Corrosion Control for Alternative Pipe Materials

In these times of intense scrutiny of the use of resources, sustainable design requirements and safety and reliability of utility structures such as water transmission mains, we can no longer tolerate the approach of installing the structures and fixing them when they leak (or fail catastrophically). Corrosion must be considered in the initial design stage for these structures, and the necessary corrosion control mitigation measures must be part of the design of the structures.

There are protocols for assessing the environment relative to its corrosivity, and making recommendations for corrosion control that are appropriate for the structure and its environment. Corrosion engineers can predict the manner in which the materials will perform with and without corrosion control measures, and advise the designers of the structures accordingly. Effective corrosion control measures have been used since the early part of the 20th century. It is not experimental. It is a perfected technology, and one that has been in use by major water utilities in the New York, Philadelphia, Baltimore and Washington, D.C. metropolitan areas.

References to Corrosion Control Requirements & Approaches

All pipe materials commonly used for water transmission facilities are subject to corrosion damage, and corrosion control measures must be considered.

Steel pipe has always been provided with coatings and cathodic protection (CP). There has never been any resistance to this practice, as this is the norm in the oil and gas industry.

Prestressed concrete cylinder pipe (PCCP), has gained the notice of the industry and the general public due to an unfortunate series of failures of the high strength stressing wire, which resulted in ruptures that buckled highways, sent rivers of water down heavily trafficked roads and caused large craters in the back yards of suburban homes. There are several high profile projects involving large diameter pipe with PCCP and steel as alternative materials, which are currently in the design stage. These alternative materials are being furnished with cathodic protection after a rigorous assessment of the soil environment into which the pipeline will be installed has indicated that it is required. The steel alternate will be furnished with a high quality coating system, making it practical to protect this large diameter pipe with groups of galvanic anodes. The PCCP, with the inherent passivating benefits of the concrete mortar coating, can also be protected by distributing groups of galvanic anodes along the pipeline.

Ductile iron pipe has also been fraught with controversy. The ductile iron pipe industry, whose products have a long history of good performance, after years of debate, have accepted the advice of the corrosion control industry to consider the corrosion control requirements based on the corrosion assessment mentioned previously.?

It is interesting that questions often arise as to the need for dielectric coatings and CP for ductile iron pipe, and for that matter, the need for corrosion protection. The following excerpt from the paper ?Causes of Underground Corrosion,? by Bernard Husock, P.E., a renowned expert in the corrosion field, clearly points out the need to protect ductile iron pipe from corrosion.

?Tests performed by the National Bureau of Standards at more than 159 sites nationwide over a period of more than 50 years and as reported in Underground Corrosion, Circular C-579 by Melvin Romanoff shows that the ferrous metals including cast iron, carbon steel, wrought iron and ductile iron corrode at essentially the same rate underground. The apparent corrosion resistance of cast iron pipe is attributed to the fact that graphitized cast iron can retain its appearance as a pipe even though much of the iron is gone.?

In terms of the need for cathodic protection, the direction regarding the need for the combination of coatings and cathodic protection is contained in the reference (see NACE standard?information below) by NACE International, the technical society dedicated to the protection of infrastructure from the effects of corrosion.

Cathodic Protection Design Approach

Cathodic protection is a well-accepted and effective method of controlling corrosion or totally eliminating it, no matter how corrosive the environment. In fact, the more corrosive the environment, the more effective the cathodic protection system is.

Any paper or treatise on cathodic protection will discuss the two types of cathodic protection system (Impressed Current and Galvanic) that are available and the advantages and limitations of each type of system. The main advantage of galvanic anode systems is the elimination of the need for a source of power to energize anodes. In addition, the installation and maintenance cost is reduced over an impressed current system and, due to the low and uniformly distributed current from the galvanic anodes, the likelihood of stray current interference from? a galvanic anode CP system is negligible. The galvanic anode system works most economically where the pipe is effectively coated and electrically isolated from other buried structures and the electrical grounding system. The great majority of cathodic protection systems that are utilized in the water utility industry are galvanic. In the wastewater industry, structures such as clarifiers, settling basins and other large structures that cannot be effectively coated and electrically isolated are protected using impressed current systems.

How Should a Utility and Design Professionals Approach Corrosion Control?

All types of critical information are gathered when a structure such as a pipeline is planned.? Routing studies are conducted to allow selection of the route with the least number of interferences and disturbance to existing facilities. Geotechnical studies are conducted to determine the mechanical characteristics of the soil so that pipe deflection and the need for joint restraints can be determined.

It is understandable that one of the most important decisions regarding the installation of a new water main or facility such as a water transmission main or wastewater treatment facility is whether? measures are required to insure that the system will provide the desired life with safety and reliability and will not fail prematurely due to corrosion.? In order to make this decision, pre-design information is required regarding the corrosion characteristics of the environment into which the structure will be installed.?

Corrosion control consultants have evaluation protocols that will allow measurement of known corrosion parameters such as soil resistivity and chemical characteristics such as chlorides, pH, and Redox potential that can be used to estimate ?time to failure? of a structure if no corrosion control measures are implemented.

The recommended practice is to perform this corrosion assessment and make recommendations for the appropriate corrosion control measures based on the results of this assessment. Corrosion control measures must be implemented in the design stages and concurrent with the installation of the structure. Retrofit of these measures after the fact is not a cost effective option and allows corrosion that degrades the structure. Cathodic protection cannot replace metal that has been lost due to corrosion. It is an irreversible process. Designing the structures with the appropriate corrosion control provisions will go a long way to preserving capital and enhancing public safety.

Paul Rothman is a senior corrosion engineer for Russell Corrosion Consultants, Inc.

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NACE International Standard Practice SP0169: Control of External Corrosion on Underground or Submerged Metallic Piping Systems

This standard points out that ?External corrosion control must be a primary consideration during the design of a piping system. Materials selection and coatings are the first line of defense against external corrosion. Because perfect coatings are not feasible, CP must be used in conjunction with coatings.? The standard also recommends practice for selecting, testing and evaluating, handling, storing, inspecting and installing external coating systems. The function of external coatings is to control corrosion by isolating the external surface of the underground or submerged pipeline from the environment, to reduce CP current requirements and to improve current distribution.

Desirable characteristics of external coatings include effectiveness as an electrical insulator and a moisture barrier, and ability to apply the coating to the pipe with a minimum of defects. Many other desirable coating characteristics are listed in the NACE standard.

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