Corrosion, Scale, and Chemical Stabilization

Corrosion, Scale, and Chemical Stabilization

Corrosion and scale are two opposite problems that operators must balance every day. Corrosion destroys pipe metal, releases contaminants into drinking water, and shortens system life. Scale (hard mineral deposits) clogs pipes, reduces flow, and insulates heat-exchange surfaces. Understanding the chemistry behind each problem lets you choose the right control strategy.

How Corrosion Happens

Corrosion in water systems is an electrochemical process. Two types are most important for operators.

Galvanic corrosion occurs when two different metals are in contact in the presence of water. One metal acts as the anode (it oxidizes and loses metal) and the other acts as the cathode (it is protected). The metal higher on the galvanic series corrodes first. A common example is copper pipe connected directly to galvanized iron pipe: the iron corrodes preferentially. Lead solder next to copper fittings is another classic galvanic pair that can release lead into drinking water.

Concentration-cell corrosion (also called differential-aeration corrosion) occurs on a single metal surface when one area has less dissolved oxygen than another. The oxygen-depleted zone becomes the anode and corrodes. This is common under deposits, biofilm, or at crevices in joints.

Key Water-Quality Factors

Several water-quality parameters drive corrosion rate:

• pH – Low pH (acidic water) aggressively dissolves metal. Most states require operators to maintain water pH in the range that minimizes lead and copper release, generally above 7.0 and often closer to 7.4–8.0, depending on other parameters.
• Dissolved oxygen (DO) – Higher DO accelerates corrosion because oxygen is the electron acceptor at the cathode. Deoxygenation is used in some industrial systems but is impractical for most drinking-water distribution.
• Chlorides – High chloride concentrations break down the thin protective oxide film (passivation layer) on metal surfaces, especially stainless steel and copper, dramatically increasing corrosion rates.
• Alkalinity – Adequate alkalinity (bicarbonate and carbonate ions) buffers pH swings and promotes the formation of a protective calcium carbonate film inside pipes. Very low alkalinity leaves water corrosive and unstable.
• Temperature – Warmer water generally corrodes metal faster because chemical reaction rates increase.

Scale Formation

Scale is the opposite of corrosion: water deposits minerals onto pipe walls. The most common scale is calcium carbonate (CaCO₃). While a thin CaCO₃ film is actually protective and desirable, heavy scale restricts flow, increases pumping costs, and can harbor bacteria. Scale also forms from calcium sulfate, magnesium compounds, and silicates in certain source waters.

Corrosion-Control Strategies

pH and alkalinity adjustment is the most fundamental tool. Raising pH toward 8.0–8.5 (within permit limits) slows the electrochemical reaction and encourages a thin protective film. Operators add sodium hydroxide, soda ash (sodium carbonate), or lime to raise pH. Alkalinity is adjusted with sodium bicarbonate or lime.

Calcium carbonate stabilization targets the Langelier Saturation Index (LSI). The LSI compares actual water chemistry to the theoretical equilibrium where water is neither depositing nor dissolving CaCO₃. An LSI near zero is balanced. Negative LSI means corrosive (scale-dissolving) water; positive LSI means scale-depositing water. Operators adjust pH, calcium, and alkalinity to move the LSI toward a slightly positive value, encouraging a thin protective coating without heavy scale.

Orthophosphate addition is a widely used corrosion inhibitor under the Lead and Copper Rule. Orthophosphate ions react with lead and copper at the pipe wall to form insoluble phosphate minerals (such as lead hydroxyapatite and copper phosphate) that coat the surface and limit metal release into the water. Typical target residuals are in the low mg/L range. The chemical feed rate must be calculated precisely to maintain that target throughout the system.

Blending of aggressive and non-aggressive source waters is another strategy when multiple sources are available.

Safety Note

When handling corrosion-control chemicals — orthophosphoric acid, sodium hydroxide, soda ash, or lime — always wear the PPE required by the Safety Data Sheet (SDS): chemical-splash goggles, face shield, acid- or caustic-resistant gloves, and a chemical-resistant apron. Caustic and acid spills on skin or eyes require immediate flushing with large volumes of water for at least 15 minutes. Before servicing any chemical feed pump, follow lockout/tagout (LOTO) procedures to isolate energy and prevent accidental start-up. Store oxidizing chemicals (such as hypochlorite) separately from acids and phosphates.

Calculating a Chemical Feed Rate

The standard concentration–flow–dose relationship is:

Feed rate (lb/day) = Flow (MGD) × Dose (mg/L) × 8.34 lb/gal

For liquid chemical feeds expressed in volume per time, operators convert using the chemical's percent concentration and specific gravity. Always verify units before calculating and check your answer against the system's established dose range.