The Blueprint of Water

Corrosion: Engineering Notes

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⟵ 07 Corrosion (Narrative) · Contents · 07 Corrosion (Problem Set) ⟶

Corrosion: Engineering Notes

“If you are reading this straight through, you can skip this section and lose nothing essential to the story. These notes are for the operators, engineers, and technicians who need to do the math.”

Why This Matters in the Field

Corrosion is electron flow. If you can account for the circuit, you can:

  • predict where corrosion will localize (cells, gradients, deposits)
  • choose control strategies intelligently (anodic vs cathodic vs oxygen removal)
  • interpret coupons/probes quantitatively (MPY)
  • anticipate risk from water chemistry (indices + chloride/sulfate ratios)

Most corrosion failures are not “mysteries.” They are batteries operating in disguise.

Core Tools & Constants

Constant / FormulaValue
Corrosion cell (Ohm’s Law)Current = Voltage / Resistance
MPY formula534 × W / (D × A × T)
MPY unitsW(mg), D(g/cm³), A(in²), T(hr)
Carbon steel density7.87 g/cm³
Larson–Skold Index (LS)(Cl⁻ + SO₄²⁻) / (HCO₃⁻ + CO₃²⁻) [meq/L]
Cl⁻ to meq/Lmg/L ÷ 35.5
SO₄²⁻ to meq/Lmg/L ÷ 48
Alk (as CaCO₃) to meq/Lmg/L ÷ 50

The Corrosion Cell

A functioning corrosion cell requires all four components:

  1. Anode (metal dissolves): Fe → Fe²⁺ + 2e⁻
  2. Cathode (electrons consumed): O₂ + 2H₂O + 4e⁻ → 4OH⁻
  3. Electrolyte Path: ionic conduction through water
  4. Metallic Path: electron conduction through metal

Circuit Logic (Ohm’s Law)

The corrosion cell is governed by the same rule as any electrical circuit.

Current = VoltageResistance

The rate of metal loss is directly proportional to the corrosion current flowing from the anode to the cathode. Increased voltage drives higher current; increased resistance reduces current.

  • Voltage increases with strong oxidizers, oxygen gradients, galvanic couples
  • Resistance increases with passivation, protective films, coatings

Oxygen Differentials

Oxygen does not have to be “high” for corrosion to be severe. It only has to be uneven.

A deposit, crevice, gasket, or biofilm creates:

  • Low Oxygen Environments (Under deposit, inside crevice): Anode
  • High Oxygen Environments (Bulk water): Cathode

Small anode + big cathode = high current density at the anode → pitting corrosion.

Measuring corrosion

Corrosion is measured as thickness lost.

The standard industry unit is Mils per Year (MPY). 1 mil = 1/1,000 of an inch.

MPY = 534 × W(D × A × T)

Where:

  • 534 = Unit Conversion Factor
  • W = Weight loss (mg)
  • D = Density of the metal (g/cm³)
  • A = Surface Area of the coupon (in²)
  • T = Time exposed (hours)

Note: The recommended time frame for coupon exposure is approximately 90 days. This allows time for passivation to occur, providing a representative sample of long-term corrosion. Shorter timeframes may significantly overestimate corrosion activity.

Interpretation:

Acceptable corrosion rates differ for open and closed systems because their operating environments are fundamentally different - specifically oxygen exposure, water chemistry stability, and contaminant levels.

General Corrosion Rates for Open Recirculating Cooling Water Systems

DescriptionCarbon SteelCopper Alloys
Excellent< 1< 0.1
Very Good1 - 30.1 - 0.25
Good3 - 50.25 - 0.35
Fair5 - 80.35 - 0.5
Poor8 - 100.5 - 1
Severe> 10> 1

General Corrosion Rates for Closed Loop Systems

DescriptionCarbon SteelCopper Alloys
Excellent< 0.2< 0.1
Good0.2 - 0.50.1 - 0.25
Moderate0.5 - 0.80.25 - 0.35
Poor0.8 - 10.35 - 0.5
Severe> 1> 0.5

The Larson-Skold Index

The Larson–Skold Index (LS) is used to estimate the corrosivity of water towards mild steel. It measures the potential for water to interfere with protective surface film formation by quantifying the ratio between: “Aggressive Anions” (chloride and sulfate) and “Protective Inhibitors” (carbonate and bicarbonate).

LS = (Cl⁻ + SO₄²⁻) / (HCO₃⁻ + CO₃²⁻)

The calculation requires milliequivalents per liter (meq/L) to be used.

Conversions (mg/L to meq/L):

  • Chloride: meq/L = mg/L ÷ 35.5
  • Sulfate: meq/L = mg/L ÷ 48
  • Alkalinity (as CaCO₃): meq/L = mg/L as CaCO₃ ÷ 50

Interpretation:

  • < 0.8: Lower corrosion risk. The passive film is likely stable.
  • 0.8 – 1.2: Potential corrosion risk. Interference of film formation likely.

  • 1.2: High corrosion risk. Localized corrosion and pitting are likely to occur.1

Galvanic Corrosion

For galvanic corrosion to happen, three things must be present simultaneously:

  1. Dissimilar Metals: Two metals with different “nobility” (electrical potential).

  2. Metal-to-Metal Contact: The metals must be touching (or connected by a wire/bolt) so electrons can flow.

  3. Electrolyte: Both metals must be exposed to a conductive liquid.

When these conditions occur, the “Less Noble” metal becomes the anode, loses electrons, and corrodes rapidly. The “More Noble” metal becomes the cathode, receives electrons, and is protected.

The simplified list below shows the relative nobility of different metals. Exact ranking depends on environment (chlorides, temperature, passivity), but the direction is reliable.

The Hierarchy (simplified and environment-dependent):

Most Noble (Protected Cathode)

  • Titanium

    • Stainless Steel (Passive)

    • Copper / Brass

    • Lead

    • Cast Iron

    • Carbon Steel

    • Aluminum

    • Zinc (Galvanizing)

    • Magnesium

    Least Noble (Sacrificial Anode)

⟵ 07 Corrosion (Narrative) · Contents · 07 Corrosion (Problem Set) ⟶

Footnotes

  1. T. E. Larson and R. V. Skold, “Laboratory Studies Relating Mineral Quality of Water to Corrosion of Steel and Cast Iron,” Corrosion, vol. 14 (1958), Illinois State Water Survey. Confirm the citation details at time of publication. ↩

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