Exterior Coating
Estimate the service life for the basic exterior base metallic coating using Equation14-5.

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.
Equation 14-6.
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.
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 |