The Evaluation of Corrosion Inhibitors for Application to Copper and Copper Alloy Archaeological ...

332 Pages·2008·21.76 MB·English

5.4.2 Experimental Predu . 181
5.4.3 Experimental Observations......................................................................................................... 182
5.4.4 Experimental Conclusion............................................................................................................ 188
5.5 Inhibiting Nantokite Corrosion with Various Inhibitor Concentrations ...............................189
5.5.1 Introduction................................................................................................................................189
5.5.2 Experimental Observations......................................................................................................... 191
5.5.3 Conclusion .................................................................................................................................196
5.6 Long Term Experiments with Varying Concentrations .......................................................197
5.6.1 Introduction................................................................................................................................197
5.6.2 Experimental Procedure.............................................................................................................. 198
5.6.3 Experimental Observations......................................................................................................... 200
5.6.4 Experimental Conclusion............................................................................................................ 204
5.7 Corrosion Inhibitor Applications of Selected Corrosion Inhibitors ......................................205
5.7.1 Introduction................................................................................................................................205
5.7.2 Experimental Procedure.............................................................................................................. 206
5.7.3 Experimental Observations......................................................................................................... 207
5.7.4 Discussion..................................................................................................................................209
5.8 Evaporation of Corrosion Inhibitors....................................................................................212
5.9 Long-term Exposure to Raised Relative Humidity and Temperature....................................214
5.9.1 Introduction................................................................................................................................214
5.9.2 Experimental Procedure.............................................................................................................. 214
5.9.3 Observation and Discussion........................................................................................................215
5.10 BTA and its Corrosion Inhibition After Prolonged Drying................................................221
5.10.1 Introduction.............................................................................................................................. 221
5.10.2 Experimental Procedure............................................................................................................221
5.10.3 Observation and Discussion...................................................................................................... 221
5.11 Summary of Experiments..................................................................................................224
CHAPTER 6
ATOMIC ABSORPTION SPECTROPHOTOMETRY OF COPPER IN CORROSION INHIBITOR
SOLUTIONS................................................................................................................................226
6.1 Introduction........................................................................................................................226
6.2 Experimental ProcedureS ....................................................................................................227
6.3 Experimental Observations ................................................................................................229
6.4 Discussion ..........................................................................................................................234
CHAPTER 7
MINERALS TREATED WITH CORROSION INHIBITORS AND THEIR COLOUR CHANGES 236
7.1 Introduction........................................................................................................................236
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7.2 Experimental Work . 242
7.3 Experimental Observations.................................................................................................244
7.3.1 Cuprite....................................................................................................................................... 245
7.3.2 Malachite...................................................................................................................................247
7.3.3 Nantokite ...................................................................................................................................249
7.4 Discussion.......................................................................................................................... 251
CHAPTER 8
COPPER ALLOY ARTEFACTS TREATED WiTH CORROSION INHIBITORS ........................258
8.1 Introduction........................................................................................................................ 258
8.2 The Treatment of Copper Alloy Arrowheads.......................................................................259
8.2.1 Benzotriazole treatment..............................................................................................................260
8.2.2 AMT Treatment .........................................................................................................................264
8.2.3 Discussion of Arrowhead Treatments Between 1992-95..............................................................265
8.3 Arrowhead No. 6679 .......................................................................................................... 266
8.4 Copper Alloy Bracelet No. 6672 .........................................................................................270
8.5 Colour Changes of Corrosion Products on Naipes ...............................................................272
8.5.1 Introduction................................................................................................................................ 272
8.5.2 Experimental Procedure..............................................................................................................274
8.5.3 Experimental Observations and Discussion .................................................................................274
8.6 Conclusion ......................................................................................................................... 278
CHAPTER 9
DISCUSSIONAND CONCLUSION ............................................................................................ 280
9.1 Selection of Corrosion Inhibitors and Suitable Corrosion Tests ...........................................280
9.2 Corrosion Testing of 8 Corrosion Inhibitors on Nantokite Covered Copper Coupons ......... .284
9.3 Atomic Absorption Spectrophotometry of Copper Minerals in Corrosion Inhibitor Solutions287
9.4 Minerals Treated with Corrosion Inhibitors and their Colour Changes ................................289
9.5 Copper Alloy Archaeological Artefacts Treated with Selected Corrosion Inhibitors ............291
9.6 Proposed Further Research ................................................................................................. 294
APPENDIX.................................................................................................................................. 297
BIBLIOGRAPHY.......................................................................................................................325
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Tables
1.1 Copr Mine1s .16
2. 1 Selected Corrosion Inhibitors..................................................................................................................40
3. 1 Corrosion Inhibition of BTA at Different pH and Different Concentrations ..............................................47
3. 2 Comparative Study..................................................................................................................................48
3. 3 Comparative Study of AP and BTA..........................................................................................................60
3. 4 Comparative Study of BTA and MBI .......................................................................................................69
3. 5 Comparative Study of Various Inhibitors.................................................................................................72
3. 6 Comparative Study of BTA, MBO, MBT, MBI, Indole, and Benzimidazole .............................................73
3. 7 Comparative Study of MBT, MB!, and BTA...........................................................................................77
3. 8 Comparative Study of BTA, MBT, MBI, and MBO................................................................................78
3. 9 Copper Dust Inhibited with MP and AP...................................................................................................81
3. 10 Copper Sheet Corroded and Inhibited with MP, AP, BTA......................................................................81
3. 11 Properties of selected corrosion inhibitors..............................................................................................82
4. 1 Specimen requirements ...........................................................................................................................91
4. 2 List of Copper Alloys..............................................................................................................................94
4. 3 Coupons Treated with CuCl2..................................................................................................................112
4. 4 Weight Changes in grams of Coupons treated with CuCl2.......................................................................122
4. 5 Treatments of Copper Coupons...............................................................................................................139
5. 1 Colour changes of Inhibitor Solutions after 24 hours treatment................................................................156
5. 2 Munsell colour notations of coupons after corrosion inhibition................................................................156
5. 3 Corrosion Inhibitor Concentration...........................................................................................................170
5.4 Colour Changes of Inhibitor Solutions Before Immersion of Nantokite Covered Coupons........................173
5. 5 Munsell Colour Notations After Corrosion Inhibition..............................................................................175
5. 6 Corrosion Protection after 24 and 48 hours at 95% RH Ambient Temperature.........................................175
5. 7 Colour Change after Corrosion Inhibition................................................................................................184
5. 8 Corrosion Inhibition in Percent after 24 and 48 hours at 95% and Ambient Temperature.........................186
5. 9 Different Inhibitor Concentrations and the Corrosion Protection in Percent.............................................189
5. 10 Colour Changes of Corrosion Inhibitor Solutions after Corrosion Inhibition...........................................191
5. 11 Surface of Coupons after Corrosion Inhibition.......................................................................................192
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5.12 Corrosion Inhibition in Percent and Comments on the Surface Appearance. 194
5. 13 Coupons Treated in Different Concentrations of Inhibitors and Exposed for 24 hours and 48 hours to
—95%RH...................................................................................................................................................... 198
5. 14 Inhibitor Concentrations Selected......................................................................................................... 199
5. 15 Weight Increase in mg During 9 days of Exposure to Elevated Relative Humidity................................ 201
5. 16 Corrosion Inhibition in Percent after 9 days of Exposure to Elevated Relative Humidity.......................201
5. 17 Weight Changes of lg Samples of Inhibitors Exposed for 4 days 20°C145% RH...................................212
5. 18 Weight Change alter Inhibition and 1 day at l0%RHandAmbientlemperature.................................215
5.19 Changes in RH and Temperature......................................................................................................... 216
5.20 Variations of Corrosion Inhibition on the 52 day .................................................................................. 217
5.2lBTADriedfor2Q,5and0days ........................................................................................................... 222
5.22 DitIèrence of Corrosion Inhibition Percent of BTA Inhibited Coupons in Various Experiments .............223
6. 1 Inhbitor Concentrations.......................................................................................................................... 225
6.2 Copper Dissolution in%........................................................................................................................ 229
6. 3 Copper Dissolution in ppm.................................................................................................................... 230
7. 1 Symbols for Colour Evaluation .............................................................................................................. 236
7. 2 Inhibitor Concentrations ........................................................................................................................ 239
7. 3 Colour Changes due to Inhibitors...........................................................................................................251
7. 4 Ecpected Colour changes on Copper Minerals........................................................................................ 257
8. 1 Conservation Treatments on Copper Alloy Arrowheads.......................................................................... 260
8. 2 Composition of Incralac......................................................................................................................... 260
8.3 Arrowheads Treated with 4JvfT............................................................................................................. 261
8. 4 Atomic Absorption analysis by Ryan (1991)........................................................................................... 263
8. 5 Atomic Asorption Analysis by Ryan (1991)............................................................................................ 267
9. 1 Selected Corrosion Inhibitors................................................................................................................. 278
9. 2 New testing procedure for the evaluation of coffosion inhibitors for copper and copper alloy archaeological
artefacts........................................................................................................................................................ 283
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Figures
1. 1 Blister Corrosion . 24
1. 2 Pourbaix diagram of Copper-Chlorine-Water...........................................................................................26
3. 1 Tautomeric Forms of BTA.......................................................................................................................42
3.2 Cuprous Benzotriazole Polymeric Chain. This Diagram is Presented as Presented by Cotton ..................44
3. 3 BTA-Cu!I Polymer ..................................................................................................................................44
3. 4 AMT Molecule........................................................................................................................................50
3. 5 Tautomeric Forms of AMT .....................................................................................................................52
3.6AMT-MetalBond...................................................................................................................................54
3. 7 AP Molecule ...........................................................................................................................................57
3.8DBMolecule...........................................................................................................................................61
3.9MBlMolecule.........................................................................................................................................63
3. 10 Bonding of the MB! Molecule...............................................................................................................65
3. 11 MB!-Copper Polymer Formation............................................................................................................67
3. 12 MBO Molecule......................................................................................................................................71
3. 13 Comparison of Benzimida.zole and 2-Mercaptobenzimidazole................................................................73
3.14 Tautomeric Forms of MBT...................................................................................................................75
3.15 MPMolecule.........................................................................................................................................79
4.1 Crossection of air abraded copper coupon...............................................................................................105
4. 2 Photomicrograph of nantokite, and cuprite layer on copper coupons........................................................124
4. 3 Photomicrograph of Paratacamite corrosion on copper coupons...............................................................124
5. 1 Coupons after 24 hours in Corrosion Inhibitor........................................................................151
5. 2 Coupons after 24 hours Corrosion Inhibition and 5 minutes Drying at 50°C...........................152
5. 3 Coupons after 24 hours at 95% RH Ambient Temperature......................................................154
5. 4 Weight Change of Coupons after CuCl2 Corrosion.................................................................155
5. 5 Weight Changes of Coupons after Inhibitor Treatment. The Third Coupon in Each Series had
ExcessInhibitor Removed.............................................................................................................155
5. 6 Weight Changes of Coupons after 24 hours and 48 hours at 95%RH......................................156
5. 7 Weight Change after Cupric Chloride Corrosion....................................................................171
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5. 8 Weight Change after 24 hours and 48 hours at 95%RH......................................................... 172
5. 9 Coupons after 24 hours in Corrosion Inhibitors...................................................................... 172
5. 10 Coupons after 24 hours in Corrosion Inhibitor Solution and Dried 5 minutes at 50°C........... 174
5. 11 Coupons after 24 hours at 95% RH and Ambient Temperature ............................................. 176
5. 12 Weight Change after Inhibition............................................................................................ 178
5. 13 Coupons after 24 hours in Corrosion Inhibitor Solution ........................................................ 183
5. 14 Coupons after 24 hours in Corrosion Inhibitor Solution and Dried 5 minutes at 50°C ........... 183
5. 15 Weight Change after Inhibitor Treatment ............................................................................ 184
5. 16 Coupons after 24 hours at 95% RH and Ambient Temperature ............................................ 185
5. 17 Coupons after 48 hours at 95% RH and Ambient Temperature ............................................. 185
5. 18 Weight increase after 24 Hours and 48 Hours at 95%RH ...................................................... 188
5. 19 Coupons after 24 hours in the Corrosion Inhibitor Solution.................................................. 193
5. 20 Coupons after Corrosion Inhibition and 5 minutes Drying at 50°C ....................................... 193
5. 21 Weight Change in mg after 24 hours in Corrosion Inhibitor Solutions .................................. 194
5. 22 Corrosion Rate in mg after 1 and 2 days at 95%RH Ambient Temperature........................... 196
5. 23 Weight Increase of Coupons During 216 hours of Exposure to Increasing Levels of Relative
Humidity....................................................................................................................................... 202
5. 24 Weight Change of Coupons with two Applications of Inhibitors, and Exposed for 6 days to
SelectedRH Levels ....................................................................................................................... 211
5. 25 Weight Loss of Inhibited Coupons........................................................................................ 216
5. 26 Variations of Corrosion Inhibition on the 52 day.................................................................. 217
5. 27 Weight Change of Coupons in mg........................................................................................ 219
5. 28 Corrosion Inhibition in Percent............................................................................................220
6. 1 Atomic Absorbtion Cu Count in ppm...................................................................................................... 229
7. 1 CIELAB System..................................................................................................................................... 237
7. 2 i L*, a*, and b* for Cuprite................................................................................................................243
7. 3 of Cuprite (AP has the Same 5.E* for I and 2 Days)........................................................................243
7. 4 E.a*, and b* for Malachite............................................................................................................244
..................................................
7. 5 of Malachite (I and 2 Day Treatments of BTA had the Same E*) 245
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7.6 L*, IS.a*, and b* for Nantokite .247
7 7 * of Nantokite....................................................................................................................................247
8.1 Copper Alloy Arrowhead Before Treatment...........................................................................................265
8. 2 Copper Alloy Arrowhead After Inhibitor Treatment................................................................................265
8. 3 Corrosion Inhibition After 1,2 Days, and 52 Days...................................................................................266
8. 4 Copper Alloy Bracelet Before Treatment................................................................................................268
8. 5 Copper Alloy Bracelet After Conservation Treatment..............................................................................268
8. 6 Naipes Before Conservation Treatment ..................................................................................................270
8. 7 Naipes After Treatment..........................................................................................................................272
8. 8 Colour Measurements After Inhibitor Treatment.....................................................................................273
8. 9 LE* Values After Inhibitor Treatment....................................................................................................274
8. 0 Deviations in L*, Aa* and Ab*, After 10 Measurements on Untreated Sample....................................275
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Acknowledgements
I cannot adequately express my gratitude to my parents, who have over the past 14
years supported me in my move to follow a career in conservation. I also wish to
express my sincere gratitude to Dr. J.F. Merkel whose wealth of knowledge in
archaeometalurgy and conservation has inspired me to do this research. As a mentor
and guide, his supervision and inspiration have fuelled to a great extent my work
during the past six years. Thanks are due to Dr. D.R. Griffiths and Dr. C.A. Price for
providing help, and Mr. K. Reeves for his assistance when it was most needed.
Thanks also to Dr.D. Saunders form the National Gallery in London for his help with
the colour assessments. I would like to thank Ronnie Gibbs and Dean Sully for proof-
reading this thesis, and all my friends who have helped me over the past years to
finish this work.
I am also grateful for the many organisations who have funded my research:
Stipendien Amt Basel Stadt, Kultur Amt Gemeinde Riehen, NFP-16 (National
Research Fund), Ciba-Geigy Stiftung, Jamggen-Phon Stifftung, Max-Geldner-
Stiftung, Freiwillige Akademische Geselschaft, Dr. Ernst Beyeler.
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Chapter 1
Introduction
The aim of this research is to evaluate corrosion inhibitors for the use in the
conservation of copper and copper alloy archaeological artefacts. The objective of
this work is to acquire an insight into the performance of copper corrosion inhibitors,
when applied to archaeological copper material. Since it seems inappropriate to
apply new corrosion inhibitors to archaeological material, new testing procedures had
to be established. The treatment of archaeological artefacts with unknown compounds
poses several problems. These had to be taken into account when assessing corrosion
inhibitors, such as:
. How effective is the compound?
Does the compound damage the corrosion products covering the metallic core?
. Do the new compounds induce unacceptable colour changes?
It is standard practice to answer these questions, by applying corrosion
inhibitors to archaeological material. As in the study, for example undertaken by
Brunner (1993), into the application of AMT as a copper corrosion inhibitor. The
wide variety of archaeological copper and the heterogeneity of the corrosion
products, does not allow a precise comparison between test results. The resulting
data are more specific to the artefact and its condition. This research focuses on
experimental procedure of corrosion testing and the reproducibility of corrosion test
results. The corrosion testing procedures undertaken, allow a direct comparison
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between corrosion inhibitors, and these test results are finally assessed by treating
archaeological copper alloy objetcs.
1.1 Copper Corrosion
Even as a copper artefact leaves the foundry, its surface has already started to
corrode. Corrosion is due to the interaction between a metal and its environment.
This includes various electrochemical reactions, initially leading to an oxide layer on a
metallic core. Copper oxides, carbonates and chlorides are the most commonly found
compounds on the surface of archaeological copper and copper alloy artefacts. The
products resulting from these deterioration processes are similar in composition to
minerals found in nature. To a degree, these minerals are usually stable with respect to
their environment. Excavated copper artefacts are usually covered by corrosion. The
stability of the corrosion products depends on the burial environment of the object.
Stable patina (noble patina) are normally found to be resistant to fluctuations in
relative humidity. This work is concerned with the features of unstable patina, also
called "Vile Patina" (Gettens, 1970). A list of commonly found copper minerals can
be found in the following table.
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