An overview of ionophore-based electrodes for ammonium sensing critically analyzing contributions in the last 20 years and with focus in analytical applications.

The local distribution of anodic and cathodic sites in a corroding cut edge using the scanning electrochemical microscope is presented. Platinum and antimony microdisks were used as tips for combined amperometric/potentiometric operation. Local changes in electrochemical activity, oxygen concentration and pH distribution associated to the onset of localized anodes and cathodes were imaged in situ, as well as their evolution with time. Local alkalization and oxygen consumption related to cathodic activity were detected above the zinc layer covered with the thinner polymeric coating, providing a direct evidence of a differential aeration process effectively operating in an asymmetrical cut edge, in addition to the galvanic coupling of aluzinc and steel. Anodic activity leading to local acidification occurred at the other aluzinc layer at all exposures.

Three newly synthesized Schiff base derivatives, sodium (E)-4-(nitrobenzylideneamino)benzoate (SNBB), sodium (E)-4-(benzylideneamino)-benzoate (SBB), and sodium (E)-4-(hydroxybenzylideneamino)-benzoate (SHBB) were investigated as pitting corrosion inhibitors for 304 stainless steel in neutral 0.1 M NaCl. Potentiodynamic polarization evidenced major shifts in pitting potential to more positive values with increasing inhibitor concentration. The scanning vibrating electrode technique (SVET) imaged metastable pitting in 0.1 M NaCl, but not in the presence of the inhibitor, indicating that it prevented pit nucleation. The inhibition performance was established under anodic polarization conditions, because only minute local anodic activity due to metastable pit formation could be observed when the steel was exposed to SNBB-containing solution, whereas the metal would undergo pit propagation at the same potential in the inhibitor-free solution. X-ray photoelectron spectroscopy (XPS) analysis evidenced chromium enrichment at weak points (pores) of the passive film at anodic polarization condition where sudden release of Fe cations is possible. In this way, the SNBB molecules will migrate to these sites to react with the Fe ions and form a chelate compound which will deposit finally at those sites and plug them, whereas no effect occurred at the open circuit potential (OCP).

Corrosion processes occurring on stainless steel 304 surfaces under anodic polarization have been characterized using scanning electrochemical microscopy (SECM) and the scanning vibrating electrode technique (SVET), complemented with conventional potentiodynamic polarization curves. Stable pit formation has been detected by SVET on the samples as result of surface modification under electrochemical control, which may include the previous electrochemical reduction of passive oxide layer if the media is not aggressive enough to induce pitting at small overpotentials. Additionally, the sample generation -tip collection operation mode of the SECM has enabled the detection of local release of iron (II) ions, as well as their conversion to iron (III), both processes being greatly affected by the potential applied to the substrate.