Bugatti - aluminium cylinder head gasket - Al Alloy - Modern Times - 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. In the Bugatti Type 37, this engine has 3 parts:

- The came shaft case above, where the camshaft coordinates the pistons.

- The cylinder block in the middle, where the cylinders slide in a linear motion.

- The crankcase block below, where the crankshaft transformes the motion from linear to rotative.

The explosion and the cylinders movement are generating a lot of heat. Therefore, the block needs to be cooled down. A cooling system circulating water or coolant between the block [hot] and a heat exchanger (or radiator) [cold]. The circulation of the liquid is often helped by a water pomp. The coolant flows through the block inside galleries in order to cool down the cylinders without wetting them. (Poulain, 1995, p.86)

The Bugatti Type 37 will be restored. When the car was dismantled, this gasket has been judged too corroded to be preserved. A new piece will be put in its place during the restoration. Therefore, this part has been classified as study material, allowing sampling for metallography.

Fig.3 shows crearly the limit where the gasket is in contact with the cylinder block ([1] edges and circular openings in the center) and where it is in contact with the coolant ([2] center).

The schematic representation below gives an overview of the corrosion layers encountered on those two sites of corrosion (Fig. 4).

 

Stratigraphy [1] Stratigraphy [2]
The metal contains the strata M1 and M2. The first stratus of metal is missing and only the core metal (M2 in strat.[1], now M1) remains.
The CP1 is a thin brown layer loosely attached to the surface of M1. It can be removed with a firm brush. The CP1 is a thick bright orange layer, really powdery and easily detached. It can be partly removed with a firm brush. The coloration is less important closer to the metal interface, where the products are white. This layer is also really fragile and friable.
In some places, CP2 develops within M1. It is colored in bright orange and can be broken in pieces with a toothpic. The CP2 is peaking through CP1 in some places, colored in green. 

 

 

Fig. 6: Stratigraphic representation in cross-section of zone 1, 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.
Fig. 7: Stratigraphic representation in cross-section of zone 2, 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.

Complementary information

During the sampling, a large part of the top layer (CP1) was lost for both zones.

Analyses performed on the gasket
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 gasket, on the bottom side (contact with the coolant [1] and with the block [2])
Metallography (unetched), BF and DF imaging.
SEM-EDS (20kV): SE and BSE imaging and semi-quantitative EDS analysis.

 

This gasket is made out of a composite sheet produced by roll-bounding a pur aluminium sheet (Table 1) on each side of an AlCu alloy core (Table 1).  Fig. 9 shows oriented inclusions on both M1 and M2, as well as a lot of small pores in M2.
Ponctual analyses on each phase appearing on Fig. 9 showed that  the roll-bounded metal M1 is a  compact (non-porous) sheet of pure Al (Table 1) with oriented inclusions of Fe and Si. The core M2 is a very porous alloy composed of Al with bigger oriented inclusions of Si, Mn, Fe, Mg.

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

Element mass %
Al 99.6
Fe 0.3
Si <0.1

Table 2: Chemical composition (mass %) of the core alloy. Method of analysis: SEM/EDS, HE-Arc Ingénierie, S.Ramseyer.

Element mass %
Al 94
Cu 3.8
Mg 2
Mn 0.7
Si 0.1

 

The SEM image of Fig. 12 shows that the corrosion products progresses through M1 showing a geometric pattern, and has a network of cracks. A cartography of this zone reveals that those products contain S, probably aluminium sulfate (Fig. 14), polluted with Fe (Fig. 13). Presence of sulfur can be explained by the proximity of this piece from the combustion chamber, as for the iron, it is probably coming from the steel block. 

In places where M1 is missing, M2 is being heavily corroded. Close to the interface, some inclusions of Cu are retained in the CP. The products are friable and tend to detatch from the surface (Fig. 11).

The schematic representations of corrosion layers of Figs. 6 and 7 integrating additional information based on the analyses carried out is given in Fig. 16.

No analyses could be carried out on the top thicker part of CP1. 

The cylinder head gasket of this Bugatti Type 37 is made out of roll-bounded aluminium. A sheet of pure aluminium (with Fe and Si traces) protect an Al-Cu core alloy (Al, Cu, Mg, Mn, Si) on both sides of this gasket. This piece is sealing the top of the cylinder block, preventing the coolant from dripping to the cameshaft or the cyinders. 

The gasket side facing the cylider block has a different corrosion form depending on wether the zone is immerged of not. The surface in contact with the cylinder block shows a geometric corrosion of aluminium sulfate, displaying a dense network of cracks. The surface immerged in the coolant is heavily corroded. The roll-bounded metal has been completely consumed, and the corrosion progresses through the core metal, retaining inclusions from the alloy in its products. This layer is very friable and tends to delaminate.

All the corrosion products are polluted with iron oxydes probably coming from the block. The immerged part is covered with a thick crust of what is thought to be a mix of iron products and other salt-precipitates. In some places, cupper-based corrosion products are peaking through this thick and poudery deposit.

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.