CORROSION PREVENTION – CATHODIC PROTECTION TRAINING BENCH
The Cathodic protection is a technique to control the corrosion of a metal surface by making it work as a cathode of an electrochemical cell. This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell. Cathodic protection systems are most commonly used to protect steel, water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms and onshore oil well casings.
The bench shall provide facilities to study the case of isolated systems, as well as the case of systems where different metals are coupled together. Particular attention shall be given to the presence or not of several kinds of insulating materials over the surfaces of the samples, in order to demonstrate the different behavior of the same material when coated or bare.
The bench shall provide suitable devices to highlight the concept of the free corrosion potential, measured with easy to use reference electrodes and means suitable to build with a certain accuracy the polarization curves.
Protective techniques shall be represented as per sacrificial anodes systems of several type of metals as per impressed current Cathodic Protection systems with the possibility to see which is the explanation of the use of constant voltage, constant current and constant potential feeders.
The bench shall be provided with measuring facilities characterized by suitable sensitivity and accuracy, in order to introduce which must be the basis of the laboratory tests to be executed, to recognize which is the correct way in order to determine the behavior of a metal in contact with the electrolyte in different conditions of temperature (thermostatic bath) and in high oxygen concentration (air insufflations pump).
A suitable multi-channel interface shall be able to connect the bench to a PC in order to record the experiment results and give the trace for further studies.
List of experiments:
1) The use of the voltmeter
The most important instrument in the field of the Cathodic Protection is the Voltmeter; typically, the digital type is the most common because of the great impedance of it allows the measurement of voltages (the potentials) due to sources with very high internal impedance.
The measurements shall follow the introduction to the electrical measurements and to the introduction to the Ohm’s law that regulates the passage of the current as into the first, as into the second species conductors (metals and electrolytes).
2) The measurement of the difference of potential of a sample into an electrolyte
This experiment shall introduce to the subject of the Cathodic Protection. The target of the discipline shall be modifying the potential (versus the reference cell) of the structure to protect by slowing the natural tendency of the metal to pass in solution.
This experiment shall emphasize the electrochemical approach to the corrosion phenomena.
3) The reference cell
This experiment shall put in relation the practical use of the three types of reference cells most common in the discipline that are the Cu/ CuSO4 reference cell, the Ag/AgCl reference cell and the Zinc reference cell.
4) The Daniel Cell
In the Daniel cell, copper and zinc electrodes shall be immersed in a solution of copper (II) sulphate and zinc sulphate respectively. At the anode, zinc will be oxidized per the following half reaction: Zn(s) Zn2+(aq) + 2e-
At the cathode, copper shall be reduced per the following reaction: Cu2+(aq) + 2e- Cu(s)
In the Daniel cell which, due to its simplicity, is often used for demonstrations, electrons that are “pulled” from the zinc travel through the wire, providing an electrical current that shall illuminate the bulb. In such a cell, the sulphate ions shall play an important role. Having a negative charge, these anions shall build up around the anode to maintain a neutral charge.
Conversely, at the cathode the copper (II) cations shall accumulate to maintain this neutral charge. These two processes will cause copper solid to accumulate at the cathode and the zinc electrode to "dissolve" into the solution.
5) The first and second species conductors
By using a simple circuit it will be possible to show the equivalence between the electrolytes and the common conductors as far the passage of the electrical current concerns.
6) Introduction to the Cathodic Protection Criteria
By using the electrolytic cell of the bench it shall be possible to reproduce the application of the NACE criteria that confirm the status of Cathodic Protection of a structure.
7) Introduction to the sacrificial anodes in Zn, Mg, and Al
By using the electrolytic cell of the bench it shall be possible to reproduce the application of the sacrificial anode to a steel structure and see in the same time the comparison in between two specimen, one in Cathodic Protection regimen, the other in free corrosion regimen.
8) Introduction to the Cathodic Protection Impressed Current System
By using the electrolytic cell of the bench it shall be possible to reproduce the application of the impressed current to a steel structure and see at the same time the comparison between two specimens, one in Cathodic Protection regimen, obtained by means of sacrificial anodes, the other driven with the impressed current system. In this experiment, it will be highlighted the the rising of the interference due to the presence of the impressive current system due to a not proper dispositions of the two systems.
9) The consumable impressed current anode (Fe)
By using the electrolytic cell of the bench it shall be possible to reproduce the application of the impressed current to a steel structure and see in time the effect of the consumption of the anode due to its passage in solution.
10) The inert impressed anode (Ti-Pt and MMO)
Not all the anodic materials pass in solution, two examples will be seen by using the Titanium Platinized anode and the Metal Oxide covered Titanium anode.
11) Resistance concept, circuit for the first and second species conductors
By using the electrolytic cell of the bench it shall be possible to produce the passage of current into the bath and in this way to demonstrate the validity of the Ohm’s Law in the field of Cathodic Protection.
Ohm's law shall apply to electrical circuits; it shall state that the current through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage across the two points) and inversely proportional to the resistance between them.
The mathematical equation that describes this relationship is: I = V/R
Where I is the current in amperes, V is the potential difference in volts and R is a circuit parameter called the resistance (measured in ohms, also equivalent to volts per ampere). The potential difference is also known as the voltage drop, and it is sometimes denoted by U, E or emf (electromotive force) instead of V.
12) Introduction to the specific resistance concept over three different first species conductors (Fe; Cu; Fe-Ni). To drive the student to the concept of resistivity, an experiment will be executed by using three geometrically identical samples of different material in order to identify the concept of specific resistance that “in fieri” is the resistivity or as inverse the conductivity concept.
13) Introduction to the concept of interference due to the presence of external electric fields on buried or submerged structures (Stray Currents). The experiment shall reproduce the effect of an external electric field on a submerged structure with the result of the formation of separated anodic and cathodic areas on the surface of the sample. It shall be the introduction to the concept of interference due to the presence of an external and interfering electric field on buried or submerged structures (Stray Currents).
14) Air presence influence on resistivity (insufflate air effect) – This experiment shall explain and demonstrate the change of the resistivity with the increase of the presence of air dissolved into the electrolyte.
15) Current density introduction and Tafel Curves construction – The concept of current density is, like the difference of potential, the main concept in the Cathodic Protection discipline and this experiment shall allow understanding that with this concept it will be possible to predict the amount of current needed to obtain the Cathodic Protection regimen over a known surface structure immersed in the electrolyte. By using the provided multi channel interface it shall be possible to record the change of the current values in the time, then build the polarization curves in a plot.
16) Temperature effect over the Current density (thermostatic cell) – This experiment shall explain and demonstrate the change of current density as a function of the temperature and introduces the concept of chemical activity.
17) Air presence influence over the Current density (insufflate air effect) – This experiment shall explain and demonstrate the change of current density as a function of the increasing of dissolved oxygen.
18) Coating and Current density – The use of coated samples shall allow demonstrating the effect of the coatings over the submerged or buried structures and it shall give the magnitude of the effect explaining that the synergy between the Cathodic Protection and the Coating of the surfaces to be protected will reduce the current density with all the relevant advantages.
List of materials:
- Bench with wheels with electrical console to connect to the mains Vac supply and
- lockable shelves to contain the hereinafter listed material. Provided with waterproof top surface.
- safety glasses and glows.
- Digital voltmeter.
- PC interface for the measurement and data storage.
- Digital voltmeter on console.
- digital ammeter on console.
- Cu/CuSO4 reference cell.
- Ag/AgCl reference cell.
- Zn reference cell.
- copper electrode
- carbon steel electrode
- transparent basin to build the electrolytic test bath
- Simple circuit with sliding resistor and lamp suitable for the insertion into the electrical circuit of
- the electrolytic cell.
- Zinc electrode
- Magnesium electrode
- Aluminum electrode
- DC feeder (each provided with constant voltage, constant current, constant potential facilities)
- Ti-Pt anodes
- MMO tubular anode
- Cu bar
- Fe bar
- Fe-Ni bar
- Resistivity fluid cell
- Waterproof resistor with thermostatic device
- Air pump with relevant sprayer
- carbon steel electrode (completely coated with epoxy compound)
- carbon steel electrode (partially coated with epoxy compound)
- various reagent in plastic cans with technical sheet as per the requirement of CE.
- Set of spare fuses.
- Set of ancillaries and connecting leads
- paper copies and CD of the manual book for training of the teacher in order to undertake the experiments.