Bugatti - aluminium water pomp - Al Alloy - France

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

Complementary information

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 (Poulain, 1995, p.86). Most of the time, the Bugatti inline cylinder engines have aluminium water pipes entering the block from below and exiting it from above, as shown on Fig.2.

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 Bugatti pipe. 

The stratum M is an aluminium alloy. Three CP strata have been identified. CP3 has a greyish mate color and is covering all of the corroded side of the pipe. Scattered over this uniform layer, blue-green (CP2) and white (CP1) spots of corrosion products can be found.

Fig. 6: Stratigraphic representation of the sample taken from the Bugatti water pipe 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. 7 and 8, credit MiCorr_HE-Arc CR, E.Granget.

Complementary information

The yellow deposit that was clugging the pipe couldn't be properly analysed (see Analyses and results below). Indeed, the deposit was way softer than the metal and suspected to contain organic compounds coming from the engine's coolant.

Analyses performed on the pipe
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).

Sample preparation
Due to a significant difference between the metal and the deposit hardness, and in order to preserve the corrosion layers, the surface preparation was realized with an alcoholic lubricant instead of water. Thus, the quality for the metal and corrosion layers have been priviledged over the deposit, as the alcohol might have corrupted the suspected organic compounds of the deposit. (See Sample: Complementary information above).

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

The Metal is slighly porous (Fig. 7, red circles). Its dendritic microstructure is revealed by observation in bright field (Fig. 8).

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

Element mass %
Al 89
Cu 7
Zn 2.5
Fe 1
Si 0.5

Point analyses of the main dendritic phase showed that it is made of Al, Cu and Zn (Table 1 and Fig. 10). Elemental chemical distribution with EDS allowed to identified the following repartition (Fig. 11): Zn is uniformely present in the alloy. There are two interdendritic phases (Fig. 12): a eutectic phase containing Fe and Cu, and a second phase containing mostly Cu. Additionally Sn and Si precipitates could be identified.

The BF and DF images (Figs. 7 and 8) show a few big cracks at the interface between CM1 and M1. The SEM image shows that they expand in a network of additional microcracks (Fig. 13). The pipe suffers from a uniform interdendritic corrosion. The Al, Cu and Zn phase is oxidising preferentially, developing then aluminium oxides, versus the interdendritic phases. In some places, pits of corrosion formed. SEM cartography showed a concentration of Cl at the base of the pit (Fig. 14: Green).

 

The schematic representation of corrosion layers of Fig. 4 integrating additional information based on the analyses carried out is given in Fig. 15.

This angled water pipe, coming from a Bugatti engine's cooling system, is made of cast Al, Cu, Zn, Fe alloy and shows a slightly porous dendritic microstructure. There are two interdendritic phases, a eutectic Fe, Cu phase and a Cu phase, as well as precipitates of Si and Sn.

One end of the pipe is heavily corroded and entirely clugged by corrosion products and other deposits on the inside. This deposit couldn't be analized.

The external surface shows a uniform interdendritic corrosion preferentially consuming the dentritic phase (Al, Cu, Zn). Pitting corrosion has been identified, with Cl pockets at the base of the pit.

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. (2018) Etude, identification des objets en aluminium patrimoniaux et classification de leurs formes de corrosion - projet EtICAL, rapport interne HE-Arc CR.