Investigation of Mg / Al alloy sacrificial anode corrosion with Scanning Electrochemical Microscopy

Corrosion of Mg/Al alloy was investigated with the scanning electrochemical microscope (SECM) using a Mg ion selective microelectrode (ISME) as measuring tip. Mg/Al alloys have practical application as sacrificial anodes to prevent the corrosion of various metals susceptible to corrosion. A typical application of such sacrificial anodes is in water boilers. A model (Mg/Al)/Fe target was fabricated using boiler sacrificial anode and high purity iron wires. Corrosion current was measured between the electrically connected galvanic pair, and Mg ion flow rate was calculated with Faraday’s law of electrolysis. However, Mg flow rate could not be calculated with this method while the twometals were left electrically-disconnected. For this reason, fixed height lateral scans and retreating scans were performed above the Mg sample with Mg ion-selective electrodes employed as SECM tips. Mg flow rate was estimated using the concentration profiles obtained with SECM. Relatively high-speed scans were possible with robust, solid contact Mg micropipette electrodes, owing to their low resistance compared to conventional micropipettes of the same size.


Introduction
As it is well known, by electrically connecting two metals with different redox character, a galvanic pair is formed.The more reactive metal has a lower redox potential than the other one.The galvanic connection can provide cathodic protection against corrosion of the less reactive metal.When the two are coupled, electrochemical oxidation takes place at the more reactive metal surface, while at the less reactive one, reduction proceeds -most often of oxygen.The more reactive metal gets dissolved, preventing the corrosion of the other.Because of this property, such protective metals are called sacrificial anodes or active anodes.The protection persists until the entire sacrificial anode is dissolved, then the less reactive metal starts to develop anodic sites as well, and starts to corrode.Because magnesium is among the most often used sacrificial anodes, and iron has a widespread use, it was worth to investigate the galvanic corrosion of this galvanic pair.Scanning Electrochemical Microscopy (SECM) is a relatively new, powerful technique, recently started to be applied in corrosion science [1,2].Because corrosion processes of metals involve the formation of metal ions, SECM technique with ion-selective microelectrode (ISME) measuring tips provides a method in corrosion sciences.Corrosion of a (Mg/Al)/Fe model galvanic pair target was studied.The Mg/Al sample was made from an actual sacrificial anode used in water boilers.
(Mg/Al)/Fe target (Mg/Al)/Fe galvanic couple target was prepared from 70/30 Mg/Al alloy (boiler sacrificial anode), and high purity Fe wires.The diameter was 0.67 mm for both metal samples.They were mounted in an epoxy resin sleeve, exposing only the disc shaped surfaces.Electrical contact was provided at the rear end of the mould.Frontal surface of the mould was first polished with sandpaper, then with 1.0, 0.3, and 0.03 µm alumina powder.

Corrosion current
Corrosion current cannot be measured directly, since the measurement itself would alter the current flow.However, it is possible to calculate it by measuring the voltage drop on the two sides of a variable resistor, which connects the Mg and Fe samples together.Plotting the voltage over the resistance (E/R) with respect to resistance, the "y" interception will be 1/i at R = 0, after taking the reciprocal, corrosion current is obtained.Using Faraday's law of electrolysis, Mg 2+ ion flow rate from the Mg/Al sample was determined.

SECM scans
Mg 2+ ion concentration profiles above the Mg sample were recorded by SECM scans.Vertical Mg 2+ ion concentration distribution was determined at different instants in time of the cor-Per.Pol.Chem.Eng.

Conclusion
Most of the SECM studies in the literature have been carried out with amperometric microtips.The applicability of new potentiometric tips has been tested in this work.SECM is a powerful tool in the field of corrosion science.Mg 2+ ion-selective electrodes combined with SECM can be used to determine Mg 2+ ion concentration distribution in a corroding system, with, and without coupling.Useful information, such as lateral and vertical concentration distribution, can be collected concerning the corrosion of metals.The gathered information can be used to calculate other parameters such as ion flow rates.This new method can be used to study the corrosion of any metal, substituting the tip with the appropriate ion-selective microelectrode.It is expected that the efficiency of corrosion protecting coatings can be tested using SECM with potentiometric tips.

Fig. 1 .
Fig. 1.Schematic design of the Mg 2+ ISME used as measuring tip in the experiments.

Fig. 2 .
Fig. 2. Mg/Al-Fe model target in epoxy sleeve.Figure also shows the variable resistor used for estimation of the corrosion current flow between the two metals.

Fig. 5 .
Fig. 5. Concentration-distance lateral scans at z = 100 µm height for different instants in time over the corroding Mg/Al sample measured with the Mg 2+ ISME tip.

Fig. 6 .
Fig. 6. 1/i plots used for the determination of corrosion current between the Mg/Al and various Fe samples of different diameters.