Cathodic Protection Training Course
The Daniel cell and such measuring techniques
The main point of the laws of thermodynamics relating to corrosion and cathodic protection is that the energy released by the chemical reaction between metals and their environments must complete a circuit to reach equilibrium. Everything must balance out.
The metal disolves, giving off energy, which we can measure using a digital voltmeter.
This energy can be called Electric Motive Force or EMF. This adds to the property known as 'electrical potential'of the eletrolyte which has disolved the metal.
This electrolyte is then charged to a higher potential than the surrounding electrolyte to which the energy is radiated.
It is VERY IMPORTANT to understand how the measurements were made to obtain the values included in the equations used by scientists. If we cannot make the same measurements in our work then we cannot apply the equations and the Pourgaix diagrams, the Nerst equations, ohms law, Kirchoff's laws etc cannot be applied to establish the corrosion status.
The Daniel Cell is a glass module that allows absolute electrical potentials to be measured.
The Daniel Cell is used to establish the relative potentials between metals in saturated solutions of their own salts.
You can see a steel coupon in acid connected to a voltmeter and the meter connected to a copper coupon in acid.The glass loop connecting the two is filled with porous material.... rolled up paper will do...and that provides a conductive path for the current to pass through
In this way they can assign numerical values to electro-chemical reactions.
Scientists can then work out theories to explain the behaviour of corrosion.
Our job is to see if these theories can be seen to work in the field.
back to Module03 index page