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Operation and maintenance instructions for platinum anode

Chapter 1 Ti anodes handling

1. The Iridium Mixed Metal Oxide coated Ti anodes (hereafter called Ir PT coated Ti anodes), whilst robust to the recommended electrolysis conditions, should be handled with care when being either installed or removed from the cell.

2. The operator should wear gloves, and oil or grease contamination of the Ir PT coating must be avoided. Grease contamination will prevent electrolysis to occur at the affected areas. The non-contaminated areas will then be active at a higher current density. As a consequence the current distribution will be non-uniform and this will be matched by non-uniform Ir PT coating wear.

3. The Ir PT coating is a porous, relatively high surface area ceramic coating, in order to provide good electrochemical characteristics. The Ir PT can easily be scratched or damaged if hard or sharp objects are rubbed or knocked against it. Therefore:

· The Ir PT coated surface must not be rested directly onto a metallic surface. Use a protected soft covering of paper or plastic material on top of the hard surface.

· The Ir PT surface must not be dragged over, or slid against any hard surface.

·  If dirt or deposits are present on the Ir PT  coating, never attempt to clean by abrasive mechanical means. For example never clean by wire brushing, sanding or emery paper or by high-pressure water jets.

Chapter 2 Ti anodes storage

1. When the Ti anodes are removed from the cell, they should immediately be thoroughly flushed with water. This is essential. If the Ti anodes are not water washed, the I rPT coating will be irreversibly damaged by sulphuric acid attack as the electrolyte dries out on the surface and the acid concentration increases.

2. Ti anodes, which will shortly be re-installed in the EGL cells, can be conveniently stored in a vertical position. A trolley type storage rack can be specially fabricated.

3. Ti anodes, which require storage for longer periods should be carefully wrapped, for example in air filled protective plastic wrapping material, to avoid scratch damage, dirty etc. to the Ir PT coating

Chapter 3 Fundamental properties of Ti anodes

1. Titanium is resistant to chemical attack in acid electrolytes, for example as used in plating applications, provided that the Ti anodes are operated according to the recommended operating procedures.

2. The reason that Titanium is not chemically corroded is that it has an extremely adherent and inert surface oxide film. Because of this thin surface Ti oxide film, which is electrically non-conducting, Ti by itself will not pass current and will therefore not act as an anode just by itself.

3. The application of the Ir PT coating to the active area of the Ti anode allows the current to flow from the Ti anode through the coated areas into the electrolyte, while the non-coated areas of the Ti anode remain inactive.

4. Ir PT coated Ti anodes can be operated at high current densities with low consumption rates of the Ir PT electro catalyst; hence long lifetimes can be achieved.

5. If small areas of the Ir PT coating are mechanically damaged, exposing the underlying Ti to the electrolyte, the Ti anode will continue to operate satisfactorily because of the protective adherent oxide film formed as described above.

6. The anodic reaction at the Ir PT coated Ti anode surface is the electrolysis (oxidation) of water into oxygen and hydrogen ions. The oxygen ions will combine to (gaseous) oxygen molecules and form bubbles that will escape from the electrolyte into the air. The hydrogen ions contribute to maintaining the pH at a sufficiently low level.

7. The Ir PT coating composition and manufacture have been optimised by MAGNETO special anodes B.V. to provide both a long lifetime and an oxygen evolution potential close to the reversible half cell potential. That is the oxygen over-potential is low, which means that the Ti anode contribution to the total cell voltage is minimised. Hence also the Ti anode contribution to the electrical power consumption is minimised.

8. Whilst the inert protective Ti oxide film, as described above, confers highly desirable properties to the Ir PT anodes in plating processes it is still essential to adhere to the operating instructions listed below.

    One of the most essential operating conditions is to maintain the Ir PT coated Ti anodes anodically polarised at any time as long as the anode is immersed in the electrolyte. The reason for this is that the electrolyte is sulphuric acid based. Sulphuric acid is a reducing acid. As long as the Ir PT Ti anode is anodically polarised the protective Ti oxide film will be maintained and the Ti anode will not be corroded. However if the Ti anode is not kept anodically polarised the reducing sulphuric acid will attack the Ti oxide film and the Ir PT TI anode will suffer from some corrosion. In effect finally the Ti metal will dissolve, forming a thicker insulating Ti oxide film between the Ir PT coating and the Ti substrate. This mechanism leads to a significant increase in the operating anodic potential due to passivation by

9. the non-conduction Ti oxide film. Eventually the anode will be totally passivated and it will cease to pass current.

10. The contrasts between the effects of reducing acids like sulphuric acid and hydrochloric acid and oxidising acids like nitric acid on the inert properties of the Ti oxide protective film is total. Titanium is fully corrosion resistant to nitric acid at any concentration and temperature because the Ti oxide protective film is stable in oxidising environments, regardless whether the Ti anode is kept polarised or not.

Chapter 4 Ti anode operating conditions

1. The Ti anodes should be maintained anodically polarised at any time as long as the Ti anodes are immersed in, or in direct contact otherwise with the electrolyte.

2. If the Ti anodes are not energised, a small positive polarising voltage (residual potential) should be impressed onto the Ti anodes. This can be achieved without any significant current flow by setting the power supply to a voltage of 2 Volts between the anode and cathode connections.

3. Start-up and shut-down procedures should include this residual potential whenever electrolyte is present in the cells.

4. Ir PT coated Ti anodes must be kept refrained from polarity reversal at any time, hence the following procedures require to be maintained. For example in steel strip plating process:

    When one side Zinc coated steel strip is being produced then both the inner carriers of each cell containing Ti anodes must be removed from the cell. If they are not removed and the anodes remain immersed in the electrolyte the Ti anodes could become cathodic opposite to the anodes in the outer

·carriers and Zinc could become plated onto the Ir PT coating of the Ti anodes in the inner carriers. The Zinc plated onto the Ir PT coated surface will cause blinding of the anode surface with consequently inhomogeneous current distribution when again both sides Zinc coated steel strip is produced.
·Consequently the outer Ti anodes on a carrier must not be allowed to become cathodic with respect to the opposing Ti anodes if differential Zinc coating loadings (hence differential current densities) are applied to the two sides of the steel strip.
·The knife-edge connection to the Ti anodes or the Ti/Cu busbars should be maintained in good condition. If they are, then the current distribution through the Ir PT coating should be reasonably uniform. The lifetime of the Ti anodes will then be maximised. The reverse will happen if the connections are not adequately maintained and contact resistances vary from individual anode to anode.

5. The current distribution across the layers is fairly uniform.The lifetime of the titanium anode is maximized.If the connection is not maintained sufficiently, the contact resistance of each anode will be different, which will cause the electrodes to be reversed.

6. The inter-electrode gap at the bottom of the Ti anodes should be less than at the top, such that the current is distributed reasonably uniform down the length of the anode.

7. The inter-electrode gap should be maintained at all times. Accidental short circuit between the Ti anode and the parts will cause irreversible damage to the Ir PT coating and will damage the Ti substrate. If the short circuit is severe the Ti anode will be destroyed.

8. Do not allow any solid particulate matter to be circulated in the electrolyte. This will cause abrasion of the Ir PT coating and shorten the Ti anode lifetime. For example do not permit partially dissolved Zn or Sn granules to be circulated through the cells.

9. Trace impurities in the electrolyte, like for example Pb, Fe and Ba, should be carefully controlled so that no significant anodic depositions will occur. Anodic depositions blind the Ti anode surface and cause inhomogeneous anodic current distribution.

10. Critical operating parameters should be strictly adhered to in order to maximise the useful Ti anode lifetime. For example (EGL): electrolyte temperature – 50-55 °C; anodic current density 15 kA/m² max.; Fluoride ions, 2 ppm max.; Chloride ions, 2 ppm max.; Zn – typical 550 g ZnSO4.7H2O / l.

11. Organic additives should not be introduced into the electrolyte. The Ir MMO coated Ti anode lifetime could be significantly reduced particularly by additives or additive oxidation products with complex forming characteristics.

12. Immediately after the electrolyte is drained from the cells, or when the Ti anodes are removed from the cells, thoroughly flush the Ti anodes with water in order to prevent from acid attack. Please be referred to earlier sections about this issue.

Chapter 5 Monitoring of Ti anode lifetime

    A continuous record of the lifetime of the Ir PT coated Ti anodes should be kept so that accurate data on accumulative current throughput in kAh/m² are available at any time.

Chapter 6 Ti anodes maintenance

1.   Handling, storage, water washing and current distribution matters are covered in previous sections of this document.

2. A visual sign of anodic deposits may be that higher voltages are required to maintain the required current output, to be confirmed by visual inspection of the Ir PT coated Ti anode surface. In case of presence of such deposits, do on any account not do any attempt to remove these deposits by mechanical means. For example do not attempt to brush or rub off the deposits as this will damage the Ir PT coating. Reference earlier comments in this document. Before the optimum cleaning method can be determined it is first necessary to determine the nature of the deposit.

Chapter 7.Deposit removal from Ir MMO coated Ti anodes

1. NEVER attempt to remove any deposits from the Ir PT coated surface of the Ti anodes by abrasive mechanical methods. For example do NOT use wire brushing, sanding paper, or high pressure water jet cleaning.

2. Hydrochloric acid – this acid is useful for removal of rust type deposits in most forms as well as calcareous deposits. Due to its reducing properties to be used at ambient temperature only and at concentrations not higher than 10% v/v of 37% HCl in water. The immersion time should be kept to a minimum, for example 10 minutes, since Ti anodes are vulnerable to slow attack by reducing acids like hydrochloric acid. The rate of attack by hydrochloric acid can be reduced by the addition of 0,1% ferric chloride to the cleaning solution before the cleaning operation is carried out.  NOTE: after cleaning with hydrochloric acid the Ti anodes must be thoroughly flushed with water and preferably be kept fully immersed in water for a minimum of 10 minutes.

3. Citric acid – a 5-10% aqueous solution of citric acid at 40-50 °C may be used for the removal of iron containing deposits. Again carefully flush the Ti anodes with water afterwards.

4. Nitric acid – because nitric acid is an oxidising acid, Ir PT coating are fully resistant to nitric acid at any concentration and temperature. But high concentration and high temperature nitric acid have strong oxidising properties. Titanium substrate would create a Ti oxide layer between titanium and part coating. Ti oxide is less conductivity. So avoid put Ir MMO anodes in high concentration and high temperature nitric acid. Low concentration and low temperature Immersion in dilute nitric acid, will remove many deposits that form soluble nitrates. for example 10%, room temperature.

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