Determine the galvanic corrosion rate in aerated 3% NaCl of the anode in a coupl

Question

Determine the galvanic corrosion rate in aerated 3% NaCl of the anode in a couple between equal areas of (a) carbon steel (iron) and 316 stainless steel and (b) between equal areas of 316 stainless steel and titanium (c) show the effect of a 15:1 ratio of cathode to anode in (a) (d) explain the difference, if any, between the galvanic corrosion rates in (a) and (b).

Penetration Density Equivalent Rate Equivalent (glem) Weight MetaVAlloy to 1 .vcm2 (mapy) Oxidation Density Equivalent Rate Equivalent (plem Weight to 1 HAJem2 (mpy) Metal/Alloy 24.62 Pure metals 27.92 29.36 25.50 8.90 8.96 103.59 32.69 59.34 23.95 11.34 20Cb3 23.98 Nickel alloys Zirconiuma 30.12 26.41 0.40 Aluminum alloys AA1100 AI/3 Mg/2. AA 3004 AU3,Mg/2 AI/3,Mg/2 C-276 Ni/2 Fe2 8.89 27.09 AA6061 AA7072 AA7075 Cu/l.Nb/4 Copper alloys CDA1IO CDA260 CDA280 8.39 Source: Adapted fronm Proposed Standand, ASTM GO1.11, with pennission, ASTM. Philadelphi 32.00 Cu/2.Za/2 modynamic parameter characteristic of an electrochemical reaction (Section 2.1.1). i, is the analogous fundamental kinetic parameter. Neither free energy (electrode potential) or i° can be calculated from first principles: both must be determined experimentally. Exchange current density is affected foremost by the nature of the surface on which it occurs. Figure 3.1 shows the half-cell electrode potential for the hydrogen reaction (6) plotted versus i Two facts must be emphasized from this fig- ure. First, the surface on which the reaction occurs has no effect on the electrode potential. Second, the i in contrast, is strongly affected by the surface. That is, AG the thermodynamic energy change is not affected by surface, but the reaction kinet- ics, as measured by ign are highly sensitive to surface properties CDA687 27.76 CDAS10 CDAS24 Cu/2.Sn/4 Cu/2.Si4 28.51 0.46 CDA700 CDATIS CDA752 Cu/2,Nin 8.94 31.46 The exchange current density for reaction (6) on mercury is only 10 A/em2 but is nine orders of magnimde higher on platinum at 10" A/cm.. In fact, platinum, is widely known as a catalyst because most reactions occur very rapidly on platinum surfaces. while the platinum itself is unreactive (Section 2.1.2). Exc Stainless steels 25·12 0.41 25.13

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