The influence of the presence of hydrogen bubbles adhered to the corroding specimen surface on the EIS estimations of corrosion rate of magnesium alloys is considered. The EIS corrosion rates for AZ31 specimens exposing a small area were four times higher than the real values determined by gravimetric or hydrogen evolution measurements. In contrast, no significant differences in the corrosion rate have been observed in the case of the AZ61 specimens. A progressive deterioration in the protective properties of the corrosion products layer formed during the test was observed only on AZ31 specimens exposing a small area.
The influence of heat treatment of magnesium alloy substrates on corrosion resistance of a sol-gel coating has been assessed during immersion tests in 0.6 M NaCl aqueous solution. Relative differences in the chemical nature of the layers were quantified by scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDX). Corrosion behaviour was evaluated by Electrochemical Impedance Spectroscopy (EIS) and hydrogen evolution measurement. Long-term immersion testing show that the sol-gel/heat treated AZ61 substrate exhibits a superior anti-corrosion property in comparison with the sol-gel/non-heated substrate. In contrast, no significant changes have been observed between the heated and non-heated samples in the case of the sol-gel coated AZ31 substrates. A link was found between lower O/Si atomic ratios observed by EDX analysis on the sol-gel coatings after the preparation process and reduced corrosion upon the coated substrates. Heat-treatment increased the protective properties of the passive film on the surface of the AZ61 substrate and hence inhibited magnesium dissolution and hydrophilic group formation during coating preparation.
The corrosion inhibition effect of the following organic additives: 2-butin-1,4-diol (BD), N,N 0 -dimethylthiourea (DMTU), N,N 0 -diethylthiourea (DETU) and N,N 0 -diisopropylthiourea (DITU), in steel pickling baths (sulphuric acid solutions) was studied by means of weight loss and electrochemical measurements like open circuit potential (OCP), polarization curves and impedance spectroscopy (EIS). A significant decrease in the corrosion rate was observed in presence of the investigated compounds depending on the nature and the concentration of the additives. The weight loss experiments show an inhibitory efficiency that reaches values beyond 98 %. The protective effect increases with the enlargement of the lateral chain size in the case of the thioureas. The electrochemical experiments show also a protective effect of the organic additives against corrosion. Their adsorptive behaviour is found to follow the Langmuir model.
The authors review their recent research work in an endeavour to interpret the influence of native oxide films on the corrosion behaviour of commercial AZ31 and AZ61 magnesium alloys and the oxidation kinetics of these alloys in air at 200ºC. The tendency of some of these thin films to be sufficiently protective in mild or weak corrosive environments was examined. For obtaining oxide films with different protective properties, some of the specimens were tested with the surface in the as-received condition, while others were tested immediately after mechanical polishing. The techniques applied to characterise thin oxide films of a thickness of just a few nanometres present on the surface of alloys has mainly been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Oxidation resistance of the alloys was quantified by thermogravimetric (TG) curves. Their corrosion rate was evaluated by Electrochemical Impedance Spectroscopy (EIS) and hydrogen evolution measurement in chloride solutions with different concentrations. Emphasis was placed on the possible effects of: (a) the different thickness of the native oxide films formed on the polished surfaces on the corrosion behaviour of the alloys; and (b) the different film homogeneity and uniformity on the oxidation results [1-5]. In parallel studies, an attempt was made to learn more about the influence of the native oxide films that covers the substrate on the subsequent growth and protective behaviour of the sol-gel coatings. In this way, the influence of heat treatment of magnesium alloy substrates on corrosion resistance of a sol-gel coating has been assessed during immersion tests in 0.6M NaCl aqueous solution. Relative differences in the chemical nature of the layers were quantified by scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDX). Long-term immersion testing showed that the sol-gel/heat treated AZ61 substrate exhibited a superior anti-corrosion property in comparison with the sol-gel/non-heated substrate. In contrast, no significant changes have been observed between the heated and non-heated samples in the case of the sol-gel coated AZ31 substrates. A link was found between lower O/Si atomic ratios observed by EDX analysis on the sol-gel coatings after the preparation process and reduced corrosion upon the coated substrates. The thermal treatment increases the protective properties of the passive film on the surface of the AZ61 substrate and hence inhibited magnesium dissolution and hydrophilic groups formation during coating preparation [5-7]. Finally, some preliminary results are shown on our studies in progress about the modification of these sol-gel coatings with different inhibitor (L-cysteine, benzotriazole and 8-hydroxyquinoline) in an attempt to improve the corrosion performance of the AZ31 and AZ61 magnesium alloys. Acknowledges: This work was supported by the Ministry of Economy and Competitiveness of Spain (Project MAT2015-65445-C2-1-R). References Feliu Jr., S., Maffiotte, C., Samaniego, A., Galván, J.C., Barranco, V., Effect of naturally formed oxide films and other variables in the early stages of Mg-alloy corrosion in NaCl solution, Electrochimica Acta, 56 (12), pp. 4554-4565 (2011). Feliu Jr., S., Maffiotte, C., Samaniego, A., Galván, J.C., Barranco, V., Effect of the chemistry and structure of the native oxide surface film on the corrosion properties of commercial AZ31 and AZ61 alloys, Applied Surface Science, 257 (20), pp. 8558-8568 (2011). Feliu, S., Maffiotte, C., Galván, J.C., Barranco, V., Atmospheric corrosion of magnesium alloys AZ31 and AZ61 under continuous condensation conditions, Corrosion Science, 53 (5), pp. 1865-1872 (2011). Feliu S. Jr, Samaniego A., Barranco V., El- Hadad A.A., LLorente I., Serra C., Galván J.C., A study on the relationships between corrosion properties and chemistry of thermally oxidised surface films formed on polished commercial magnesium alloys AZ31 and AZ61, Applied Surface Science, 295, pp 219-230 (2014). Feliu S. Jr., El hadad A.A, Barranco V., Llorente I., García-Galván F.R., Jiménez-Morales A., Galván J.C., Native Oxide Films on AZ31 and AZ61 Commercial Magnesium Alloys: Corrosion Behaviour Resistance, Effect on Conversion Coating Growth and Sol-gel Thin Film Formation. In “New Trends in Alloy Development, Characterization and Applications”. Editor: Zaki Ahmad. Publisher: InTechOpen, September 2015. El hadad A.A., Barranco V., Samaniego A., Llorente I., García-Galván F.R., Jiménez-Morales A., Galván J.C., Feliu Jr. S., Influence of substrate composition on corrosion protection of sol-gel thin films on magnesium alloys in 0.6 M NaCl aqueous solution, Progress in Organic Coatings 77 (11) 1642–1652 (2014). Diaz, L., García-Galván, F.R., Llorente, I., Jiménez-Morales, A., Galván, J.C., Feliu Jr, S., Effect of heat treatment of magnesium alloy substrates on corrosion resistance of a hybrid organic-inorganic sol-gel film, RSC Adv., 5, 105735-105746 (2015).
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