Degrigny, C. Et Al. Character is at Ion Corrosion Product Layers on Atmospherically Corroded Ferrous Objects. 2007

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1. INTRODUCTION

Corrosion product (CP) layers that form in the atmos-

phere on ferrous cultural heritage objects are not so well

documented in the literature. Until recently the corrosion

model of Stratmann and Streckel [1] based on wet dry cy-cles, where lepidocrocite seems to be the only reactive

phase in the corrosion product layer has been accepted.

More recently Neff et al. [2] showed that this model is

questioned by the study of objects such as those found on

historic open-air monuments since lepidocrocite is not

found in contact with the metal surface. The phases iden-

tified with micro-Raman spectroscopy (Raman) were

goethite (main product in contact with the remaining

metal surface), an hydrated iron oxy-hydroxide (ferrihy-

drite) which is poorly crystallised, lepidocrocite confined

in small zones and cracks next to the surface and aka-

ganeite in cracks and closer to the metal interface. Mon-

nier et al. [3] seem to confirm these observations at nano

and micro scales using other techniques such as X-ray

Absorption Spectroscopy (XAS), Transmission Electron

Microscopy (TEM), Electron Energy Loss Spectroscopy

(EELS) and micro-X-Ray Diffractometry (XRD).

In this paper we document the CP layers that formed

on a selection of historic atmospherically corroded fer-

rous body armours on display at the Palace Armoury,

Valletta, Malta. This collection was chosen by Heritage

Malta for the EU PROMET project. Understanding the

composition of the CP stratigraphies was required in or-

der to reproduce CP layers on artificial coupons that

were used to simulate real objects and to test innovative

corrosion protection systems (PS) developed within the

PROMET project.

Characterising the CP layers occurring on metal ob-

jects is often the first step in any metal conservation work.

Techniques employed in this project range from non-in-

vasive visual observation (naked-eye and optical mi-croscopy (OM) documented via digital photography) and

elementary analysis (X-Ray Fluorescence (XRF)) to

non-destructive examination (observation and elemen-

tary analysis of embedded cross-sections via metallo-

graphic microscopy and Scanning Electron Microscopy

(SEM) associated with Energy Dispersive Spectrometry

(EDS)). To structurally document the CP layers two oth-

er techniques were used: Synchrotron-Radiation (SR)-

micro()XRD and Raman.

Past and present PSs remaining on these objects were

also investigated using Fourier Transform Infra-Red

spectroscopy (FTIR).

2. THE PALACE ARMOURY COLLECTION

The Palace Armoury ranks among the worlds great-

est arms & armour collections. The collection situated in

the Grandmasters Palace in Valletta, dates from the 15th

to the 18th century, representing many European arms

and armour workshops. It is certainly one of the most vis-

ible physical symbols of the past glories of the Order of St

John.

Excluding some parade armour pieces and full suits of

armour belonging to the Grand Masters, the majority of

the armour exhibits are field armour belonging to in-

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Characterisation of Corrosion Product Layers

on Atmospherically Corroded Historic Ferrous Objects:Application to the Armour of the Palace Armoury, Valletta, Malta

Christian Degrigny, Daniel Vella, Stavroula Golfomitsou and James Crawford

Heritage Malta, Conservation Division

Old Royal Naval Hospital, Bighi CSP 12,Kalkara, Malta

Phone: +35621807675, Fax [email protected]

Corrosion product layers developing on atmospherically corroded historic ferrous objects have not beenstudied as thoroughly as those on archaeologically buried ferrous objects. This paper aims to describe the

methodology and scientific techniques employed to chemically and structurally characterise the corrodedsurfaces of a number of low-carbon steel and iron armour elements at the Palace Armoury, Valletta (Malta)that were selected by Heritage Malta within the EU PROMET project. The study of corrosion product lay-ers developed on Palace Armoury objects was an essential aide in confirming the similarity of the corrosionproducts formed on the steel coupons manufactured to simulate real objects that were used to test the cor-rosion protection systems developed within the project.

Keywords: steel, iron, armour, corrosion products, analytical techniques

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fantry being much less decorated. The Palace Armoury isrenowned for the fact that it is one of a few armouries inEurope that has survived "in-situ". In 1975 the armoury wastransferred from the first floor of the Palace (a site it had

occupied since inception) to the ground floor level (origi-nally the Palace stables). This transfer has possibly acceler-ated the degradation processes of the objects since thewalls of the "new" exhibit halls suffer from the severe risingdamp, salt efflorescence and elevated relative humidity of-ten associated with ground floor locations. Furthermore,the atmosphere inside the armoury is uncontrolled and ob-jects freestanding in the exhibition halls or attached to thewalls are suffering from dust deposition. The collection islocated in an urban and maritime environment with heavytraffic pollution entering from the main entrance as well aschlorides coming from the Mediterranean atmospherethat is also characterized by large daily fluctuations of rela-tive humidity and temperature.

The collection studied within the EU PROMET pro-ject corresponds to iron-based armour elements stylisti-cally dating from 16-17th c. Recently the collection indisplay cases was rearranged according to its typology. Itwas an occasion to inspect the objects exhibited in show-cases, attached to the walls or freestanding inside thehallway (Fig. 1 and 2) as well as the objects kept in storageinside the reserve collection.

2.1 Selection of representative objects

A preliminary condition survey, emphasising corro-sion PSs and corrosion phenomena, of the exhibited ar-mour (display cases, freestanding and wall-attached) and

the reserve collection was performed. This was undertak-en through visual observation of the objects (without thepossibility of direct handling). Survey sheets detailing ar-mour type, and probable manufacture techniques, ap-plied coatings, surface deposits, corrosion forms and con-servation interventions were filled for each object exam-ined. The survey data was then used to construct adatabase (work in progress). The database should help usto statistically analyze the collection and confirm andquantify its main conservation problems in addition tohighlighting deterioration correlations (e.g. corrosion as-sociated with certain PSs, decorative techniques etc.) thatwould otherwise not be apparent when evaluating a singleobject or small groups of objects.

Ten armour elements representative of the collectionwere chosen for further study. A thorough examination ofthe core material of these objects was carried out. Fig. 3summarises these 10 elements. The microstructure of asample from all objects is described in an earlier study [4]and is summarized in Table 1 (from [5]). As can be ob-served from Table 1, the majority of objects are fabricat-ed in steel and exhibit a ferrite-pearlite microstructure.One of the objects is manufactured from a phosphoriciron (PA RC 165).

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Figure 1 - Free-standing exhibits in the hall

Figure 2 - Exhibits in a glazed showcase

Figure 3 - Armour elements representative of the PalaceArmoury collection and selected for scientific investigation

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Characterisation of Corrosion Product Layers on Atmospherically Corroded Historic Ferrous Objects

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Table 1 - Summary of morphological features of corrosion layers obtain via SEM observations of the cross-sections. Type Iand II fragments both exhibit an exterior and interior surface. In type II fragments the exterior surface is masked by an ad-jacent plate (from Vella et al. [5])

2.2 Non-invasive examination of the collection

The development of analysis instrumentation under

the PROMET project facilitated the use of XRF on the

collection and was used to confirm that most objects were

fabricated in steel (and not phosphoric iron) and added

detail (such the presence of trace elements) to these pre-

liminary results. State of the art milli and micro-beam

XRF spectrometers were utilized for this work. These

non-invasive investigations were carried out on a larger

number of representative objects from the collection and

also more decorated armour. Our strategy for the selec-

tion of armour elements was once again based on a pre-

liminary visual observation of the objects exhibited inside

the showcases. After a preliminary selection of objects

made by the Heritage Malta (HM) PROMET team, and

discussion with the curator of the collection as regards

the relevance of our choice, a final selection was made

and submitted to the Demokritos PROMET team (Insti-

tute of Nuclear Physics, "Demokritos" Research Institute,

Greece) in charge of the XRF examination.

Examination with the milli-beam XRF spectrometer

was conducted first in-situ in the exhibition hall. Spot

analyses allowed us to confirm that most of the objects

were iron-based (with some containing traces of phos-

phorus), but also to determine the nature of the gilding

processes on non-field armour (Fig. 4). In some cases,

further investigation was carried out with the micro-beam

XRF spectrometer.

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C. Degrigny et al

Figure 4 - Examination of Jean Parisot de la Valettes ar-mour, circa 1560 with a milli-beam XRF spectrometer

3. MORPHOLOGY AND ANALYSIS OF CORRO-

SION PRODUCT LAYERS

The samples extracted previously for metallographicstudies of the armour [4] were reused to characterise the

corrosion product layers. A repolishing with 1000 siliconcarbide paper and 3 and 1 micron diamond paste was re-

quired to recover a surface unaffected by the metallo-graphic sample preparation by acid-etching.

3.1 Observation and analysis of cross-sections

Embedded fragment cross-sections were observedwith an optical microscope (Olympus model BX50) un-

der incident light source. This allowed a preliminary as-sessment of the CP layers with respect to colour and in-

tactness of the layers (Table 1). The embedded cross-sec-

tions were then mounted onto aluminium stubs viadouble-sided carbon tape and sputter coated with

graphite. Silver paste was applied between the samples

and stubs for better electrical contact. Samples were ob-

served with a LEO 1430 SEM. Observation of iron CPs bybackscatter (BS) detector allowed the possibility to dis-tinguish between different iron oxide phases that show

significant variation in atomic mass content largely affect-

ed by iron content (for example magnetite (72wt % Fe)and the iron oxyhydroxides (62wt % Fe)). Associated En-

ergy Dispersive Spectrometry/X-ray analysis was provid-

ed by an Oxford Link spectrometer with an ATW2 win-dow, operated by INCA software.

Figure 5 - Examination of pauldron element PA RC 166on cross-section. Sampling area (a/). Observation under

binocular and selection of the zone to further investigateunder SEM-EDS (b/), SEM (c/) from a detail of b/) andEDS analysis (d/)

The cross-sections were first observed at low magnifi-cation (x100-x300). This allowed a general assessment of

the CP layer(s). Corrosion product layer intactness anddepth of penetration into the core metal were evaluated(Table 1). To aid detection of any endogenous or exoge-nous elements present X-ray analyses were performed atdifferent points perpendicular to the corrosion layer(s)

(that are horizontally oriented to the metal). Higher mag-nifications (x500-x2500) were required to investigate em-bedded secondary phases (slag inclusions) and corrosiondroplets possibly associated with akaganeite phenome-non. EDS X-ray elemental mapping was carried out atthese high magnifications.

Figure 5 shows the results obtained from pauldron el-ement PA RC 166 that is heavily corroded on one of its

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edges (Fig. 5a). The regions of interest to study under

SEM were preselected through the binocular observation

of the cross-section (Fig. 5b). Corrosion product layers

that have not been disrupted by previous interventions

were preferred for further investigation. It has to be not-

ed here that objects in the PA collection were treated on

many occasions over the last centuries to remove the CP

layers that were constantly forming. It is only in the recent

years that CPs have been partially retained. Protection

systems (of organic origin) were removed (mostly) and

replaced with fresh material. It is very common to find re-

mains of past protective applications between strata of

iron oxides (Fig. 6).

Figure 7 - Examination of the cross-section of Fig. 5 withRaman (a/ and detail b/), 1: internal layer and 2: exter-

nal layer. On spectra of goethite and akaganeite fluores-

cence phenomena are observed at high wavelength (cred-

it J. Monnier, LPS)

Samples were observed under an Olympus micro-

scope using x100 Leitz objective lens. The diameter of the

analysed area was ca. 3mm. The mRaman technique was

optimized for the characterization of iron corrosion prod-

ucts based on the work carried out by Neff et al. [2]. Fig-

ure 7 shows some results obtained on the cross-section

from armour plate PA RC 166 (Fig. 5). The area analysed

is very near to the one examined in Fig. 5c and d. The in-ternal part of the layer contains both ferrihydrite (hydrat-

ed iron oxyhydroxide) and goethite and the external part

akaganeite. Chlorine (probably from sea aerosols) was in-

deed detected through EDS analysis, but Raman spec-

troscopy allows us to identify the iron oxide phases con-

taining chloride. The Raman spectra (particularly in the

external part of the layer) clearly show a fluorescence ef-

fect (deviation of the base-line signal at higher wave-

lengths) due to the presence of organics, cracks and dust

(characteristic of external layers). This fluorescence phe-

nomenon when associated to the presence of chlorides

(akaganeite detected by Raman) and calcium (detected

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Figure 6 - Remains of protection system (polyurethane?)in corrosion layers formed on the surface of pauldron PA

316. Cross-section observed under binocular

SEM-EDS observation/analysis gave interesting in-formation on the homogeneity of the CP layers (identifi-cation of one or several phases with BS observation), thelocation of the original surface (OS) (through thestudy/analysis of the slag inclusions and the elements ofatmospheric pollutants or particle deposition (Ca) as in-dicated on Fig. 5 c. and d.). Indeed the location of vanadi-um, a component and therefore marker of the slag inclu-sions, shows that the OS is likely to be above these. Fur-thermore highly concentrated zones of the CP layerpolluted with Ca are normally above the OS. Comple-mentary examination was carried out through a COST

G8 Short Term Scientific Mission (STMS) with Ramanat Laboratoire de Dynamique Interactions et Ractivit(LADIR), CNRS labs, Thiais, France using a notch-based spectrometer LabRam Infinity (Jobin Yvon-Hori-ba). Analytical radiation (green, 531.89nn) was producedby a frequency-doubled Nd:YAG laser.

1/

2/

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C. Degrigny et al

by EDS) show that the external layer has been subjected

to different interventions (cleaning, application of new

PS) and important pollution in comparison to the inter-

nal layer that seems intact and is constituted of a mixture

of goethite and ferrihydrite.

3.2 Complementary investigation of microsamples

The examination described above and carried out on

small fragments sampled from one location on the objects

are essential. Still they do not give an overall description

of the CP layers that form on the whole surface of the ob-

jects. Although they were chosen carefully the small frag-

ments are not always extracted from the most representa-

tive (as regards corrosion product morphologies) area of

the artefact and one must remember that most armour el-

ements are composed of many separate armour plates.

The extraction process results in fact from a discussion be-

tween the team of conservation scientist / conservatorsand the curator in charge of the collection who together

try to limit sampling to the minimum. From the ten sam-ples previously taken from the objects selected only a few

gave significant results in terms of CP layers. Other ob-

jects were without CP layers or covered with thick protec-

tion layers and loosely bound corrosion products that

formed afterwards on top of them (see Fig. 6 and Table 1).

To have a more realistic idea of the homogeneity /

heterogeneity of the corrosion product layers forming on

the objects, twenty to thirty microsamples were taken

from each artefact and analysed using SR-XRD. These

microsamples were taken from different locations when

the CP layer was very thin and in the case of thicker CP

layers samples were taken from the same spot in differentdepths to get the whole local stratigraphy of the corrosionproduct layers and to be able to compare the results ob-

tained with those of Raman spectra. The use of SR-

XRD instead of conventional XRD was appropriate due

to the very small amount of sampled material and the

very large number of microsamples to investigate (more

than 300) since SR drastically shortens X-ray data acqui-

sition time.

SR-XRD measurements were performed at station

9.6 of the Synchrotron Radiation Source at Daresbury

Laboratory (UK). Powdered corrosion product mi-

crosamples were transferred onto Scotch MagictapeTM

(pressure sensitive tape) that does not provide back-ground interference) stretched over 5mm internal diame-

ter stainless steel washers that served the purpose of sam-ple holders (Fig. 8a). The sample holders were mounted

on the goniometer (Fig. 8b). X-ray exposure time for all

samples was 60 seconds and the X-ray wavelength was

0.87 . Two-dimensional diffraction patterns were ac-

quired using a CCD detector. The data were polar trans-

formed and azimuth integrated using the ESRF program

FIT2D [6]. Reference data from the JCPDS PDF cards

were used to identify the corrosion products.

Figure 8 - In-situ microsampling from PA objects and posi-tioning of the microsamples under binocular on tapestretched over thin metal sample holders (a/). Mounting ofthe holders on the goniometer at Daresbury Laboratory (b/)

Results on pauldron PA 316 confirm the presence ofakaganeite and goethite on the outside of the artefact but

calcite, lepidocrocite and magnetite are identified as wellwhile magnetite, hematite and wstite (black layer) + ak-aganeite, goethite and lepidocrocite (on top) are foundon the interior surfaces facing the wearer of the armour.

3.3 The nature of the protection systems applied

From reports of experts that examined the armour el-ements of the PA, it is clear that different protection sys-tems have been applied on the objects at different occa-sions. Czerwinski and Zygulski [7] recommended that theuse of polyurethane varnish (employed since 1900)should be discontinued in favour of acrylic resin (ParaloidB72) and/or beeswax mixed with paraffin. A close exami-nation of some objects indicates the presence of "wax-like" material (simply in terms of appearance) while oth-ers have a yellow appearance due to colouring of PS ap-plied, possibly also increased by the natural ageing. Thebackplate (PA 329) seems to have two PSs (Fig. 9).

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Figure 9 - Waxy and yellow appearance of the external

surface of backplate PA 329 suggesting the presence of adouble layer of protection systems ("wax/grease" +"polyurethane")

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The protection systems applied on both sides of the

ten objects have been studied via FTIR. Fig. 10 shows the

spectra obtained from the pauldron (PA 316) and Table 2

gives a summary of all results obtained on both sides of

the objects [8]. It can be concluded that a preliminary lay-

er of a polyurethane PS was applied on all objects. It was

observed that the PS had failed (i.e. permitted corrosion)

but it was not removed. An extra layer of wax, or other

petrochemical derived coating such as grease (another

common PS applied especially on the PA reserve collec-

tion), was applied onto the failed PS layer as a means of

temporary protection. Based on this result we understand

now that the investigation of the CP layers is further com-

plicated by the presence of a double layer of organic ma-

terial. The PS still visible at the interface between the

metal surface and the CP layers on Fig. 6 could be the re-

mains of the polyurethane PS. The CP layers above may

contain though microcrystalline wax or grease. Thesetransparent organic layers might be the reason behind the

fluorescence phenomenon observed on the Raman

spectra (Figure 7).

3.4 Analysis of decorations under corrosion product layers

Nine of the ten objects selected within this study are

without surface decoration. One though (gorget PA RC

25) has visible black decoration (most probably from

acid-resist etching) under the CP layers (Fig. 11). None of

the techniques described earlier are really appropriate to

determine in a non-invasive way the chemical composi-

tion of the black decoration (applied pigmentation and/or

traces remaining from the possible acid used).

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Figure 10 - FTIR spectra showing the possible presenceof both polyurethane Rylard "varnish" and Renaissance"wax" on pauldron PA 316, (from Lemasson [8])

Object External surface Internal surface

Polyurethane Wax Polyurethane Wax

varnish varnish

PA316 Yes Yes Yes Yes

PA317 Yes Yes Yes (?) Yes

PA329 Yes Yes Yes Yes

PA-RC 20 Yes Yes Yes (?) YesPA-RC 25 Yes Yes Yes (?) Yes

PA-RC 29 Yes (?) Yes Yes ?) Yes

PA-RC 80 Yes Yes Yes (?) Yes

PA-RC 88 Yes Yes No Yes

PA-RC 165 Yes Yes Yes (?) Yes

PA-RC 166 Yes (?) Yes Yes (?) Yes

Table 2 - FTIR analyses of the protection systems appliedon both sides of the 10 objects from the PA (Lemasson [8])

Figure 11 - Black decoration at the surface of gorget PARC 25 and visible through the corrosion layer

Spot analysis with milli-beam XRF was carried out

first by the Demokritos PROMET team, but no clear dif-

ference could be observed between non decorated anddecorated areas. A more thorough examination was per-

formed by the team using the micro-beam XRF spec-

trometer developed within the PROMET project and we

could observe an increase of the baseline of the spectra in

medium energy that seems to indicate the presence of an

organic binder. Furthermore small amounts of Pb and Cucould be detected. Interpretation of these results is still in

progress.

4. DISCUSSION

Not all samples have corrosion product layers intact

(as indicated on Table 1). In the case of objects PA 329,

PA RC 80, PA 317 the CP layers have been disturbed

(possibly during cleaning of the artefact) and a mixture of

loose CPs and other soiling material was left behind. Thisresulted from a problem during sample preparation since

grinding and polishing gave rise to constant loss of mater-

ial from these layers creating holes. For these samples,therefore we cannot really talk of CP layers but more CP

deposits. In the sample extracted from object PA 329 itseems that all the CPs were removed from the external

surface and a thick layer of protective coating was ap-

plied. For the interior surface (i.e. facing the wearer), a

thick layer of coating was applied directly over porous

CPs. This also seems to be the case for object PA 317.The original exterior surface (OS) of the object was lost

(since the CPs have been disturbed) and the new original

surface (i.e. the surface remaining from last intervention)

would lie at the surface of the metal.

The OS is likely to be preserved in areas that present

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C. Degrigny et al

intact CP layers. In the case of artefact PA RC 20, the PSlayer was observed running within an intact CP layer. Inthis case the original surface might be found just under-neath the protective application. In the case of pauldron

PA RC 166 the OS is lost, but some remains of the formerCP layer (mixture of goethite and ferrihydrite) can befound under a disturbed superficial layer that containsmixtures of "particles" or deposits (calcite (Ca) and Cl-based). Other internal markers would need to be assessed(slag inclusions) in CP layers in order to establish a theo-retical position of the OS.

5. CONCLUSION

The corrosion product layers formed on the iron-

based historic armour elements of the Palace Armoury

collection are of different types. The ones that have not

been disrupted by continuous interventions seem to cor-respond to the existing model defined by experts in

France on iron-based elements inserted in historic monu-

ments. The analyses performed indicate though that the

heavy pollution in Valletta, presence of high amount of

chlorine and past interventions on objects have in most

cases modified the CP layers. Still it is quite easy to differ-

entiate the more recent deposits formed after these last

interventions from the "original" CP layers.

This information is very precious for the conservator

since it gives him/her some guidelines on how to clean the

metal surface in order to remove the non adherent de-

posits and keep the traces of original corrosion productlayers. It is on these types of surfaces that the innovative

protection systems designed within PROMET will be ap-plied.

As a general remark, the investigation work per-

formed showed not only the importance of clustering dif-

ferent analytical techniques to thoroughly describe a

complicated CP system, but the necessity of first defining

a strategy to properly use the diagnostic tools that are lo-

cally available and to apply for funds (e.g. EU funding) to

use other tools that are only available internationally.

ACKNOWLEDGEMENTS

The authors are grateful to the following: the Euro-pean Commission through its 6th Framework Pro-gramme, priority INCO, for the funding of this research

and through its Research Infrastructure Action under the

6th Framework programme "Structuring the European

Research Area" and The Council for the Central Labora-

tory of the Research Councils (UK) for their financial

support that made the SR-XRD experiments possible as

well as Dr Manolis Pantos for valuable assistance at the

SR-XRD facility at Daresbury ; the EU COST pro-

gramme for the STSM at Laboratoire Pierre Se (LPS),

CEA-CNRS Saclay, Gif-sur-Yvette (France) funded un-

der COST G8. Many thanks are addressed to Dr.

Philippe Dillmann and Ms. Judith Monnier from the LPSfor performing and interpreting the Raman spectra and

the interpretation of the SR-XRD spectra obtained atDaresbury laboratory.

We would like to thank the Demokritos PROMET

team, Dr. Andreas Karydas, Dr. Charalambos.Zarkadas

and Ms.Vicki Kantarelou, who brought to Malta their

portable milli and micro-beam XRF spectrometers that

were so useful for the non-invasive examination of the ob-

jects of the Palace Armoury. Finally we would like to

thank Mr. Emmanuel Magro-Conti and Mr. Michael

Stroud, curators at Heritage Malta for allowing access to

the Palace Armoury collection.

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Figure 12 - Schematic representation of typical corrosionlayer on archaeological iron artefacts corroded in the air(from Neff et al. [2])

The results obtained from the scientific investigationof remaining former CP layers are very much similar tothose obtained by Neff et al. [2] and Monnier et al. [3].The same CPs were analysed on our objects as those typi-cally found on iron-based reinforced systems inserted onhistoric monuments. A schematic representation is givenon Fig. 12 ([2]) that we can refer to. In our case chlorinat-

ed phases were not found at the metal interface, but atthe external surface of CP layers and the system is com-plicated by the presence of remaining PSs. Furthermore,the presence of phases such as magnetite, hematite andwstite could be attributed to the manufacturing of theobject: for example the exposure to high temperaturesduring forging. The presence and continued preservationof these high temperature forming oxides on the armourinterior is probably attributable to the tendency of ar-mours and restorers alike to only polish the exterior sur-face that is visible when being worn.

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[5] Vella, D., Degrigny, C. and Grech, M.: "A prelimi-nary study of the morphology of corrosion develop-ing onto historic steel armour artefacts from thePalace Armoury Collection, Valletta, Malta", in Pro-ceedings of the 14th triennial meeting of ICOM-CC,The Hague, Netherlands, pp. 317-325 (2005).

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[7] Czerwinski, A. and Zygulski, Z.: "The Palace Ar-moury of Valletta, UNESCO document, (1969).

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Documents

Degrigny, C. Et Al. Character is at Ion Corrosion Product Layers on Atmospherically Corroded Ferrous Objects. 2007