The role of surface films and dissolution products on the negative difference effect for magnesium

Abstract

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. Show more