The role of surface films and dissolution products on the negative difference effect for magnesium: Comparison of Cl^- versus Cl^- free solutions

The anodic dissolution and associated hydrogen evolution of high purity Mg (80 ppmw Fe) was studied as a function of potential in unbuffered 0.6 M NaCl (pH ≈ 8.5), 0.6 M NaCl saturated with Mg(OH)₂ (pH ≈ 10.25), 0.1 M MgCl₂ (pH ≈ 5.6), 0.1 M Na₂SO₄ (pH ≈ 5.5), and a 0.1 M Tris(hydroxymethyl)aminomethane hydrochloride (TRIS, pH ≈ 7.25) buffer solution via simultaneous mass loss, hydrogen volume collection, potentiostatic and potentiodynamic polarization, and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The negative difference effect (NDE) was substantial in the unbuffered Cl^- containing environments where Mg(OH)₂ formed on the surface and weak in 0.1 M Na₂SO₄. In contrast, the 0.1 TRIS buffered solution exhibited a positive difference effect and the absence of thick corrosion films. Mg(OH)₂ films formed on samples in Cl^- spalled off easily whilst the Mg(OH)₂ films formed in 0.1 M Na₂SO₄ were more tenacious, which suggests that film stability plays significant role in the NDE.