Corrosion control is important for both design of new infrastructure and continued utilization of existing infrastructure.  Many systems have been deemed structurally unsound or replaced due to corrosion.  In some cases, these systems had not reached 50% of their design life.  These structures could have been in service today if proper corrosion mitigation measures had been applied.  For the structures under consideration in the context of this paper, uninterrupted service lives in excess of 50-plus years should be realized.

The responsibility for the implementation and direction of an effective corrosion control program lies with a number of individuals.  It is necessary to recognize that the corrosion engineering expertise necessary for implementation of an effective program is typically not within the utility staff.  Therefore the principals, operators and planners within the utility should be cognizant of the corrosion process, support the identification and resolution of problems as they arise and allocate resources for the corrosion control program.  Corrosion engineering expertise is available from various consulting engineering firms.

Cause of Corrosion

Corrosion activity occurs as a natural reaction between a metallic structure and its environment.  This paper primarily addresses corrosion of metallic structures, steel, ductile iron and cast iron.  These are the common materials of construction for the infrastructure of concern: piping systems, storage tanks and water treatment equipment/structures.

With available oxygen and water, exposed metal will under go a process called oxidation.  When a metal experiences oxidation, the basic element of construction ie. iron, reacts with the hydroxide ion to form corrosion films.  The most readily observed corrosion films are rust, iron oxide and copper patina.  As the base material is consumed to form the corrosion films, the base material experiences corrosion degradation, leading to loss of material.

Corrosion is established as direct current, DC, circuits.  Alternating current, AC, is not typically a factor in the corrosion exposure of water piping.  DC circuits are defined by the relationship called Ohms Law, E=IR, where E is the driving voltage of the circuit, I is the current magnitude and R is the resistance of the circuit.  The greater the current flow in the corrosion circuit, the greater the metal loss.  Ferrous materials corrode at the rate of 20 pounds per ampere-year.  How the corrosion circuit is established and where corrosion would be experienced is complex, however some examples presented in this paper should be useful.    

General Corrosion

General corrosion will occur on structures exposed to several environments or electrolytes encountered in the water treatment, storage, and transmission and distribution industry.  Electrolytes include raw and treated water, salt water and fresh water, soil of many varieties,  atmospheric rain and air borne contaminants.  Corrosion aggressiveness of these electrolytes is influenced by many contaminants contained in the electrolyte:

• Chlorine from water treatment
• Chloride ions from road de-icing
• Marine environments, salt water
• Variations in soil
• Clay soil
• Organic soil
• Air borne contaminants effecting pH

These are some of the primary contributors of corrosion normally encountered in the industry.  The effect that these contaminants generally have on environment corrosivity is a reduction in the environment resistivity.  Lower resistivity, or circuit resistance, would produce larger corrosion currents for a given circuit.  This can be seen in a modified Ohms Law where I=E/R.  As the resistance decreases for a given voltage, the value of current increases.  Field experience would support this relationship: salt water environments are very corrosive as the resistivity is very low, conversely little corrosion would be found in uncontaminated dry sand.

Soil resistivity, the reciprocal of conductivity, has been used for years as an indicator of the corrosivity of soil.  The lower the resistivity, the easier current will flow through the soil.  Of the measurable soil characteristics, resistivity is generally accepted as the primary indicator of soil corrosivity

In addition to the structure’s environmental exposure influencing corrosion activity, there are external factors that influence corrosion of the structure.  Often, these are more aggressive than corrosion exposure from the pipe environment.  For simplicity, as related to the water industry, these are galvanic corrosion and stray current corrosion.  

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