Water main failures are very expensive for municipalities because they typically result in expenses associated with repair costs, flood damage, and loss of revenue to affected businesses. Water main failures also interrupt the operation of vital services, such as medical care and fire fighting operations. Currently, millions of dollars are spent annually by industry and by municipalities on the repair of failed components of the water distribution infrastructure, such as components that are made from gray cast iron or “gray iron” pipe.
The rate of municipal water main failure is expected to increase as the existing gray iron infrastructure continues to age. The cost of repairing damages caused by broken water mains (and subsequent flooding damage) may become an important item in many municipal budgets. The development of a non-destructive sensing technique to detect defects in the water distribution infrastructure to prevent catastrophic failure of water distribution infrastructure components would result in tremendous savings.
A non-destructive testing technique could be used in a program that is designed to detect localized corrosion before actual failures occur. A typical program would include identification of microstructure (gray iron, ductile iron, or other), identification of corrosion mechanisms, determination of the extent of internal and external corrosion (maximum and minimum wall thickness), determination of degradation and distribution of the magnetic properties of the metal, and an analysis of data and determination of preventative action. The monitoring of pipe corrosion would be continued for a few years beyond the application of corrosion control measures.
The metallurgy of gray iron is disclosed in detail in the publication entitled “Iron—A Unique Engineering Material,” by D. E. Krause, Gray, Ductile and Malleable Iron Castings—Current Capabilities, ASTM STP 455, Philadelphia, Pa., ASTM: 1969, p. 3. The most important elements in gray iron, aside from iron, are carbon and silicon. The silicon content affects the carbon distribution in the metal.
Unlike the carbon in ductile iron and steel, which is disbursed as graphite spheroids and pearlite, respectively, the carbon in gray iron is present in flake form. These flakes form in the eutectic cell boundaries during cooling of the cast metal. The resulting graphite flakes form a continuous matrix throughout the gray iron.
A gray iron sample that includes an increased amount of silicon will have a decreased amount of carbon in the eutectic phase. Such a sample will have an increased amount of carbon in the form of pearlite and a decreased amount of graphite flakes. The lower content of graphite flakes results in an increase in tensile strength.
Typically, gray iron component failure is attributed to graphitic corrosion or graphitization. Graphitization occurs when the metallic constituents of gray iron are selectively removed or converted into corrosion products. Graphitization leaves behind the graphite matrix of the gray iron in the shape of the original casting. Graphitic corrosion is particularly insidious because graphitized pipe may appear perfectly sound upon visual inspection despite being embrittled and prone to premature failure under load.
Graphitic corrosion is one example of the dealloying of a metal. During dealloying, one component of an alloy is selectively dissolved, leaving other components behind. The preferential attack on iron in gray iron results from the fact that graphite is located at a highly noble or corrosion resistant position in the galvanic activity series. The relative position of two metals in the galvanic activity series determines which will most readily participate in electrochemical reactions, such as corrosion.
Pipes that are subject to graphitization may appear sound and may conduct water adequately. However, the metallic portion of a pipe wall may be significantly thinner in various places along the wall. Graphitized regions of the pipe wall will be brittle and subject to failure under load as the result of temperature variation, heavy traffic, or shock.
The galvanizing of iron with zinc inhibits corrosion because iron is nobler in the activity series than zinc. Therefore the zinc plating layer is preferentially attacked, greatly extending the service life of the iron substrate.Similarly, graphite is far more noble than iron, so that the graphite matrix within the gray iron can act as the cathode in an electrochemical reaction under the right conditions of soil composition and moisture. The iron in gray iron samples that are subject to an electrochemical reaction will undergo anodic attack. In such samples, the graphite matrix will survive, while the iron is dissolved away.
Our overall approach to corrosion risk assessment, locating “hot spots” and corrosion mitigation:
Our soil lab performs more than four thousand soil corrosivity measurements per year. Based on soil chemistry and corrosivity we can determine an estimated remaining life by predictive modeling.
Detection and Measurement of of Graphitization
We also design and install cathodic protection systems to protect these assets, which can add 20 years to their remaining life. I have attached our publication regarding the cast iron graphitization sensor, a Matergenics’ brochure, and several publications
The properties of iron that could be used to detect graphitization or other localized corrosion phenomena include ductility, electrical resistivity, or acoustic properties, such as ultrasonic sound velocity or attenuation. However, assessing ductility, by nature, involves destruction of the sample. Acoustical methods cannot be used with coated pipes due to the fact that it requires surface contact with bare, clean metal.
The publication entitled “Development of a Cast Iron Graphitization Measurement Device,” NYGAS Technology Briefs, Issue 99-690-1, January 1999, discloses a meter that uses eddy currents to measure the electrical resistivity of a sample surface. Eddy current methods require sophisticated control circuitry and precisely tuned components. The eddy current device necessarily consumes a considerable amount of power to generate the RF signal that it uses to induce eddy currents in the sample.
Ultrasonic measurement of acoustic properties requires a very clean interface between the probe and the pipe for purposes of acoustic transmission and impedance matching, so that it is poorly suited for use with exposed, buried pipe which is often wet or dirty. Accordingly, there is a need for an improved non-destructive testing method and apparatus for detecting the graphitization of gray iron. The sensor development in in progress at Matergenics corrosion testing laboratories.
Matergenics Approach-Graphitization Corrosion Sensor and Calibration Graph
Recent Publications By Matergenics Experts
C2018-11140 2: Corrosion Risk and Mitigation in Building Water Systems (2018 NACE Publication)
Water Corrosion Risk 2: Corrosion Risk Assessment and Failure Analysis in Industrial Water Systems
Client Involvement: No evaluation can be complete without input from the project owner or maintenance personnel. These individuals live with the project on a daily basis and typically have historical data related to water main/project use, prior maintenance history, cbreaks, etc. In addition, a successful repair strategy can only be developed if the expectations of the client are clearly understood. For example, if the structure will be replaced in one year, it is not beneficial to the client to evaluate the structure and develop repairs for a 20-year anticipated life. We can design cathodic protection system to increase the life of the water main based on soil resistivity and condition assessment of the water main
We respond to all inquiries promptly by sending a technical proposal detailing scope of work and estimated costs. Please call Dr. Zee at 412-952-9441, Ed Larkin at 412-788-1263 or James Datesh for additional information, and how we may be able to assist you in the lab or at the project site. Alternatively, you can send your request to firstname.lastname@example.org. Call us today for a quote!