and Prevention

Introduction

The degradation of municipal infrastructure is rarely the result of sudden catastrophic force; rather, it is the slow, relentless electrochemical and biological breakdown of assets. For water and wastewater engineers, the total annual direct cost of corrosion is estimated in the billions, yet it remains one of the most frequently underestimated factors in plant design and collection system planning. The challenge of Corrosion and Prevention is not merely about selecting a thicker pipe wall or applying a generic coating; it is about understanding the complex interplay between wastewater chemistry, microbiology, and metallurgy.

A critical oversight in many capital improvement projects is the treatment of corrosion control as a line-item afterthought rather than a fundamental design parameter. In wastewater environments, the shift from aerobic to anaerobic conditions in force mains can turn a collection system into a generator of sulfuric acid, destroying concrete manholes and steel components downstream. In potable water systems, neglecting the Langelier Saturation Index (LSI) or stray currents can lead to premature failure of distribution piping and storage tanks.

This article serves as a technical guide for the selection, specification, and lifecycle management of corrosion control technologies. It covers applications ranging from headworks and digesters to distribution pumping and storage. By focusing on engineering fundamentals regarding Corrosion and Prevention, utility decision-makers can extend asset life by decades, reduce unplanned outages, and optimize total cost of ownership (TCO).

How to Select / Specify Corrosion Control Strategies

Specifying the correct corrosion protection system requires a multi-dimensional analysis of the operating environment. A “one-size-fits-all” specification often leads to catastrophic delamination of coatings or rapid consumption of sacrificial anodes. The following criteria provide a framework for engineering robust Corrosion and Prevention strategies.

Duty Conditions & Operating Envelope

The first step in specification is accurately defining the chemical and physical stress the asset will endure. Engineers must evaluate parameters beyond average daily values.

• Chemical Exposure (pH & H2S): In wastewater headspaces, H2S levels can fluctuate between 5 ppm and 500+ ppm. Biogenic sulfide corrosion occurs when Thiobacillus bacteria convert H2S gas into sulfuric acid, dropping surface pH to near 1.0. Specifications must account for this extreme acidity, requiring materials like PVC liners or novolac epoxies rather than standard coal tar epoxies.
• Temperature: Reaction rates for corrosion roughly double for every 10°C increase in temperature. High-temperature industrial discharges or thermophilic digestion processes require coatings with higher glass transition temperatures (Tg) to prevent softening and permeation.
• Flow and Abrasion: High-velocity grit in headworks or sludge lines causes erosion-corrosion. The protective oxide layer on metals (like stainless steel) can be stripped away by abrasion, accelerating attack. In these zones, hardness and abrasion resistance take precedence over pure chemical resistance.
• Intermittent Wet/Dry Cycles: Tanks with fluctuating levels create a splash zone that is particularly aggressive due to high oxygen availability and wet/dry concentration cycles. This zone often requires a reinforced coating system compared to the submerged zone.

Materials & Compatibility

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology

Material selection is the primary line of defense. The compatibility matrix must include the substrate and the process fluid.

• Concrete Substrates: Standard Portland cement is highly susceptible to acid attack. For high-H2S environments, specifications should consider Calcium Aluminate Cement (CAC) or antimicrobial concrete additives. When coating concrete, the surface tensile strength must be verified to ensure it can support the coating system without cohesive failure.
• Metallurgy: While Type 316L Stainless Steel is the industry workhorse, it is susceptible to pitting in high-chloride environments (e.g., coastal plants or ferric chloride dosing lines). Duplex stainless steels (2205) or super-austenitic grades may be required.
• Dissimilar Metals: Galvanic corrosion is a common failure mode in pump stations where stainless steel anchor bolts contact ductile iron flanges or carbon steel supports. Dielectric isolation kits (sleeves and washers) must be specified at these interfaces.

Hydraulics & Process Performance

Process hydraulics directly influence corrosion potential. Turbulence strips volatile gasses (like H2S) out of solution, creating corrosive headspaces. Laminar flow in force mains can promote slime layer growth and sulfide generation.

• Turbulence Management: Design drop structures and weirs to minimize free-fall turbulence where possible to keep H2S in solution.
• Velocity Constraints: Ensure scouring velocities (>2 ft/s or 0.6 m/s) are maintained to prevent solids deposition, which creates anaerobic zones conducive to under-deposit corrosion and sulfate-reducing bacteria (SRB) activity.

Installation Environment & Constructability

A specification is only as good as its installability. Many high-performance coatings fail due to impossible application conditions.

• Moisture & Dew Point: Most high-performance coatings cannot be applied if the substrate temperature is less than 5°F (3°C) above the dew point. In humid lift stations, this window may never open without active dehumidification. Specifications must include climate control requirements.
• Confined Space Access: Surface preparation (abrasive blasting) requires significant equipment and ventilation. If access is limited, surface-tolerant coatings or mechanical preparation methods (SSPC-SP3 or SP11) might be necessary, though they generally offer lower performance than white metal blast cleaning (SSPC-SP5).
• Cure Times: Fast-return-to-service applications (e.g., emergency manhole repairs) may require polyurea or polyurethane hybrid systems that cure in minutes, whereas epoxies may require days.

Reliability, Redundancy & Failure Modes

Read moreAdvanced Oxidation Processes in Wastewater Treatment: Efficiency and Innovation

Engineers must plan for the eventual failure of the primary protection system.

• Cathodic Protection (CP) Redundancy: For critical buried pipelines, impressed current CP systems should have redundant rectifiers. Galvanic anode systems should be designed with test stations to monitor consumption rates.
• Holiday Detection: Failure often starts at pinholes. Specifications must mandate high-voltage spark testing (holiday detection) on 100% of the coated surface area before acceptance.
• Corrosion Allowances: For metallic tanks and pipes, a “corrosion allowance” (e.g., adding 1/8 inch to wall thickness) provides a mechanical safety factor if the coating system fails.

Maintainability, Safety & Access

Maintenance teams need safe access to inspect and repair protective systems.

• Test Stations: Buried assets with CP must have above-grade test stations for measuring structure-to-soil potential.
• Coating Colors: Use contrasting colors for primer, intermediate, and topcoats. This aids in application inspection and allows operators to visually identify depth of wear during future inspections.
• Safety in Application: Isocyanates in polyurethanes and solvents in epoxies present respiratory hazards. Specifications must outline ventilation and PPE requirements during application.

Lifecycle Cost Drivers

The cheapest initial option is rarely the most cost-effective solution for Corrosion and Prevention.

• CAPEX vs. OPEX: A PVC liner in a concrete pipe has a high CAPEX but near-zero maintenance. A coal tar epoxy coating has low CAPEX but may require recoating every 7-10 years.
• Asset Replacement: The cost of bypass pumping during a rehab project often exceeds the cost of the repair itself. Investing in a 50-year solution (e.g., FRP liners) avoids the massive mobilization costs of repeated interventions.
• Energy Impact: Corroded pipes have higher friction factors (C-factors drop), increasing pumping energy costs. Preventing internal tuberculation maintains hydraulic efficiency and lowers electrical OPEX.

Comparison of Corrosion Control Technologies

The following tables provide a comparative analysis of common corrosion protection methodologies used in municipal water and wastewater applications. These are intended to guide engineers in selecting the most appropriate technology based on environmental constraints and lifecycle expectations.

Engineer & Operator Field Notes

Successful Corrosion and Prevention programs rely heavily on field execution. A perfect specification can be rendered useless by poor application or neglected maintenance.

Commissioning & Acceptance Testing

Commissioning a coating or protection system is as critical as commissioning a pump. Do not accept a visual inspection alone.

• Surface Preparation Verification: Before any coating is applied, verify the surface profile using replica tape or a comparator. For concrete, verify the pH is neutral and moisture content is within spec (ASTM D4263 plastic sheet test).
• Dry Film Thickness (DFT): Use calibrated magnetic gages (SSPC-PA2) to ensure the specified thickness is met. Under-thickness leads to premature permeation; over-thickness can cause cracking or solvent entrapment.
• Adhesion Testing: Perform pull-off adhesion tests (ASTM D4541) on witness panels or non-critical areas. A coating that looks good but has no bond will fail under hydrostatic pressure.
• High-Voltage Holiday Testing: For immersion service, spark testing detects pinholes invisible to the naked eye. This is mandatory for aggressive wastewater environments.

Common Specification Mistakes

Engineers often recycle specifications, leading to outdated or inappropriate requirements.

• “Or Equal” Ambiguity: Corrosion products vary wildly in chemistry. Specifying “100% Solids Epoxy or Equal” allows contractors to bid inferior hybrids. Specify performance criteria (permeability, adhesion, abrasion resistance) rather than just generic types.
• Ignoring Dew Point: Failing to specify dehumidification in the bid documents for tank or manhole rehab projects guarantees delays or poor adhesion.
• Neglecting Stripe Coats: Edges, welds, and corners are where coatings pull away due to surface tension. Specifications must require a “stripe coat”—a brush-applied layer on all edges before the full spray application.

O&M Burden & Strategy

Operations teams play a vital role in Corrosion and Prevention through vigilance and environment management.

• Chemical Dosing Optimization: If using nitrate or iron salts for H2S control, ensure dosing is paced to flow. Under-dosing allows corrosion to restart; over-dosing wastes budget.
• Cathodic Protection Logs: Rectifiers should be checked monthly for voltage and current output. A sudden drop in current may indicate a broken cable or depleted anode groundbed.
• Visual Inspections: Schedule annual tank inspections. Look for blistering (osmotic failure), rust staining (coating breakdown), or cracking in concrete. Early touch-ups can extend the system life by 10 years.

Troubleshooting Guide

• Blistering in Coating: Usually caused by osmotic pressure due to soluble salts left on the surface before coating (poor cleaning) or solvent entrapment.
• Concrete Spalling with Rust Stains: Indicates carbonation or chloride ingress has reached the rebar, causing the steel to expand and crack the concrete cover.
• Rapid Pitting in Stainless Steel: Often due to MIC or chloride attack. If occurring under deposits, increase cleaning frequency or flow velocity.

Design Details & Standards

Engineering robust Corrosion and Prevention systems requires adherence to specific calculations and industry standards.

Sizing Logic & Methodology

When designing active corrosion control or chemical inhibition, quantitative analysis is required.

Predicting Sulfide Generation (Z-Formula):
Engineers should estimate potential sulfide generation in force mains to determine the severity of the environment. The Pomeroy-Parkhurst equations or the “Z” formula can estimate H2S buildup based on BOD, temperature, and retention time.
General Rule: If predicted dissolved sulfide > 0.5 mg/L, significant corrosion and odor control measures are required.

Cathodic Protection Current Demand:
To size a CP system, calculate the total surface area and multiply by the current density requirement for the material/environment.
Typical Current Densities:

• Bare Steel in Soil: 10-30 mA/m²
• Coated Steel in Soil: 0.1-1.0 mA/m² (Current is only needed for holidays)
• Steel in Moving Wastewater: 50-100+ mA/m²

Specification Checklist

Ensure your Section 09 or 13 specifications include:

• Surface Preparation Standards: Explicitly reference SSPC-SP10 (Near-White Metal) or SSPC-SP13 (Concrete).
• Climatic Conditions: Max relative humidity, min/max air and surface temps, dew point spread.
• Quality Assurance: Hold points for inspection (Pre-blast, Post-blast, Post-prime, Final).
• Warranty: Specific terms regarding blistering, delamination, and MIC resistance.

Standards & Compliance

Reference these governing bodies to ensure compliance and safety:

• AMPP (Formerly NACE/SSPC): The primary authority. Relevant standards include NACE SP0169 (External Corrosion Control) and NACE SP0188 (Discontinuity/Holiday Testing).
• ASTM: ASTM D4263 (Moisture in Concrete), ASTM D4541 (Adhesion).
• AWWA: C210 (Liquid-Epoxy Coating Systems), D102 (Coating Steel Water Storage Tanks).
• NSF/ANSI 61: Mandatory for any material in contact with potable water.

Frequently Asked Questions

What is Microbiologically Influenced Corrosion (MIC)?

MIC is corrosion caused or accelerated by microorganisms. In wastewater, the most common form involves Thiobacillus bacteria oxidizing hydrogen sulfide gas into sulfuric acid on concrete surfaces, rapidly degrading the cement paste. In metal piping, sulfate-reducing bacteria (SRB) creates biofilms that generate sulfides against the metal surface, causing deep, localized pitting even in stainless steel. MIC prevention requires antimicrobial materials or rigorous chemical control.

How does the Langelier Saturation Index (LSI) affect corrosion prevention?

LSI measures the calcium carbonate stability of water. A negative LSI indicates corrosive water that will dissolve calcium carbonate (protective scale), exposing metal pipe walls to oxidation. A positive LSI indicates scale-forming water. Water utility engineers aim for a slightly positive LSI (+0.2 to +0.5) to deposit a thin, protective layer of calcium carbonate without clogging pipes, acting as a natural form of Corrosion and Prevention.

What is the difference between Cathodic and Anodic protection?

Cathodic protection makes the protected structure the cathode of an electrochemical cell (lowering its potential), effectively stopping metal loss. It is widely used for pipelines and tanks. Anodic protection makes the structure the anode but maintains it in a “passive” voltage range where a stable oxide film forms. Anodic protection is rare in municipal water but common in handling extremely corrosive industrial acids (like sulfuric acid storage).

When should impressed current cathodic protection (ICCP) be used over galvanic anodes?

Galvanic anodes (sacrificial zinc or magnesium) are simple and require no external power, but they have limited driving voltage. They are ideal for well-coated, smaller structures or localized hotspots. Impressed Current (ICCP) uses a rectifier to drive current and is necessary for large bare structures, long pipelines, or high-resistivity soils where galvanic anodes cannot generate enough current to overcome the resistance.

How often should protective coatings be inspected in wastewater plants?

Formal inspections should occur every 1-2 years for immersion service. However, “walk-through” visual checks should be part of monthly routines. Look for rust staining (running rust), blistering, or peeling. For potable water tanks, AWWA recommends a comprehensive washout and inspection every 3-5 years. Early detection of coating failure prevents substrate damage and expensive structural repairs.

Is Type 316 Stainless Steel immune to corrosion in wastewater?

No. While 316L is resistant to general corrosion, it is susceptible to pitting and crevice corrosion in the presence of chlorides (salts) and stagnant conditions. If flow stops and solids settle, the area under the deposit becomes oxygen-depleted, breaking the passive film and allowing rapid corrosion. Engineers must specify pickling and passivation after fabrication to restore the protective oxide layer.

Conclusion

Effective Corrosion and Prevention in water and wastewater infrastructure is not a static product selection but a dynamic engineering discipline. It requires a thorough understanding of the specific environment—whether it is the headspace of a sewer manhole or the invert of a water main. Engineers must move beyond “boilerplate” specifications and advocate for robust materials, proper surface preparation, and active monitoring systems.

By prioritizing lifecycle costs over initial capital expenditure, utilities can avoid the premature failure of critical assets. The integration of proper material selection, rigorous construction quality assurance (QA/QC), and proactive operations strategies forms the defense necessary to protect public health infrastructure for generations.