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)

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Credit HE-Arc CR.

Fig. 1: Sacrificial anode (left) from submarine "Mesoscaph" (right) (www.verkehrshaus.ch),

The artefact could be a weight or sacrificial anode of a submarine (Fig. 1). It is surrounded by a whitish brown-grey corrosion crust. The broken metal has a greyish shiny colour. 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

Outdoor atmosphere

Swiss Museum of Transport, Luzern, Lucerne

Swiss Museum of Transport, Luzern, Lucerne

VHS-8339

N/A

Complementary information

The anodes were produced by Horton Maritime.

Credit HE-Arc CR.

Fig. 3: Micrograph of the cross-section of the fragment sampled from the sacrificial anode showing the location of Figs. 4 to 8,

The sample (Fig. 3) shows a cross-section from the fragment of the sacrificial anode (Fig. 2). It has a silvery appearance. The thickness of the corrosion crust is variable. Dimensions: L = 17mm; W = 14mm.

Zn Alloy

Cast and annealed

VHS-Mq-1

HE-Arc CR, Neuchâtel, Neuchâtel

Swiss Museum of Transport, Luzern, Lucerne

07/09/2009 metallography

Complementary information

None.

Analyses performed:
Metallography (unetched), Vickers hardness testing, SEM/EDS.

None.

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 1 2 98


Table 1: Chemical composition (mass %) of the metal. Method of analysis: SEM/EDS, 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

Al

Complementary information

None.

Extensive intergranular corrosion / cracking has developed in the metal structure (Figs. 4, 5). The metal is covered by a corrosion crust that is hardly visible in bright field and which contains remnant metal (Fig. 4). On most of the sample the corrosion crust is uniform. In areas we see cracks (Fig. 6) appearing as brown lines separating the corrosion crust (Fig. 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 (Table 2 and Fig. 8). 

 

Elements

 O Al Zn Total
Light-grey corrosion part 23 < 77 98
Dark grey corrosion part 38 <1 68 107


Table 2. Chemical composition (mass %, <: below the detection limit) of the corrosion layer from Figs. 6 and 8. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Credit HE-Arc CR.

Fig. 6: Micrograph showing the metal - corrosion products interface from Fig. 5 (detail), unetched, bright field,

Credit HE-Arc CR.

Fig. 7: Micrograph (same as Fig. 6) corresponding to the stratigraphy of Fig. 4, unetched, polarised light. We observe in dark-grey the metal, in white the corrosion crust separated by brown cracks including remnant metal,

Credit Empa.

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

Multiform - intergranular

Unknown

Complementary information

None.

Fig. 9: Stratigraphic representation of the fragment sampled from the sacrificial anode in cross-section (dark field) 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 Fig. 5, Credit HE-Arc CR.

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. Extensive intergranular corrosion/cracking has developed in the metal structure.

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