Hans Holtan

The corrosion of magnesium in aqueous solution containing chloride ions

Abstract Measurements have been performed in order to relate different aspects of the corrosion of magnesium. Magnesium is shown to be a naturally passive metal that undergoes pitting corrosion when exposed to chloride ions in non-oxidizing solutions. By polarizing the metal in the negative direction an arrest on the current density/potential curve is found, probably corresponding to an activation potential. This potential determines the negative limit of the passive range. The “negative difference effect” is proposed to be a direct result of the breakdown of passivity.

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.

Use of potentiokinetic methods for the determination of characteristic potentials for pitting corrosion of aluminium in a deaerated solution of 3%NaCl

Abstract Three different potentiokinetic methods have been used to determine the pitting potential ( E p ) and the protection potential against pitting ( E pp ) for unalloyed aluminium in a deaerated solution of 3% NaCl at 0 and 30°C. These three methods are: (1) Continuous potentiokinetic (potentiodynamic) method, (2) quasi-stationary potentiokinetic method, and (3) stationary potentiokinetic method. From the results of these measurements it is found that the potentials are strongly dependent on which one of these three methods is being used. Also, under the special conditions and the use of the stationary potentiokinetic method, it seems like two different potentials do not exist, as has been assumed in earlier work in the same field.

Determination of characteristic pitting potentials for aluminium by use of the potentiostatic methods

Abstract Potentiostatic methods were used to determine the pitting potential, E p , and the protection potential against pitting, E pp , for unalloyed aluminium in a de-aerated solution of 3 %NaCl at 0 and 30°C. The results show that the pitting potentials determined by these methods are more active than the pitting potentials determined by potentiokinetic and galvanokinetic methods. 1,2 The results also prove the existence of only one characteristic potential for pitting corrosion on aluminium as indicated in earlier papers. The temperature has a marked influence on the pitting potential which decreases with increasing temperature.

Cathodic polarization of aluminium in acetate-buffered chloride media

Abstract Polarization behaviour of 1S aluminium has been studied in chloride solutions containing equal molar amounts of acetic acid and sodium acetate. The effects of stirring and buffer capacity have been tested. The oxide film becomes highly permeable to the passage of protons when acetate is added compared to solutions of neutral pH containing NaCl alone. The transport of Al 3+ to the solution is negligible in relation to the proton flux towards the metal in the presence of a cathodic field, and the film is thus protective against significant metal dissolution. As the potential is decreased to more negative values, a limiting-current behaviour is observed followed by film breakdown below a certain threshold potential. The limitation is ascribed to the blockage of the paths for proton transport through the film by the hydrogen gas diffusing away from the metal surface. Possible causes of cathodic film breakdown are discussed.

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.

Use of Galvanokinetic Methods for the Determination of Characteristic Potentials for Pitting Corrosion on Aluminum

Abstract Three different galvanokinetic methods have been used to determine the pitting potential (Ep) and the protection potential against pitting (Epp) for unalloyed aluminum in a deareated solut...

Improving the Corrosion Resistance of Aluminum Alloys by Cathodic Polarization in Aqueous Media

Abstract Corrosion of aluminum alloys is often closely related to the type, amount, and properties of intermetallic phases present in the matrix. The phases that contain iron as a component are especially detrimental to corrosion resistance. These phases can be dissolved preferentially, thereby achieving a relatively particle-free surface, by various chemical and electrochemical means. An efficient method appears to be cathodic polarization within a certain potential range in unbuffered salt solutions of neutral pH. It is shown that the method leads to an appreciable improvement of the corrosion resistance of alloys in the 1000, 3000, and 6000 series. The particles are removed physically as a result of crevice corrosion of the matrix adjacent to their surface. The rate of removal depends on the extent to which the cathodic reaction can be depolarized on the particles relative to the substrate. The mechanism is investigated by electrochemical and microanalytical means.

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.