Corrosion Inhibitors

Overview

Corrosion inhibitors are chemical additives formulated to protect metallic components in secondary coolant systems from electrochemical degradation. These compounds establish protective barriers through adsorption, passivation, or precipitation mechanisms that reduce metal loss rates from typically 5-20 mils per year (uninhibited) to less than 1 mil per year when properly maintained.

Primary Functions

Metal Surface Protection

• Formation of protective oxide layers on steel, copper, and aluminum surfaces
• Blocking of anodic sites where metal dissolution occurs
• Suppression of cathodic reactions that drive corrosion current
• Prevention of galvanic cell formation between dissimilar metals

System pH Stabilization

• Buffering capacity to maintain pH within 7.5-10.5 range for glycol systems
• Neutralization of acidic degradation products from coolant breakdown
• Prevention of pH excursions that accelerate corrosion rates
• Maintenance of inhibitor effectiveness across temperature ranges

Deposit Control

• Sequestration of dissolved metals to prevent precipitation
• Dispersion of corrosion products and sludge formation
• Scale inhibition from hard water contamination
• Foam suppression in systems with high fluid velocities

Inhibitor Chemistry Fundamentals

Electrochemical Basis

Corrosion in coolant systems proceeds through coupled anodic and cathodic reactions:

Anodic Reaction (metal dissolution): Fe → Fe²⁺ + 2e⁻

Cathodic Reaction (electron consumption): O₂ + 2H₂O + 4e⁻ → 4OH⁻

Corrosion inhibitors function by disrupting these reactions through one or more mechanisms:

Anodic Inhibitors

• Form protective oxide films (Fe₂O₃, Fe₃O₄) at anodic sites
• Increase polarization resistance at metal dissolution points
• Examples: nitrites, molybdates, orthophosphates
• Risk: insufficient concentration can cause localized pitting

Cathodic Inhibitors

• Block oxygen reduction or hydrogen evolution reactions
• Precipitate insoluble compounds on cathodic areas
• Examples: zinc compounds, polyphosphates
• Lower risk profile than anodic types

Mixed Inhibitors

• Affect both anodic and cathodic processes simultaneously
• Provide broader protection across pH ranges
• Examples: organic acid technology (OAT), hybrid formulations
• Most common in modern HVAC applications

Inhibitor Types and Formulations

Nitrite-Based Inhibitors

Traditional inhibitor chemistry for ferrous metal protection in closed-loop systems.

Chemical Composition

• Sodium nitrite (NaNO₂) as primary active ingredient
• Typical concentration: 1200-3000 ppm in 25-50% glycol solutions
• Borax or other buffering agents for pH control
• Azoles (tolyltriazole, benzotriazole) for copper protection

Protection Mechanism Nitrite ions oxidize ferrous ions at anodic sites to form protective gamma-Fe₂O₃:

2Fe²⁺ + 2NO₂⁻ + 3H₂O → 2Fe(OH)₃ + N₂O + 2H⁺

This reaction consumes nitrite and generates nitrogen compounds, requiring periodic replenishment.

Performance Characteristics

• Excellent ferrous metal protection at proper concentrations
• Effective temperature range: -30°F to 250°F
• pH stability: 8.5-10.5 optimal range
• Depletion rate: 10-30% annually depending on system conditions

Limitations

• Nitrite depletion through oxidation and biological activity
• Formation of nitrosamines under certain conditions
• Incompatibility with some elastomers at high concentrations
• Not recommended for potable water systems or open loops

Application Requirements

• Initial charge: 2500-3000 ppm nitrite for new systems
• Minimum maintenance level: 1200 ppm
• Test frequency: quarterly for critical systems
• Replenishment when levels drop below 1500 ppm

Molybdate-Based Inhibitors

Alternative anodic inhibitor providing broad-spectrum protection.

Chemical Composition

• Sodium molybdate (Na₂MoO₄) as primary component
• Typical concentration: 200-800 ppm molybdate ion
• Often combined with azoles, nitrates, or phosphates
• Buffering agents to maintain alkaline pH

Protection Mechanism Molybdate forms protective ferric molybdate complexes:

Fe + MoO₄²⁻ + O₂ → FeMoO₄ (protective layer)

This passive film is self-healing and provides excellent long-term stability.

Performance Characteristics

• Lower toxicity than nitrite-based formulations
• Excellent high-temperature stability (up to 350°F)
• Synergistic effects when combined with other inhibitors
• Low depletion rate: 5-15% annually

Advantages Over Nitrites

• No nitrosamine formation concerns
• Better biological stability
• Lower required concentrations
• Enhanced protection for aluminum alloys

Cost Considerations

• Higher initial material cost than nitrite systems
• Lower lifecycle cost due to reduced replenishment frequency
• Preferred for long-term installations with minimal maintenance access

Organic Acid Technology (OAT)

Advanced inhibitor formulation using organic carboxylates as primary protection mechanism.

Chemical Composition

• Sebacic acid, 2-ethylhexanoic acid, or other carboxylic acids
• Concentration: 0.5-2.0% by volume in coolant
• pH adjusters (typically caustic) to maintain 7.5-9.0 range
• Defoamers and colorants as functional additives

Protection Mechanism Organic acids form protective coordination complexes with metal surfaces:

R-COO⁻ + Fe²⁺ → Fe(R-COO)₂ (absorbed layer)

Unlike nitrites, OAT inhibitors are not consumed by the protection mechanism, providing extended service life.

Performance Characteristics

• Extended service life: 5-10 years in properly maintained systems
• Minimal depletion over time
• Excellent protection for aluminum and copper alloys
• Lower electrical conductivity than inorganic inhibitors

System Requirements

• Clean system required before charging
• Not compatible with traditional inhibitor residues
• Requires dedicated test procedures
• May not protect solder joints as effectively as azoles

Application Scenarios

• New construction with homogeneous metallurgy
• Systems designed for extended service intervals
• Applications requiring low conductivity fluids
• Geographic locations where nitrite disposal is restricted

Hybrid Organic Acid Technology (HOAT)

Combination formulations providing benefits of both traditional and OAT chemistries.

Chemical Composition

• Organic acid base (carboxylates) at 0.5-1.5%
• Low-level inorganic inhibitors (molybdate, silicate, or phosphate)
• Azole compounds for copper/brass protection
• pH buffering to maintain 7.5-9.5 range

Synergistic Protection Hybrid formulations combine:

• Fast-acting inorganic inhibitor response
• Long-term organic acid stability
• Broad metallurgy compatibility
• Reduced replenishment requirements

Performance Advantages

• Service life: 3-5 years typical
• Protection across -60°F to 300°F temperature range
• Compatible with mixed-metal systems
• Lower total cost of ownership than traditional inhibitors

Design Considerations

• Suitable for retrofit applications with proper flushing
• Compatible with most elastomers and gasket materials
• Requires specific test methods (cannot use nitrite test strips)
• May require vendor-specific replenishment products

Inhibitor Concentration Requirements

Dosing Calculations

Inhibitor concentration depends on coolant type, system metallurgy, and operating conditions.

Glycol-Based Coolants

For ethylene or propylene glycol systems:

Inhibitor dose (gallons) = System volume (gallons) × Target concentration (%) ÷ Inhibitor strength (%)

Example for 1000-gallon system requiring 2500 ppm nitrite using 40% inhibitor: 2500 ppm = 0.25% by weight Dose = 1000 × 0.0025 ÷ 0.40 = 6.25 gallons

Adjustment Factors

• Increase by 20-30% for systems above 200°F operating temperature
• Increase by 50% for systems with significant aluminum content
• Reduce by 10-15% for closed systems with minimal fresh water makeup

Concentration Limits

Minimum Effective Levels

Maximum Recommended Levels Exceeding recommended concentrations can cause:

• Precipitation and deposit formation
• Seal and gasket degradation
• Increased fluid viscosity
• Cost inefficiency without performance benefit

Critical Thresholds

• Below minimum: exponential increase in corrosion rate
• Above maximum: diminishing returns and potential side effects
• Operating band: maintain within ±20% of target for optimal protection

Testing and Monitoring Procedures

Field Test Methods

Nitrite Testing Standard colorimetric test strips or titration kits provide rapid field assessment:

1. Collect sample from system drain or test port
2. Add reagent drops per manufacturer instructions
3. Compare color change to reference chart
4. Record result in ppm nitrite

Accuracy: ±10% for concentrations 500-3000 ppm Frequency: monthly for first year, quarterly thereafter

pH Measurement Critical parameter affecting inhibitor performance:

1. Calibrate pH meter with buffer solutions
2. Measure sample at ambient temperature
3. Temperature compensate reading if meter lacks auto-compensation
4. Acceptable range: 7.5-10.5 depending on inhibitor type

Out-of-range pH indicates:

• Low pH (<7.0): glycol degradation, inhibitor depletion
• High pH (>11.0): excessive inhibitor, contamination

Electrical Conductivity Indirect measure of total dissolved solids and inhibitor concentration:

• Typical range: 1000-3000 μS/cm for inhibited glycol
• Sudden increase: contamination or over-inhibition
• Gradual decrease: inhibitor depletion

Laboratory Analysis

Complete Coolant Analysis Comprehensive testing provides detailed system health assessment:

Recommended Test Frequency

• New systems: baseline test after 30-60 days operation
• Established systems: annual comprehensive analysis
• Problem systems: quarterly until stabilized
• Critical applications: semi-annual minimum

Interpretation Guidelines

Metal contamination thresholds (ppm):

• Iron: <50 ppm acceptable, >100 ppm indicates active corrosion
• Copper: <10 ppm acceptable, >20 ppm indicates copper corrosion
• Aluminum: <10 ppm acceptable, >30 ppm indicates severe attack
• Chloride: <25 ppm preferred, >100 ppm requires water treatment

Corrosion Rate Assessment

Coupon Testing Install pre-weighed metal coupons in system for direct corrosion measurement:

Corrosion rate (mils/year) = (534 × W) ÷ (D × A × T)

Where:

• W = weight loss (mg)
• D = metal density (g/cm³)
• A = coupon area (in²)
• T = exposure time (hours)

Acceptance Criteria Per ASTM D1384:

• Ferrous metals: <1 mil/year
• Copper: <0.2 mil/year
• Aluminum: <1 mil/year
• Solder: <3 mil/year

Coupon exposure period: minimum 30 days, 90 days preferred

System Material Compatibility

Metallurgy Considerations

Ferrous Metals Carbon steel, cast iron, ductile iron components:

• Primary concern in most closed-loop systems
• Require anodic or mixed inhibitors for protection
• Susceptible to pitting in chloride-contaminated systems
• Protection mechanism: passive oxide layer formation

Copper and Copper Alloys Brass fittings, bronze pumps, copper heat exchangers:

• Susceptible to dezincification in brass alloys
• Require azole inhibitors (benzotriazole, tolyltriazole)
• Sensitive to high pH (>10.5) and ammonia contamination
• Protection: formation of stable azole-metal complexes

Aluminum Alloys Heat exchangers, evaporators, manifolds:

• Most challenging metal to protect in mixed systems
• Require pH control between 7.5-9.0 maximum
• Molybdate and OAT inhibitors provide superior protection
• Avoid silicate inhibitors (gel formation risk)

Stainless Steel Austenitic grades (304, 316) in specialty applications:

• Generally excellent corrosion resistance
• Susceptible to crevice corrosion in chloride environments
• Compatible with all common inhibitor types
• May experience stress corrosion cracking in hot chloride service

Galvanic Compatibility

When dissimilar metals contact in conductive fluid, galvanic corrosion accelerates at the anodic (less noble) metal.

Galvanic Series in Inhibited Glycol (Anodic/least noble to cathodic/most noble)

1. Magnesium alloys
2. Aluminum alloys
3. Carbon steel
4. Cast iron
5. Lead
6. Tin
7. Brass (yellow)
8. Copper
9. Bronze
10. Stainless steel (300 series)

Design Mitigation Strategies

• Minimize surface area ratio between dissimilar metals (favor large anode)
• Use dielectric unions between dissimilar metals in threaded connections
• Specify inhibitor packages designed for mixed metallurgy
• Consider cathodic protection for critical components
• Maintain inhibitor levels above minimum thresholds

Elastomer and Seal Compatibility

Inhibitor chemistry affects long-term seal performance.

Compatible Elastomers

Incompatible Materials

• Natural rubber: swelling and degradation in glycol
• SBR (styrene-butadiene): poor glycol resistance
• Polyurethane: hydrolysis in alkaline inhibited fluids

Testing Requirements For critical applications, conduct compatibility testing per ASTM D471:

• Immerse seal material in actual coolant formulation
• Test at maximum operating temperature for 168 hours minimum
• Measure volume change, hardness change, tensile strength retention
• Acceptance: <10% volume change, <10% property degradation

pH Buffering and Control

Buffer Chemistry

pH buffering agents maintain system alkalinity and prevent acid formation from glycol oxidation.

Common Buffering Compounds

• Borax (sodium tetraborate): 200-500 ppm
• Sodium hydroxide: pH adjustment only, no buffering capacity
• Potassium hydroxide: similar to sodium hydroxide
• Phosphates: effective but risk of precipitation

Reserve Alkalinity Measure of buffering capacity to neutralize acid formation:

Test method: Titrate sample with standardized acid to pH 4.0 endpoint Typical values: 5-15 ml of 0.1N HCl per 10 ml sample Low reserve alkalinity (<3 ml) indicates depleted buffering capacity

pH Control Strategies

Target pH Ranges by Inhibitor Type

pH Drift Causes and Corrections

Decreasing pH (acidification):

• Cause: glycol oxidation producing organic acids
• Indicator: increasing iron and copper levels
• Correction: add alkaline buffer, verify inhibitor levels, check for air ingress

Increasing pH (alkalinization):

• Cause: fresh inhibitor addition, makeup water contamination
• Indicator: sudden pH jump after service
• Correction: dilute with deionized water, verify proper dosing

Inhibitor Depletion Mechanisms

Chemical Consumption

Oxidative Depletion Nitrite inhibitors are consumed by oxidation reactions:

4NO₂⁻ + O₂ + 2H₂O → 4NO₃⁻ + 4OH⁻

Depletion rate factors:

• Temperature: doubles for every 20°F increase above 180°F
• Oxygen concentration: proportional to air ingress rate
• System volume-to-surface area ratio: smaller systems deplete faster

Biological Degradation Nitrite-reducing bacteria convert nitrite to nitrogen gas:

2NO₂⁻ + 3H₂ → N₂ + 2OH⁻ + 2H₂O

Prevention:

• Maintain nitrite concentration above 1500 ppm (bacteriostatic level)
• Use biocides in systems with fresh water makeup
• Minimize organic contamination sources

Physical Losses

System Leakage Inhibitor loss proportional to coolant volume loss:

Annual depletion (%) = (Makeup volume ÷ System volume) × 100

Example: 50 gallons makeup on 1000-gallon system = 5% inhibitor loss

Adsorption on Surfaces Initial system startup adsorbs inhibitors onto metal surfaces:

• Expect 10-20% concentration drop in first 90 days
• New systems require higher initial charge
• Stabilizes after protective films establish

Replenishment Procedures

Calculation Methods

Simple Dosing Approach Based on measured inhibitor level:

Replenishment volume = System volume × (Target % - Measured %) ÷ Concentrate %

Example:

• 2000-gallon system measures 1500 ppm nitrite
• Target concentration: 2500 ppm
• 40% inhibitor concentrate available

Volume needed = 2000 × (0.25% - 0.15%) ÷ 40% = 5 gallons

Proportional Addition Method Add small volumes and retest:

1. Calculate theoretical dose
2. Add 50% of calculated volume
3. Circulate system for 24 hours
4. Retest and adjust as needed

Addition Procedures

Best Practices

1. Add inhibitor concentrate to pump suction or expansion tank
2. Never add directly to hot equipment
3. Allow minimum 8 hours circulation before testing
4. Document date, quantity, and test results
5. Verify pH stability after addition

Precautions

• Add slowly to avoid local concentration spikes
• Ensure system is circulating during addition
• Wear appropriate PPE (gloves, eye protection)
• Prevent cross-contamination between inhibitor types

Design Considerations

New System Specifications

Initial Fill Recommendations

• Use deionized or softened water for mixing (TDS <100 ppm)
• Pre-charge inhibitors to upper end of recommended range
• Include 20-30% excess for initial adsorption losses
• Verify pH and inhibitor levels before commissioning

System Volume Calculation Account for all wetted components:

Total volume = Piping volume + Equipment volume + Expansion tank

Piping volume (gallons) = π × (ID/2)² × Length × 7.48 ÷ 144

Include:

• Chillers, heat exchangers, air handlers
• Coils, fan coils, terminal units
• Buffer tanks, thermal storage
• Expansion tanks (pressurized systems only include acceptance volume)

Makeup Water Quality

Maximum Contaminant Levels

Water Treatment Options

• Deionization for critical systems
• Reverse osmosis for large volumes
• Softening for moderate hardness
• Filtration to remove particulates

Standards and References

ASHRAE Guidelines

ASHRAE Handbook - HVAC Systems and Equipment

• Chapter on hydronic heating and cooling
• Inhibitor selection criteria
• Water quality requirements
• Testing and maintenance protocols

ASHRAE Standard 188 Legionellosis prevention in building water systems:

• Requires water management programs
• Documentation of chemical treatment
• Regular testing and monitoring
• Applies to some large chilled water systems

Industry Standards

ASTM D1384 Standard Test Method for Corrosion of Heat Transfer Coolants:

• Glassware corrosion test method
• 88 hours at 190°F ± 2°F
• Evaluates six common metals
• Pass/fail criteria for each metal

ASTM D3306/D4985 Automotive coolant specifications applicable to HVAC:

• Reserve alkalinity requirements
• Corrosion inhibitor performance
• Stability and compatibility testing

ASTM D2570 Simulated service corrosion testing:

• Cyclic temperature exposure
• More severe than D1384
• Better prediction of field performance

Regulatory Considerations

Environmental Compliance

• Disposal regulations for glycol and inhibitors vary by jurisdiction
• Some areas restrict nitrite discharge to sanitary sewers
• Molybdate may have discharge limitations
• OAT formulations generally have fewer restrictions

Safety Data Sheets Maintain SDS for all coolant products:

• Required by OSHA Hazard Communication Standard
• Includes composition, hazards, and emergency procedures
• Update when formulations change

Potable Water Protection

• Backflow prevention required on all makeup connections
• Reduced pressure zone (RPZ) devices for glycol systems
• Annual testing of backflow preventers
• Documentation per local plumbing codes

Best Practices Summary

Selection Criteria

Choose inhibitor type based on:

1. System metallurgy (ferrous, copper, aluminum content)
2. Operating temperature range
3. Expected service life and maintenance access
4. Regulatory constraints on disposal
5. Compatibility with existing infrastructure
6. Total lifecycle cost including testing and replenishment

Maintenance Protocol

Quarterly Tasks

• Visual inspection for leaks
• pH measurement
• Primary inhibitor concentration test
• System makeup volume documentation

Annual Tasks

• Comprehensive laboratory analysis
• Review and trend historical data
• Adjust inhibitor levels as needed
• Verify expansion tank pre-charge

Five-Year Tasks

• Consider complete coolant replacement
• System flush if contamination is severe
• Evaluate upgrade to extended-life inhibitors
• Update documentation and procedures

Troubleshooting Guide

Low Inhibitor Concentration

• Check for system leaks and repair
• Verify makeup water quality
• Test for biological activity
• Calculate and add replenishment dose

High Metal Levels

• Identify corrosion source (iron vs. copper)
• Verify pH is in acceptable range
• Ensure inhibitor concentration is adequate
• Check for galvanic couples
• Consider system flush if levels are extreme

pH Instability

• Test reserve alkalinity
• Check for water contamination
• Verify glycol concentration
• Look for signs of oxidation or degradation
• Add buffering agents as needed

Foam or Deposits

• Verify inhibitor concentration is not excessive
• Check for contamination (oils, dirt)
• Test water hardness
• Consider system filtration or partial drain-and-fill

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This comprehensive technical document provides HVAC professionals with detailed guidance on corrosion inhibitor selection, application, testing, and maintenance for secondary coolant systems. The content emphasizes electrochemical principles, practical calculations, and compliance with industry standards.