Ductile Iron Pipelines - Materials Performance

October 2008 MATERIALS PERFORMANCE 49

CASE HISTORY

Investigating Polyethylene-

Encased Ductile Iron

PipelinesDANIEL W. CRABTREE AND MARK R. BRESLIN,

Ductile Iron Pipe Research Association, Birmingham, Alabama

T is article presents several case histories where

ductile iron (DI) pipeline sections have been

investigated to ascertain the corrosion control benefi ts

of polyethylene (PE) encasement. T e procedures and

results of the investigations are evaluated and

discussed. T ese investigations demonstrate the

overall eff ectiveness of PE encasement as a corrosion

control system for DI pipe.

For more than 50 years, ductile iron

(DI) pipe has been the most com-

mon material for water transmis-

sion and distribution systems.

Strength, durability, and reliability make

DI pipe the industry standard.

A ferrous material, DI pipe is subject

to corrosion when installed in aggressive

environments. When corrosive environ-

ments are encountered and identifi ed, the

iron pipe industry recommends appropri-

ate corrosion control. The most com-

monly used form of corrosion control for

DI pipe involves the use of polyethylene

(PE) encasement.1-4 An American Water

Works Association (AWWA) Engineering

and Construction Division survey re-

ported 95% of the utilities polled used PE

encasement for their corrosion protection

of DI pipe.5

Many tools and procedures are avail-

able to help in the identifi cation of cor-

rosive soil conditions and their subse-

quent consequences. Parameters such as

soil resistivity, pH, moisture content,

oxidation-reduction potential, sulfi des,

chlorides, sulfates, etc., can all be mea-

sured and evaluated by the experienced

corrosion engineer.6-8

After pipelines are installed, there are

also tools to evaluate the condition of the

pipe. Water utility break records and main-

tenance reports can give valuable insight

concerning the condition of a pipe system.

Survey methods and techniques such as

pipe-to-earth potential measurements,

close interval surveys, cell-to-cell surveys,

side-drain technique, etc., have also been

used.9-10 DI pipelines generally are not

electrically continuous because of their

rubber-gasketed, bell-and-spigot installa-

tion design. For this reason, test stations

generally are not installed for DI pipeline

systems. These are important consider-

ations in choosing and evaluating a condi-

tion survey procedure for DI pipelines.

Presented are case histories of condi-

tion assessment investigations at three

50 MATERIALS PERFORMANCE October 2008

Investigating Polyethylene-Encased Ductile Iron PipelinesM A T E R I A L S S E L E C T I O N & D E S I G N

prominent water utilities that practice

corrosion control of their DI pipe systems:

BexarMet Water District, San Antonio,

Texas; Charleston Water System, South

Carolina; and Onondaga County Water

Authority, Syracuse, New York. The

predominant system of corrosion control

utilized by these three utilities is PE en-

casement in accordance with the ANSI/

AWWA C105/A21.5 Standard.6

Procedures

At each of the investigation sites, there

were minimal traffi c control issues and

reasonable excavation access. Utility

personnel had determined the location of

the pipeline at each site and fl agged it

accordingly. All other underground utili-

ties in the vicinity of the investigation

route were also located and identifi ed.

Discussions with other utilities’ personnel

and fi eld observations determined that no

sources of potential stray current were in

the general area.

Since these pipelines, like most DI

pipelines, were not installed with joint-

continuity bonds or test stations, cell-to-

cell potential surveys and side-drain

techniques were performed to search for

corrosion. The cell-to-cell survey inter-

vals were 5 ft (1.5 m) directly over the

pipeline, with a perpendicular distance of

10 ft (3 m) on both sides of the pipeline

for side-drain measurements. A high

impedance voltmeter and two matching

copper/copper sulfate (Cu/CuSO4) half

cells were used for the survey. Data from

the survey were directly entered into a

notebook computer and used to generate

a fi nished graph for evaluation and to

select locations for excavation.

In theory, the cell-to-cell potential

survey is intended to identify areas of ac-

tive corrosion that would correspond to

locations where the PE encasement was

damaged. On the survey graph, such

areas should be located where the cell-to-

cell potential shifts from positive to nega-

tive. All excavation/inspection sites were

generally from locations on the survey

graphs with the greatest magnitude of the

positive-to-negative shift (Figure 1).

In the area of specifi c condition assess-

ment locations, the in situ soil resistivity

was measured. In situ four-pin soil resis-

tivities have limited use when determining

the specifi c corrosivity of soils around

buried pipelines. These types of measure-

ments cannot be taken closely parallel to

the pipeline under investigation or the

results can be skewed. Proper procedures

for these resistivity measurements require

Typical cell-to-cell potential survey graph. Theoretical anodic areas of the positive-to-negative shift were chosen as excavation/inspection sites for all of the investigations.

Twenty-year-old PE-encased 16-in DI pipe at an “anodic” location from the cell-to-cell survey in San Antonio, Texas.

FIGURE 1

FIGURE 2

October 2008 MATERIALS PERFORMANCE 51

M A T E R I A L S S E L E C T I O N & D E S I G N

that the pins be perpendicular to the

pipeline and no closer than 15 ft (4.6 m)

from the pipe for a 10-ft pin spacing.9

Representative soil samples were obtained

from each excavation site and tested for

resistivity, pH, and oxidation-reduction

potential, along with qualitative tests for

sulfi des, chlorides, and moisture.

At each of the excavation sites, the soil

was carefully removed from around the

pipe to evaluate the condition of the PE

encasement. A representative sample of

the exhumed PE material was tested for

thickness, elongation, and tensile strength

to determine if it conformed to the re-

quirements of the ANSI/AWWA C105/

A21.5 standard, which was in effect at the

time of its installation. After inspection

and removal of the installed PE encase-

ment material, the pipe surface was

cleaned and examined. The examination

procedures included wire brushing, prob-

ing with a pointed hammer, measuring

pipe-to-earth potentials, and recording

the depth of any corrosion-related pitting.

At the conclusion of the inspections, new

PE encasement was installed and the pipe

inspection site backfi lled.

Results

BexarMet Water District—San Antonio, Texas

The district personnel made available

~1 mi (1.6 km) of right of way containing

a PE-encased 16-in (406-mm) diameter

DI water main for the condition assess-

ment work. The pipeline was installed in

1986. Three specifi c sites were identifi ed

for excavation and inspection using the

cell-to-cell survey and side drain mea-

surements.

No damage was observed to the in-

stalled PE encasement at these inspection

locations. The soil test results indicated an

aggressive environment, but the exposed

16-in DI pipe was found to be in excellent

condition beneath the PE wrap. The pipe

surface was moist and clean, with only

minor surface oxidation present (Figure

2). Upon sounding and probing of the

pipe surface, it was revealed that no pit-

ting or graphitization had occurred.

Charleston Water System—Charleston, South Carolina

System personnel made available ap-

proximately a half-mile (0.8-km) section

of PE-encased 12-in (305-mm) DI water

main. The pipeline was installed in 1984.

Two specifi c sites were identifi ed for ex-

cavation and inspection.

No damage to the installed PE encase-

ment was observed at either of the two

Charleston, South Carolina: Twenty-two-year-old PE-encased 12-in DI pipe at a location where the cell-to-cell survey indicated an anodic area.

FIGURE 3

The 18-year-old PE encasement in Syracuse, New York, was cleaned to allow for close inspection of any damage that may have occurred during its installation.

FIGURE 4

52 MATERIALS PERFORMANCE October 2008

locations. The soils were found to be ag-

gressive, but the exposed 12-in diameter

DI pipe was found to be in excellent

condition beneath the PE wrap (Figure

3). The pipe surface displayed only minor

surface oxidation. Sounding and probing

of the pipe revealed no pitting or graphi-

tization.

At the second excavation site, an

abandoned 3-in (76-mm) cast iron pipe-

line was discovered ~4 ft (1.2 m) north of

the 12-in DI pipe. Records indicated that

the 3-in pipeline was installed in the

1950s and was, therefore, ~55 years old.

The 3-in pipe did not have any corrosion

protection and had experienced corro-

sion-related pitting. At a small area

cleaned for inspection, the deepest corro-

sion pit measured 0.18 in (4.57 mm),

which would equate to a corrosion rate

of 3.3 mpy.

Onondaga County Water Authority—Syracuse, New York

Authority personnel made available

approximately one mile of right of way

containing a PE-encased, 8-in (203-mm)

DI water main. The pipeline was installed

in 1988. Three specifi c sites were identi-

fi ed for excavation and inspection.

No damage was observed to the in-

stalled PE encasement at each of the

identifi ed inspection locations (Figure 4).

The soil samples were found to exhibit

corrosive characteristics, but the exposed

8-in diameter DI pipe was found to be in

excellent condition beneath the PE wrap.

The pipe surface displayed only minor

surface oxidation (Figure 5). Sounding

and probing of the pipe revealed no pit-

ting or graphitization.

Additional Results from the Investigations

• All tests of the PE samples procured

from the excavations/inspections

exceeded the requirements of the

ANSI/AWWA C105/A21.5 Stan-

dard, which was in effect at the time

of the installations of the pipelines.

• In situ four-pin resistivity values in

accordance with ASTM G57 Stan-

dard11 (including Barnes Method

layering results) ranged from 2,700

to 4,690 Ω-cm at the San Antonio

sites, 3,925 to 4,120 Ω-cm at

Charleston, and 1,200 to 8,500 Ω-

cm in Syracuse.

• Soil box resistivity results in accor-

dance with ASTM G187 Standard12

(from soil samples obtained at all

excavation sites and representative

of the soil environment in contact

with the pipelines) ranged from

1,400 to 1,480 Ω-cm at San

Antonio, 1,440 to 1,480 Ω-cm at

Charleston, and 680 to 980 Ω-cm at

Syracuse.

• For all specifi c results of these inves-

tigations see NACE International

paper number 127, “Investigating

DI Pipelines,” which was presented

at the Pipeline Integrity Symposium

during CORROSION/2007 in

Nashville, Tennessee.

ConclusionsThese investigations were performed

at eight inspection sites with the coopera-

tion of three utilities. At each site, PE

encasement had provided corrosion pro-

tection to DI pipe in corrosive soil condi-

tions. These results mirror numerous

reports, publications, and tests that indi-

cate PE encasement has provided viable

protection for millions of feet of gray and

DI pipe since its fi rst use in 1958.1,3-4,13-17

As with any method of corrosion pro-

tection, proper material specifi cations

and installation procedures must be

maintained to ensure the system’s integ-

rity. Appropriate national and interna-

tional standards are readily available to

achieve this result.6,18-21

Specifi c conclusions from these 20-

year-old case histories are as follows:

• Cell-to-cell potential surveys and

side-drain technique measurements

are not reliable in locating corrosion

activity on PE-encased DI pipe. At

eight different locations from all

three water systems, the potential

tests indicated active corrosion

where none was found.

• In situ four-pin soil resistivity mea-

surements consistently provide

higher values (less corrosive) than

Syracuse, New York: Eighteen-year-old PE-encased 6-in DI pipe at a location where the cell-to-cell survey indicated an anodic area.

FIGURE 5

M A T E R I A L S S E L E C T I O N & D E S I G N Investigating Polyethylene-Encased Ductile Iron Pipelines

October 2008 MATERIALS PERFORMANCE 53

those of representative soil box re-

sistivities. Whenever soil resistivity

values are presented, the method

used in their determination must be

referenced.

• Soil corrosivity tools such as the 10-

point Soil Evaluation System6 and

The Design Decision Model†7 deter-

mined that the soils at each of the

investigation sites were aggressive to

DI pipe. These methods, when used

correctly, have provided and con-

tinue to provide benefi cial assistance

in the determination of corrosion

control for DI pipe.

• At several of the inspection locations,

moisture under the PE encasement

was encountered but no corrosion-

related pitting was discovered.

• It has been demonstrated at these

three investigation locations that

PE encasement can be an effective

system of corrosion protection for

DI pipe.

AcknowledgmentsThe authors wish to thank Johnnie

Terrazas, P.E. (BexarMet Water District),

Chris Sordelet (Charleston Water Sys-

tem), John Van Deusen (Onondaga

County Water Authority), Dale Linde-

muth, P.E. (Corrpro), and Bill Foulds,

P.E. (NACE Corrosion Specialist—

retired) for their contributions and coop-

eration with this work.

References

1 T.F. Stroud, “Corrosion Control Mea-sures for Ductile Iron Pipe,” CORRO-SION/89, paper no. 585 (Houston, TX: NACE, 1989).

2 E.C. Bell, A.E. Romer, “Making Baggies Work for Ductile Iron Pipe,” “Corrosion and Corrosion Control Research of Iron Pipe,” Pipelines 2004, ASCE Annual Conference, August 1-4, 2004, San Diego, CA.

3 Ductile Iron Pipe Research Association, “Cast Iron Pipe Century and Sesquicen-tury Club Records and Correspon-dence,” ongoing.

4 “Corrosion Protection for Ductile Iron Pipe by Polyethylene Sleeves,” Kubota Ltd. Report, October 1986.

5 American Water Works Association, AWWA Engineering and Construction Division Survey, October 2000 Main-stream, Denver, CO.

6 ANSI/AWWA C105/A21.5 (latest revision), “Polyethylene Encasement for Ductile Iron Pipe Systems” (New York, NY: ANSI and Denver, CO: AWWA).

7 DIPRA, “The Design Decision Model™ for Corrosion Control of Ductile Iron Pipelines,” December 2004, Ductile Iron Pipe Research Association, Birmingham, AL

8 D.H. Kroon, D. Lindemuth, S. Samp-son, T. Vincenzo, “Corrosion Protection of Ductile Iron Pipe,” CORRO-SION/2004, paper no. 46 (Houston, TX: NACE, 2004).

9 A.W. Peabody, Control of Pipeline Corrosion (Houston, TX: NACE, 1967).

10 Appalachian Underground Short Course, “Advanced Course,” West Virginia University, Morgantown, WV, 1993.

11 ASTM G57, “Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method” (West Conshohocken, PA: ASTM).

12 ASTM G187, “Standard Test Method for Measurement of Soil Using the Two-Electrode Soil Box Method” (West Conshohocken, PA: ASTM).

13 A.M. Horton, “Protecting Pipe with Polyethylene Encasement, 1951-1988,” AWWA Water World News 4 (1988): pp. 26-28.

14 R.W. Bonds, L.M. Barnard, A.M. Horton, G.L. Oliver, “Corrosion and Corrosion Control of Iron Pipe—75 Years of Research,” Journal AWWA 6 (2005).

15 DIPRA, “Inspection Report—Cathodically Protected Ductile Iron Pipe Encased in Loose Polyethylene Film—Dickinson, ND,” April 21, 2004 (Bir-mingham, AL).

16 DIPRA, “Inspection Report—Cathodically Protected Ductile Iron Pipe

†Trade name.

Encased in Loose Polyethylene Film—Lafourche Parish, Louisiana,” May 28, 2003 (Birmingham, AL).

17 T.F. Stroud, “Infrastructure: Is the Prob-lem Being Blown Out of Proportion?” Ductile Iron Pipe News, Fall/Winter (1985): p. 9.

18 ASTM A 674 (latest revision), “Polyeth-ylene Encasement of Ductile Iron Pipe Systems” (West Conshohocken, PA: ASTM).

19 ISO 8180 (latest revision), “Ductile Iron Pipes—Polyethylene Sleeving” (Geneva, Switzerland: ISO).

20 JDPA Z 2005 (latest revision), “Polyeth-ylene Sleeves for Corrosion Protection of Ductile Iron Pipes” (Tokyo, Japan: JDPA).

21 BS 6076 (latest revision), “Tubular Poly-ethylene Film for Use as Protective Sleeving for Buried Iron Pipes and Fittings” (London, U.K.: BSI).

This article is based on CORROSION/2007

paper no. 127, presented in Nashville, Tennessee.

DANIEL W. CRABTREE is the research coordinator for the Ductile Iron Pipe Research Association (DIPRA), 245 Riverchase Pkwy. E., Ste. O, Birmingham, AL 35244, where he has been employed for 30 years. He is a 25-year NACE International member and a NACE-certifi ed Corrosion Specialist and Cathodic Protection Specialist. Currently, he is the chairman of the ASTM G01.10 Subcommittee on Corrosion in Soils.

MARK BRESLIN is a registered professional engineer in the state of Alabama. He is currently the staff engineer for DIPRA, where he has been employed for 17 years. He has been a member of NACE International for 17 years and is certifi ed as both a Corrosion Specialist and Cathodic Protection Specialist. He has a B.S. degree in mechanical engineering.

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