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EIS studies of the corrosion of Mg alloys in NaCl solutions

A summary of the available Rp data measured by the EIS for various Mg alloys is given below:

Some remarks concerning the corrosion of Al-Mg alloys

(Working notes)

1. Virtually any Al-containing Mg alloy consists of two main phases (the very important influence of minor additives is ignored at this stage):

α-phase: essentially pure magnesium

β-phase: Mg17Al12 (calculated 55.7wt% Mg)

2. Considering the equilibrium phase diagram for the system Al-Mg, β-phase should be more exactly referred to as γ-phase, because a congruent compound is found at ~ 50% Al:

Is it in equilibrium?

An interesting discussion took place in a meeting with students.

One student asked if gaseous oxygen is in equilibrium with the oxygen dissolved in water. The issue is that at ambient conditions (25oC and 1 atm) the solubility of oxygen in water is very low, about 40 mg/L. Therefore, if oxygen is formed in a chemical reaction in the aqueous phase, it volaties practically completely. If, for example, we take 1 liter of 20% aqueous H2O2 and add an appropriate catalyst (say, MnO2), the fast reaction will occur:

About the software for fitting EIS. Part 5

Part 1

Part 2

Part 3

Part 4

EIS Spectrum Analyzer is free of pay for commercial use, which is of course an advantage. It works fairly good for simple impedance spectra, but with our non-trivial case it could not result in anything satisfactory:

About the software for fitting EIS. Part 4

Part 1

Part 2

Part 3

ZSimWin Demo is a nice program for fitting equivalent circuits. The demo version allows to see only plot and the value of Chisquare, but the visual results are really impressive:

Equivalent circuit #1: R(RC)(RQ[RL])

Chisq = 8.19E-4

ZSim

About the software for fitting EIS. Part 2

(Part 1)

So, we are starting from IviumSoft. The latest version available for download to the moment of writing this post is 2.402.

General remarks, all positive:

(+) The program is available for free fownload.

(+) New releases appear every couple of months.

(+) No installation is necessary. You just unzip the downloaded archive and launch IviumSoft.exe.

(+) A comprehensive help system is present.

(+) Elegant intuitive interface, almost no additional "polish" of plots is necessary.

About the software for fitting EIS

One of the most frequent problems with the interpretation of results of Electrochemical Impedance Spectroscopy (EIS) is fitting experimental results to an equivalent scheme. I will try to compare here some programs, both free and commercial, with which I have some experience. All these programs will be tested on a EIS spectrum measured in a corrosion experiment, in which a specimen of a magnesium alloy was immersed into salt water until a stable corrosion potential had been achived. The experimental EIS spectrum is given below:

A story of one false discovery

Two postgraduates (each in the framework of his/her research) had to compare corrosion rates of magnesium alloys in salt water measured by several techniques. Both obtained weird results, which can be summarized as follows:

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