Exterior Coating

Estimate the service life for the basic exterior base metallic coating using Equation14-5.
EquationObject338405
Equation 14-5.
The basic exterior service life (L
e
) for 18-gage corrugated galvanized metal pipe is provided in the table following Equation 14-6 for pH values of 7.3 and lower and using the equation for pH values in excess of 7.3.
EquationObject339406
Equation 14-6.
Exterior Durability for 18-Gage CMP (years)
pH
Resistivity (ohm-cm)
1,000
1,500
2,000
2,500
3,000
4,000
5,000
7,500
10,000
7.3
54.8
59.6
63.1
65.8
67.9
71.4
74.1
78.9
82.4
7.0
34.6
39.4
42.9
45.6
47.7
51.2
53.9
58.7
62.2
6.5
23.9
28.8
32.2
34.9
37.1
40.5
43.2
48.0
51.5
6.0
18.0
22.9
26.3
29.0
31.2
34.6
37.3
42.1
45.6
5.8
16.2
21.0
24.5
27.2
29.3
32.8
35.5
40.3
43.8
5.5
13.8
18.6
22.1
24.8
26.9
30.4
33.1
37.9
41.4
5.0
10.4
15.3
18.7
21.4
23.6
27.0
29.7
34.5
38.0
Heavier gage metal has more sacrificial metal and, therefore, a longer anticipated life under given conditions. The table below provides coating thickness/gage multipliers for use in Equation 14-1 and Equation 14-2 for the respective gage and metallic coating. The resulting values are not exact but allow a systematic comparison of relative durability of the various metals and gages used in design.
Thickness Multipliers for Steel Conduit
Gauge
Item 460 - CMP
Item 461 - Structural Plate
Thickness
Factor
Thickness
Factor
in. (mm)
Galv
Alt 2
in. (mm)
Galv
18
0.052 (1.32)
1
3.6
**
**
16
0.064 (1.63)
1.3
3.9
**
**
14
0.079 (2.01)
1.6
4.2
**
**
12
0.109 (2.77)
2.2
4.8
0.109 (2.77)
2.24
10
0.138 (3.50)
2.8
5.4
0.138 (3.50)
2.84
8
0.168 (4.27)
3.4
6
0.168 (4.27)
3.54
7
**
**
**
0.188 (4.78)
3.81
5
**
**
**
0.218 (5.54)
4.42
3
**
**
**
0.249 (6.32)
5.05
1
**
**
**
0.280 (7.11)
5.68