Galvanic corrosion Fe/Zn (Evans)

Files:
Coupled corrosion (Evans).EXP
Coupled corrosion (Evans) 07.CRV

Coupled corrosion occurs when two different materials subject to corrosion are in contact with each other and with the same solution. The less noble material (which is the more reactive) will suffer from oxidation (corrosion itself) while the more noble material will undergo a cathodic reaction (reduction process) as predicted by the thermodynamic potentials of these materials. Coupled corrosion is also called bimetallic corrosion or galvanic corrosion. Evans experiments determine the corrosion current (and therefore the corrosion rate) which is expected to take place at "rest" potential. This rest potential is also called a mixed potential. This corrosion current and this mixed potential are experimental values that take into account the surfaces of the two materials. To run the experiment properly, the more electronegative element will be the anode (zinc for a zinc/iron system) to be used as the working electrode (WORK). The less electronegative element will be the cathode (iron for a zinc/iron system) to be used as the counter electrode (AUX). A reference electrode (REF) must be used.

SolutionKCl 0.0552 M in water at room temperature
WORK Zinc (Zn plate)
REF Calomel electrode (XR110)
AUX Iron (Fe plate)

The current step is 1.01 µA with a current drift of 3 mV/min and a maximum time of 30 sec.

1) Display: Type = "Evans"

The WORK measured versus the REF (Potential) is displayed and in addition the potential of the AUX measured versus the REF (Auxiliary) is displayed in dashed lines versus the TOTAL current.

2) Display: Type = Normal X = Current Y1 = Potential Y2 = Auxiliary

The WORK measured versus the REF (Potential) is displayed in Y1 and in addition the potential of the AUX measured versus the REF (Auxiliary) is displayed in Y2 versus the current DENSITY = current/WORK area. This is the difference with the "Evans" display.

3) Display: Type = Evans and estimate the corrosion rate with "Processing Evans plot"

Evans (galvanic corrosion)-------------- 30-12-1997
Working area (cm²) :0.385
Auxiliary area (cm²) :0.196
Potential : -1009.4 mV
Current : 13.561 µA

The curve can also be processed with a modification of the area values to extrapolate the estimation of the galvanic corrosion current. If you enter, for instance, a 1 cm² value for the working electrode area, you obtain a slightly different estimation for the corrosion current.

4) Display: Type = Evans and estimate the corrosion rate with "Processing Evans plot" with a different surface for the electrodes.

Evans (galvanic corrosion)-------------- 20-01-1999, 11:14:35
Working area (cm²) :1
Auxiliary area (cm²) :0.196
Potential : -1009.2 mV
Current : 13.559 µA

For Zinc/Iron, the Zinc is the sacrificial anode. The Coupled corrosion (Evans) master method plots an Evans diagram in real time. The Evans diagram plots the potential of the two materials as a function of the imposed current. It is possible to process the experimental values and to modify the areas to evaluate the corrosion current at rest potential.