Sacrificial anode VHS-8339 - Zn Alloy - Modern Times

Sacrificial anode VHS-8339 - Zn Alloy - Modern Times

Sacrificial anode VHS-8339

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

Contact the author Share Print
/artefacts/218/
Credit HE-Arc CR.

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

Complementary information

The anodes were produced by Horton Maritime.

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 locations of Figures 4 to 8,

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.

Credit HE-Arc CR.

Fig. 4: Micrograph of the metal sample from Fig. 3 (reversed picture, detail), unetched, bright field. Extensive intergranular corrosion is visible.The rectangle marks Fig. 6,

Credit HE-Arc CR.

Fig. 5: Micrograph of the metal sample from Fig. 3 (detail), etched,

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.

Credit HE-Arc CR.

Fig. 6: Micrograph showing the metal - corrosion products interface from Fig. 4 (detail), unetched, bright field,

Credit HE-Arc CR.

Fig. 7: Micrograph (same as Fig. 6), unetched, polarised light. We observe in dark-grey the metal, in white the corrosion crust separated by brown cracks including remnant metal,

Credit HE-Arc CR.

Fig. 8: SEM image, BSD-mode, and elemental chemical distribution of most of the area of Fig. 6 (reversed picture). Method of examination: SEM/EDX, Laboratory of Analytical Chemistry, Empa,

Uniform - intergranular

?

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

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