Domed cap nut - Zn Al Sn Cu Alloy - Modern Times - France

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Mathea. Hovind (University of Oslo, Department of archaeology, conservation and history (IAKH-UiO), Oslo, Oslo, Norway)

Complementary information

Nothing to report.

The schematic representation below (Fig. 3) gives an overview of the corrosion layers encountered on the object from a first visual macroscopic observation.

Fig. 5: Stratigraphic representation of the sample taken from the domed cap nut in cross-section using the MiCorr application. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation. Stratigraphy representative of an area with intact metal plating (M1). S1 represents Fe-polluted soil while CP1 is a green translucent corrosion product, credit MiCorr_UiO-IAKH, M.Hovind.
Fig. 6: Stratigraphic representation of the sample taken from the domed cap nut in cross-section using the MiCorr application. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation. Area where the metal plating is missing. CP1 corresponds to CP2 in Fig. 3, CM1 represents the corroded metal phase, while M1 corresponds to the bulk metal (M2 in Fig. 3 and Fig. 5), credit MiCorr_UiO-IAKH, M.Hovind.

Complementary information

The fact that the artefact was considered as test material enabled extensive sampling that would not otherwise be possible.

Analyses performed:
Metallography: m
icroscope: Leica DMi8 (a metallographic, inverted, reflected light microscope) with magnification up to 500X. Camera: Olympus SC50 connected to the software “Olympus Stream”, version 1.9.4. Illumination modes: bright field and cross-polarized light. The metal is unetched.
SEM-EDS: i
nstrument: Jeol 6400; voltage: 20 kV; working distance: 18 and 24mm; sample preparation: palladium depot.

The metal of the domed cap nut (M2) consists of a Zn/Al/Pb/Cu/Sn alloy (Table 1) externally covered by a nickel-based plating (M1) (Figs. 7 and 8) containing some cobalt (Co). The bulk metal (M2) has a dendritic microstructure appearing light grey in bright field, while the interdendritic phase appears white (Fig. 9). The latter is Zn-rich, while the dendritic phase consists of approximately equal amounts of Zn and Al (Table 2, Fig. 11). Observation in SEM (BSE-mode) reveals the presence of Pb-nodules, visible as white irregular spots with a scattered distribution, in addition to opaque grey patches which are rich in Al and Fe (Fig. 10).

Elements

Zn

Al

O

C

Pb

Cu

Sn

Si

Fe

mass%*

54

23

11

6

3

2

1.5

0.3

0.2

Table 1: Chemical composition of the metal (M2). Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr_HEI Arc, C.Csefalvay. *The value is the calculated average of three analyses of the same feature, but in different areas.

                                   Elements mass %

Phase*

Zn Al O C Pb Cu Sn Si Fe

Dendrites

31

32

25

6

2

2

1

0.2

0.1

Interdendritic phase

86

0.7

2

6

3

2

0.8

0.2

0.1

Table 2: Chemical composition of the matrix consisting of dendrites and an interdendritic phase. Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr_HEI Arc, C.Csefalvay. *The value is the calculated average of three analyses of the same feature, but in different areas.   

Complementary information

The domed cap nut is nickel plated (6-7 micrometers).

The dendritic phase is heavily oxidized compared to the interdendritic phase (Table 2 and Fig. 12). The corrosion has developed throughout the entire metal body, generating cracks (Fig. 9). The cracks are Zn and O-rich (Fig. 12) and could be composed of zinc carbonate. It could be a case of zinc pest, an intergranular corrosion phenomenon known to cause disintegration in poor quality alloys (Selwyn 2004, Zhang 2011).

The external corrosion products and deposits (Fig. 3) were documented but not analyzed as they were considered to be mainly exogenous in origin and not the main reason behind the deterioration of the object. The orange deposit (S1) is probably soil from Fe-rich environment, while the thin green layer (CP1) is most likely consisting of an oxide of Ni from the nickel plating (M1). The white corrosion product (CP2) can either be a salt from the environment, corrosion products of Zn/Al or Zinc carbonate.

Complementary information

Nothing to report.

The schematic representation of corrosion layers integrating additional information based on the analyses carried out is given in Fig. 13.

The domed cap nut is made of nickel plated Zn/Al/Pb/Cu/Sn alloy. It has a dendritic microstructure, indicative of production by casting (probably a die cast). The deterioration of the metal could be due to absence of cohesion between the different phases which enabled the penetration of oxygen during the manufacture of the alloy and the formation of internal corrosion products, eventually leading to expansion of its internal structure (zinc pest)

References on object and sample

References sample
1. Selwyn, L. (2004) Metals and corrosion: A handbook for the conservation professional. Ottawa: Canadian Conservation Institute, 155-156.
2. Zhang, X. G. (2011) “Zinc”. In. Revie R. W. ed. Uhlig’s Corrosion Handbook, 3rd ed. Toronto, ON: John Wiley & Sons, 879 – 892.   

References on analytic methods and interpretation