Coffee filter - Al Alloy - Modern Times - France

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Complementary information

Nothing to report.

Stratigraphic representation: none.

Fig. 5: Stratigraphic representation of the fragment sampled from the coffee filter 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, C.Degrigny.

Complementary information

A second sample was taken and gave similar results.

Analyses performed: 
Metallography, SEM/EDS.

The metal is a relatively pure aluminium alloy with numerous elongated inclusions (Fig. 6). From their chemical composition they can be interpreted as Al3Fe intermetallic compounds (Fig. 7). Inter- and transgranular corrosion has developed so extensively that some grains (not elongated) are outlined (Fig. 8). 

Complementary information

Nothing to report.

The average thickness of the corrosion layer (CP1) is about 50 micrometers, but may be thinner or thicker depending on the area. Intergranular corrosion developed locally to extend through the entire thickness of the metal. EDS-SEM analysis indicates that the metal is, as expected, covered by a layer rich in Al and O containing chlorides (red spots on Fig. 9) and surprisingly Na (Figs. 10 and 11). Chlorides do not seem to form active corrosion. A new examination carried out after 4 months (Fig. 12) shows new forms of alteration with a local enrichment of Na, C and O (Na2CO3?).

Complementary information

Nothing to report.

Corrected stratigraphic representation: none.

This aluminium alloy has a composition similar to a primary aluminium with an Al content between 99 and 99.8 mass%. The main impurity is Fe forming intermetallic (Al3Fe) inclusions. The metal was stamped and punctured. It is covered by a relatively thick corrosion layer (probably aluminium oxide) which developed under superficial filiform corrosion. Extensive intergranular corrosion has developed locally within the metal. Chlorides have been identified but the progress of the corrosion might be due to Na, C and O-rich compounds (Na2CO3?).

References on object and sample

References object
1. Degrigny, C. (2018) Etude, identification des objets en aluminium patrimoniaux et classification de leurs formes de corrosion - projet EtICAL, rapport interne HE-Arc CR.
2. 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.

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

References on analytic methods and interpretation