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CA Branch 1 study materials
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CA Branch 1: Corrosion Control and Water Quality Stability

CA Branch 1 · Work through your exam, one topic at a time.

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Corrosion Control and Water Quality Stability

This domain covers the chemical principles of corrosion, the Lead and Copper Rule (LCR), and strategies to maintain water quality stability in distribution systems. The exam tests your ability to interpret corrosion indices, select inhibitors, and apply LCR requirements.

Corrosion Fundamentals

  • Corrosion is the electrochemical oxidation of metal. In water systems, it is influenced by pH, alkalinity, dissolved oxygen, chloride-to-sulfate mass ratio (CSMR), and temperature.
  • Galvanic corrosion occurs when dissimilar metals are connected. Stray current corrosion can result from electrical systems.
  • Scaling (passivation) can protect pipe surfaces; however, excessive scale reduces capacity.

Corrosion Indices

  • Langelier Saturation Index (LSI) predicts calcium carbonate scaling potential: LSI = pH – pHs. Positive LSI indicates scaling; negative indicates corrosion. Typical target is slightly positive (0 to +0.5) for corrosion control.
  • Ryznar Stability Index (RSI) = 2pHs – pH. Values 6–7 are balanced; >7 corrosive; <6 scaling.

Lead and Copper Rule (LCR)

  • Action levels: lead 0.015 mg/L (15 ppb), copper 1.3 mg/L. Compliance is based on the 90th percentile of first-draw samples from high-risk sites.
  • If action level exceeded, corrosion control treatment (CCT) is required; options include pH/alkalinity adjustment, phosphate inhibitors (orthophosphate, polyphosphate), or silicate inhibitors.
  • Lead Service Line Replacement (LSLR) requirements: if lead action level exceeded, systems must replace a percentage of lead service lines annually, with consumer notification.
  • Public education is required within 60 days of a lead action level exceedance.

Water Quality Stability

  • Disinfectant residual must be maintained throughout distribution; free chlorine residual typically 0.2–4.0 mg/L; chloramine residual 0.5–4.0 mg/L.
  • Nitrifcation can occur with chloramines, reducing residual and increasing nitrite/nitrate. Monitoring and breakpoint chlorination can control it.
  • Temperature affects corrosion rates and biological activity; warmer water accelerates both.

Worked Example

A water has pH 7.2, calcium hardness 80 mg/L as CaCO3, alkalinity 60 mg/L as CaCO3, and total dissolved solids 200 mg/L. The calculated pHs is 7.8. What is the LSI? Is the water corrosive or scaling?

  • LSI = pH – pHs = 7.2 – 7.8 = –0.6.
  • Negative LSI indicates the water is corrosive (undersaturated with respect to calcium carbonate).
  • To achieve a slightly positive LSI, the operator could increase pH or alkalinity, or add a corrosion inhibitor.

Common Traps

  • Confusing LSI sign: negative LSI = corrosive, positive = scaling.
  • Forgetting that LCR compliance is based on 90th percentile, not average.
  • Assuming all corrosion control requires phosphate; pH/alkalinity adjustment may suffice.
  • Overlooking nitrification when using chloramines, which can mask residual loss.
  • Neglecting that lead service line replacement has specific annual replacement rates and notification requirements.