5.3 Corrosion Control & Cathodic Protection

Corrosion vs. Scaling

Maintaining the structural integrity of the distribution system and ensuring water quality requires balancing corrosion and scaling.

Corrosion is the electrochemical deterioration of a metal as it reacts with its environment. Inside a water main, corrosive water dissolves the metal of the pipe, leading to leaks, breaks, and red water complaints (due to dissolved iron). More critically, corrosive water can leach lead and copper from residential plumbing and fixtures, posing severe health risks.

Scaling, on the other hand, is the precipitation of dissolved minerals (primarily calcium carbonate) onto the interior surfaces of the pipe. While severe scaling can reduce the inner diameter of the pipe and increase friction loss (lowering the C-factor), a very thin, uniform layer of scale is actually beneficial because it acts as a physical barrier between the corrosive water and the metal pipe.

The Langelier Saturation Index (LSI)

The Langelier Saturation Index (LSI) is a mathematical tool used by operators to predict the scale-forming or corrosive tendencies of water. It is based on the difference between the water's actual pH and its calculated saturation pH ($pH_s$).

$LSI = pH_{actual} - pH_s$

The calculation for $pH_s$ considers the water's calcium concentration, total alkalinity, temperature, and total dissolved solids (TDS).

• LSI < 0 (Negative): The water is undersaturated with calcium carbonate. It will dissolve scale and is considered corrosive.
• LSI = 0: The water is in equilibrium.
• LSI > 0 (Positive): The water is oversaturated with calcium carbonate and will deposit scale.

Operators typically aim for a slightly positive LSI (e.g., +0.1 to +0.3) to promote the formation of a protective microscopic eggshell-like coating of calcium carbonate on the pipe walls.

Lead and Copper Corrosion

The Lead and Copper Rule (LCR) requires water systems to monitor tap water in homes. Since lead and copper rarely occur naturally in source water, their presence is almost entirely due to the corrosion of service lines, solder, and brass fixtures in the plumbing.

Factors that increase the corrosivity of water toward lead and copper include low pH, low alkalinity, high dissolved oxygen, and high levels of chloride and sulfate. When water sits stagnant in pipes overnight, the contact time increases, allowing more metals to leach. This is why first-draw samples are critical for LCR compliance.

Chemical Corrosion Inhibitors

When adjusting pH and alkalinity (e.g., by adding lime or sodium hydroxide) is insufficient to control corrosion, water systems may add chemical inhibitors. These chemicals form a protective film on the pipe interior, passivating the metal surface.

1. Orthophosphates: These are the most widely used inhibitors for lead and copper control. They react with dissolved metals to form an insoluble, protective coating (like lead phosphate) on the pipe wall. They are highly effective even in waters with low hardness and alkalinity.
2. Polyphosphates: Primarily used for sequestering (binding) iron and manganese to prevent colored water problems. They can also provide some corrosion control by forming a protective film, but they eventually revert to orthophosphates in the system.
3. Silicates: Sodium silicate is used in some systems to form a thin, durable silicate film on the pipe. They raise the pH of the water, which provides a secondary corrosion control benefit.

Cathodic Protection

While internal corrosion is controlled through water chemistry, external corrosion of buried metal pipes and storage tanks is managed using coatings and cathodic protection. Corrosion is an electrochemical process involving an anode (where metal is lost), a cathode (where metal is protected), an electrolyte (the soil or water), and a metallic return path.

Cathodic protection turns the entire structure into a cathode, preventing it from losing metal. There are two main types:

1. Galvanic (Sacrificial) Anodes: This method uses a metal that is more electrically active (higher on the galvanic series) than the pipe material. For example, bags of magnesium or zinc are buried near a steel pipe and connected to it by a wire. The magnesium acts as the anode and corrodes, sacrificing itself to protect the steel cathode. These anodes must be periodically replaced.
2. Impressed Current: Used for larger structures like elevated storage tanks or long pipelines where sacrificial anodes would be insufficient. This system uses an external DC power source (a rectifier) to force electrons into the structure, making it a cathode. The anodes used in these systems (often high-silicon cast iron or mixed metal oxides) do not rapidly corrode.

Proper application of coatings (like epoxies or polyurethanes) in conjunction with cathodic protection provides the best defense against external corrosion.

Dielectric Unions

Galvanic corrosion occurs when two dissimilar metals are physically connected in the presence of an electrolyte (like water). For instance, connecting a copper pipe directly to a galvanized iron pipe will cause the iron to corrode rapidly. To prevent this, operators install dielectric unions—fittings that contain a non-conductive plastic or rubber washer to physically separate and electrically isolate the two metals.