The negative-difference effect during the localized corrosion of magnesium and of the AZ91HP alloy

Abstract

Foram realizados testes galvanostáticos em soluções de NaCl com magnésio puro e com a liga AZ91HP, tendo sido medido o volume de hidrogênio desprendido durante os testes. A fim de estudar o efeito da diferença negativa (NDE), uma idéia foi desenvolvida para separar o desprendimento de hidrogênio (H.E) na superfície passiva do desprendimento dentro dos pites. A análise dos dados volumétricos mostrou que a corrente de redução de hidrogênio dentro dos pites, sobre o Mg, é muito mais alta do que nos pites sobre a liga AZ91HP. Supondo que a razão entre a taxa de redução de hidrogênio e a taxa de dissolução de magnésio, expressa pelo fator k, não muda com a polarização, foi encontrado um valor de aproximadamente 0,5 para o Mg puro. Para a liga AZ91HP foi alcançado somente um valor em torno de 0,3.Galvanostatic tests with pure magnesium and with the AZ91HP alloy were performed in NaCl solutions, and the volume of hydrogen evolved during the tests was measured. In order to examine the negative-difference effect, a concept was developed to separate the hydrogen evolution (H.E.) on the passive surface and the H.E. inside the pits. An analysis of the volumetric data showed that the current density of H.E. inside the pits is much higher for Mg than for the AZ91HP alloy. With the assumption, that the ratio between the hydrogen reduction rate and the rate of the anodic metal dissolution, expressed by the factor k, does not change with the polarization, a k value of ca. 0.5 for pure Mg was found, while for the AZ91HP alloy this value was only ca. 0.3.

The Influence of MgH2 on the Assessment of Electrochemical Data to Predict the Degradation Rate of Mg and Mg Alloys

“…Furthermore, the formation of Mg + did not explain the discrepancy between measured and calculated HER in the study of Abidin et al [20]. This possibly explains the findings of Dietzel et al [22] and Weber et al [6], where the HER in pits is much higher than at the surface. Lee et al [23] tried to interpret the NDE by a chunk breakage and the creation of a protective film at the surface, but they did not explain how the surface film is chemically composed.…”

“…Nowadays, Mechanism 2 is often used as an approach to explain the NDE with the existence of Mg + ions as stable intermediates [2,5,6,20], which produce hydrogen, according to Equation (4) [21]. However, Mg + might be adsorbed or solved in the vicinity of the surface and not able to take part in the electron transfer connected to the bulk.…”

Inhibitor-doped sol–gel coatings for corrosion protection of magnesium alloy AZ31

“…The dissolution is only partially blocked by the formed corrosion products, as MgO has a smaller molar volume compared to pure Mg, as characterized by a Pilling-Bedworth ratio lower than one. Thus, the formed Mg-oxide film is always porous and the resulting low pitting potential, more negative than the that of the hydrogen evolution reaction, will allow pits to grow with a fairly constant fraction of the dissolution current compensated by hydrogen evolution reaction (HER) inside the pit [9]. On the other hand, intermetallic phases act as local cathodes of microgalvanic cells with acceleration of corrosion.…”

Metal–Air Technology

“…This effect has been the subject of research dating from the 1950s with varying authors [32,33] attempting to explain the NDE by means of electrochemical reaction mechanisms. Atrens and Dietzel [34] discusses the two mechanistic approaches to the magnesium NDE which are (i) the unipositive Mg + ion NDE mechanism and (ii) the magnesium self-corrosion mechanism.…”