Kemal Nisancioglu

Measurement of the critical pitting potential of aluminium

Abstract The electrochemical methods for the measurement of the critical pitting potential have been analyzed in terms of their applicability to aluminium and its alloys. Interpretation of data for these alloys may follow different guidelines than those developed for more noble alloys like steel in order to obtain meaningful results. Present work and a literature survey show that most methods give fairly accurate results with careful data analysis with the exception of fast-scan methods. Experimental difficulties associated with different methods are reviewed. The critical pitting potential of aluminium varies appreciably with chloride concentration, other factors such as surface treatment, temperature, and small additions of alloying elements being of minor importance. The significance of the critical potential for aluminium is discussed in view of this result.

Cathodic polarization of commercially pure aluminium

Abstract Cathodic corrosion, particularly alkaline pitting and repassivation, of 1S aluminium has been investigated by potential-controlled methods in unbuffered chloride media. The results are mainly for de-aerated solutions; the influence of dissolved oxygen and stirring, which are likely to be present in a practical situation, are also discussed. The metal undergoes stable pitting and then uniform etching as the applied potential is decreased below − 1.35 V(SCE). At potentials more positive than this threshold value, metal dissolution still occurs possibly near the cathodic impurities on the surface due to local alkalinization resulting from hydrogen evolution; but, the surface repassivates slowly as the cathodic sites are covered up by the oxide film. The polarization behaviour of the partially passive surface is studied by fast-scan potential measurements and is associated with hydrogen evolution kinetics. The measured current decays exponentially with time during repassivation at constant potential. This is explained in terms of a model based on film growth controlled by metal dissolution.

Anodic Activation of Aluminum by Trace Element Tin

Trace elements in Group IIIA-VA activate aluminum alloys anodically in chloride environment. The present focus is on trace element Sn. Surface segregation by heat treatment and resulting surface activation on model AlSn alloys, containing 30 to 1000 ppm of Sn, were characterized, respectively, by use of surface-analytical and electrochemical techniques. Tin segregated by annealing for 1 h annealing at 300oC, thereby causing significant activation, as characterized by pitting potential depression and high anodic currents produced. Activation was attributed to the segregated submicron particles. By increasing the annealing temperature to 600oC, tin was incorporated in solid solution. This reduced surface activation to an insignificant level for the AlSn alloy containing 30 ppm Sn. However, alloy containing 1000 ppm Sn remained active. Removal of the active surface layers resulting from the segregation of the activating metals passivated the surface. However, Sn in solid solution affected the entire material.

Correlation of the protection potential and the ohmic potential drop

Abstract A simple model based on the ohmic potential drop is proposed to explain the difference between the critical pitting and protection potentials. Results are compared with the experimental data of Wilde and Williams.

Theoretical Problems Related to Ohmic Resistance Compensation

Current interruption, potential pulse methods, and the AC impedance technique are well accepted means of measuring the ohmic potential drop to an electrode for ohmic resistance compensation. An interpretation of the measured quantities is given, and the relationship between the measured and true parameters is established. Theoretical calculations indicate that a significant discrepancy may exist between the measured and the true corrosion rate, although the data are corrected for the ohmic resistance in the solution by conventional means. The cause of the error is a nonuniform ohmic potential drop to the electrode surface, and its magnitude is determined by geometry, position of the reference electrode, solution conductivity, and corrosion rate. Such errors in practice are specific to low-conductivity media and occluded cells with gaps or crevices. The magnitude of expected error is calculated for a few typical geometries. Possible ways of correcting the error are discussed. These include probe design, measurement technique, and methods of data analysis.