1. BASICS CORROSION Cells
CHAPTER 2: Part II
BASICS CORROSION Cells

2. Corrosion Cell
Corrosion is an electrochemical process involving the flow of electrons and ions. Corrosion occurs at the anode, Protection occurs at the cathode.
Electrochemical corrosion involves the transfer of electrons across metal/electrolyte interfaces.
Any corrosion cell consists of four parts:
Anode;
Cathode;
Electrolyte;
Metallic Path.

3. Corrosion Cell
For corrosion to occur, there must be a current flow and a completed circuit, which is then governed by Ohm’s law:
I = E/R
The cell potential calculated here represents the peak value for the case of two independent reactions.

4. Types of Local Cell Formations
Three main types of local cell formations leading to corrosion are encountered in practice:
Dissimilar electrode cells
Concentration cells:
Salt concentration cell
Differential aeration cell
Differential temperature cells

5. Dissimilar Electrode Cells
Dissimilar electrode cells may be formed when two dissimilar metals are in contact in the same electrolyte.
In practice, a copper pipe connected to a steel pipe.
This is often referred to as galvanic coupling, in which the less noble metal becomes the anode.
Dissimilar phases and impurities, grain boundaries, and scratches or abrasions on the same metal.

6. Concentration Cells
Concentration cells are formed when the electrodes are identical but are in contact with solutions of differing composition.
A salt concentration cell forms when one electrode is in contact with a concentrated solution and the other electrode with a dilute solution.
The electrode in contact with the dilute solution will be anodic.
A differential aeration cell forms when the identical electrodes are exposed to solutions of identical chemical composition that differ in oxygen content.
The electrode in contact with the less aerated or oxygenated solution will act as the anode.
A metallic tank half filled with water tends to corrode just below the water line because of the lower oxygen concentration compared to the area just above it near the water line.

7. Differential Temperature Cells
Differential temperature cells are formed when electrodes of the same metal, each of which is at a different temperature, are immersed in an electrolyte of the same initial composition.
For iron immersed in dilute aerated sodium chloride solutions, the hot electrode is initially anodic to the colder metal, but the polarity may reverse with the progress of corrosion.

8. Oxygen Concentration Cells
The oxygen-reduction reaction that occurs in neutral or basic solutions, plays a significant role in many corrosion processes.
This occurs when differences in dissolved oxygen content exist at one area on the metal surface relative to another.
Corrosion occur at the anodic or low oxygen concentration site.

9. Metal Ion Concentration Cells
When a significant difference in metal ion concentration exists over a metal surface.
In a corrosive environment more metal ions go into solution at the low-concentration area.
The current flow generated by this process can result in plating out metal ions at the cathodic or high metal ion concentration region.

10. Potential–pH Diagrams
Potential–pH diagrams, also known as Pourbaix diagrams, are graphical representations of the stability of a metal and its corrosion products as a function of the potential and pH (acidity or alkalinity) of the aqueous solution.
Such diagrams are constructed from calculations based on the Nernst equation and the solubility data for various metal compounds.
The potential–pH diagram for an Fe–Η2Ο system is below:

11. Potential–pH Diagrams

12. Potential–pH Diagrams
The potential–pH diagram shows three clear-cut zones:
Immunity zone.
Corrosion zone.
Passive zone.
Such diagrams can be used for:
Predicting the spontaneous direction of reactions
Estimating the stability and composition of corrosion products
Predicting environmental changes that will prevent or reduce corrosion

13. Polarization
Polarization refers to the potential shift away from the open circuit potential (free corroding potential) of a corroding system.
If the potential shifts in the "positive" direction (above Ecorr), it is called "anodic polarization".
If the potential shifts in the "negative" direction (below Ecorr), it is called "cathodic polarization".
For all metals and alloys in any aqueous environment, cathodic polarization always reduce the corrosion rate.

14. Polarization
Cathodic protection is essentially the application of a cathodic polarization to a corroding system.
For a non-passive system (e.g. steel in seawater), anodic polarization always increases the corrosion rate.
For systems showing active-to-passive transition, anodic polarization will increase the corrosion rate initially and then cause a drastic reduction in the corrosion rate. Anodic protection is essentially the application of anodic polarization to a corroding system.

15. Polarization

16. Polarization

17. Corrosion Rate Measurement
Measurement of corrosion is essential for the purpose of material selection.
Corrosion rate measurement may become necessary for the evaluation and selection of materials for a specific environment, a given definite application, or for the evaluation of a new or old metal or alloys to determine the environments in which they are suitable.

18. Corrosion Rate Expressions
Corrosion involves dissolution of metal, as a result of which the metallic part loses its mass (or weight) and becomes thinner.
Corrosion rate expressions are therefore based on either weight loss or penetration into the metal.
The most widely used corrosion expressions are:
g/m2.day (gram/square meter . day)
mm/y (millimeter/year)
mpy (mils penetration/year)

19. Corrosion Rate Calculation
The expression is readily calculated from the weight loss of the metal specimen during the corrosion test by the empirical formula:
mm/y = 87.6 x (W / DAt)
where W is weight loss (mg), D is density of metal (g/cm3), A is area of specimen (cm2), t is exposure time (h),

20. Example problem
A sheet of carbon steel one meter wide by three meter long has lost 40 g to corrosion over the past six months.
Convert that mass loss to a penetration rate of the steel in mm units.
What would be the total corrosion current associated with such a corrosion rate? (carbon steel density = 7.8 g/cm3) 
Answer: 
The surface area is 3 m2 and the exposure duration is six months.
corrosion rate = 40 g / (3 m2 x 6 months x 30 days /month)
Corrosion rate= g m-2 day-1
Converting into mm/y means x 0.365/d
Corrosion rate = mm/y.

21. Example problem
A sheet of carbon steel one meter wide by three meter long has lost 40 g to corrosion over the past six months.
Convert that mass loss to a penetration rate of the steel in mm units.
What would be the total corrosion current associated with such a corrosion rate? (carbon steel density = 7.8 g/cm3) 
Answer: 
In order to estimate the total corrosion current, convert into mA cm2 
CR(mA cm-2) = CR(mm/y) x nd/M
Corrosion current = ( x x 2 x 7.8)/56
Corrosion current = mA cm-2.
The total current = mA cm-2 x cm2 = 9 mA.

22. Measuring Potential
Sketch below shows the actual electrochemical current flow that exists in a corrosion cell.
In corrosion and cathodic protection work, conventional current flow is used.
This is a flow of current in the direction of the positive ion transfer (so called positive current).

23. Use of Voltmeters
An analysis of meter polarity connection and sign displayed allows the determination of the direction of conventional current flow.
When measuring voltage across a circuit, the voltmeter is connected in parallel across the element.
For example, the voltmeter in sketch below is connected in parallel to Resistor R2 of the external circuit.

24. Use of Voltmeters
The measurements commonly made in cathodic protection surveys are:
Structure-to-electrolyte potential
Driving voltage of a galvanic anode system
Rectifier voltage output
Voltage drop across a pipe span
Voltage across a current shunt

25. Polarity Sign
Most digital meters will display a negative sign for a negative reading and no sign for a positive reading.
When a voltmeter is connected across a metallic element, the voltage display is positive when the positive terminal of the voltmeter is upstream of the current flow as illustrated in sketch below.
When measuring the voltage difference of dissimilar metals, the sign is positive when the positive terminal of the voltmeter is connected to the more noble metal

26. Polarity Sign

27. Sign Convention
When the positive terminal of voltmeter is connected to the more noble metal and the negative terminal to the more active metal, then the reading is positive.
Current flow from the active to the noble metal through the electrolyte and from the noble to the active metal through the metallic path.
Structure-to-electrolyte readings are considered negative to the reference electrode.
When the reference electrode is connected to the negative terminal, the voltmeter produces a negative reading.

28. Reference Electrodes (Half-Cells)
Reference electrodes, or half-cells, are important devices that permit measuring the potential of a metal surface exposed to an electrolyte. An example is a structure-to-soil potential measurement.
There are several potential benchmarks in common use, but all of them are related to a basic standard.
The standard hydrogen electrode half-cell is used in most circumstances in which potential measurements are to be made.
Other combinations of metal electrodes in solution with a specific concentration of ions are used.
The reference cell must be stable and capable of producing reproducible data.

29. Reference Electrodes (Half-Cells)
Common reference electrodes and potential with respect to the standard hydrogen electrode:
Standard hydrogen electrode (SHE) (E=0.000 V) activity of H+=1
Saturated calomel electrode (SCE) (E= V saturated)
Copper-copper(II) sulfate electrode (CSE) (E= V)
Silver chloride electrode (E= V saturated)

30. Copper-Copper Sulfate Electrode
Copper sulfate reference electrodes (CSE) are the most commonly used reference electrode for measuring potentials of underground structures and also for those exposed to fresh water.