Some remarks on the monitoring of the storage / treatment of iron based alloys artefacts & Introduction to the E vs time Droplet Technique (EDT)

Transcription

1 Some remarks on the monitoring of the storage / treatment of iron based alloys artefacts & Introduction to the E vs time Droplet Technique (EDT) corr as a spot test for metal artefacts Christian Degrigny

2 Giving simple but reliable tools to conservators To monitor the storage / stabilisation of iron based artefacts in alkaline solutions To get a preliminary idea of the composition of metal artefacts

3 Meaning of E corr measurements E corr all ½ electrochemical reactions on a metal surface in contact with a solution - Oxidation / dissolution of the metal (x 4) M M n+ + ne - I a -Reductionof the dissolved oxygen (O 2d ) in the solution: (x n) O 2d + 2H 2 O + 4e - 4OH - I c V At E corr, I a +I c =0 Practically, E corr (V/SHE)-E ref. (V/SHE) = V (read on the multimeter) E corr (V/SHE) = V + E ref (V/SHE)

4 Storage / treatment of Iron based artefacts Arc Antique Swivel gun recovered from the Mediterranean Sea

5 Storage / treatment of Iron in alkaline solutions: from theory Non corroded Fe in KOH solution: passivation observed

6 to practice Arc Antique Artificially corroded steel coupons Max. extraction = solution reach M

7 When the thickness of corrosion layers KOH KOH Turgoose 1994, STEP EU program thin corrosion layers KOH KOH thick corrosion layers CL Remaining metal CL CL CL KOH CL KOH Corroded Fe in KOH solution

8 DRASSM Ingots from St Marie de la Mer (1 st cent. BC) Iron ingots mechanically cleaned on site Arc Antique Part of the cargo left to dry outdoor Stabilisation in 1% w/v KOH solution Arc Antique Weeping on the surface of ingots

9 Slow diffusion of KOH Quick diffusion of chlorides slow re-increase of Ecorr after III step? -Cl - compete Questions with OH -? - KOH still diffuses within the cracks of the ingot and cannot passivate the whole metal surface?

10 Composite iron based swivel gun, Malta MCR Objectives? active corrosion? thickness of the corrosion layer? setup of a simple stabilisation process training of Maltese professionals

11 Voltmeter MCR Contact on the artefact Reference electrode

12 E corr ph Ecorr -0.8 ph Time (hours) Time (hours) CHLORIDES (ppm) C Cl - Conductivity (ms) Λ TIME (hours) Time (hours)

13 Conclusion Monitoring of Ecorr vs time is a simple technique to transfer Need to monitor other parameters: Concentration of Cl- extracted, ph, conductivity and to relate them to the evolution of Ecorr values Such applied research can easily and systematically be carried out by conservators under the supervision of a corrosion scientist

14 Use of E corr measurement as a spot test to determine the qualitative composition of metal artefacts: an update Claire Galea, Vera Ruvinskaya & Roslyn DeBattista Diagnostic Science Laboratories Heritage Malta, MALTA

15 Objectives Need in conservation for simple tools to determine the nature of materials Spot tests for metals: destructive (use of aggressive chemicals, sampling) Design of an innovative spot test without any sampling and using no corrosive solutions Measurement of E corr with a droplet of solution

16 Principle Artefact connected to the V output of a multimeter Reference electrode placed above the metal (does not touch it) connected to COM output A droplet of solution is placed in between Immediate reading of a potential on the multimeter Recording vs. time (5 to 10 minutes) Reference electrode artefact COM V I multimeter

17 Preliminary tests: electrochemical behaviour of brass in alkaline solution E/SHE 7 14 Copper, Zinc and brass alloys Solution: sodium sesquicarbonate (ph=10) Cu I P C Normally both Cu and Zn should passivate What about brass? E/SHE Zn C I 7 14 C

18 Metal coupons Electrochemical behaviour of Cu, Zn and brass plates Testing the reproducibility of the results Comparison between Cu, Zn and brass Cu Brass Cu No staining is observed after both tests on brass or Cu compared to Zn Zn

19 Electrochemical behaviour of real artefacts Series 1 Series 2 Series 3 Series 4 Series 5 Series 6

20 Conditions 3 recordings Metal degreased with acetone (non polished) or locally cleaned with abrasive cotton (OUATOR), degreased with acetone, rinsed under running water and dried Cleaning area: usually backside and as small as possible Contact to the metal with aluminium foil and crocodile clamp SEM-EDS analysis performed in // drop of solution

21 Compilation of analytical results Artefacts Item Composition Category Standard brass Cu 66.2% / Zn 33.8% III Series 1 Item 2 Cu 64.4 / Zn 35.6 II Series 2 Item 3 Cu 62.9 / Zn 37.1 I Item 4 Cu 64.7 / Zn 35.3 II Item 5 Cu 66.7 / Zn 33.3 III Series 3 Item 1 Cu 63.9 / Zn 35 / Sn 1.1 II + Sn Item 4 Cu 62.7 / Zn 36.3 / Sn 1 I + Sn Series 4 Item 1 Cu 72.2 / Zn 27.8 V Item 2 Cu 72 / Zn 28 V Series 5 Item 1 Cu 67.2 / Zn 32.8 IV Item 2 Cu 70.4 / Zn 27.5 / Sn 1.2 / Ni 0.9 V + Sn + Ni Series 6 Item 1 Cu 68.1 / Zn 31.6 / Sn 0.3 IV

22 Comparison between E corr vs. time plots for the 5 categories Ecorr (V/Ag-AgCl) Zn I3, S2 (I) I2, S1 (II) I4, S2 (II) Brass st I5, S2 (III) Brass artefacts only -350 I1, S5 (IV) I1, S6 (IV) -400 I1, S4 (V) I4, S3 (V) -450 time (seconds)

23 Conclusion 1 The «E corr -Drop Test» (EDT) technique: - seems to give reproducible results on polished coupons - seems to provoke no damage on the surface - can be used for a qualitative analysis of metal artefacts - requires a small metal surface and a small volume of solution: it is a spot test

24 In the case of brass - Artefacts were chosen from visual appearance (our results show that they are indeed rather similar) - Electrolyte: sodium sesquicarbonate - Not corrosive: only a slight transformation of the oxide surface is observed (passivation, when polished)

25 Construction of a database for copper based alloys: EDT as a spot test Systematic application of the technique to certified copper based alloys: - EU IMMACO: As copper, tin bronze, tin brass, lead bronze and quarternary bronze - EU Eureka E2210! Bronzart: Bronzes 1, 5, 6, 8, 9, 11, 12 - ISTN: silver bronzes 1 to 3 Experiments carried out with two solutions: sodium sesquicarbonate and mineralised water

26 Alloy Cu Ag As Sn Si Zn Pb Ni Bi Arsenic copper Tin bronze 93 7 Bronze Bronze Bronze Silver bronze Silver bronze Silver bronze (Tin) brass Pure brass Bronze Bronze Lead bronze Quaternary bronze Bronze Bronze

27 110 Sodium sesquicarbonate Arsenic copper (certified) Cu 95.4% As 4.6% E corr(v/she) Passivation Time (seconds) 175 Transformation of the oxide layer followed by passivation after 1 min; passivation slows down after 5 min Ecorr(V/She) Time (seconds) Mineralised water

28 130 Sodium sesquicarbonate Silver bronze 2 (certified) Cu 90.4% Ag 5.3% Sn 4.5% Ecorr(V/She) Rapid passivation Time (seconds) Corrosion E corr(v/she) Time (seconds) Mineralised water

29 Quaternary bronze (certified) Cu 78.9% Sn 7.2% Zn 6% Pb 7.9% Ecorr(V/She) Sodium sesquicarbonate Passivation Transformation of the oxide layer followed by slow passivation after 1.5 min Ecorr (V/She) Time (seconds) Time (seconds) Mineralised water

30 Applications Artefact SS Time (se c o nd s) 150 MW Time (seconds) Similar to Bronze 6 (Cu 87.3%, Sn 9.5% and Ni 3.2% Bronze 6 E corr(v/she) Time (seconds) Ecorr(V/She) Analysis EDS: Cu 83.7%, Sn 9.6%, Zn 6% and Ni 0.7% (effect of Sn, not much an effect of Zn) SS Time (seconds) MW

31 Artefact 2 0 SS MW Time (se c o n ds) Time (se c o nds) Similar to Quaternary bronze: Cu 78.9%, Sn 7.2%, Zn 6% and Pb 7.9% Ecorr(V/She) 80 Ecorr (V/She) SS Time (seconds) -100 MW Time (seconds) Analysis EDS: Cu 90.3%, Al 6.7% and Fe 3%

32 Conclusion 2 The database can be a useful tool to give a first indication of the composition of materials - uncertainty when the alloy contains more than one element - possibilities of misinterpretation Need then to insert more data (alloys and perhaps the behaviour of alloys in a 3 rd solution

33 Testing the EDT technique between different operators: Students in conservation science and conservation, technicians, conservators Preliminary tests: Pure metals Mineralised water

34 Ecorr(V/She) Time(seconds) Drop Vera Ingrid Ingrid Ros Ros Ros Ros Ros Ros Ros5.9-1 Ros5.9-2 Sn Ag Drop Vera-1 Drop Vera-2 Drop Vera-3 Ecorr(V/SHE) Ingrid Ingrid Ros Ros Time(seconds) Ros Ros Ros Series11

35 DropRos Ros Ecorr(V/She) Ros Ros Ros Ros Ros Ros5.9-1 Cu Al Tima(seconds) Ecorr(V/SHE) Time(seconds) Drop Vera 1 Vera2 Vera3 Ingrid17.07 Ros Ros Ros Ros Ros Ros Ros5.9-1

36 Conclusion 3 Importance of the protocol to clean the metal surface (reproducible polishing conditions are essential) Still some metals are more reliable than others: Sn more; Cu, Ag and Al less

37 Further work Testing the final version of the protocol by different professionals Optimisation: limit the size of the drop to the minimum (volume, distance the distance between the two electrodes) Construction of the database for copper based alloys

38 Final conclusion Simple electrochemical techniques are ideal tools to setup collaborative work between conservation / corrosion scientists and conservators (including students in conservation) Still some applied research has to be carried out in the field to fully understand the behaviour of CH metals in solutions commonly used by professionals to test, store or treat artefacts