The influence of technological factors on electrochemical characteristics of cast magnesium cathodic protective elements

• V. Kechin¹, E. Lyublinski² and A. Kireev¹

¹ Vladimir State University, ul. Gor’kogo, 87, Vladimir, 600000 Russian Federation
² COR/SCI, LLC, 6421 Dorset Ln., Solon, OH 44139, USA

Abstract: Cathodic protection systems with magnesium cast sacrificial anodes are widely used for corrosion protection of metallic structures in sea water and soil. The accumulated experience in the application testifies to the dispersion of current capacity (CC) values up to 20%. As a result, the service life and economic efficiency of protection is unpredictable. Magnesium alloys have low CC (45–60%). Studies have revealed that uncontrolled content of oxygen and hydrogen play an important role in stabilizing the CC of magnesium alloys. Oxygen present in magnesium alloys in the form of suspended oxide inclusions changes the basic electrochemical properties of anodes. In open melting, the oxygen content (in terms of MgO) in magnesium melts is 0.001–0.02%. It has been established that the alloy components should contain less than 0.01% of oxygen to ensure maximum and stable CC. The solubility of hydrogen in the interaction with magnesium leaps in the transition of metal from liquid to solid state, contributing to the formation of gas porosity in the cast metal. The increase of hydrogen content in magnesium melts from 5 to 28 cm³/100 g leads to reduction of CC values from 1310–1340 to 1100–1260 A·hr/kg. A similar result was observed on the change of the magnesium alloys CC depending on the gas content. It is also established that the morphology of the crystalline structure of the cast sacrificial anodes has a significant influence on the stabilization of the magnesium alloys’ CC. It is shown that the crushing of the cast structure of magnesium anodes from high purity alloys of the Mg–Al–Zn system provides not only maximum CC values but also reduces the spread of these values. Thus, in order to improve and stabilize the main electrochemical properties of the sacrificial anodes, technological processes have been developed that ensure in the process of the production of anodes the control of the required purity of melts on the maximum allowable content of cathodic impurities (Fe, Cu, Ni) and the restriction in the process of melting oxide inclusions and hydrogen with the mandatory conditions of casting, ensuring the grinding of the structure of cast anodes.

Keywords: magnesium, compounds, impurities, structure, electrochemical properties

Int. J. Corros. Scale Inhib., 2019, 8, no. 3, 600-612 PDF (1 206 K)
doi: 10.17675/2305-6894-2019-8-3-10

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