Sacrificial anode - Zn Alloy - Modern Times

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

Complementary information

The anodes were produced by Horton Maritime.

Stratigraphic representation: none 

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

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.

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.

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