Sacrificial anode VHS-8339
Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)
Fig. 1: Submarine and the anode (www.verkehrshaus.ch),
Half of possibly a weight or sacrificial anode. It is surrounded by a whitish brown-grey corrosion crust, the broken metal has a greyish shining colour, whereas the cut metal part is silvery. Dimensions: L = 4.9cm ; WT = 95g.
Submarine part
Submarine “Mesoscaph” from Auguste Piccard
The sacrificial anodes (?) might have been added when the submarine was used in the sea.
Modern Times
1970 _ 1974
Outdoor atmosphere
Swiss Museum of Transport, Luzern, Lucerne
Swiss Museum of Transport, Luzern, Lucerne
VHS-8339
Not conserved
The anodes were produced by Horton Maritime.
Stratigraphic representation: none
The sample shows a cross-section from the sacrificial anode. The thickness of the corrosion crust is variable. Dimensions: L = 17mm; W = 14mm.
Zn Alloy
Cast and annealed
VHS-Mq-1
Empa (Marianne Senn)
Swiss Museum of Transport, Luzern, Lucerne
07/09/2009 metallography
Analyses performed:
Metallography (unetched), Vickers hardness testing, SEM/EDX.
The remaining metal is an almost pure zinc alloy (Table 1). The oxygen content is not from the original alloy, but is due to secondary corrosion. The metal grains are visible without etching and present a polygonal structure (Figs. 4 and 5). The structure is recrystallised after annealing. The recrystallization of zinc alloys begins at room temperature.
Elements |
Zn | Al | O | Total |
---|---|---|---|---|
Metal | 95 | 0.8 | 1.6 | 97 |
Table 1: Chemical composition (mass %) of the metal. Method of analysis: SEM/EDX, Lab Analytical Chemistry, Empa.
Recrystallized structure (polygonal grains)
Zn
Extended intergranular corrosion has developed in the metal structure (Figs. 4, 5 & 6). The metal is covered by a corrosion crust that is hard to see in bright field and which contains remnant metal (Fig. 5). On most of the sample the corrosion crust is uniform. In areas we see cracks appearing as brown lines separating the corrosion crust (Figs. 6 and 7). In bright field the corrosion crust appears grey containing dark-grey zones (Fig. 6). Under polarized light, the corrosion crust appears white with darker parts including remnant metal (Fig. 7). It contains Zn and O as well as S along some cracks (Fig. 9). The cracks appear in brown.
Elements |
O | Al | Zn | Total |
---|---|---|---|---|
Light-grey corrosion part | 23 | < | 77 | 98 |
Dark grey corrosion part | 38 | 0.6 | 68 | 106 |
Table 2. Chemical composition (mass %) of the corrosion layer from Fig. 6. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.
Uniform - intergranular
?
Corrected stratigraphic representation: none
The artefact is possibly either a weight or a sacrificial anode. However, it is made of a cast and annealed zinc alloy which makes the interpretation as a weight implausible. In contrast an interpretation as a sacrificial anode is more likely. It is known that zinc alloy sacrificial anodes are used to protect marine propellers especially in salt water. The thick corrosion layer seems to consist of oxides or hydroxides. The origin of the sulphur along some of the cracks is unclear.
References on object and sample |
References object 1. Auskunftsblatt der Sammlung des Verkehrshauses der Schweiz, Inventarnummer VHS-8339.
References sample 2. MIFAC-métal cat. 29. |
References on analytic methods and interpretation |
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