Corroded metal sheet FN 655.13.01
Marianne. Senn (Empa, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)
Metal (iron-based) sheet with middle rib. The shape is no longer discernible (Fig. 1). The metal is covered by a thick corrosion crust. Dimensions: L = 4.5cm; WT = 35g.
Metal sheet
Steinmöri, Neftenbach / Dorf Neftenbach, Zurich, Switzerland
Excavation of the Roman villa, 1986-1990, phase S2, StbII.2 (2nd/3rd century AD)
Modern Times
Product of the 20th century AD introduced accidentally into a Roman layer
Soil
Kantonsarchäologie, Dübendorf, Zurich
Kantonsarchäologie, Dübendorf, Zurich
FN 655.13.01
N/A
None.
None.
A longitudinal cut has been made through the sheet (Fig. 2). Large parts of the corrosion crust fell off during sampling (Fig. 3).
Fe Alloy
Forged with final cold work
NEF 655
HE-Arc CR, Neuchâtel, Neuchâtel
Kantonsarchäologie, Dübendorf, Zurich
2000, metallography
None.
Analyses performed:
Metallography (nital etched), Vickers hardness testing, LA-ICP-MS, SEM/EDS.
None.
The remaining metal is an iron containing elevated (more than 1g/kg) concentrations of Co and Ni (Table 1), small round slag inclusions and cracks resulting from corrosion and deformation (Fig. 4). The composition of the round, elongated slag inclusions is similar to wüstite-FeO (Table 2). Iron reduced in the direct smelting process never contains such pure compounds and only FeO inclusions. In modern steels FeO occurs in low carbon alloys and Armco-iron (Schumann 1991, 474). After etching, the cross-section shows a heavily deformed ferritic microstructure from cold working (Fig. 6). The cracks have the same orientation as the deformation. The average hardness of the metal is HV1 195.
| Elements | Al | Ti | V | Cr | Mn | P | Co | Ni | Cu | As | Mo | Ag | Sn | Sb | W | Ni/Co |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Median, mg/kg | < | < | < | < | 20 | 300 | 1100 | 1200 | 800 | 200 | < | < | 60 | 10 | < | 1.1 |
| RSD % | - | - | - | - | 3 | 17 | 6 | 5 | 35 | 15 | 13 | - | 35 | 45 | - | 4 |
| Detection limit mg/kg | 6 | 9 | 19 | 17 | 2 | 77 | 1 | 4 | 2 | 3 | 3 | 1 | 1 | 1 | 3 | - |
Table 1: Chemical composition of the metal sheet. Method of analysis: LA-ICP-MS, Lab of Inorganic Chemistry, ETH.
|
Elements |
O | Fe | Total |
|---|---|---|---|
| Round inclusion 1 | 21 | 75 | 96 |
| Round inclusion 2 | 20 | 74 | 95 |
| Round inclusion 3 | 20 | 75 | 96 |
| Round inclusion 4 | 20 | 73 | 93 |
Table 2: Chemical composition (mass %) of some of the round inclusions of Fig. 4. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.
Credit HE-Arc CR.
Heavily deformed ferritic microstructure
Fe
Co, Ni
None.
The corrosion is massive and large parts of the outer corrosion layers have been lost during the sample preparation. The heavily cracked corrosion crust represents about one third of the thickness of the sample (Fig. 3) and is located on one side of the metal (the rest having been lost during the cutting process). It does not show well defined layers. Despite this, three areas can be distinguished (Figs. 6 and 7). Adhering to the metal (area 1), we find an orange-red-brown layer enriched in Cl (CP3 in Fig. 9, Fig. 8 and Table 3). The dark or violet middle area 2 is richer in Fe (CP2 in Fig. 9). The outer area 3 is red-brown (CP1 in Fig. 9) and strongly contaminated by soil material (rock fragment inclusion and elements like Si, Al, P etc.). This layer is enriched in O (Fig. 9).
|
Elements |
Location | O | Na | Al | Si | P | Cl | Fe | Cu | Mo | Total |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Area 1 (CP3) | Dark-brown | 31 | < | < | < | < | < | 67 | < | < | 99 |
| Dark-brown | 35 | < | < | < | < | 1 | 64 | < | < | 100 | |
| Red-brown | 38 | < | < | < | < | 2 | 64 | < | 1 | 105 | |
| Red-brown | 36 | < | < | < | < | 3 | 67 | < | < | 106 | |
| Red-brown | 39 | < | < | < | < | 1 | 63 | < | < | 103 | |
| Orange-brown | 35 | 1 | < | < | < | < | 61 | < | < | 97 | |
| Area 2 (CP2) | Orange-brown | 31 | < | < | < | < | 1 | 63 | < | < | 95 |
| Violet-brown | 30 | < | < | < | < | < | 75 | 1 | 1 | 107 | |
| Brown | 30 | < | < | < | < | < | 77 | < | 1 | 108 | |
| Violet-brown | 35 | < | < | < | < | < | 68 | < | < | 103 | |
| Yellow | 33 | < | < | < | < | < | 67 | < | < | 100 | |
| Violet-brown | 32 | < | < | < | < | < | 74 | < | < | 106 | |
| Area 3 (CP1) | Quartz inclusion | 54 | < | < | 52 | < | < | < | < | < | 106 |
| Mixture brown | 42 | < | < | 2 | 1 | < | 58 | < | < | 103 | |
| Mixture brown | 39 | < | 2 | 4 | < | < | 56 | < | < | 101 | |
| Mixture brown | 35 | < | < | 1 | < | < | 59 | < | < | 95 |
Table 3: Chemical composition (mass %) of the corrosion products from near the metal (area 1, CP3) to the outer surface (area 3, CP1). Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.
Credit HE-Arc CR.
Fig. 6: Micrograph of the corrosion layer from Fig. 3 showing the metal - corrosion crust interface (detail, rotated by 45°) and corresponding to the stratigraphy of Fig. 9, unetched, bright field. The areas 1 (for CP3), 2 (for CP2) and 3 (for CP3) selected for elemental chemical distribution (Fig. 8) are marked by red rectangles,
Credit HE-Arc CR.
Credit HE-Arc CR.
Fig. 8: SEM images, BSE-mode, and elemental chemical distribution of the selected areas from Figs. 6 and 7 (reversed picture rotated by 270°, details). First column: mapping on area 1 (CP3) where an inner layer enriched in Cl appears near the metal surface. 2nd column: area 2 (CP2) where iron oxides prevail. 3rd column: area 3 (CP1) where the outer O-rich layer includes rock fragments rich in Si. Method of examination: SEM/EDS, Lab. Anal. Chem. Empa,
Uniform - transgranular
Unknown
None.
None.
This iron-based sheet is entirely cold worked and hard compared to an annealed metal. The corrosion is massive and masks the shape of the object. The presence of Cl adjacent to the metal surface indicates that the corrosion front is potentially active. The very thick top corrosion layers may have slowed down the corrosion. The object was mentioned as being Roman in Senn Bischofberger 2005, however the chemical composition of the slag inclusions shows that it is a product of the 20th century AD which has been accidentally introduced into a Roman layer.
References on object and sample
References object
1. Rychener, J. (1999) Der römische Gutshof in Neftenbach. Katalog, Tafeln und Tabellen. Monographien der Kantonsarchäologie Zürich 31/2 (Zürich und Egg), 138.
References sample
2. Senn Bischofberger, M. (2005) Das Schmiedehandwerk im nordalpinen Raum von der Eisenzeit bis ins frühe Mittelalter. Internationale Archäologie, Naturwissenschaft und Technologie Bd. 5, (Rahden/Westf.), 137-138.
References on analytic methods and interpretation
3. Schumann, H. (1991) Metallographie. Leipzig.
