5.8 Corrosion Control Treatment & Water Stability
Key Takeaways
- Corrosion Control is a named sub-topic on the treatment exam and Corrosion and Corrosivity are named sub-topics in two separate distribution exam categories.
- The Langelier Saturation Index is the measured pH minus the calcium carbonate saturation pH, where a positive value indicates scale-forming water and a negative value indicates corrosive water.
- The three corrosion control strategies are pH and alkalinity adjustment, inhibitor addition using orthophosphate or silicate, and calcium carbonate precipitation.
- Orthophosphate forms a low-solubility lead phosphate scale and is the most widely used corrosion control treatment for lead and copper in California.
- Galvanic corrosion between dissimilar metals, such as a lead goose-neck bonded to new copper, accelerates release of the less noble metal and is a reason partial lead service line replacement is disfavored.
Why Corrosion Control Is an Operator Responsibility
The Expected Range of Knowledge lists Corrosion Control in the treatment Water Treatment Processes category and lists Corrosion (Equipment Operation/Maintenance) and Corrosivity (Water Quality/Water Sources) as separate distribution sub-topics. Corrosion is also the mechanism behind the entire Lead and Copper Rule: lead in drinking water almost never comes from the source; it comes from the plumbing, and how much of it dissolves is a function of finished water chemistry that the operator controls.
Corrosion costs show up four ways: metals in the water (lead, copper, iron, zinc), loss of pipe (main breaks, pinholes, tuberculation), loss of hydraulic capacity (a tuberculated main can lose half its C-factor), and customer complaints (red water, blue-green staining, metallic taste).
The Electrochemistry
Corrosion is an electrochemical cell with four required parts. Break any one and corrosion stops.
| Component | Role | In a water main |
|---|---|---|
| Anode | Metal oxidizes and dissolves; electrons released | The pit, the scratch, the less noble metal |
| Cathode | Electrons consumed, typically reducing oxygen | The surrounding intact surface |
| Electrolyte | Ion path | The water itself; conductivity accelerates it |
| Metallic path | Electron path | The pipe wall or a bonding wire |
Factors That Increase Corrosion Rate
| Factor | Effect |
|---|---|
| Low pH | Hydrogen ions strip protective scale; strongly increases lead and copper solubility |
| Low alkalinity | Poor buffering, so pH swings; less carbonate available to build protective scale |
| High dissolved oxygen | Supplies the cathodic reaction |
| High temperature | Roughly doubles reaction rate every 10 °C - hot water lines corrode faster |
| High velocity | Erosion corrosion, especially in copper above about 5-8 ft/s |
| High total dissolved solids / conductivity | Better electrolyte |
| High chloride and sulfate relative to alkalinity | The Larson ratio; chloride and sulfate are aggressive to iron and to lead solder |
| Free chlorine vs chloramine | Changes oxidation-reduction potential and can destabilize existing scale |
| Dissimilar metals in contact | Galvanic cell |
| Microbial activity | Microbiologically influenced corrosion under tubercles and biofilm |
Saturation Indices
Langelier Saturation Index (LSI)
where pH_s is the pH at which the water would be exactly saturated with calcium carbonate, computed from calcium hardness, total alkalinity, total dissolved solids, and temperature.
| LSI | Interpretation |
|---|---|
| Positive (> 0) | Supersaturated - water tends to deposit calcium carbonate scale |
| Zero | At equilibrium - neither dissolving nor depositing |
| Negative (< 0) | Undersaturated - water tends to dissolve calcium carbonate, i.e. it is corrosive |
Many systems target a slightly positive LSI, roughly +0.2 to +0.5, to encourage a thin protective film without scaling heat exchangers and meters.
[!IMPORTANT] LSI is a directional indicator, not a corrosion rate. It tells you whether calcium carbonate will dissolve or deposit. It says nothing about lead or copper solubility in a low-calcium water, and a positive LSI does not guarantee lead control. This is why the Lead and Copper Rule requires demonstrated optimal corrosion control treatment based on actual tap monitoring rather than on an index value.
Related Indices
- Ryznar Stability Index (RSI) = 2pH_s − pH. Below about 6 indicates scaling; above about 7 indicates corrosive; above 8.5 strongly corrosive. Note that the RSI scale runs the opposite direction from LSI, which is a reliable exam trap.
- Aggressive Index (AI) = pH + log(alkalinity × calcium hardness). Used for asbestos-cement pipe; below 10 is highly aggressive, 10-12 moderately, above 12 non-aggressive.
- Calcium Carbonate Precipitation Potential (CCPP) quantifies how much CaCO₃ in mg/L would precipitate or dissolve, which is more useful operationally than LSI's yes/no direction.
- Larson Ratio = (chloride + sulfate) / alkalinity, in equivalents. Above about 0.5 indicates water aggressive to iron.
The Three Corrosion Control Strategies
1. pH and Alkalinity Adjustment
The cheapest and often the most effective lever. Raising pH into the 7.5 to 8.5 range and raising alkalinity to at least 30 to 60 mg/L as CaCO₃ greatly reduces lead and copper solubility and stabilizes the pH so it does not drift in the distribution system.
| Chemical | Raises pH | Raises alkalinity | Adds calcium | Notes |
|---|---|---|---|---|
| Sodium hydroxide (caustic soda) | Yes | Yes | No | Fast, precise; freezes near 12 °C at 50% strength; strong caustic hazard |
| Soda ash (sodium carbonate) | Yes | Yes | No | Easier handling than caustic; adds sodium |
| Lime (calcium hydroxide/oxide) | Yes | Yes | Yes | Adds calcium hardness; slurry feed, scaling and housekeeping issues |
| Sodium bicarbonate | Slightly | Yes | No | Adds buffer capacity with minimal pH change |
| Limestone contactor / calcite | Yes | Yes | Yes | Passive, self-limiting; good for small systems |
| CO₂ stripping (aeration) | Yes | No | No | Removes carbonic acid; free where aeration already exists |
2. Corrosion Inhibitors
- Orthophosphate (phosphoric acid, zinc orthophosphate, sodium orthophosphate) is the workhorse. It forms a low-solubility lead orthophosphate and copper orthophosphate scale on the pipe wall. Typical residual 0.5 to 3.0 mg/L as PO₄, held continuously - the film takes weeks to months to build and dissolves if the feed stops. Effective across a wide pH range but works best at pH 7.2 to 7.8.
- Blended ortho-polyphosphate provides both film formation and sequestration of iron and manganese, but the polyphosphate fraction can strip existing scale if the blend or dose is wrong.
- Sodium silicate raises pH and forms a silicate film; used where phosphorus discharge to a wastewater plant is a concern, since orthophosphate fed to the drinking water system eventually arrives at the wastewater plant and increases its phosphorus load.
- Zinc orthophosphate adds zinc, which helps in some waters but raises the zinc load on the receiving wastewater plant.
3. Calcium Carbonate Precipitation
Deliberately operating at a modest positive CCPP so a thin calcium carbonate layer forms. Effective in hard waters; ineffective in soft, low-calcium waters, and risky where scaling would foul meters and heat exchangers.
Field Diagnosis
| Symptom | Mechanism | Typical response |
|---|---|---|
| Red water, rusty staining, higher iron at the tap than at the plant | Iron corrosion and tubercle release in unlined cast iron | Raise pH/alkalinity, orthophosphate, unidirectional flushing, main lining or replacement |
| Blue-green staining of fixtures, metallic taste | Copper corrosion, usually acidic and low-alkalinity water, aggravated by high temperature | Raise pH to 7.5-8.0, raise alkalinity, orthophosphate |
| Pinhole leaks in copper, usually cold-water lines | Pitting corrosion, sometimes chloride- or microbially driven | Water chemistry adjustment; investigate velocity and grounding |
| Elevated lead only at some homes | Lead service lines, lead solder, or brass fixtures at those addresses | Optimal corrosion control treatment, service line inventory and replacement |
| Loss of C-factor and pressure in old mains | Tuberculation | Cleaning and lining, or replacement |
| Localized deep pitting near an electrical service | Stray current corrosion | Investigate grounding and cathodic interference |
| Accelerated attack at a lead-to-copper joint | Galvanic corrosion | Avoid dissimilar metal contact; use dielectric unions |
[!WARNING] Partial lead service line replacement can make lead levels worse. Cutting a lead line and connecting new copper creates a galvanic couple in which the lead becomes the anode, and the physical disturbance also releases accumulated scale. This is the technical reason regulators now push full service line replacement rather than partial replacement, and the reason customers must be given flushing instructions and a filter after any service line work.
A water has a measured pH of 7.2 and a calculated calcium carbonate saturation pH of 8.1. What does the Langelier Saturation Index indicate?
A utility feeds orthophosphate for lead control and then suspends the feed for six weeks during a chemical supply interruption. What is the most likely consequence?
A crew replaces the utility-side portion of a lead service line with copper but leaves the customer-side lead pipe in place. Why can this increase lead at the tap?