Coated cast iron fragment - Grey cast iron - Modern Times - Switzerland

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Mathea. Hovind (University of Oslo, Department of archaeology, conservation and history (IAKH-UiO), Oslo, Oslo, Norway)

Complementary information

Nothing to report.

The schematic representation below (Fig. 3) gives an overview of the corrosion layers encountered by visual macroscopic observation.

Fig. 6: A MiCorr stratigraphy of the coated metal. To be compared with Fig. 10, credit MiCorr_UiO-IAKH, M.Hovind.
Fig. 7: A MiCorr stratigraphy of the paint layer from Fig. 5, credit MiCorr_UiO-IAKH, M.Hovind.

Complementary information

The fact that the fragment was considered a test material enabled extensive sampling that would not otherwise be possible.

Metallography

Microscope: Leica DMi8 (a metallographic, inverted, reflected light microscope) with magnification up to 500X. Camera: Olympus SC50 connected to the software “Olympus Stream”, version 1.9.4. Illumination modes: bright field and cross-polarized light.

SEM-EDS

Instrument: Jeol 6400; voltage: 20 kV; working distance: 18 and 24mm; sample preparation: palladium depot.    

The metal is a grey cast iron with characteristic graphite flakes (Fig. 8). It contains C in addition to P, Mn and some Si (Fig. 9). Its matrix consists of a P-rich eutectoid and a dendritic phase; the latter containing a significant amount of C, but less P than the aforementioned eutectoid (Table 1).

The metal is covered by a coating of alternating layers of paint (appearing light grey, black and bright orange under polarized light, NMM1-NMM3 in Fig. 10). Punctual analyses by SEM-EDS (Table 2) as well as elemental mapping (Fig. 12) reveal that the light grey layer (NMM1) contains a considerable amount of Ba in addition to S, Si and Al. The black layer (NMM2) contains Fe, Si, Mg and Al. The latter is visible as rectangular flakes (see NMM2 in Fig. 11). This layer could consist of Al flakes and FeO in an organic binder (C and O). The underlying, bright orange layer (NMM3) contains Fe and O in addition to a considerable amount of Pb, some Si, Zn and Ca. This layer is probably a preparatory paint consisting of red lead (Pb3O4) and Zn oxide (ZnO). C and O were detected in all layers and are probably components of the binder (Table 2 and Fig. 12).

Furthermore, punctual analyses were carried out in order to determine the chemical composition of the paint flake. Three punctual analyses were carried out for each colour (light grey, black and orange in Fig. 5). The colours correspond to the paint layers previously mentioned, but their composition is slightly different (Table 2, Table 3):

- The light grey layer (similar to NMM1) contains Pb, C and O. Its composition is significally different from NMM1 as it contains neither Ba, S or Al.

The black layer has a similar composition to NMM2 (C, Fe, O and Al).

- The orange layer is even more rich in lead than NMM3 and contains Ba. This layer is probably a preparatory layer of red lead (Pb3O).

Elements

mass%

Fe

P

C

Mn

Si

V

Ca

S

Mg

O

Eutectoid phase

83

10

5

0.7

0.6

0.1

0.1

0.1

0.1

-

Dendritic phase

93

0.4

4

0.3

2

0.1

0.1

0.1

-

-

Table 1: Chemical composition of the metal. Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr_HEI Arc, C.Csefalvay.

 

Elements

mass%

C

O

Fe

Ba

Pb

Si

Al

S

Zn

Ca

Mg

Na

Cl

P

Mn

K

Ti

NMM1

Light grey

42

23

0.6

22

-

4

4

5

0.1

0.1

0.5

-

-

-

-

-

-

NMM2

Black

48

26

18

0.4

-

2.1

2

0.2

0.1

0.3

1.2

0.3

0.2

0.1

-

0.1

0.1

NMM3

Orange

21

26

27

0.3

17

2

0.8

-

2

2

0.2

0.9

0.6

0.4

0.2

0.1

-

Table 2: Chemical composition of the paint layers attached to the metal sample (Fig. 4). Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr_HEI Arc, C.Csefalvay.

 

Elements

mass%

C

O

Fe

Ba

Pb

Al

Si

Zn

Mg

Na

P

Ca

S

Mn

Cl

K

Ti

Light grey layer

22

13

0.2

1.0

63

-

0.4

0.9

-

0.2

0.1

0.1

-

0.1

-

-

-

Black layer

49

29

15

0.2

-

3

2

0.1

1

0.2

0.1

0.3

0.2

-

0.1

0.1

-

Orange layer

26

17

0.1

13

42

-

0.5

0.6

-

0.2

0.4

0.2

 

-

-

-

-

Table 3: Chemical composition of the paint layers from the paint sample (Fig. 5). Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr_HEI Arc, C.Csefalvay.   

Complementary information

Nothing to report.

The metal is covered by a thin graphitized layer (CP1) as well as a corroded metal phase (CM1). The latter is located just beneath NMM3 and appears dark grey under polarized light (Fig. 10). It consists mainly of Fe and O with Pb and Ca (Table 4, Fig. 12). Pb can probably be explained by the proximity to the Pb-rich paint layer NMM3 (Table 2). The presence of Ca is more interesting however, as it might originate from past exposure to the environment, implying that the paint was not applied immediately after fabrication. The composition of CP1 was not analyzed.

Elements

mass%

Fe

O

C

Pb

Si

P

Ca

Na

Mn

Cl

Mg

Zn

K

Al

Ba

CM1

42

32

8

6

4

2

2

1

0.9

0.7

0.5

0.5

0.4

0.3

0.3

Table 4: Chemical composition of the corroded metal phase (CM1) from Fig. 10. Method of analysis: SEM-EDS. Lab. of Electronic Microscopy and Microanalysis, Néode, HEI Arc, credit MiCorr, HEI Arc_C.Csefalvay.

The rain gutter fragment is a grey cast iron with a microstructure consisting of graphite flakes and a P-rich eutectic phase. It is coated by several layers of paint, up to 12 in total. What appears to be a Pb-based primer is located right next to the metal, followed by a black layer consisting of aluminium flakes and iron oxide in an organic binder and superimposed by a light grey, Ba- and Pb-rich paint layer. Inhibitive primers often contain red lead in linseed oil (Hare 2011:979). The uppermost layer is dark in colour and corresponds to the black paint layer previously mentioned.

It seems likely that the rain gutter fragment has been treated with a system of at least three different paints which have all been applied several times, hence the relatively thick coating. The efficiency of the paint system is proven by the fact that internal corrosion is much more prominent in areas that are not protected by paint system. Corrosion control by application of paint films is a well-known treatment of metals (Hare 2011:971, Selwyn 2004:109). The use of an anti-corrosion system consisting of several layers of paint with different colours is known to facilitate monitoring as a break in the coating will be readily visible. Additionally, the presence of more than one paint layer will minimize the risk for a hole in the coating and help avoid localized corrosion (Selwyn 2004:105). 

References sample:

1. Selwyn, L. (2004). Metals and corrosion: A handbook for the conservation professional. Ottawa: Canadian Conservation Institute.

2. Hare, C. H. (2011) “Corrosion control of steel by organic coatings”. In. Revie R. W. ed. Uhlig’s Corrosion Handbook, 3rd ed. Toronto, ON: John Wiley & Sons, p. 971 – 983.