related to the location of the flat-band potential. © 2008 Elsevier Ltd. All rights reserved.

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.

The formation and breakdown of the passive films on stainless steels are mainly controlled by ionic and electronic transport processes. Both these processes are in part controlled by the electronic properties of the oxide film. Consequently, it is vital to gain a detailed perception of the electronic properties of the passive films together with structural and compositional information for a comprehensive understanding of mechanisms behind passivity and localised corrosion. As a step towards this goal the passive films formed on two main austenitic stainless steels AISI 316L and AISI 304L in borate solution were characterised by in situ Raman spectroscopy and photocurrent spectroscopy coupled with electrochemical measurements. This revealed the formation of an Fe-Cr spinel as the dominant constituent in the passive films with more Cr enrichment in the oxide film on 316L than that of 304L. Bandgap readings and semiconductivities of the two stainless steels suggested that three different applied potential regions existed; 800 mV(SCE) to 300 mV(SCE), 200 mV(SCE) to −300 mV(SCE) and below −500 mV(SCE).

The photocurrent transient effects of passive films grown on austenitic stainless steels in borate solutions have been investigated and explained by extending a mid-bandgap states model that was previously developed for titanium oxide. Tunneling of electrons trapped in mid-bandgap states into the conduction band was proposed as the reason behind the slow relaxation of photocurrent once the light is turned off. In addition, it was found that surface recombination effects started to appear at potentials where Cr(III) is expected to occur in the passive film, supporting earlier suggestions that this species can act as recombination centers.