Corroded metal sheet FN 655.13.01 - Fe Alloy - Modern Times - Switzerland

Corroded metal sheet FN 655.13.01 - Fe Alloy - Modern Times - Switzerland

Corroded metal sheet FN 655.13.01

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

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Credit HE-Arc CR.

Fig. 1: Iron (and steel) sheet (after Senn Bischofberger 2005, 137),

Iron 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.

Iron (and steel) sheet

Steinmöri, Neftenbach / Dorf Neftenbach, Zurich, Switzerland

Excavation of the Roman villa, 1986-1990, phase S2, StbII.2 (2nd/3rd century AD)

Modern Times

20th century (1986 _ 1990)

Soil

Kantonsarchäologie, Dübendorf, Zurich

Kantonsarchäologie, Dübendorf, Zurich

FN 655.13.01

Not conserved

Complementary information

Nothing to report.

Credit HE-Arc CR.

Fig. 2: Location of sampling area,

Stratigraphic representation: none.

Credit HE-Arc CR.

Fig. 3: Micrograph of the cross-section showing the location of Figs. 5 to 9,

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

Empa (Marianne Senn)

Kantonsarchäologie, Dübendorf, Zurich

2000, metallography

Complementary information

Nothing to report.

Analyses performed:
Metallography (nital etched), Vickers hardness testing, LA-ICP-MS, SEM/EDS.

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. 5). 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 iron. 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. 5. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Credit HE-Arc CR.

Fig. 5: Micrograph of metal sample from Fig. 3 (inverted picture, detail), unetched, bright field. In white the metal with round slag inclusions as well as cracks filled with corrosion products,

Credit HE-Arc CR.

Fig. 6: Micrograph of metal sample from Fig. 3 (detail), etched, bright field. The heavily cold worked metal shows elongated grains with nearly invisible grain boundaries and cracks parallel to the grains,

Heavily deformed ferritic microstructure

Fe

Ni

Complementary information

Nothing to report.

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. 7 and 8). Adhering to the metal (area 1), we find an orange-red-brown layer enriched in Cl (CP3 in Fig. 4, Fig. 9 and Table 3). The dark or violet middle area 2 is richer in Fe (CP2 in Fig. 4). The outer area 3 is red-brown (CP1 in Fig. 4) 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 < < < < 0.7 64 < < 100
Red-brown 38 < < < < 2.2 64 < 0.6 104
Red-brown 36 < < < < 3.3 67 < < 106
Red-brown 39 < < < < 1.3 63 < < 104
Orange-brown 35 1.3 < < < < 61 < < 98
Area 2 (CP2) Orange-brown 31 < < < < 0.7 63 < < 95
Violet-brown 30 < < < < < 75 0.8 1.0 108
Brown 30 < < < < < 77 < 0.6 108
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 < < 1.6 0.7 < 58 < < 102
Mixture brown 39 < 1.9 4.3 < < 56 < < 102
Mixture brown 35 < < 0.9 < < 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.

Uniform - transgranular

?

Complementary information

Nothing to report.

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Figs. 7 and 8.

Corrected stratigraphic representation: none.

Credit HE-Arc CR.

Fig. 7: 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. 4, unetched, bright field. Areas 1 (inner layer, CP3) to 3 (outer layer, CP1) are indicated,

Credit HE-Arc CR.

Fig. 8: Micrograph (same as Fig.7) and corresponding to the stratigraphy of Fig. 4, polarised light. The inner layer (CP3) is red-brown, area 2 is brown-violet (CP2) and the outer pale brown area contains rock fragment inclusions (CP1),

Credit Empa.

Fig. 9: SEM images, BSE-mode, and elemental chemical distribution of the selected areas from Fig. 8 (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,

This iron 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.