K. Ogle

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.

Factors Affecting MoO42– Inhibitor Release from Zn2Al Based Layered Double Hydroxide and Their Implication in Protecting Hot Dip Galvanized Steel by Means of Organic Coatings

Zn2Al/-layered double hydroxide (LDH) with intercalated MoO4(2-) was investigated as a potential source of soluble molybdate inhibitor in anticorrosion coatings for hot dip galvanized steel (HDG). The effect of solution pH, soluble chlorides, and carbonates on the release kinetics of the interleaved MoO4(2-) ions from the LDH powder immersed in solutions containing different anions was studied by X-ray diffraction, in situ attenuated total reflectance infrared (ATR-IR) spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The effect of the solution composition on the total release and the release kinetics was demonstrated. Less than 30% of the total amount of the intercalated MoO4(2-) was released after 24 h of the immersion in neutral 0.005-0.5 M NaCl and 0.1 M NaNO3 solutions whereas the complete release of MoO4(2-) was observed after 1 h in 0.1 M NaHCO3 or Na2SO4 and in alkaline solutions. The in situ ATR-IR experiments and quantification of the released soluble species by ICP-AES demonstrated the release by an anion exchange in neutral solutions and by the dissolution of Zn2Al/-LDH in alkaline solutions. The anion exchange kinetics with monovalent anions was described by the reaction order n = 0.35 ± 0.05 suggesting the diffusion control; for divalent anions, n = 0.70 ± 0.06 suggested the control by a surface reaction. Dissolution of Zn from coated HDG with and without Zn2Al/-MoO4(2-) fillers, leaching of MoO4(2-) from the coating, and the electrochemical impedance spectroscopy response of the coated systems were measured during the immersion in 0.5 M NaCl solutions with and without 0.1 M NaHCO3. Without carbonates, the release of soluble MoO4(2-) was delayed for 24 h with no inhibiting effect whereas with 0.1 M NaHCO3 the immediate release was accompanied by the immediate and strong inhibiting effect on Zn dissolution. The concept of controlling the inhibition performance of LDH hybrid coatings by means of the environment composition is discussed.

A Mathematical Model for Cathodic Delamination of Coated Metal Including a Kinetic pH–Porosity Relationship

A mathematical model for the delamination of coating from active metal was developed in the presence of multiple electrochemical reactions, homogeneous reactions, and precipitation of corrosion product. The computational results illustrate that the propagation of potential front is controlled by the production of OH - ions along the metal-coating interface, while that of porosity front is limited by the rate constant of bond-breakage reactions. The movement of the two fronts are close when the time constant associated with breaking bonds is small as compared to that of diffusion and migration processes. When the time constants of bond-breakage and coating degradation are large, the velocity difference between the potential and porosity fronts becomes large. The delamination rate and delamination mechanism predicted from the model are in good agreement with experimental interpretations. A systematic sensitivity analysis for the fitting parameters used in the program was conducted and the results show that the computations are most sensitive to the parameters in the porosity-pH relation.

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...