P. Volovitch

Mg Dissolution in Phosphate and Chloride Electrolytes: Insight into the Mechanism of the Negative Difference Effect

Magnesium (Mg) dissolution was investigated using an online measurement of dissolved Mg and simultaneous time-resolved volumetric analysis of hydrogen gas evolution. Testing was performed at open-circuit potential and under anodic polarization in neutral, unbuffered chloride and buffered phosphate solution. In the unbuffered chloride electrolyte, electrochemical dissolution occurs with a stoichiometry of n = 2, and a significant enhancement of the corrosion rate at open-circuit dissolution rate was observed following anodic polarization. The extent of the enhancement increased with anodic polarization time requiring at least 500 s to become detectable. In a phosphate buffer solution, neither the corrosion rate enhancement nor the negative difference effect (NDE) were observed. The kinetics of hydrogen evolution in nonbuffered solutions tends to support a mechanism involving the activation of the cathodic reaction related to the disruption of a protective surface film.

Revisiting the Electrochemical Impedance Spectroscopy of Magnesium with Online Inductively Coupled Plasma Atomic Emission Spectroscopy

The electrochemical impedance of reactive metals such as magnesium is often complicated by an obvious inductive loop with decreasing frequency of the AC polarising signal. The characterisation and ensuing explanation of this phenomenon has been lacking in the literature to date, being either ignored or speculated. Herein, we couple electrochemical impedance spectroscopy (EIS) with online atomic emission spectroelectrochemistry (AESEC) to simultaneously measure Mg-ion concentration and electrochemical impedance spectra during Mg corrosion, in real time. It is revealed that Mg dissolution occurs via Mg(2+) , and that corrosion is activated, as measured by AC frequencies less than approximately 1 Hz approaching DC conditions. The result of this is a higher rate of Mg(2+) dissolution, as the voltage excitation becomes slow enough to enable all Mg(2+) -enabling processes to adjust in real time. The manifestation of this in EIS data is an inductive loop. The rationalisation of such EIS behaviour, as it relates to Mg, is revealed for the first time by using concurrent AESEC.

The effect of absorbed hydrogen on the dissolution of steel

Atomic hydrogen (H) was introduced into steel (AISI 1018 mild steel) by controlled cathodic pre-charging. The resultant steel sample, comprising about 1 ppmw diffusible H, and a reference uncharged sample, were studied using atomic emission spectroelectrochemistry (AESEC). AESEC involved potentiodynamic polarisation in a flowing non-passivating electrolyte (0.6 M NaCl, pH 1.95) with real time reconciliation of metal dissolution using on-line inductively coupled plasma-atomic emission spectroscopy (ICP-OES). The presence of absorbed H was shown to significantly increase anodic Fe dissolution, as evidenced by the enhanced detection of Fe2+ ions by ICP-OES. We discuss this important finding in the context of previously proposed mechanisms for H-effects on the corrosion of steels.

On the Origin of the Second Anodic Peak During the Polarization of Stainless Steel in Sulfuric Acid

The origins of the second anodic current peak in polarization curves of AISI 430 (UNS S43000) stainless steel in deaerated 0.1 M sulfuric acid (H2SO4) have been investigated by potentiodynamic pola...

Investigating the structure of the surface film on a corrosion resistant Mg-Li(-Al-Y-Zr) alloy

The surface film formed on an ultra-lightweight Mg-Li(-Al-Y-Zr) alloy was investigated. Previous research reported that this body-centered cubic (bcc) Mg-Li(-Al-Y-Zr) alloy demonstrated high corros...