DOI: 10.1002/adem.200600206 Experimental Measurement and Computer Simulation of Galvanic Corrosion of Magnesium Coupled to Steel** By Jimmy X. Jia, Guangling Song and Andrej Atrens* The study of the galvanic corrosion of magnesium be- comes increasingly important as the use of magnesium alloys increases rapidly in the auto and aerospace industries due to their advantages of light-weight, adequate mechanical prop- erties and moderate cost. Corrosion, however, limits the ap- plication of magnesium alloys. [1–7] A number of different mechanisms are important for the corrosion. [8–14] But galvanic corrosion is probably the most important for magnesium be- cause magnesium is the most active structural metal and con- sequently may suffer serious corrosion when joined to all other common metals of construction, such as aluminum or steel. [1,2,15] Fasteners and their galvanic corrosion is of major concern in automotive applications. [16–18] Skar [17] showed that 6000 series aluminum alloy fasteners caused negligible galva- nic corrosion of magnesium in the salt spray test. However, steel fasteners are desired for many applications due to their inherently better mechanical properties. Poor compatibility with magnesium was shown by aluminium-coated steel fasteners [18] whereas steel fasteners with zinc or tin-zinc alloy coatings were compatible with magnesium in salt-water ex- posure. [16] To be able to design a structural component, incor- porating galvanic corrosion, it is useful to be able to simulate the galvanic corrosion distribution qualitatively. The research presented in this paper has been undertaken as part of a pro- gram to explore that aim. The total corrosion in the area of galvanic corrosion can be considered to be made up of the fol- lowing two components: (1) galvanic corrosion and (2) self corrosion. The galvanic corrosion is that part of the corrosion, which is directly caused by the coupling of the magnesium to a steel fastener. The self-corrosion is defined as the extra cor- rosion. Both the galvanic corrosion and the self-corrosion may take the form of more or less general corrosion, or the form of localized corrosion or pitting corrosion. Prior studies of galvanic corrosion of magnesium have been scarce. The earliest study, started in the 1950s by Tee- ple, [19] investigated the influence of location and climate. This study revealed that different locations produced different corrosion rates because of the different electrolyte properties of the condensed film on the metal surface. [19] Atmospheric galvanic corrosion could be detrimental for magnesium in one location whilst it was almost harmless in another loca- tion. This provided helpful information to select magnesium for a particular location. However, the study was time con- suming. It is often not practical to wait years to have the test results for each particular service location. Limited studies have addressed the effect of electrolyte on the galvanic corro- sion of magnesium. More effort has been focused on general corrosion, particularly on the influence of the ion species in solution, and the influence of cathodic impurity elements in the alloy. [20–28] The influence of the electrolyte involves many factors such as pH, composition, concentration of the ions and the conductivity of the electrolyte. The influence of con- ductivity on galvanic corrosion has been studied in terms of “macroscopic” and “microscopic” galvanic cells, [29] and in terms of the Wagner number. [30,31] Recent research revealed the “alkalization”, “passivation” and “poisoning” effects dur- ing salt spray exposure. [15] In addition, the galvanic cell ge- ometry significantly influences the galvanic corrosion beha- viour. Geometrical factors include the anode/cathode area ratio, the insulation distance between the anode and cathode, electrolyte film thickness and the shapes of the anode and cathode. [32] Furthermore, there could be more than one galva- nic couple such as many steel fasters for one magnesium part, and there could be an interaction of the current caused by each galvanic couple. Various methods have been employed to measure galvanic corrosion. Galvanic corrosion of magnesium has been mea- sured in terms of weight loss and pitting depth. [33–37] The weight loss and pitting depth methods actually measure the total corrosion rate, which is the sum of general and galvanic corrosion. These methods are not accurate in terms of just the galvanic corrosion. The method of surface potential scanning provides the surface potential distribution, [38] but the poten- tial is not directly related to the galvanic corrosion rate. The present study measured the galvanic current, which directly relates to the galvanic corrosion rate. The BEM is a relatively new numerical method and is be- coming more important with recent developments in com- puter technology. Numerical methods have been utilized in COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2007, 9, No. 1–2 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 65 – [*] Dr J. X. Jia, Dr G. Song, Prof. A. Atrens CRC for Cast Metals Manufacturing (CAST) Division of Materials, School of Engineering The University of Queensland St Lucia, QLD 4072, Australia E-mail: a.atrens@minmet.uq.edu.au [**] The authors would like to acknowledge the financial support of the Cooperative Research Centre for Cast Metals Manufactur- ing (CAST). CAST was established and is funded in part by the Australian Government’s Cooperative Research Centres Program.