Armature from a clock movement - Zn Al Sn Cu Alloy - Modern Times - Switzerland

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

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 metal is a Zn-Al-Sn-Cu alloy (Table 1) with an average hardness of HV1 105.  A fine dendritic structure is observed (Fig. 4) which consists of clearly separated Zn, Al-Cu and Sn-rich phases (Figs. 5 and 6). Pb is associated with Sn but also forms tiny nodules. 

 

Elements

Zn Al Sn Cu Pb Total
Metal 67 17 11 4 <1 99

Table 1: Chemical composition (mass %) of the metal (oxygen not shown). Method of analysis: SEM/EDX, Lab of Electronic Microscopy and microanalysis, IMA (Néode), HEI Arc.

The Zn-rich phase is heavily oxidised (Fig. 6). The corrosion has developed throughout the entire metal body, generating cracks (Fig. 7). The cracks are Zn,O and C-rich (Fig. 9) and could be composed of zinc carbonate.

Corrected stratigraphic representation: none

The armature of the clock mechanism is constituted of a ZnAlSnCu alloy. The absence of cohesion between the different phases has led to the penetration of O during the manufacturing of the alloy. Small original cracks have developed further eventually causing the armature to break. This phenomenon, known since the 1920’s, is called the Zn pest. It develops mainly on cast objects, starting with localised modifications (blisters and pits, Cramer and Covino 2005). It appears that the armature was made of an unsuccessful experimental alloy that was used for a prototype. The same armature element was later produced in a more stable alloy.

References on object and sample

References object

1. Seemater, V. (2009) Conservation-restauration d’une horloge à mouvement électrique, rapport interne HE Arc CR.

 

References sample

2. Seemater, V. (2009) Conservation-restauration d’une horloge à mouvement électrique, rapport interne HE Arc CR.

References on analytic methods and interpretation

3. Cramer, S.D., Covino Jr., B.S. volume editors. (2005). ASM Handbook, 13B, 37.