Iron-based bar - Fe 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 iron-based bar 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. This representation can be compared to Figs. 9 and 10, 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.
SEM-EDS: i
nstrument: Jeol 6400; voltage: 20 kV; working distance: 18 and 24mm; sample preparation: palladium depot.   

The metal is a wrought iron consisting of Fe, with some P and C (Fig. 6). The presence of Si is due to slag inclusions. The inclusions appear elongated* (Fig. 7) and filled with phases appearing light grey, medium grey and dark grey both in bright field and in SEM in BSE-mode (Figs. 7 and 8). Punctual analysis by SEM-EDS (Table 1) revealed that the light grey phase consists mainly of Fe and O with some C and has a composition similar to Wüstite (FeO). The medium grey phase has a similar composition but contains more P and C in addition to Si (Table 1). This phase is probably Wüstite in a Fe-P matrix. The dark grey phase corresponds to the glassy matrix and contains significally higher concentrations of Si and P, in addition to the usual Fe and O (Table 1). The relatively high Si-content indicates that this phase might be Fayalite (FeSiO4) in a Fe-P matrix.

Smaller inclusions/nodules are evenly distributed throughout the metal (Fig. 7 and 8). They appear dark grey and have a composition similar to the dark grey phase in the elongated inclusions (Table 1).

Elements mass %

Phase / nodule

Fe

O

P

Si

C

V

S

Mn

Al

Cr

Mg

Ca

Light grey phase



Medium grey phase


Dark grey phase



Nodules

83

 

74

 

 52

 

49

12

 

15

  

22

 

27

0.1

 

4

 

13

 

11 

0.2

 

3

 

8

 

5

2

 

3

 

3

 

6

2

 

0.8

 

-

 

- 

0.1

 

0.7

 

0.2

 

1

0.4

 

0.6

 

 -

 

0.9

0.2

 

0.3

 

 0.1

 

0.1

0.4

 

0.1

 

 -

 

-

-

 

-

 

 0.2

 

-

-

 

-

 

 0.1

 

-

Table 1: Chemical composition of the different phases in the slag inclusions and the nodules in the metal matrix. Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr_HEI Arc, C.Csefalvay.

As the section was cut across the iron bar – it is the cross section of the inclusions that are visible. Thus, their length and direction cannot be deduced directly from the sample.

Complementary information

Nothing to report.

The corrosion crust is relatively thick and consists of two layers: CP1 and CP2. The latter is a dense product layer appearing light grey under both bright field and polarized light (Figs. 9 and 10). The outermost layer (CP1) is a porous crust, appearing dark grey under bright field and bright orange under polarized light (Figs. 9 and 10). The corroded metal (CM1) appears as isolated areas of corrosion within the sound metal.

The composition of the corrosion products shows a varying content of Fe and O throughout the crust with an increasing O-content towards the surface of the corrosion layer CP1 (Table 2). Elemental mapping by SEM-EDS (Fig. 11) shows that Ca and Mg are present in cracks which penetrates the outer corrosion crust (CP1). These elements are probably exogenous and could originate from the layer of soil (S1) that is covering the metal surface.

Elements mass %

Layer

Fe

O

C

Ca

Si

P

S

Mg

Al

V

Cr

Mn

CP1

57

36

5

1.0

0.3

0.3

0.3

0.3

0.1

-

-

-

CP2

68

27

3

0.5

0.4

0.4

0.1

0.1

-

-

-

-

CM1

70

25

3

0.1

0.5

0.5

0.3

0.1

0.1

0.3

0.1

0.1

Table 2: Chemical composition of the corrosion layers from Figs. 9 and 10. Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr_HEI Arc, C.Csefalvay.

Complementary information

Nothing to report.

The schematic representation of corrosion layers integrating additional information based on the analyses carried out in Fig. 12.

The artefact is a wrought iron with evenly distributed inclusions of what appears to be wüstite in a fayalite matrix. Wrought iron containing slags was readily available until World War II, after which it was substituted by low-carbon steels (Selwyn 2004:96). This indicates that the artefact can be dated no later than the first half of the 20th century. As regards the production method, it has been suggested (Dr. Phil. M. Senn, 2018, personal communication the 26th of April) that the artefact was produced by puddling, an indirect process for the conversion of pig iron to wrought iron, while decreasing the level of impurities (Selwyn 2004:112-113).

The corrosion products on the surface of the iron bar are typical for iron exposed outdoors with varying contents of Fe an O in addition to a layer of Ca-containing soil.  

References on object and sample

References sample
1. Selwyn, L. (2004) Metals and corrosion: A handbook for the conservation professionalOttawa: Canadian Conservation Institute.

References on analytic methods and interpretation