Section 4: Composite artefacts (PDF 20858kb) - National Museum of ...

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Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4-8 October2004ABN 70 592 297 9673.1.3. Compositional differences within the lead of toilet 124.00010There are several bands of light and dark coloured metal visible on the body (seeFigure 4). It is possible that lead of different compositions may have been used to make thelead component. This may represent different pours during the casting process, perhaps dueto crucible size or other factors. Toilet 124.0009 only shows one different coloured band andnone are visible on toilet 124.00318. We hope to be able to analyse the various metalcompositions in the future.3.1.4. Effects of chloridesThe chlorides diffusing out of the ceramic increases the chloride content in thedesalination water. The increase in chloride level may have an influence on the corrosion rateby increasing the conductivity of the water and breaking down the protective films initiallyproduced by other seawater constituents (Burns, 1948). The other two toilets had far lessceramic remaining and this would have led to less chloride being present in the tap water inthe tank.4. Potential measurementsTo try to understand the electrochemical state of the toilet, a single set of corrosionpotential (Ecorr) measurements were taken using a Saturated Calomel Electrode (SCE) as thereference electrode. Using a Digital Multimeter set on D.C voltage, the negative (COM)terminal was connected to the toilet with an alligator clip above the water line, and the SCEwas connected to the positive (V) terminal. This set up makes the toilet negative to the SCE.[When using digital multimeters it is also possible to take the readings by connecting thenegative terminal to the electrode and the positive terminal to the structure because digitalmeters will display negative readings. However when using an analogue multimeter our setup is essential as analogue multimeters do not have a negative scale, and therefore userswould not get a reading if the more conventional connection format was followed.] The SCEwas then positioned over various parts of the toilet and the potentials recorded at eachlocation. These readings were taken for both toilets 124.00009 and 124.00010 and the resultsare summarised in Table 1 below. The SCE is +241mV with respect to (wrt) the StandardHydrogen Electrode (SHE). Standard electromotive and Pourbaix data are given wrt the SHEin theoretical literature (Pourbaix, 1974) but the SCE is often used in real applicationsbecause of its sturdy nature. Values in cathodic protection literature are generally quoted wrtthe Saturated Copper/Copper Sulphate (Cu/CuSO4) electrode, which is 74mV wrt SCE(Jones, 1996). Where possible all values in this paper have been converted to SCE.The toilets were immersed in Melbourne tap water for the desalination process.Analysis of Melbourne water based on the average of July 1999 to June 2004 data(Melbourne Water, 2004) gives the following parameters:Total dissolved solids (TDS) mg/L 35 to 90pH range: 6.9 to 7.8Hardness range mg/L 12.0 to 25.0Total alkalinity (as CaCO3) mg/L 11.3 to 14.0Specific Conductance µS/cm 55 to 138Sulphates mg/L 0.8 to 8.3In applying these parameters using the median value of the range and assuming a temperatureof 25 o C in the room, a calculated Langelier Saturation Index (LSI) returns a value of -2.60.© Published by the National Museum of Australia www.nma.gov.au469
Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4-8 October2004ABN 70 592 297 967The LSI is the difference between the pH of a water sample, and the pH at which that watersample would be saturated in calcium carbonate (RMCC, 1999). A negative LSI means thatthe water dissolves calcium carbonate, and therefore passivation due to PbCO 3 film formationmay be reduced. While the levels of sulphate in Melbourne water are very low, PbSO 4 couldhave been be formed on the lead by residual sulphates from the earlier inhibition attempt.However since corrosion continued it seemed likely that this layer was now either absent ordamaged.The resistivity of the water gives an indication to the rate of corrosion that may occur.The resistivity of Melbourne tap water calculated from the Specific Conductance parameter(Melbourne Water, 2004) ranges from 7,246 to 18,181 Ωcm. The corrosivity of the waterbased on the resistivity therefore ranges from moderate to slight (British Standards Institute,1973).The initial corrosion potentials were measured over various parts of the toilets (seeTable 1). The range of corrosion potentials was -320 to -340mV wrt SCE across the stabletoilet 124.00009. For the corroding toilet, 124.00010, readings with the SCE ranged from -285mV to -225mV over the exterior surface of the metal. Two equilibrium potential-pHdiagrams were consulted (Turgoose, 1985), see Figures 3 and 4. However, since thecomposition of the desalination solution was not the same as either of these diagrams theycould only be used as an indication of the corrosive behaviour of the two toilets. Since thepotential range across the non-corroding toilet is much less (20mV) compared to thecorroding toilet (60mV), it is possible that the higher emf is causing the corrosion and shiftingthe potential of the toilet into a corrosive zone. It is unclear what created this higher emf.Figure 3 – Pourbaix diagram for the Pb-H2O system. (Turgoose, 1985). Used withpermission of the United Kingdom Institute for Conservation.Figure 3 – Pourbaix diagram for the Pb-H2O system. (Turgoose, 1985). Used withpermission of the United Kingdom Institute for Conservation.© Published by the National Museum of Australia www.nma.gov.au470
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