The galvanic corrosion of magnesium alloy AZ91D coupled to a steel fastener was studied using a boundary element method (BEM) model and experimental measurements. The BEM model used the measured polarization curves as boundary conditions. The experimental program involved measuring the total corrosion rate as a function of distance from the interface of the magnesium in the form of a sheet containing a mild steel circular insert (5 to 30 mm in diameter). The measured total corrosion rate was interpreted as due to galvanic corrosion plus self corrosion. For a typical case, the self corrosion was estimated typically to be $ 230 mm/y for an area surrounding the interface and to a distance of about 1 cm from the interface. Scanning Kelvin Probe Force Microscopy (SKPFM) revealed microgalvanic cells with potential differences of approximately 100 mV across the AZ91D surface. These microgalvanic cells may influence the relative contributions of galvanic and self corrosion to the total corrosion of AZ91D.
The study of the galvanic corrosion of magnesium becomes 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 properties and moderate cost. Corrosion, however, limits the application of magnesium alloys. [1][2][3][4][5][6][7] A number of different mechanisms are important for the corrosion. [8][9][10][11][12][13][14] But galvanic corrosion is probably the most important for magnesium because magnesium is the most active structural metal and consequently 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][17][18] Skar [17] showed that 6000 series aluminum alloy fasteners caused negligible galvanic 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 exposure. [16] To be able to design a structural component, incorporating 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 program to explore that aim. The total corrosion in the area of galvanic corrosion can be considered to be made up of the following 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 corrosion. 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 Teeple, [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 location. This provided helpful information to select magnesium for a particular location. However, the study was time consuming. 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 corrosion 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][21][22][23][24][25][26][27][28] The influence of the ele...
The boundary element method (BEM) was used to study galvanic corrosion using linear and logarithmic boundary conditions. The linear boundary condition was implemented by using the linear approach and the piecewise linear approach. The logarithmic boundary condition was implemented by the piecewise linear approach. The calculated potential and current density distribution were compared with the prior analytical results. For the linear boundary condition, the BEASY program using the linear approach and the piecewise linear approach gave accurate predictions of the potential and the galvanic current density distributions for varied electrolyte conditions, various film thicknesses, various electrolyte conductivities and various area ratio of anode/cathode. The 50-point piecewise linear method could be used with both linear and logarithmic polarization curves.
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