A shell-and-tube exchanger specified for a decade starts weeping at year three. The cooling water passed its hardness check and the tower looked clean, yet the mild-steel tubes are pitted through and the admiralty brass has gone copper-red at the inlets. Corrosion is the quiet line item on a cooling system. It does not announce itself until a tube fails, a pump seizes, or a heat exchanger loses duty, and by then the repair is a shutdown rather than a dosing change. The lever that prevents most of it is a corrosion inhibitor matched to your metals, your water, and the rest of the treatment program.
The short version: A corrosion inhibitor is a chemical dosed into water to slow metal loss by maintaining a thin protective film on the metal surface. Inhibitors fall into three groups by how they interrupt the corrosion cell: anodic (passivating) film-formers such as molybdate and the legacy chromate and nitrite; cathodic (precipitating) film-formers such as zinc; and mixed / adsorption film-formers, the azoles that protect copper and brass. Yellow metals are protected by azoles: tolyltriazole (TTA), benzotriazole (BTA), or mercaptobenzothiazole (MBT). Mild steel is protected by molybdate, phosphonates, and zinc, almost always blended, and phosphonates such as HEDP and PBTC double as scale and corrosion inhibitors. Real programs are multi-component blends tuned to the water and the metallurgy, verified with corrosion coupons.
How a corrosion inhibitor actually works
Aqueous corrosion is electrochemical: metal dissolves at anodic sites (iron goes to Fe²⁺) while a reduction reaction, usually oxygen reduction, consumes the electrons at cathodic sites. The two half-reactions are coupled, so an inhibitor only has to interfere with one of them to slow the whole cell, and it does that by maintaining a film. The Veolia cooling-water corrosion handbook classifies inhibitors by which half-reaction they suppress.
Anodic (passivating) inhibitors reinforce an oxide film over the anodic areas and stall metal dissolution, protecting at economical concentrations with a tenacious film. They carry a real trade-off: anodic inhibitors are sometimes called “dangerous” because underdosing can leave bare anodic spots while the cathode keeps running, concentrating attack into deep pits. Molybdate is the modern example; chromate and nitrite are the legacy ones.
Cathodic (precipitating) inhibitors form an insoluble film over the cathodic areas, limiting the oxygen and water reaching the surface. Zinc is the common one, depositing a hydroxide film where pH rises at the cathode. Cathodic inhibitors do not drive pitting when underdosed, but their protection ceiling is lower, so they are usually paired with other chemistries.
Mixed / adsorption film-formers chemisorb across the whole surface and damp both half-reactions. The azoles sit here, bonding to copper and building a thin protective layer over the entire metal face.
Yellow-metal corrosion inhibitors: the azoles
“Yellow metals” means copper and its alloys: brass, admiralty brass, bronze, and the copper-nickel tubing common in condensers. Azoles are the standard protection. They chemisorb onto the metal and coordinate with copper to build a thin, polymer-like film that blocks aggressive ions like chloride and sulfate and stops copper from dissolving. Copper release is not only a yellow-metal problem: dissolved copper plates onto mild steel and aluminum elsewhere in the loop and sets up galvanic pitting there, so a few ppm of azole on the brass quietly protects the steel too. Three azoles do most of the work:
- Tolyltriazole (TTA). Tolyltriazole / methylbenzotriazole is the most widely used copper-alloy inhibitor in cooling water, with better thermal stability and oxidizing-biocide resistance (chlorine, bromine) than benzotriazole, so it dominates open towers that run an oxidizer.
- Benzotriazole (BTA). Benzotriazole is the original copper film-former and a strong one, but it is more readily consumed by oxidizing biocides than TTA, so it sees heavy use in closed and oxidizer-free loops.
- Mercaptobenzothiazole (MBT). 2-mercaptobenzothiazole films through sulfur and nitrogen chelation with copper and acts quickly, so it is often paired with TTA to passivate fresh copper fast. It is less chlorine-stable than TTA, so it works as a partner, not the sole azole.
Dosed at a few ppm of active, azoles are tracked by a residual test that keeps the copper film topped up rather than depleting.
Mild-steel corrosion inhibitors: molybdate, phosphonate, zinc
Mild (carbon) steel is the bulk metallurgy in most cooling systems and the metal most programs are built around. Three chemistries carry it, usually together.
Molybdate. Sodium molybdate is an anodic passivator that adsorbs onto the iron-oxide layer at anodic sites much as chromate once did, but without hexavalent chromium. The molybdate mechanism literature notes it needs dissolved oxygen or another oxidizer to passivate the steel, and because it carries no oxidizing character of its own it blends readily with organics. It is effective but not cheap, so it often runs at modest levels alongside an azole and a phosphonate.
Zinc. Zinc is a cathodic inhibitor that lays down a hydroxide film at cathodic sites and performs in aggressive water (low hardness, low alkalinity, high chloride and sulfate) where hardness-dependent film-formers struggle. Discharge limits on zinc usually cap the dose, so it acts as a booster within a blend, not a standalone.
Phosphonates. The phosphonates do double duty as scale and corrosion inhibitors, which is why they appear in nearly every modern program. HEDP (etidronic acid) is a general-purpose threshold inhibitor that sequesters hardness ions and helps form protective films on steel. PBTC (phosphonobutane-tricarboxylic acid) holds up better under high-stress water (high calcium, high temperature, oxidizing biocide), where HEDP can be degraded. In a corrosion role the phosphonates are typically combined with zinc or inorganic phosphate to build the cathodic film. Their scale-control side is covered in our scale inhibitors and antiscalants guide.
Selecting an inhibitor by metal and role
There is no single corrosion inhibitor for a mixed-metallurgy system. You select by the metal you are protecting and the role the chemical plays in the cell.
| Target metal / role | Inhibitor chemistry | Mechanism class | Notes |
|---|---|---|---|
| Copper / brass / admiralty / bronze (yellow metals) | Tolyltriazole (TTA), benzotriazole (BTA), mercaptobenzothiazole (MBT) | Mixed / adsorption film-former | TTA most chlorine- and heat-stable; BTA for closed/oxidizer-free loops; MBT for fast passivation of fresh copper, usually with TTA |
| Mild / carbon steel | Sodium molybdate | Anodic (passivating) | Needs dissolved oxygen; replaces chromate without Cr(VI); often run at modest dose in a blend |
| Mild / carbon steel (aggressive, low-hardness water) | Zinc | Cathodic (precipitating) | Holds up in high-chloride / low-alkalinity water; discharge-limited, used as a booster |
| Mild steel + scale control together | HEDP, PBTC (phosphonates) | Threshold / film-former (dual scale + corrosion) | PBTC for high-stress / oxidizer conditions; HEDP general-purpose; paired with zinc or phosphate for the corrosion film |
| Whole mixed-metal system | Blend: azole + molybdate / phosphonate / zinc + dispersant polymer | Mixed program | The real-world answer; balanced to water chemistry and cycles |
The pattern: azoles for the yellow metals, an anodic or cathodic inhibitor for the carbon steel, a phosphonate for scale and the steel film, then a dispersant and a biocide around them.
The legacy chemistries: chromate and nitrite
Older cooling programs leaned on chromate and nitrite, and you still meet both in the field, so it helps to know why the industry moved.
Chromate delivered excellent, low-cost anodic control for decades and set the benchmark molybdate is still measured against. It also introduced hexavalent chromium, Cr(VI), a recognized toxicant, into the water and blowdown. On toxicity and disposal grounds it has been largely phased out of open cooling systems; the water-treatment literature describes a near-total move away from chromate after the toxicity of Cr(VI) was recognized (Veolia handbook, above). Confirm the current regulatory status for your jurisdiction before relying on any chromium chemistry.
Nitrite is an effective anodic inhibitor for ferrous metals and still appears in some closed loops, often borate-buffered. Its weakness in open systems is biological: nitrifying bacteria oxidize nitrite to nitrate, which both consumes the inhibitor and feeds microbial growth, so an open-system nitrite program needs a biocide and close monitoring. Many open recirculating systems now run molybdate, phosphonate, and zinc blends with an azole instead.
This is a neutral summary of why the chemistries shifted, not a regulatory determination; treat the legacy-versus-modern decision as site-specific and confirm it against current rules and your discharge permit.
Dosing and monitoring: prove it with coupons
An inhibitor program is only as good as the film it maintains, and the film depends on holding the right residual at the metal surface. Two things keep a program honest.
Residual control. Each active is dosed to a target and verified by its own test: azole residual for the copper film, molybdate and phosphonate levels for the steel side. If a feed pump drifts or cycles swing, the film thins and corrosion accelerates, so feed control and routine testing matter as much as product choice.
Corrosion coupons. Pre-weighed coupons of your system metals (typically mild steel and copper) are mounted in a bypass rack, exposed to the flowing water for a set period (commonly 30 to 90 days), then cleaned, reweighed, and reported as a corrosion rate in mils per year (MPY). As general field benchmarks, mild-steel rates under about 3 MPY are usually considered acceptable and under 1 MPY good, while copper-alloy rates under about 0.5 MPY are acceptable and under 0.2 MPY good. Watch for pitting as well as the average rate: localized pitting can fail a tube quickly even when overall MPY reads low, the failure mode an underdosed anodic inhibitor produces.
The practical recommendation: run a coupon rack with both steel and copper, hold inhibitor residuals to target with reliable feed control, and review the program whenever a coupon comes back above your MPY threshold or shows pitting.
Real programs are blends, not a single chemical
A cooling or process-water treatment is not one inhibitor. A typical open-recirculating program runs an azole for the yellow metals, one or two steel inhibitors (molybdate, phosphonate, zinc), a dispersant polymer, and a biocide, all balanced to the pH, hardness, alkalinity, chloride, temperature, and cycles the system runs at. The right choice depends on the metallurgy, the water chemistry, and the rest of the program; the chemistries here are the building blocks, but the program is the engineering, confirmed by dosing and coupons on your own system.
Sourcing corrosion inhibitors
RawSource supplies the corrosion-inhibitor building blocks for water-treatment formulators and end users: yellow-metal azoles (TTA, BTA, MBT), sodium molybdate for the steel side, and the dual-function phosphonates (HEDP and PBTC), in drums, IBCs, and bulk with CoA documentation. Tell us your metallurgy, makeup-water chemistry (hardness, chloride, alkalinity), cycles, and biocide regime, and request a sample to qualify performance on your own loop. For program context, see our cooling tower water treatment guide and the broader water treatment chemicals guide.
Frequently asked questions
What is a corrosion inhibitor?
A corrosion inhibitor is a chemical dosed into water in small concentrations to slow the rate at which metal corrodes, by maintaining a thin protective film on the metal surface. In cooling and process water it interrupts the electrochemical corrosion cell at the anode, the cathode, or both, so the metal loses far less material over time.
What is the best corrosion inhibitor for cooling water?
There is no single best one, because a cooling system usually contains more than one metal. The practical answer is a blend: an azole such as tolyltriazole (TTA) for copper and brass, plus a steel inhibitor (molybdate, a phosphonate, or zinc) for the mild steel, balanced to your water chemistry and cycles and confirmed with corrosion coupons.
What is a yellow metal corrosion inhibitor?
A yellow-metal corrosion inhibitor protects copper and its alloys: brass, admiralty brass, bronze, and copper-nickel. The standard chemistries are azoles, namely tolyltriazole (TTA), benzotriazole (BTA), and mercaptobenzothiazole (MBT). They chemisorb onto the copper surface and form a thin film that blocks chloride and sulfate attack and limits copper dissolution, which also protects steel elsewhere in the loop from galvanic pitting.
What is the difference between anodic and cathodic corrosion inhibitors?
Anodic (passivating) inhibitors form a film over the anodic sites where metal dissolves, and include molybdate and the legacy chromate and nitrite. Cathodic (precipitating) inhibitors form a film over the cathodic sites where oxygen is reduced, and include zinc. Anodic inhibitors protect strongly at economical doses but can drive localized pitting if underdosed; cathodic inhibitors avoid that pitting risk but offer lower overall protection, which is why program blends combine both.
Why have chromate and nitrite inhibitors been replaced in many systems?
Chromate gave excellent corrosion control but introduced hexavalent chromium (Cr(VI)), a recognized toxicant, and has been largely phased out of open cooling systems on toxicity and disposal grounds; confirm the current regulatory status for your site. Nitrite is still used in some closed loops but in open systems is oxidized to nitrate by nitrifying bacteria, which consumes the inhibitor and feeds microbial growth. Many open systems now use molybdate, phosphonate, and zinc blends with an azole instead, and verify performance with corrosion coupons reported in mils per year (MPY).
Editorial note. This article is general technical guidance for water-treatment, cooling-water, and process-water professionals. The right corrosion-inhibitor chemistry, dose, and residual target depend on your specific metallurgy, makeup-water chemistry, cycles of concentration, temperature, biocide regime, and discharge requirements, and must be validated on your own system, including with corrosion coupons; the Certificate of Analysis governs the grade you buy. Regulatory status for legacy chemistries such as chromate and nitrite varies by jurisdiction and application and must be confirmed against current rules and your discharge permit. Several of these chemicals are corrosive or otherwise hazardous; review the current Safety Data Sheet (SDS) and use appropriate PPE before handling. Products are sold for industrial and professional use only and are not offered for potable or drinking-water treatment unless separately specified and documented. Nothing here is a medical, health, safety, or environmental claim. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.
