Scrape Potential Measurements on Aluminum Alloys

Scrape potential measurements were made on aluminum alloys Type 1100 and 7075 in solutions of NaNO₃, Na₂SO₄, K₂SO₄, NaClO₄, NaCl, and Na₂CrO₄. The measured parameters,(V_sc), the scrape potential, (V_ss), the steady state potential, (ΔV), the differences between the steady state potential and the scrape potential, and the rate at which the alloy returns to the steady state potential from the scrape potential (dV/dt), are all influenced by the nature of the anion in solution. The scrape potential is also a function of electrolyte concentration, and all parameters vary with alloy composition. A comparison of results in Na₂CrO₄ and NaCl indicate that the scrape potential is more negative and the rate dV/dt is considerably higher in the solution containing the passivating ion. These studies support the thesis that anions enter into the earliest steps of the metal dissolution reaction in a specific way by directing the kinetics of formation and the composition of specific surface complexes or compounds.