Corrosion and Scale Inhibitors for Heat Exchangers: Selection and Optimization

The Cost of Inaction

An unscheduled heat exchanger failure is not just the cost of a new unit. It means production stoppage, emergency replacement, product losses, and penalties from contractors. According to AMPP (formerly NACE International), a single heat exchanger failure incident costs an enterprise $10,000–100,000 – depending on the equipment type, production scale, and downtime duration.

Meanwhile, 85% of heat exchanger failures are related to two reasons: deposits (scale) and corrosion. Both problems are manageable. A correctly selected chemical program prevents them entirely or minimizes them, extending equipment life from 5–7 to 15–20 years.

Two Enemies of the Heat Exchanger

Scale: Unwanted Thermal Insulation

Salts of calcium and magnesium dissolved in water convert to an insoluble form when heated and precipitate on heat transfer surfaces. Three main types of deposits:

Calcium carbonate (CaCO₃) — the most common type. Precipitates intensively at temperatures >60°C. Dissolves in acids, making it relatively easy to remove by chemical cleaning. Forms in water with temporary (carbonate) hardness >4 mg-eq/l.

Calcium sulfate (CaSO₄) — forms at >95°C and is practically insoluble in acids. Removal requires special reagents based on EDTA or converters that transform gypsum into a soluble form. Occurs in systems with high sulfate hardness or when using sulfuric acid acidification.

Silicate deposits (SiO₂) — the most dangerous type. Form when silicon concentration >120 mg/l and are almost resistant to chemical dissolution. Mechanical removal damages the surface. Prevention is the only effective approach.

The thermal conductivity of scale is 20–40 times lower than that of steel. A CaCO₃ layer only 1 mm thick increases energy consumption by 7–10%. At 3 mm — by 25–30%. At 5 mm — metal wall overheating reaches critical values: deformation, microcracks, emergency shutdown. For a 10 MW boiler, even 1 mm of scale costs $15,000–25,000 in annual additional fuel expenses.

Corrosion: Metal Loss

Heat exchanger corrosion has several mechanisms, each requiring a specific approach:

Oxygen corrosion. Dissolved oxygen (O₂ > 0.02 mg/l) causes pitting corrosion — localized depressions that eat through the wall. The most dangerous type: pitting 2–3 mm deep with a total wall thickness of 4 mm means a leak. Rate — up to 1.5 mm/year in hot water without treatment.

Carbonic acid corrosion. CO₂ dissolves in water, forming carbonic acid (H₂CO₃). pH drops to 5.5–6.5, and carbon steel corrodes at a rate of 0.3–1.0 mm/year. Typical for condensate lines and boiler return circuits.

Galvanic corrosion. Occurs in bimetallic systems — for example, copper heat exchanger tubes and steel pipelines. The less noble metal (steel) corrodes at an accelerated rate. Must be considered when selecting inhibitors: protecting one metal should not enhance the corrosion of another.

Under-deposit corrosion. Under a layer of scale or biofouling, an area with oxygen deficiency and reduced pH is created. Under-deposit corrosion is 5–10 times more intense than on a clean surface. Therefore, anti-scale and anti-corrosion programs are inextricably linked.

For a general overview of industrial water problems and reagent classification, see the article “Reagents for Industrial Water Treatment”.

Scale Protection: Scale Inhibitors

Phosphonates: Threshold Effect

Phosphonate inhibitors are the basis of anti-scale programs. The most common active substances:

• HEDP (1-hydroxyethylidene diphosphonic acid) — effective against CaCO₃ at temperatures up to 95°C. Chlorine-stable, which is important for chlorinated systems. Dosing: 5–15 mg/l.
• ATMP (aminotrimethylene phosphonic acid) — works at higher temperatures (up to 110°C). Effective against sulfate deposits. Dosing: 5–20 mg/l.
• PBTC (phosphonobutane tricarboxylic acid) — best thermal stability among phosphonates (up to 130°C). Resistant to oxidizing biocides.

The mechanism of action is the threshold effect: phosphonate in a substoichiometric concentration (tens of times less than the amount of salts in water) blocks the active sites of crystal growth. CaCO₃ nuclei cannot grow to a critical size and are carried away by the flow.

Polymeric Dispersants: Crystal Modification

Polymers work in parallel with phosphonates, providing a second line of defense:

• Polymaleates — modify the crystal structure of CaCO₃, converting dense calcite into loose vaterite that does not adhere to surfaces. Dosing: 3–10 mg/l.
• Polyacrylates — disperse microcrystals in the water volume, preventing their agglomeration and precipitation. Effective at hardness >10 mg-eq/l. Dosing: 5–15 mg/l.
• Copolymers (maleic + acrylic acid, or with the addition of sulfonated monomers) — combine both mechanisms. Better efficiency at high temperatures and increased mineralization.

Combined Programs

In practice, an anti-scale program is always combined: phosphonate (threshold effect) + polymer (dispersion) + pH adjustment. Typical dosing depends on the quality of the source water:

For recirculating cooling systems with a concentration factor of 3–5x, dosing increases proportionally.

Corrosion Protection: Inhibitor Programs

Molybdate Programs

Sodium molybdate (Na₂MoO₄) is a modern standard for corrosion inhibition in closed heating systems and HVAC circuits. Advantages: low toxicity (unlike chromates, which are prohibited), stability over a wide pH range (7.0–10.0), effective protection of carbon steel and cast iron.

Dosing: 50–200 mg/l as Mo (depending on water aggressiveness). Ensures a corrosion rate of < 0.05 mm/year under optimal conditions. Often combined with sodium nitrite (NaNO₂) for a synergistic effect — so-called molybdate-nitrite inhibition.

Phosphate-Zinc Programs

Traditional approach for open cooling systems: orthophosphate (PO₄³⁻) forms a protective iron phosphate film on the steel surface, zinc (Zn²⁺) provides cathodic protection. Effective dosing: 5–15 mg/l PO₄, 1–3 mg/l Zn.

Limitations: zinc is classified as a pollutant in discharge waters (MCL 0.5–1.0 mg/l), which creates problems with environmental regulations. Many enterprises are switching to zinc-free programs. Phosphate deposits are possible if pH > 8.5 is exceeded or overdosed.

Organic Film-Forming Inhibitors

New generation polymeric inhibitors — an alternative to phosphate-zinc programs. Form a monomolecular film on the metal surface without the risk of secondary deposits. Effective for multi-metallic systems (steel + copper + aluminum). Main classes: hydroxyphosphonoacetic acid (HPA), polyhydroxystearic acid (PHSA), tolyltriazole (TTA) for copper alloys.

Protection of Different Metals

There is no universal inhibitor for all metals. Each metal requires its own approach:

• Carbon steel — molybdates, phosphates, nitrites, polymeric inhibitors. Optimal pH: 9.0–10.5 for closed systems, 7.5–9.0 for open systems.
• Copper alloys — azoles (benzotriazole BTA, tolyltriazole TTA). Dosing 2–5 mg/l. Without azoles, copper dissolves and redeposits on steel surfaces, causing galvanic corrosion.
• Aluminum — narrow safe pH range (6.5–8.5). Corrodes in both acidic and alkaline environments. Silicates (10–20 mg/l SiO₂) are the most effective inhibitor.

More details on the mechanisms of action and classification of corrosion inhibitors can be found in the article “Corrosion Inhibitors for Oil and Gas Pipelines”.

Chemical Cleaning of Fouled Heat Exchangers

Even with an ideal chemical program, periodic cleaning is necessary. The question is how often and with what to clean.

Acid Cleaning

The main method for removing carbonate deposits. The choice of acid depends on the type of scale and the heat exchanger material:

• Hydrochloric acid (HCl, 3–10%) — rapid dissolution of CaCO₃, but aggressive to metal. Mandatory with an acid corrosion inhibitor (0.3–0.5%). Do not use for stainless steel — risk of chloride stress corrosion cracking.
• Citric acid (3–5%) — a milder alternative. Safe for stainless steel and copper alloys. Dissolves CaCO₃ and iron oxides. Forms soluble chelate complexes with Ca²⁺ and Fe³⁺ ions.
• EDTA (disodium salt, 3–5%) — most effective for mixed deposits (carbonate + silicate + iron oxides). Works at pH 4–10 depending on the target ion. More expensive, but safer for equipment.

Duration of acid cleaning: 4–8 hours at 50–70°C with circulation. Control: pH of the solution, concentration of Ca²⁺ and Fe²⁺ in the cleaning fluid. Cleaning is complete when the ion concentration stabilizes.

Alkaline Cleaning

For removing biological fouling, oil contamination, and silicate deposits. Typical composition: NaOH (1–3%) + surfactant (0.1–0.3%) + dispersant. Temperature: 60–80°C, duration: 4–12 hours.

For recirculating cooling systems, where biological fouling is a key problem, alkaline cleaning is combined with shock dosing of biocide.

Passivation After Cleaning

A critical step that is often ignored. After acid cleaning, the metal surface is activated — the corrosion rate is maximal in the first hours. Passivation restores the protective oxide layer:

• Sodium nitrite (NaNO₂, 0.5–1.0%) at pH 9.0–10.0, exposure 2–4 hours — for carbon steel.
• Citric acid with ammonia (pH 3.5–4.0, 80–90°C, 2 hours) — forms a protective iron citrate film on steel surfaces.

Without passivation, the corrosion rate after cleaning can exceed 2–3 mm/year during the first weeks — even with a standard inhibitor present.

Monitoring: Keeping a Finger on the Pulse

A chemical program without monitoring is money wasted. Three levels of control:

Corrosion coupons. Weighted metal samples are installed in the flow for 30–90 days, then removed and weighed. The mass difference gives the average corrosion rate. The method is simple, inexpensive, but provides results with a delay. Standard — ASTM D2688, NACE SP0775.

Online probes (LPR, ER). Linear polarization resistance (LPR) and electrical resistance (ER) probes measure corrosion in real time. LPR gives the instantaneous corrosion rate, ER — cumulative metal loss. Equipment cost: $2,000–10,000, but pays for itself with one prevented incident.

Water analysis. Regular control of key parameters:

Definitions of terms (pitting corrosion, threshold effect, concentration factor) — in the glossary of industrial chemistry.

FAQ

Which anti-scale agent should I choose for a boiler house with hard water?

For water with hardness >7 mg-eq/l, a combined program is recommended: phosphonate ATMP or PBTC (10–20 mg/l) + polymeric dispersant based on a maleic and acrylic acid copolymer (8–15 mg/l). At feedwater temperatures >95°C, choose PBTC — it is more thermally stable than HEDP and ATMP.

How often should a heat exchanger be cleaned?

The frequency depends on water quality and the effectiveness of the chemical program. With proper inhibition — once every 2–3 years. Without chemical treatment in hard water (>7 mg-eq/l), cleaning may be needed every 6–12 months. Indicators of necessity: pressure drop increase >20%, heat transfer reduction >10%, increase in boiler flue gas temperature.

Can one inhibitor be used for steel and copper simultaneously?

No. Inhibitors for carbon steel (molybdates, nitrites) do not protect copper alloys. For bimetallic systems, a comprehensive program is required: molybdate or phosphate for steel + azole (benzotriazole or tolyltriazole, 2–5 mg/l) for copper. Without azole, copper dissolves and redeposits on steel surfaces, accelerating galvanic corrosion.

What is passivation and why is it needed after cleaning?

Passivation is the formation of a thin protective oxide layer on the metal surface after acid cleaning. Without passivation, "bare" metal corrodes at a rate of 2–3 mm/year during the first weeks. Standard procedure: treatment with sodium nitrite solution (0.5–1.0%) at pH 9–10 for 2–4 hours. Passivation is mandatory — skipping this step negates the effect of cleaning.

SVK Experience

At one of Dnipro's heating enterprises, we reduced the corrosion rate from 0.8 to 0.05 mm/year simply by changing the inhibitor program — without replacing equipment. Coupon tests according to NACE SP0775 confirmed the result after 90 days.

SVK develops comprehensive heat exchanger protection programs for boiler houses, HVAC systems, and industrial cooling. The approach begins with source water analysis in our laboratory — we determine hardness, alkalinity, chloride, sulfate, silicon, and dissolved gas content to select the optimal inhibitor combination.

Test Drive Program: receive samples of anti-scale and corrosion inhibitor for testing on your equipment for 30 days. We provide technical support — from dosing setup to interpreting monitoring results. Ask for a consultation — we will select a solution tailored to your system parameters.

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Read also:

• Reagents for Industrial Water Treatment
• Corrosion Inhibitors for Oil and Gas Pipelines
• Biocides for Recirculating Cooling Systems
• Glossary of Industrial Chemistry: 30 Terms