Scale & Corrosion Inhibitors

Scale and corrosion are the two most common and costly problems in industrial water systems. They often occur simultaneously, and left unmanaged, they degrade heat transfer efficiency, accelerate equipment failure, and increaseenergy and maintenance costs. Understanding how they form, and how inhibitors work, is the first step toward controlling them.

What Is Scale and How Does It Form?

Scale is a mineral deposit that builds up on heat transfer surfaces inside boilers, cooling towers, heat exchangers, and associated piping. It forms when dissolved minerals, primarily calcium carbonate, calcium sulfate, magnesium silicate, and silica, exceed their solubility limits and precipitate out of solution.

The conditions that accelerate scale formation include:

• High temperatures, which reduce the solubility of many minerals
• High cycles of concentration, which increase dissolved solids in recirculating water
• High pH (alkalinity), particularly relevant to silica scaling
• Slow flow rates, which reduce turbulence and allow deposits to settle

Even a thin layer of scale has a large impact on system performance. Scale has very low thermal conductivity, roughly 20 to 40 times lower than steel, so an 1/8-inch deposit can reduce heat transfer efficiency by 20 to 25 percent, forcing equipment to work harder to achieve the same output.

What Is Corrosion and How Does It Occur?

Corrosion is the electrochemical degradation of metal surfaces by water and its dissolved constituents. In industrial water systems, the primary corrosion mechanisms are:

• Oxygen pitting: Dissolved oxygen attacks metal surfaces, creating deep, localized pits that can penetrate pipe walls and vessel surfaces
• Carbonic acid attack: Carbon dioxide dissolves in water to form carbonic acid, which lowers pH and aggressively corrodes steel, copper, and other metals
• Galvanic corrosion: Dissimilar metals in contact with each other and water create an electrochemical cell, where the less noble metal corrodes preferentially
• Under-deposit corrosion: Scale and biological deposits create oxygen-depleted zones beneath them, accelerating localized corrosion

Corrosion produces iron oxide (rust) and other byproducts that foul heat exchangers, plug spray nozzles, and deposit in boilers, creating a cycle where corrosion feeds scale and scale feeds corrosion.

How Scale Inhibitors Work

Scale inhibitors interrupt mineral precipitation through several mechanisms depending on their chemistry:

Threshold Inhibition

Certain phosphonate and polymer compounds prevent scale from forming even when they are present in concentrations far below what would be needed to chemically react with the minerals present. A small amount of inhibitor adsorbs onto crystal nuclei and distorts their growth, preventing them from reaching a critical size and precipitating. This is called the threshold effect.

Crystal Modification

Some inhibitors allow crystals to form but alter their shape, making them irregular and unable to adhere to surfaces. These modified crystals remain dispersed in the water and are carried out of the system rather than depositing on equipment.

Dispersion

Polymer dispersants adsorb onto mineral particles and impart a negative surface charge, causing particles to repel each other and remain suspended in the water column. This is particularly effective for iron oxide and silt that enters the system from corrosion or makeup water.

Common scale inhibitor chemistries include polyphosphates, phosphonates (such as HEDP and PBTC), polyacrylates, and maleic copolymers. Each has different performance characteristics across temperature ranges, pH, and specific mineral types, program selection should match the chemistry of your makeup water and the operating conditions of your system.

How Corrosion Inhibitors Work

Corrosion inhibitors function by forming a protective barrier on metal surfaces, either as a physical film or a passivating chemical layer, that separates the metal from the corrosive water.

Anodic Inhibitors

These compounds, including molybdates and orthophosphates, react at the anodic sites on the metal surface to form a stable, insoluble oxide film. They are effective at relatively low concentrations but carry a risk: if concentration drops below a critical threshold in the presence of oxygen and chlorides, they can actually accelerate localized corrosion rather than prevent it. Careful monitoring is required.

Cathodic Inhibitors

Cathodic inhibitors interfere with the reduction reactions at cathodic sites, reducing corrosion current. Zinc salts are a classic example, precipitating as zinc hydroxide at cathodic sites to form a protective film.

Film-Forming Inhibitors

Certain organic inhibitors, particularly azoles such as benzotriazole (BZT) and tolyltriazole (TTA), form a monomolecular film on copper and copper alloy surfaces. These are standard components of any cooling water program that includes copper heat exchangers or brass components.

Mixed Inhibitors

Most industrial programs use blended inhibitor packages that target both anodic and cathodic reactions simultaneously, providing broader protection across mixed-metallurgy systems.

Program Design Considerations

Effective inhibitor programs are not one-size-fits-all. Key variables that shape program selection include:

• Makeup water chemistry (hardness, alkalinity, chlorides, sulfates, silica)
• System metallurgy (all-steel, mixed copper/steel, stainless, admiralty brass)
• Operating temperatures and pressures
• Cycles of concentration
• Regulatory constraints on discharge chemistry

The Langelier Saturation Index (LSI) and Ryznar Stability Index are commonly used tools to characterize the scaling or corrosion tendency of a water sample. A well-designed program controls both indices within target ranges to balance scale prevention and corrosion protection.

Common Questions

Can I run my system without scale and corrosion inhibitors?

Technically yes, some facilities rely entirely on water softening or other pretreatment to manage mineral content. But most systems still need corrosion inhibitors, and any gap in pretreatment makes inhibitors the last line of defense. The cost of inhibitor chemistry is almost always a fraction of the cost of a heat exchanger replacement or a boiler retubing.

How do I know if my inhibitor program is working?

The most direct indicators are corrosion coupon data (measured metal loss rates), deposit weights from periodic tube inspections, and regular water analysis. Inhibitor residual testing confirms that active chemistry is present in the system at the correct concentration.

Are scale and corrosion inhibitors safe to discharge?

This depends on the specific chemistry and local discharge regulations. Phosphorus-based programs are regulated in many jurisdictions due to eutrophication concerns in receiving water bodies. Molybdate programs face scrutiny in some areas. A compliant program requires a clear understanding of your permit conditions and chemistry selection that supports them.