What are the common types of boiler corrosion inhibitors?

Boiler corrosion inhibitors are chemical substances used to prevent or reduce corrosion in boiler systems. Here are the common types, categorized by their mechanisms and applications:

1. Oxygen Scavengers (Deoxygenators)

Function: Remove dissolved oxygen in boiler water, as oxygen is a primary cause of corrosion (e.g., pitting corrosion).
Common Types:

• Sodium Sulfite: Reacts with oxygen to form sodium sulfate. Suitable for low-to-medium pressure boilers.

• Reaction: $2N{a}_{2}S{O}_3+O_2\to 2N{a}_{2}S{O}_4$.

• Hydrazine: A powerful oxygen scavenger for high-pressure boilers. Also acts as a passivating agent (forms a protective iron oxide layer).

• Note: Toxic and requires careful handling; increasingly replaced by safer alternatives.

• Hydrogen Sulfite (Bisulfite) Salts: Used in systems where sulfite may increase total dissolved solids (TDS).

• Organic Oxygen Scavengers:

• Hydroxymethylcellulose (HMC) or ** carbohydrazide**: Safer alternatives to hydrazine, suitable for high-pressure boilers.

• Amino acids (e.g., morpholine): Act as both scavengers and pH adjusters.

2. pH Buffers (Alkalinity Maintainers)

Function: Raise and stabilize the pH of boiler water to prevent acidic corrosion (e.g., from carbon dioxide or dissolved salts).
Common Types:

• Sodium Hydroxide (Caustic Soda): Increases pH but must be controlled to avoid caustic embrittlement.

• Sodium Phosphate (e.g., trisodium phosphate): Maintains alkalinity and precipitates calcium/magnesium ions as non-adherent sludge (also acts as a scale inhibitor).

• Ammonia or Amines (e.g., morpholine, cyclohexylamine): Volatile bases that neutralize carbon dioxide in steam condensate systems.

3. Film-Forming Inhibitors

Function: Adsorb onto metal surfaces to form a protective molecular film, blocking corrosive agents (e.g., oxygen, water).
Common Types:

• Tannins: Organic compounds from plant extracts, forming a protective layer in low-pressure boilers.

• Azoles (e.g., benzotriazole, tolyltriazole): Effective for protecting copper and copper alloys (e.g., in condenser tubes).

• Sulfonic Acids or Amines: Organic inhibitors that form hydrophobic films on steel surfaces.

4. Scale Inhibitors (Dispersants)

Function: Prevent the formation of mineral scales (e.g., calcium carbonate, iron oxides), which can trap corrosive agents and promote localized corrosion.
Common Types:

• Polyphosphates (e.g., sodium hexametaphosphate): Sequester metal ions (e.g., Ca²⁺, Mg²⁺) to prevent precipitation.

• Polymers (e.g., acrylic polymers, polyacrylates): Disperse scale-forming particles to inhibit deposit formation.

• Phosphonates: Strong chelators for calcium and magnesium, effective in high-hardness waters.

5. Volatile Corrosion Inhibitors (VCIs)

Function: Released as vapors to protect steam and condensate systems, especially in idle or shutdown boilers.
Common Types:

• Diethylaminoethanol (DEAE): Neutralizes acidic condensate from carbon dioxide.

• Cyclohexylamine: Provides vapor-phase protection in steam lines and condensate return systems.

6. Mixed Inhibitor Formulations

Many commercial inhibitors are blends of the above types to address multiple corrosion mechanisms. For example:

• Oxygen scavenger + pH buffer + film former: A typical combination for comprehensive protection in industrial boilers.

• Antiscalant + corrosion inhibitor: Used in water with high mineral content.

Key Considerations for Selection

• Boiler Type: Low-pressure vs. high-pressure systems require different inhibitors (e.g., hydrazine for high-pressure, sulfite for low-pressure).

• Water Quality: Hardness, pH, dissolved gases (O₂, CO₂), and TDS levels influence inhibitor choice.

• Material Compatibility: Copper alloys require specific inhibitors (e.g., azoles), while carbon steel may use amines or phosphates.

• Environmental and Safety Standards: Toxicity (e.g., hydrazine) and disposal requirements limit some inhibitors in modern applications.

By targeting the primary corrosion mechanisms (oxygen attack, acidic conditions, scale formation), these inhibitors help extend boiler lifespan and improve operational efficiency.