The applicability of the pH-microscopy with microelectrodes based on H+-ion-selective liquid membranes is shown for phosphating solutions. Linear potential response is found at room temperature up to 60 degreesC with Nernst factors of -58 and -69 mV/pH, respectively. The investigation of an electrode surface activity by pH-imaging is demonstrated with surface scans above special designed test electrodes. The formation of pH-gradients due to the corrosion of steel in diluted phosphoric acid is examined in dependence on nitrite (accelerator) concentration, time and distance to the corroding surface DeltapH = f(CNO2, t, z). In a distance of I mum above the corroding steel surface the pH-shift within 100 s is DeltapH = 0.08 with c(NaNO2) = 2.9 mM. The pH-shift increases up to DeltapH = 0.8 with c(NaNO2) = 11.6 mM. The simulation of corrosion caused pH-gradients by electrochemical generated pH-gradients on an inert microdisk electrode allows the quantitative determination of the additional H+-consumption by nitrite. Therefore, the theory of spherical steady-state diffusion fields around microdisk electrodes is used. (C) 2003 Elsevier Ltd. All rights reserved.