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Any coating which provides a barrier to the moisture and oxygen in the air will help protect carbon steel from corrosion. A properly painted surface will provide a barrier, but it is subject to scratching from contact with hard objects. Figure 4 illustrates how rust can grow and damage a painted surface when corrosion begins because the paint barrier is broken by a scratch.

Figure 5 illustrates the cathodic protection provided when a galvanized surface is scratched.

DUPLEX SYSTEMS usually require painting over galvanizing. Some of our customers have specified a duplex system. This is more expensive but it can be justified for certain corrosive environments or for appearance. The American Galvanizing Association suggests the following “rule of thumb” to estimate the service life of a duplex system.

(Duplex System Service Life) = 1.5 * (Service Life: HDG Only) + (Service Life: Paint Only)

The synergistic multiplier of 1.5 is based on the barrier protection the paint provides for the galvanized surface.

At Piping Technology and Products Inc., many customers have returned painted variable and constant spring supports which could no longer function due to corrosion. Costs must be considered during the specification of coatings for pipe supports. The owner and operator of a facility should consider life-cycle costs. Pipe supports are usually a relatively small percentage of the total cost of installing and operating a power plant, petrochemical plant, paper mill or other major facility. The small additional cost of hot dip galvanizing the carbon steel components of pipe supports is almost always a wise investment.

For more information you may want to contact the following organizations:

	American Galvanizers Association-AGA
	12200 E. Illif #204  Aurora, CO  80014
	ph 800-468-7732

	National Association of Corrosion Engineers-NACE
	1440 S. Creek Dr.  Houston, TX  77084
	ph 713-492-0535

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