Stabilisation of CH artefacts An overview. Christian Degrigny

Transcription

1 Stabilisation of CH artefacts An overview Christian Degrigny

2 A bit of history, choice of solution The same as for the cleaning overview could be said here

3 Why a stabilisation treatment? DRASSM Ingots left to dry in storage area Iron ingots from the 1 st century BC Mediterranean Sea DRASSM site Part of the cargo left to dry outdoor

4 Exposure to the atmosphere Droplets (Fe 2+, Fe 3+, Cl - ) Rupture of the equilibrium HR and O 2 - Drying of corrosion products and - Crystallisation of salts - Oxidation of some salts - If the artefact contains chlorides: development of active corrosion - Formation of akaganeite (β-feooh ): mechanical effect on the corrosion layer, unstable, - Weeping phenomenon, droplets

5 Stabilisation without polarisation Uncleaned Cleaned Storage in KOH solution

6 On clean iron-based alloys V Passivation observed

7 On corroded iron-based alloys E corr bare Passivation corroded time Transformation of the oxide layer

8 confirmation in practice Artificial steel coupon Max. extraction = solution reach M

9 Marine wrought iron EVTEK shackle stock shank fluke arm Finnish, 19th c. Active or not?

10 FeOOH Cl -? Fe 3 O 4 Fe EVTEK EVTEK EVTEK EVTEK Humid chamber

11 EVTEK Set-up of the electrical contact EVTEK EVTEK Immersion of the anchor Checking of the good electrical contact

12 Monitoring Ecorr vs time E corr measurem ent anchor w ith stock in 1% (w/v) KOH E (mv/ag-ag-cl) Quick diffusion of KOH through thin corrosion layers T im e (ho urs ) Transformation of the corrosion layer Passivation And no extraction of chloride: stable artefact

13 Metals with thick corrosion layers? thin corrosion layers thick corrosion layers KOH KOH KOH KOH CL Remaining metal CL CL KOH CL KOH CL Corroded Fe in KOH solution

14 Iron ingots (Mediterranean sea) Weeping on the surface of ingots High reactivity of the inside of one ingot in the atmosphere

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

16 How chlorides interfere with the behaviour of iron in KOH solutions? E (V) Passivation of Fe in KOH solution Interference with Cl - Further perturbations t (time)

17 Marine iron-based swivel gun Multimeter in NaOH Ref. electrode HM Swivel gun recovered from Maltese waters

18 Full monitoring of the stabilisation E corr ph C Cl - Λ

19 speed up of stabilisation processes Cathodic polarisation

20 2 Voltage (V) Intensity (A) I 2,5 1,8 1,6 1,4 1,2 voltage 2 1,5 0.95V/Ag-AgCl 1 0,8 0,6 I 1 0,4 0,5 0,2 Chlorides (ppm) st wash (14 samples) time (h) nd wash (26 samples) Electrolyte changed First wash Second wash Linear (First wash) Linear (Second wash) Square root of time (hours)

21 Use of pulsed currents E c =-0.75V/SHE -preventing hydrogen bubbling -accelerating the stabilisation process by favouring rapid processes Q Q Cl total = 0,29 Q Q Cl total = 0,17

22 Application Mass treatment: 23 iron bullets from a grape shot EVTEK EVTEK EVTEK

23 EVTEK Cathodic polarisation in 1% KOH at 0.95V/Ag-AgCl

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26 Pitting corrosion of copper Stabilisation of copper-based alloys EDF-Valectra Cl - Cu 2 (OH) 3 Cl, atacamite Cu CuCl, nantokite Pitting corrosion: autocatalytic corrosion process - metal dissolved inside the pit : excess of positive copper ions. - chlorides migrate from the environment or the metal bulk inside the pit to maintain electroneutrality, and nantokite (CuCl) is formed. - Inside the pit, the metal is hydrolysed, resulting in low ph inside the pit. - Low ph and high concentration of chlorides: acceleration of corrosion

27 In sodium sesquicarbonate CCl Bell - Ecorr and Cl- with time in sodium sesquicarbonate 1% Ecorr CCl EVTEK Ecor r time (h) 0

28 AWM Stabilisation of aluminiumbased alloys Pitting corrosion sensitivity according to the alloys [NaCl]=10-3 M

29 Risk of pitting corrosion ph=5.4, [NaCl]=10-3 M

30 Cathodic corrosion on aluminium alloys in NaCl solutions Al e- 2H 2 O + 2e - H 2 + 2OH - ph Al 2 O 3 is dissolved preferably above active sites (Al 3 Fe and Al 2 Cu) Al 2 O 3 + 2OH - AlO 2- + H 2 O H 2 ph : Al is dissolved Under the H 2 bubble

31 Optical monitoring

32 Modification of the Pourbaix diagram 6082 alloy with the fields of pitting corrosion and cathodic corrosion in neutral ph added. [NaCl]=10-3 M Field of stability

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34 Marine composite objects Porthole, Finnish wreck, 19th c. Cleaning of the wood: soaking in 1% EDTA disodique followed by electrophoresis in tapwater (2 times): appearance of the wood more natural EVTEK Impregnation of the wood iin PEG 400 and PEG 4000

35 Leather/wooden clog Day Solution Iron conc. (mg/l) Iron Content (mg) 1 1 % Na 2 EDTA EVTEK % triammonium citrate Stored in tap water and treated with electrophoresis % Na 2 EDTA

36 EVTEK In tap water Current (ma) Voltage current voltage 50 1st side 2nd side time (hours)

37 with metal left - + Rifles, 17th c. X XX X X XCl - X X X X X XX X XX X X stabilisation of the barrels in un-buffered KNO3 (+ addition of HNO3 to keep the ph neutral)

38 Further treatment of the wood Protection Loss of protection -400 Stabilisation of the wooden part: Impregnation in PEG Hostacor Potential (mv) / ESS Protection 4. New PEG PEG Hostacor 3. Contamination Weeks 25

39 Better monitoring of the cathodic potential EVTEK EVTEK Dry porthole, 19th c., Finnish waters Glass Stabilisation in a neutral unbuffered solution (1% (w/v) NaNO 3 ) Stabilility of the ph? EVTEK rubber

40 Preparation of the artefact EVTEK Preliminary mechanical cleaning EVTEK Electrical contact

41 Simulating the treatment in NaNO3 Corroded nails Bare nail EVTEK Slightly polished nails

42 Bare iron nails Potential corresponding to the highest extraction of chlorides The following reaction is favoured: 2 H 2 O+ O 2 d + 4 e - 4 OH -

43 Slightly polished iron nails The following reaction is favoured: 2 H 2 O+ O 2 d + 4 e - 4 OH - x FeO(OH) + x e - x Fe + x OH -

44 Corroded iron nails Opposite behaviour than before: with corrosion layers and an increase of the cathodic potential, the ph tends to decrease

45 Application on the porthole Monitoring of all electrochemical parameters Anodic potential ph Cathodic potential

46 Low! Decrease of ph Addition of NaOH (day) Night

47 Conditions modified During the 5 first days. Same conditions Afterwards: anodic Control and Ea=1.1V/Ag-AgCl and Ec=-0.5V/Ag-AgCl

48 Stabilisation of corroded Pb artefacts E (V/HSE) Jacquard looms (Musée des Arts et Métiers, Paris) -1.3V Pb 2+ Pb I (ma) Cathodic polarisation of a lead coupon carbonated artificially in Na2SO4 0.5M (objective: treatment of composite artefacts), vb= 1mV/s

49 Chronoamperometric curve at 1.3V/HSE in Na 2 SO 4 0.5M. Counter-electrode: Pb

50 Application Conservation of corroded lead seals still attached to parchments with silk threads Original surface lost

51 Simulation of the treatment of real composite artefacts: -lead: fragments from lead seals -silk threads: naturally died silk coupons - counter-electrode: lead plates Dissolution of the dies without any protection

52 Protection of the textile threads Loss of adherence Cathodic reduction Paper impregnated with paraffine

53 Simulation of a treatment on artificial materials Immersion test Enveloppe + CDD after immersion test

54 Preparation of a simulated composite Absorbing paper hemp Lead silk

55 Simulation of the protection of both the silk and the parchment

56 Design of a special support for the artefact once protected

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58 Optimisation: use of Perspex rods to give a better strength

59 Packing the organic materials in PE envelops sealed with cyclododecane Parchment from the French National Archives Reinforcing of the protected composite with Perspex rods