7.3 Corrosion Control, Saturation Indices & the Lead and Copper Rule
Key Takeaways
- A positive Langelier Saturation Index means water tends to deposit calcium carbonate scale, a negative index means it tends to dissolve it and is corrosive, and zero means the water is at equilibrium.
- The Lead and Copper Rule action levels are 0.015 mg/L for lead and 1.3 mg/L for copper at the 90th percentile of first-draw tap samples, and an action level is not a maximum contaminant level.
- Lead and copper compliance samples are first-draw, one-liter samples collected after at least six hours of stagnation from taps at high-risk sites.
- Orthophosphate is the most widely used corrosion inhibitor because it forms a low-solubility lead and copper phosphate film on pipe walls.
- The Lead and Copper Rule Revisions add a 0.010 mg/L trigger level, mandatory service line inventories, and school and childcare sampling, all of which raise the operational stakes on stable water chemistry.
7.3 Corrosion Control, Saturation Indices & the Lead and Copper Rule
Corrosion is the deterioration of a material by reaction with its environment. In a water system it is simultaneously an asset problem (pipe failures, red water, loss of capacity) and a public health problem (lead and copper leaching into drinking water at the tap). The Need-to-Know Criteria list "corrosion control chemicals (e.g., phosphates)" among the chemicals to ensure proper use of, and "corrosion control (e.g., Langelier Saturation Index, phosphate)" among laboratory analyses.
What Drives Corrosion
| Factor | Effect |
|---|---|
| Low pH | Strongly increases corrosivity; the single most influential factor |
| Low alkalinity | Poor buffering, so pH swings easily; less carbonate available to form protective scale |
| High dissolved oxygen | Supports the cathodic reaction |
| High temperature | Roughly doubles reaction rate per 10°C rise; hot water lines corrode faster |
| High total dissolved solids and chloride | Increases conductivity, accelerating electrochemical corrosion |
| High velocity | Erosion corrosion, especially at elbows and in copper |
| Dissimilar metals in contact | Galvanic corrosion — the less noble metal sacrifices |
| Microbial activity | Microbiologically influenced corrosion; sulfate-reducing bacteria under tubercles |
Galvanic corrosion deserves particular attention because it is common at partial repairs. When copper is joined directly to galvanized steel or to a lead service line, the more active metal corrodes preferentially. A dielectric union breaks the electrical path and is the standard remedy.
Common Corrosion Indicators at the Tap
- Blue-green staining on fixtures → copper corrosion
- Red-brown water and staining → iron corrosion, often from unlined cast iron
- Black water → often manganese, but can be sulfide corrosion
- Metallic taste → copper or iron
- Pinhole leaks in copper tubing → pitting corrosion, sometimes velocity-driven
Saturation Indices
The classic tool is the Langelier Saturation Index (LSI), which compares actual pH to the pH at which the water would be exactly saturated with calcium carbonate.
where pH_s is the saturation pH 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; balanced |
| Negative (< 0) | Undersaturated; water tends to dissolve calcium carbonate and is corrosive |
Worked example. Finished water has pH 7.4 and a calculated pH_s of 8.1.
The water is undersaturated and corrosive. Raising pH toward 8.1 with caustic soda, lime, or soda ash moves it toward balance.
[!IMPORTANT] LSI predicts a tendency, not a rate, and it says nothing directly about lead. A slightly positive LSI is the usual target, but a strongly positive index causes scaling that reduces pipe capacity and fouls water heaters. Related indices you may see include the Ryznar Stability Index and the Aggressiveness Index, which is used for asbestos-cement pipe.
The Lead and Copper Rule
Lead in drinking water almost never comes from the source. It comes from lead service lines, lead solder (banned in 1986 but present in older plumbing), and brass fixtures. Copper comes from copper tubing. Because the metal enters the water in the customer's plumbing, the rule is written around tap sampling, not plant sampling.
Action Levels
| Metal | Action level | Basis |
|---|---|---|
| Lead | 0.015 mg/L (15 ppb) | 90th percentile of first-draw tap samples |
| Copper | 1.3 mg/L | 90th percentile of first-draw tap samples |
[!WARNING] An action level is not a maximum contaminant level. Exceeding it is not automatically a violation; it triggers required actions — corrosion control treatment steps, public education, water quality parameter monitoring, and in some circumstances service line replacement. The maximum contaminant level goal for lead is zero, because no level of lead exposure is considered safe.
Sampling Protocol
Compliance samples must be:
- First-draw, collected after the water has stood motionless in the plumbing for at least 6 hours
- One liter in volume
- Taken from a cold water tap normally used for drinking, typically the kitchen or bathroom
- Collected from high-risk (Tier 1) sites: homes with lead service lines, copper piping with lead solder installed before the 1986 ban, or known lead pipe
Do not remove or clean the aerator before sampling, and do not flush the line first — both defeat the purpose of capturing the water that actually sat against the plumbing. The 90th percentile is computed from the set of results, so one high house does not by itself create an exceedance, and a well-designed site pool matters.
Corrosion Control Treatment
pH and Alkalinity Adjustment
Raising pH and alkalinity reduces lead and copper solubility and supports formation of a protective carbonate scale. Chemicals used:
- Sodium hydroxide (caustic soda) — raises pH, adds no hardness
- Lime (calcium hydroxide) — raises pH and adds calcium
- Soda ash (sodium carbonate) — raises pH and alkalinity
- Carbon dioxide — used to lower pH when overshoot occurs
Corrosion Inhibitors
- Orthophosphate is the workhorse. It forms a low-solubility lead phosphate and copper phosphate film on the pipe wall. Typical doses run about 1 to 3 mg/L as PO₄, and maintaining pH in the effective range matters as much as the dose itself.
- Blended ortho-polyphosphate provides both film formation and some sequestration of iron and manganese.
- Silicates are used less commonly.
[!NOTE] The passivating film takes weeks to months to establish and it is fragile. Changing source water or disinfectant type without evaluating the corrosion chemistry can destabilize an existing film and release accumulated lead. This is the mechanism behind the most serious lead-in-water incidents in the United States, and it is why any source change, or a switch between free chlorine and chloramine, demands a corrosion control review first.
The Lead and Copper Rule Revisions
The LCRR added requirements operators must know:
- A trigger level of 0.010 mg/L for lead, below the 0.015 mg/L action level, which initiates planning and optimization before an exceedance occurs
- A mandatory service line material inventory, made publicly accessible
- Find-and-fix requirements when an individual sample exceeds the action level
- Sampling in schools and childcare facilities
- Revised service line replacement obligations, with an emphasis on full rather than partial replacement
Partial lead service line replacement can temporarily increase lead release by disturbing the pipe and creating a galvanic couple between the new copper and the remaining lead, which is why full replacement is strongly preferred.
Finished water has a pH of 7.1 and a calculated saturation pH of 7.9. What does this indicate about the water, and what is the appropriate corrective direction?
A utility collects lead compliance samples and the 90th percentile result is 0.018 mg/L. Which statement correctly describes the regulatory consequence?
A system with an established orthophosphate corrosion control program plans to switch from its long-standing groundwater source to a new surface water supply. What is the principal risk?