Corrosion

Freshly cut lead has a bright finish, but it rapidly tarnishes in the air with the formation of a blue-grey film of lead carbonate and lead sulfate. In damp conditions a white deposit of lead carbonate is produced, and in cladding this can both be aesthetically unacceptable and cause some staining of the adjacent materials. The effect can be prevented by the application of patination oil after the lead has been fixed. Lead is generally resistant to corrosion due to the protection afforded by the insoluble film; however, it is corroded by organic acids. Acidic rainwater run-off from mosses and lichens may cause corrosion and contact with damp timbers, particularly oak, teak and western red cedar should be avoided by the use of building paper or bitumen paint. Trapped condensation under sheet lead may cause significant corrosion, so consideration must be given to

Table 5.11 Lead sheet colour codes to BS EN 12588: 1999 and typical applications

European Designation

Colour code Thickness range (mm)

green

green

yellow

blue

red

black

white

orange

1.50-1.75

1.75-2.00

2.00-2.50

2.50-3.00

3.00-3.50

3.50-6.00

4

5

6

7

8

1.80

2.24

2.65

3.15

3.55

20.4

25.4

30.1

35.7

40.3

Lead codes

Nominal thickness (mm) Nominal weight (kg/m2)

Typical Application:

Flat roofing Pitched roofing Vertical cladding Soakers

Hip and ridge flashings Parapets, box/tapered valley gutters Pitched valley gutters Weatherings to parapets Apron and cover flashings Chimney flashings the provision of adequate ventilation underneath the decking which supports the lead. Dew points must be checked to ensure that condensation will not occur and be trapped under the lead sheet in either new work or renovation. Generally lead is stable in most soils; however, it is attacked by the acids within peat and ash residues. Electrolytic corrosion rarely occurs when lead is in contact with other metals, although within marine environments aluminium should not be used in association with lead. Corrosion does occur between wet Portland cement or lime products and lead during the curing process; thus in circumstances where the drying out will be slow, the lead should be isolated from the concrete with a coat of bitumen paint.

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