External Corrosion Prevention

Figure 1

Protection of Ductile Iron Pipe from External Corrosion

A Rubber Gasket Joint (no electrical bond) retains water provide uniform resistance to current flow.
B 8 mil thick polyethylene shields against stray loose wrap or tube.
C Wrap and contained water provide uniform environment along metal. Dielectric polyethylene shields against stray currents.
D Any sulfate reducing bacteria in water soon deplete life support (nutrient) and die off.
E Thin shoot of water loses corrosivity as a result of initial reactions.

Extensive burial tests and field experience have consistently demonstrated that ductile iron pipe resists soil corrosion. This is explained in part by the character of the products of initial corrosion. Graphite (14%) and the oxides of iron (47%) and silicon (25%) along with several minor compounds in the corrosion product form a tough matrix that remains tightly in place, inhibiting the progression of corrosion. The uniformity of the ductile iron microstructure also seems to contribute to corrosion resistance. In fact, long term comparative corrosion rate studies in the United States, France, and Great Britain have shown that the soil corrosion rate in ductile iron pipe is an average of 29% less than in its predecessor gray cast iron pipe. These studies included over 30 tons of pipe specimens in various corrosive soils.

Ductile iron pipe can be attacked in certain highly corrosive soils or concentrations of stray direct current. But because of its configuration, metallurgical structure, and the quality of corrosion product, ductile iron is the easiest of all metal pipes to protect.

For insurance against external corrosion the installaiton of loose, 8-mil thick polyethylene sheeting or tubing around the pipe has proven completely effective and costs a fraction of adherent coatings and/or cathodic protection. If the soil where ductile iron pipe is being installed is found to be severely corrosive, either by experience or analysis according to Appendix A of ANSI/AWWA C105/A21.5, loose polyethylene encasement should be specified. The function of this system is explained in Figure 1.

Loose polyethylene encasement for ductile iron pipe is also an effective shield against stray current. In addition it provides a uniform environment for the pipe and controls corrosion resulting from sulfatereducing bacteria. More than 10 million feet of polyethylene-wrapped ductile iron pipe is being used in the most severely corrosive soils in the United States. Some of this pipe is more than 25 years old. Yet, there have been virtually no corrosion failures to date. The few failures that have occurred were the result of neglect such as not re-wrapping the pipe after making a service tap or failing to repair large tears in the wrap during installation.

Ductile iron pipe is not especially vulnerable to stray direct current and not at all to long-line current because of the retained electrical resistance offered by rubber-gasket joints. However, in extreme applications, such as crossings with cathodically-protected pipelines or installations proximate to impressed current ground beds, 8-mil polyethylene shields against the assimilation of direct current, as shown in Figure 2.

Polyethylene Encasement Installation

Various installation methods have been developed for this corrosion-prevention System.

Installation procedures commonly used for wrapping ductile iron pipe with 8-mil polyethylene are as fallows:

Step 1. Cut polyethylene tubing about two feet longer than pipe length. Gather the polyethylene tube on the spigot end of the pipe being assembled

Step 2. Using shallow holes to accommodatesystem. the wrapped hell, pull the gathered tubing Installation procedures commonly used for over the bell and fix it in place with tape, hand or string.

Step 3. Take up slack in the tubing along the pipe barrel and secure in place. Step 4A, 4B. Overlap the tubing from the next pipe length and secure in similar fashion.

Step 4A, 4B. Overlap the tubing from the next pipe length and secure in similar fashion.



Polyethylene Encasement Precautions:
Regardless of the method chosen, there are a few precautions that are an important part of successful installation:

Make sure that no soil or other foreign material is allowed in the annular space between the polyethylene film and the pipe.

Bedding and backfill must be free of any debris that might damage the polyethylene.

Avoid damage to the polyethylene by tamping or other construction operations.

It is necessary that any metallic service connection be electrically insulated from the pipe to ensure the integrity of the system. This is accomplished by providing a dielectric bushing for the corporation stop connection and taping or wrapping the service line with polyethylene for a length of three feet from the tap. It is also important to resecure the polyethylene film around the tap.

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