Hispano-Suiza water pump - Al Alloy - France

Granget. Elodie (, None) & . (MNAM (Musée National de l'Automobile de Mulhouse), Mulhouse, Alsace, France)

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A combustion engine transforms thermal energy into kinetic energy. This process is imperfect and releases a lot of heat through the block of the engine. Therefore, these parts need to be cooled down. A cooling system circulating water between the block [hot] and a heat exchanger (or radiator) [cold] is frequently used to fulfill this function. The water flow in this circuit can be optimised by the use of a water pump. The water is sucked in from one pipe and pushed out from another one (Poulain, 1995, p.86).

This artefact is part of the "Materials for study Library" that the museum collected. It can therefore be subject to sampling.

The schematic representation below gives an overview of the corrosion layers encountered on the corroded pipe of the fastening part of the pump. 

The CP stata show big cracks and can be removed with a bit of scratching (severable). CP2 has a green-blueish color. Over most of the corroded part there is a thick red-orange deposit. This is probably coming from the corroded steel screws, used to fasten the parts of the water-pump.

Fig. 8: Stratigraphic representation of the sample taken from the fastening part of the Hispano water pump in cross-section 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. 9, credit MiCorr_HE-Arc CR, E.Granget.

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During the sampling, the orange colored deposit layer (D1 on Fig. 6) was lost.

Analyses performed on the body and fastening parts
XRF with portable X-ray fluorescence spectrometer (Niton XL3t 950 Air Goldd+ analyser Thermo Fischer (voltage 50V, General metals mode with acquisition times 20s(main)/20s (Low) /20s (Light).

Analyses performed on the cross-section sampled from the corroded exit pipe
Metallography (unetched), BF and DF imaging.
SEM-EDS (20kV): SE and BSE imaging and semi-quantitative EDS analysis.

The porosity of this aluminium-copper alloy (Table 1) has been confirmed by optical (Figs. 9 and 10) and SEM observations on the cross-section of the fastening part's fragment (Fig. 11).

Table 1: Chemical composition (mass %) of the metal. Method of analysis: SEM/EDS, HE-Arc Ingénierie, S.Ramseyer.

Element mass %
Al 95
Cu 4
Sn 0.5
Fe 0.5
Si 0.1

 

Ponctual analyses on each phase appearing on Fig. 11 showed that the main phase is composed of Al and the interdendritic phases are composed of Al, Cu and Fe as well as Sn precipitates. This distribution is given through EDS cartographies (Figs. 13 to 15).

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About the water pump
The body of the pump is casted in Al-Si alloy and the inner parts (turbine and other mechanical shafts) are made out of bronze, brass and steel.

The SEM image of Fig. 12 clearly shows a network of cracks throughout CM1 and CPs. These cracks are not marking any clear separation between strata, but seem to affect all corrosion layers. The main Al phase is preferentially oxidized, while the interdendritic phase seems to be preserved in the corrosion products (Figs. 12 to 14). Na and K but no Fe are detected in the corrosion layers CM1 and CPs (Fig. 15). This surface is in contact with the coolant (glycol + additives, usually mixed 1/1 in water), which sometimes contain Na or K-based corrosion inhibitors. A cluster of CaCO3 was identified on the very surface of the sample (Fig. 16). CaCO3 can sometimes precipitate if the water used to mix the coolant is hard.

The schematic representation of corrosion layers of Fig. 6 integrating additional information based on the analyses carried out is given in Fig. 17.

The heavily corroded exit pipe of the fastening part of this Hispano-suiza water pump is made out of an Al-Cu cast alloy with traces of Sn, Fe and Si(?). The corrosion is through the dendritic microstructure: the main phase (Al) is preferentially oxidized, while the interdendritic phase (Fe, Sn, Cu) and Sn precipitates are preserved in the corrosion products. This component of the engine being in contact with the coolant might explain the presence of Na, K and Ca in the corrosion layers.

References on object and sample

References object
1. Poulain, P. and J-M. (1995) Voitures de collection : Restauration Mécanique Editions Techniques pour l’Automobile et l’Industrie (ETAI), Paris.

2. Granget, E. (2020) La corrosion des alliages d’aluminium des circuits de refroidissement à eau de véhicules en contexte patrimonial : Utilisation d’outils open-access dans l’établissement d’un diagnostic des altérations d’un corpus de véhicules conservés au Musée National de l’Automobile de Mulhouse (Collection Schlumpf), Rapport interne MNAM.

References sample
3. Granget, E. (2020) La corrosion des alliages d’aluminium des circuits de refroidissement à eau de véhicules en contexte patrimonial : Utilisation d’outils open-access dans l’établissement d’un diagnostic des altérations d’un corpus de véhicules conservés au Musée National de l’Automobile de Mulhouse (Collection Schlumpf), Rapport interne MNAM.

References on analytic methods and interpretation

4. Vargel, C. (2004) Corrosion of Aluminium, Elsevier.

5. Degrigny C. and Schröter J. (2019) Aluminium Alloys in Swiss Public Collections: Identification and Development of Diagnostic Tools to Assess Their Condition, in METAL 2019, proceedings of the ICOM-CC Metal WG interim meeting, eds. C. Chemello, L. Brambilla, E. Joseph, Neuchâtel (Switzerland), 408-415.