Corrosion inhibitors for water treatment are 8 chemicals in 4 classes, from nitrite, molybdate, zinc and silicate salts to azoles and filming amines, that slow the corrosion of steel, copper and aluminum in cooling, closed-loop, condensate and distribution water. Untreated once-through cooling water can lose up to 100 mpy (2.5 mm/y) of metal, while treated water stays at 10 mpy (0.25 mm/y) or less, so which inhibitor protects which metal?
The 8 inhibitors are 4 inorganic salts (sodium nitrite, sodium molybdate, zinc salts, sodium silicate), 3 azoles (tolyltriazole, benzotriazole, mercaptobenzothiazole) and 1 filming amine (octadecylamine). They form the fifth of the 16 families of water treatment chemicals.
Doses run from 600 to 1,200 mg/L (ppm) of nitrite as NO2 in closed loops to about 1 mg/L of benzotriazole as active in cooling towers, and coupons prove the result.
Every value on this page is checked against primary sources (NSF listings, eCFR, AWWA, ASTM, EPA, IAPWS and the handbooks named in the text); see our methodology and fact-checking process.
What Is a Corrosion Inhibitor in Water Treatment?#
A corrosion inhibitor in water treatment is a chemical, such as sodium nitrite, sodium molybdate or tolyltriazole, dosed at a controlled residual so that a passive, precipitated or adsorbed film slows the anodic or cathodic reaction on the metal. It neither removes oxygen nor neutralizes carbonic acid; oxygen scavengers and neutralizing amines do that.
Vapor-phase, coating and concrete inhibitors belong to other fields, and acid cleaning uses its own inhibitors, compared under chemical cleaning acids and inhibitors.
Which metals and systems do corrosion inhibitors protect?#
Corrosion inhibitors protect 4 metal groups in water systems: carbon steel, copper and its alloys, aluminum and galvanized zinc, in cooling towers, closed loops, condensate lines and drinking-water mains. The 4 groups need different chemistries.
- Carbon steel: nitrite, molybdate, orthophosphate, zinc or silicate.
- Copper and brass: an azole such as tolyltriazole.
- Aluminum: pH below 9.0, ideally 7.8 to 8.3.
- Galvanized zinc: pH 6.5 to 8.0 during passivation.
How do corrosion inhibitors differ from scale inhibitors and phosphates?#
Corrosion inhibitors slow metal loss, while scale inhibitors keep mineral salts such as calcium carbonate in solution at sub-stoichiometric doses.
Phosphonates and polyphosphates do both, which is why combined scale and corrosion products are compared on scale inhibitors for water treatment.
Polyphosphates and phosphonates act as cathodic inhibitors only with calcium present (Suez water handbook), and in drinking water polyphosphates alone are not effective lead and copper inhibitors (US EPA).
Orthophosphate, zinc orthophosphate and blended phosphates, the main drinking-water corrosion chemicals, have their own family under phosphates in water treatment.
How Do Corrosion Inhibitors Work?#
Corrosion inhibitors work in 3 ways: anodic inhibitors build or stabilize a passive oxide film, cathodic inhibitors precipitate a film that blocks oxygen reduction, and adsorbing inhibitors such as azoles and filming amines bond a thin barrier to the metal.
Anodic inhibition and passivation#
Anodic inhibitors passivate the metal: oxidizing types such as nitrite build the oxide film themselves, while non-oxidizing types such as molybdate and orthophosphate need dissolved oxygen to do it (Suez water handbook). What happens when the dose falls short? Uncovered anodes pit.
Pitting and galvanic couples are explained under corrosion in water systems.
Cathodic inhibition and precipitation films#
Cathodic inhibitors precipitate at the cathode, where oxygen reduction raises the local pH 1 to 2 units above the bulk water, so zinc hydroxide and calcium phosphate deposit exactly where oxygen would be reduced (Veolia Water Technologies Handbook, chapter 24).
Adsorbed and filming barriers#
Adsorbing inhibitors bond a molecular film to the metal: azoles chemisorb on cuprous oxide on copper, and filming amines such as octadecylamine form a water-repelling layer on steel in steam and condensate. Sodium silicate builds its film over existing corrosion products within 2 to 3 weeks.
Why dose, pH, oxygen and deposits decide inhibitor performance#
An inhibitor protects only at its working residual, pH and oxygen level: molybdate needs at least 1 ppm dissolved oxygen when no nitrite is present, and deposits or biofilm keep every inhibitor away from the metal underneath. The 3 conditions are listed below.
- Residual: above the protective threshold, stated with its basis.
- Oxygen and pH: inside the chemistry's window, such as pH 7.0 to 9.0 for molybdate.
- Clean surface: no deposits or biofilm, whose anaerobic zones turn anodic (microbiologically influenced corrosion).
8 Types of Corrosion Inhibitors in 4 Classes#
The 8 corrosion inhibitors used in water treatment fall into 4 classes: 4 anodic, cathodic and mixed inorganic inhibitors, 3 azole copper inhibitors, 1 filming amine and the organic class, whose members are blend components or have pages in other families.
Table 1 compares the 4 classes by members, mechanism, metals and systems.
| Class | Members on this site | Mechanism | Main metals | Typical systems |
|---|---|---|---|---|
| anodic, cathodic and mixed corrosion inhibitors | sodium nitrite, sodium molybdate, zinc salts, sodium silicate (plus a chromate history section) | passive oxide (anodic); precipitated cathodic film; silica film | carbon steel | closed loops, cooling towers, drinking water |
| copper and yellow-metal corrosion inhibitors (azoles) | tolyltriazole, benzotriazole, mercaptobenzothiazole | chemisorbed film on Cu2O | copper, brass | cooling towers, closed loops |
| filming amines | octadecylamine (plus OLDA and oleylamine sections) | hydrophobic adsorbed film | carbon steel | steam and condensate |
| organic corrosion inhibitors for water systems | none counted (HPAA sits with the phosphonates; benzoate and carboxylate blends) | adsorbed film | carbon steel | closed loops, low-halogen cooling |
1. Anodic, cathodic and mixed inorganic inhibitors (4)#
Anodic, cathodic and mixed inorganic inhibitors are salts that passivate steel or precipitate a cathodic film; this class has 4 inhibitors: sodium nitrite, sodium molybdate, zinc salts and sodium silicate. Chromate once led this class at 400 to 600 mg/L as CrO4 (Suez water handbook) and is now banned in comfort cooling towers.
Sodium nitrite#
Sodium nitrite (NaNO2) is the oxidizing anodic inhibitor of closed loops, dosed at 600 to 1,200 mg/L (ppm) as NO2 for iron and steel and consumed by nitrifying and nitrite-reducing bacteria. In open systems it oxidizes to nitrate.
Sodium molybdate#
Sodium molybdate (Na2MoO4) is a non-oxidizing anodic inhibitor that needs dissolved oxygen and runs at 200 to 300 mg/L (ppm) as MoO4 in closed loops and about 3 mg/L as Mo in open cooling. It works synergistically with nitrite, zinc and phosphonates.
Zinc salts (zinc sulfate, zinc chloride)#
Zinc sulfate and zinc chloride are cathodic precipitating inhibitors, and alkaline cooling programs keep about 0.5 mg/L (ppm) of insoluble zinc above the soluble level at pH 7.6 or higher.
Sodium silicate#
Sodium silicate (water glass, SiO2:Na2O ratio typically 3.22) is a film-forming inorganic inhibitor that needs 2 to 3 weeks to protect and works in drinking water above 15 to 20 mg/L (ppm) as SiO2.
2. Copper and yellow-metal inhibitors: azoles (3)#
Copper and yellow-metal inhibitors are azoles and thiazoles that chemisorb on cuprous oxide; this class has 3 inhibitors: tolyltriazole, benzotriazole and mercaptobenzothiazole. Excess chlorine deactivates the film.
Tolyltriazole (TTA)#
Tolyltriazole (TTA) is the azole for copper heat exchangers, dosed at 10 to 50 mg/L (ppm) as sodium salt in closed loops and supplied as solid or 50 % sodium tolyltriazole solution.
Benzotriazole (BZT)#
Benzotriazole (BZT, C6H5N3) is the parent azole for copper and yellow metals, dosed at about 1 mg/L (ppm) as active in zinc-based cooling programs.
Mercaptobenzothiazole (MBT)#
Mercaptobenzothiazole (MBT, C7H5NS2) is a thiazole copper inhibitor, supplied mainly as a 50 % sodium salt solution for closed circuits and glycol and largely replaced by benzotriazole derivatives in cooling water.
3. Filming amines (1)#
Filming amines are fatty amines that adsorb on steel in steam and condensate as a hydrophobic film; this class has 1 entity page, octadecylamine, with OLDA and oleylamine as sections. IAPWS TGD8-16(2019) defines all 3.
Filming amines differ from the volatile amines that neutralize carbonic acid, compared under neutralizing amines for condensate treatment.
Octadecylamine (ODA)#
Octadecylamine (ODA, C18H39N) is the reference film-forming amine, fed to the steam header at 2 to 20 mg/L (ppm) in condensate circuits (Suez water handbook); long-term overdosing forms gel-like deposits (IAPWS).
4. Organic corrosion inhibitors (blend components, no entity page here)#
Organic corrosion inhibitors are carbon-based molecules, such as HPAA, benzoate and carboxylate blends, that adsorb on steel; none has an entity page in this family. All-organic cooling programs run at pH 8.7 to 9.2.
HPAA, dosed at 5 to 15 mg/L (ppm) as active in cooling water, degrades rapidly with chlorine and is described with the other phosphonate scale inhibitors.
Which Corrosion Inhibitor for Which System? Selection by Application and Metal#
Corrosion inhibitor selection depends on 3 factors: the metals in contact with the water, the system type (open, closed, steam or potable) and the oxidizing biocide or disinfectant in use, and corrosion coupons confirm the choice.
Table 2 lists the default inhibitors and one key number per system.
| System | Default inhibitors | Key number |
|---|---|---|
| corrosion control in cooling water systems | phosphate, zinc or molybdate plus an azole and a polymer | ≤ 1 mpy steel, ≤ 0.1 mpy copper rated excellent |
| white rust and galvanized steel passivation | phosphate passivation | zinc pH 6.5 to 8.0; 3 to 5 ppm as PO4 polyphosphate after 5 to 6 days |
| closed loop water treatment | nitrite, molybdate, azole blends | makeup < 5 % of system volume per year |
| chilled water system treatment | nitrite or molybdate plus an azole | chilled loops run below 22 °C (72 °F) |
| hot water heating system treatment | nitrite, molybdate, silicate | silicate only below 82 °C (180 °F) |
| glycol system treatment and inhibited glycol | inhibited glycol blends with an azole | minimum 25 % inhibited glycol |
| condensate system treatment and return-line corrosion | filming amines | feedwater iron 0.5 to 0.05 ppm in 2 months (combined treatment) |
| boiler corrosion | oxygen scavengers and amines (other families) | n/a |
| corrosion control treatment for lead and copper | orthophosphate, zinc orthophosphate, silicate | LCRI ≥ 0.5 or 1.0 mg/L as PO4 at the tap |
By metal: carbon steel, copper, aluminum and galvanized zinc#
Each metal sets its own inhibitor and pH window: carbon steel takes nitrite or molybdate at pH 9.0 to 10.5 in closed loops, copper needs an azole, aluminum needs pH below 9.0 and galvanized zinc needs pH 6.5 to 8.0 during passivation. Loops with aluminum carry nitrite up to 10,000 mg/L as NO2, and new galvanized zinc forms white rust above pH 8.2 to 8.3.
Open recirculating cooling towers#
Open cooling towers run combined programs, phosphate, zinc or molybdate for steel plus an azole for copper and a polymer for deposits, with alkaline phosphate programs at 2 to 10 ppm inorganic PO4 and pH 8.0 to 9.2 (Veolia Water Technologies Handbook, chapter 31).
Blowdown, cycles and biocides belong to the full program described under cooling tower water treatment.
Closed chilled-water, hot-water and glycol loops#
Closed loops hold the highest inhibitor residuals of any water system, typically nitrite, molybdate or both with an azole, because makeup stays below 5 % of the system volume per year and the dose stays in the loop. Chilled loops run below 22 °C (72 °F), and glycol loops hold at least 25 % inhibited glycol.
Boiler condensate and steam lines#
Corrosion inhibitors play a narrow role in boilers: filming amines fed to the steam header protect condensate lines, while dissolved oxygen and carbonic acid are handled by other chemical families.
Dose ratios per mg/L of oxygen by pressure are compared under oxygen scavengers for boiler water.
Drinking-water distribution: lead and copper corrosion control#
Drinking-water systems control lead and copper corrosion with orthophosphate, zinc orthophosphate or sodium silicate, and LCRI requires at least 0.5 or 1.0 mg/L orthophosphate as PO4 in tap samples where orthophosphate is the treatment (40 CFR 141.82(f)(4), 2024-10-30).
Corrosion control in water treatment is the State-set utility program that combines pH and alkalinity adjustment with an inhibitor; LCRI also sets a minimum distribution pH of 7.0.
Action levels (lead 0.010 mg/L from 2027-11-01, copper 1.3 mg/L) are explained under the Lead and Copper Rule (LCR, LCRR, LCRI).
Corrosion Inhibitor Dosage Ranges by Chemical#
Corrosion inhibitor doses span 3 orders of magnitude, from about 1 mg/L (ppm) of benzotriazole as active in a cooling tower to 1,200 mg/L of nitrite as NO2 in a closed loop, and every range carries its own basis.
Table 3 gives dose, basis, control test and condition per inhibitor and system (mg/L = ppm).
| Inhibitor | System | Dose or residual (mg/L) | Basis | Control test | pH or condition |
|---|---|---|---|---|---|
| Sodium nitrite | closed loop, iron and steel | 600 to 1,200 (about 900 to 1,800 as NaNO2) | as NO2 | ceric sulfate or permanganate titration, or kit | pH above 7.0; below 9.0 with aluminum |
| Sodium nitrite | closed loop, steel-copper couples; with aluminum | 5,000 to 7,000; up to 10,000 | as NO2 | same | same |
| Sodium nitrite | brine loop | about 2,000 | as NO2 | same | pH 7.0 to 8.5 |
| Sodium molybdate | closed loop | 200 to 300 (120 to 180 as Mo; about 258 to 386 as Na2MoO4) | as MoO4 | Hach Method 8036 | pH 7.0 to 9.0; not above 500 ppm calcium |
| Sodium molybdate | open cooling (inhibitor; tracer) | about 3; about 0.5 | as Mo | molybdate | with phosphate or zinc programs |
| Zinc salts | alkaline cooling | about 0.5 insoluble zinc above the soluble level | as Zn | total and filtered zinc | pH 7.6 or higher |
| Zinc salts | drinking water | product MUL 5 to 15 (sulfate), 6 to 15 (chloride) | as product | zinc | zinc ≤ 2.0 mg/L in finished water |
| Sodium silicate | drinking water | effective above 15 to 20; maintenance 4 to 8 | as SiO2 | silica | silica is a water quality parameter (40 CFR 141.87) |
| Sodium silicate | closed loop | 50 to 100 alone; 10 to 25 as aluminum or copper adjunct | as SiO2 | silica | below 82 °C (180 °F) |
| Tolyltriazole | closed loop | 10 to 50; 5 to 10 free azole in one owner specification | as sodium salt; free azole | free azole (UV) | pH 8.5 to 10.5 |
| Tolyltriazole | open cooling | minimum 1 free azole in one owner specification | as free azole | free azole (UV) | chlorination deactivates |
| Benzotriazole | open cooling, zinc programs | about 1 | as active | free azole | chlorination deactivates |
| Mercaptobenzothiazole | closed loops with copper, glycol | in inhibitor blends; no dose with a stated basis | n/a | n/a | largely replaced by benzotriazole derivatives |
| Octadecylamine | steam condensate circuits | 2 to 20 | as film-forming amine | film-forming amine test, mg/kg as ODA | ≤ 3 ppm in food-contact steam (21 CFR 173.310) |
| HPAA (phosphonate family) | open cooling | 5 to 15 | as active | Hach Method 8007, factor 1.64 | halogen sensitive |
| Orthophosphate (phosphates family) | drinking water at the tap | 1.0 to 3.0 (EPA reference, not a limit) | as PO4 | orthophosphate (EPA 365.1) | pH 7.2 to 7.8 |
Why does the dose basis matter: NO2 vs NaNO2, Mo vs MoO4?#
The same molybdate residual reads 120 mg/L as Mo, 200 mg/L as MoO4 or about 258 mg/L as Na2MoO4, so a target without its basis can be off by more than a factor of 2. The factors are Mo x 1.67 = MoO4 and Mo x 2.15 = Na2MoO4 (Veolia Water Technologies Handbook, chapter 32). Likewise 600 mg/L as NO2 equals about 900 mg/L as NaNO2.
Worked example: a molybdate dose for a closed loop#
Worked example: raising a 20 m³ (5,283 US gal) loop to 250 mg/L as MoO4 needs 5.0 kg (11.0 lb) of MoO4, which is 18.4 kg (40.5 lb) of 35 % sodium molybdate liquid containing 27.2 % MoO4. The inputs are listed below.
- Loop volume: 20 m³ (5,283 US gal), starting residual 0 mg/L.
- Target: 250 mg/L as MoO4, so 250 g/m³ x 20 m³ = 5,000 g of MoO4.
- Product: 35 % Na2MoO4 liquid = 27.2 % MoO4; 5.0 kg / 0.272 = 18.4 kg.
The chemical dosage calculator turns any mg/L target into kg, lb and pump settings.
How Is Corrosion Inhibitor Performance Controlled and Tested?#
Corrosion inhibitor performance is controlled in 2 layers: residual tests keep the inhibitor in range, and corrosion coupons or probes prove that metal loss stays below its target.
Corrosion coupons and corrosion-rate benchmarks#
Corrosion coupons measure inhibitor performance over about 90 days, and an open cooling system rates excellent at 1 mpy (0.025 mm/y) or less on carbon steel and 0.1 mpy (0.0025 mm/y) or less on copper alloys (AWT, Boffardi 2000). Exposures of 30 days or less read high, and ASTM D2688-23 is the current weight-loss method.
Racks need 0.9 to 1.5 m/s (3 to 5 ft/s); below 0.6 m/s (2 ft/s) corrosion and fouling accelerate (Chem-Aqua TB2-009).
Table 4 lists the AWT rating steps in mpy, with mm/y in parentheses.
| Step | Open: carbon steel | Open: copper alloys | Closed: carbon steel | Closed: copper alloys |
|---|---|---|---|---|
| 1 | excellent ≤ 1 (≤ 0.025) | excellent ≤ 0.1 (≤ 0.0025) | excellent ≤ 0.2 (≤ 0.0051) | excellent ≤ 0.1 (≤ 0.0025) |
| 2 | very good 1 to 3 (0.025 to 0.076) | very good 0.1 to 0.25 (0.0025 to 0.0064) | good 0.2 to 0.5 (0.0051 to 0.0127) | good 0.1 to 0.25 (0.0025 to 0.0064) |
| 3 | good 3 to 5 (0.076 to 0.127) | good 0.25 to 0.35 (0.0064 to 0.0089) | moderate 0.5 to 0.8 (0.0127 to 0.0203) | moderate 0.25 to 0.35 (0.0064 to 0.0089) |
| 4 | moderate to fair 5 to 8 (0.127 to 0.203) | moderate to fair 0.35 to 0.5 (0.0089 to 0.0127) | poor 0.8 to 1 (0.0203 to 0.0254) | poor 0.35 to 0.5 (0.0089 to 0.0127) |
| 5 | poor 8 to 10 (0.203 to 0.254) | poor 0.5 to 1 (0.0127 to 0.0254) | very poor to severe ≥ 1 (≥ 0.0254) | very poor to severe ≥ 0.5 (≥ 0.0127) |
| 6 | very poor to severe > 10 (> 0.254) | very poor to severe > 1 (> 0.0254) | n/a | n/a |
UK HSG274 Part 1 (2nd edition, 2024) sets carbon steel generally below 5 mpy (0.13 mm/y), preferably below 2 mpy (0.05 mm/y). Rack design is covered under corrosion coupons and corrosion monitoring.
How is a corrosion rate calculated in mpy and mm/y?#
A coupon corrosion rate in mpy equals 534 times the weight loss in mg divided by metal density in g/cm³, exposed area in square inches and exposure hours, and 1 mpy equals 0.0254 mm/y.
The formula assumes uniform corrosion, so reports also state alloy, exposure time and pitting.
The corrosion rate calculator converts coupon weight loss to mpy and mm/y.
Residual tests for each corrosion inhibitor#
Each corrosion inhibitor has its own residual test: nitrite by ceric sulfate or permanganate titration, molybdate by Hach Method 8036, azoles by a UV free-azole test, silicate as silica and zinc as total and filtered zinc. The 5 tests are listed below.
- Nitrite: titration or kit, reported as NaNO2, NO2 or NO2-N.
- Molybdate: reported as Mo, MoO4 or Na2MoO4.
- Azoles: UV test; online, ion chromatography at 214 nm.
- Silicate: silica by ASTM D859-16(2021)e1.
- Zinc: total and filtered.
Test frequencies are listed under closed loop water testing and control ranges.
Which Standards and Regulations Apply to Corrosion Inhibitors?#
Corrosion inhibitors are governed in 4 layers: NSF/ANSI/CAN 60 certification for drinking water, AWWA and EN product standards, limits on the inhibitor ions in drinking water and discharges, and the 21 CFR 173.310 list for steam.
NSF/ANSI/CAN 60 categories and maximum use levels for corrosion inhibitors#
NSF/ANSI/CAN 60 lists corrosion inhibitors in 7 chemical categories, from sodium silicate to blended corrosion inhibitors, and has no category for sodium nitrite, molybdate, azoles or filming amines, so none of these is certified for drinking water (NSF listings, 2026-10-20).
Maximum use levels are product-specific and stated as product: sodium silicate 10 to 353 mg/L (most frequent 42), blended corrosion inhibitors 10 to 80 mg/L, zinc sulfate 5 to 15 mg/L and zinc chloride 6 to 15 mg/L, with finished-water zinc capped at 2.0 mg/L.
How these levels are set is explained on NSF/ANSI/CAN 60.
AWWA and EN product standards for corrosion inhibitors#
Sodium silicate is the only inhibitor of this family with both an AWWA and an EN standard, ANSI/AWWA B404-22 and EN 1209:2003, while zinc orthophosphate follows ANSI/AWWA B506-25.
Nitrite, molybdate, the azoles and octadecylamine have none.
Current editions are listed under AWWA B-series standards.
Drinking-water limits for inhibitor ions and metals#
Nitrite is the only inhibitor ion with a US enforceable drinking-water limit, 1 mg/L as N (40 CFR 141.62), which makes backflow from a nitrite closed loop into potable water a compliance risk. The 4 limits are listed below.
- Nitrite: MCL 1 mg/L as N (US); 0.50 mg/L at the tap, 0.10 mg/L ex works (EU).
- Molybdenum: no MCL; lifetime health advisory 0.04 mg/L (EPA, 2018).
- Zinc: secondary MCL 5 mg/L (40 CFR 143.3), not enforceable.
- EU lead and copper: lead 10 µg/L until 2036-01-12, then 5 µg/L; copper 2.0 mg/L.
The EU values sit in the EU Drinking Water Directive 2020/2184.
Discharge limits, biosolids and the chromate ban#
Chromate, the historic anodic inhibitor, may not be distributed for comfort cooling towers since 1990-02-20 or used in them since 1990-05-18 (40 CFR 749.68), and industrial process cooling towers under 40 CFR 63.402 use no chromium-based chemicals.
Steam-electric tower blowdown is limited to 1.0 mg/L total zinc and 0.2 mg/L chromium (40 CFR 423.13(d)), and biosolids ceilings are molybdenum 75 and zinc 7,500 mg/kg dry weight (40 CFR 503.13).
Permit disclosure is explained under NPDES permits and chemical discharges.
21 CFR 173.310 status for boiler and steam systems#
Two chemicals of this family appear in 21 CFR 173.310: sodium silicate without a numeric limit and octadecylamine at no more than 3 ppm in steam, excluding steam that contacts milk and milk products.
Sodium nitrite, sodium molybdate, zinc salts and the azoles are not listed in 21 CFR 173.310.
The full list sits under 21 CFR 173.310 boiler water additives.
Complete List of Corrosion Inhibitors: All 8 Chemicals in One Table#
The complete list of corrosion inhibitors below gives all 8 chemicals with their CAS number, class, function, systems and NSF/ANSI/CAN 60 status.
| # | Corrosion inhibitor | CAS | Formula | Class | Function | Systems | NSF/ANSI/CAN 60 (NSF listings, 2026-10-20) |
|---|---|---|---|---|---|---|---|
| 1 | sodium nitrite | 7632-00-0 | NaNO2 | inorganic, anodic (oxidizing) | steel inhibitor | closed chilled, hot-water, glycol and brine loops | no category |
| 2 | sodium molybdate | 7631-95-0 (dihydrate 10102-40-6) | Na2MoO4 | inorganic, anodic (non-oxidizing) | steel inhibitor; tracer | closed loops; open cooling | no category |
| 3 | zinc salts (zinc sulfate, zinc chloride) | 7733-02-0 (heptahydrate 7446-20-0, monohydrate 7446-19-7); zinc chloride 7646-85-7 | ZnSO4; ZnCl2 | inorganic, cathodic | steel inhibitor | alkaline zinc cooling; drinking-water blends | MUL 5 to 15 (sulfate), 6 to 15 (chloride) mg/L as product |
| 4 | sodium silicate | 1344-09-8 (metasilicate 6834-92-0) | Na2O·nSiO2 | inorganic, film-forming | steel, lead and copper inhibitor; Fe/Mn sequestrant | drinking water; closed loops | MUL 10 to 353, most frequent 42 mg/L as product |
| 5 | tolyltriazole (TTA) | 29385-43-1 (sodium salt 64665-57-2) | C7H7N3 | azole | copper inhibitor | cooling towers; closed loops | no category |
| 6 | benzotriazole (BZT) | 95-14-7 | C6H5N3 | azole | copper inhibitor | cooling towers; closed loops | no category |
| 7 | mercaptobenzothiazole (MBT) | 149-30-4 (sodium salt 2492-26-4) | C7H5NS2 | thiazole | copper inhibitor | closed circuits with copper; glycol | no category |
| 8 | octadecylamine (ODA) | 124-30-1 | C18H39N | filming amine | condensate inhibitor | steam and condensate | no category |
"No category" means NSF/ANSI/CAN 60 has no chemical category for the substance, not that it is unsafe. Every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.
Who Supplies Corrosion Inhibitors and Corrosion Control Programs?#
Scale and corrosion inhibitors are the fastest-growing product segment of water treatment chemicals, at a 4.8 % CAGR in Grand View Research's 2025 estimate of a USD 35.8 billion market.
Drinking-water supply runs only within the NSF/ANSI/CAN 60 categories, while nitrite, molybdate and azoles reach industrial systems as single chemicals or blends.
Makers and distributors by region are listed under water treatment chemical suppliers.
A program provider reports residuals and coupon mpy; a chemical supplier quotes by strength and basis, volume and container (drum, IBC or bulk), freight distance, contract term and any NSF/ANSI/CAN 60 listing required.
How to compare program providers is covered under water treatment companies.
Are Corrosion Inhibitors Safe to Handle and Discharge?#
Corrosion inhibitors range from oxidizing, toxic-if-swallowed sodium nitrite to corrosive liquid silicates and aquatic-toxic zinc salts, so each product needs its SDS, its own storage and a discharge check against zinc, molybdenum or nitrite limits.
Table 6 summarizes the GHS hazard statements and main incompatibility of each inhibitor (ECHA, checked 2026-10-20).
| Inhibitor | GHS hazard statements | Basis | Key incompatibility |
|---|---|---|---|
| Sodium nitrite | H272, H301, H400 | ECHA; see the supplier SDS | acids (nitrogen oxides); ammonium salts on heating |
| Sodium molybdate | not classified by most notifiers; a minority notify H315, H319, H332, H335 | notified (ECHA C&L) | calcium above 500 ppm in closed loops; brine antifreeze |
| Zinc sulfate | H302, H318, H400, H410 | harmonised (CLP Annex VI) | high pH precipitates zinc |
| Sodium silicate | H315, H318 (majority of notifications) | notified (ECHA C&L); metasilicate harmonised H314, H335 | acids (gel) |
| Tolyltriazole, benzotriazole | see the supplier SDS | n/a | oxidizing biocides |
| Mercaptobenzothiazole | H317, H400, H410 (MBT); NaMBT solution H314, H317, H318, H400, H410 | harmonised (MBT); notified (NaMBT) | oxidizing halogen biocides |
| Octadecylamine | H304, H315, H318, H373, H400, H410 | harmonised (CLP Annex VI) | acids |
What is the most common corrosion inhibitor in water treatment?#
The most common corrosion inhibitor depends on the system: orthophosphate-based treatment in drinking water, nitrite and molybdate in closed loops, phosphate and zinc programs with an azole in cooling towers, and filming amines in condensate.
What is the best corrosion inhibitor?#
No single corrosion inhibitor is best: the best choice protects every metal in the system at the target coupon rate, such as 1 mpy (0.025 mm/y) or less on carbon steel in a cooling tower, at the lowest cost per volume treated.
How long does a corrosion inhibitor last?#
A corrosion inhibitor lasts as long as its residual: closed loops hold nitrite or molybdate for months because makeup stays below 5 % of the volume per year, while open towers lose inhibitor continuously with blowdown. After a biocide shot, supplementary nitrite follows 5 to 7 days later (BSRIA BG 50/2021).
What does a corrosion inhibitor need in order to work?#
A corrosion inhibitor needs 2 conditions: a residual above its protective threshold, because under-dosed anodic inhibitors such as nitrite leave anodes open to pitting, and a clean surface free of deposits and biofilm.
Which chemicals must never be mixed with corrosion inhibitors?#
Sodium nitrite must never meet acids, which release nitrogen oxides, or ammonium salts under heat, which form explosive ammonium nitrite, and oxidizing biocides fed at the same point degrade azoles.
Every incompatible pair is listed under water treatment chemical compatibility.
Is the safety data sheet the same for every corrosion inhibitor product?#
No: hazard classes vary by product and form, for example sodium silicate's corrosivity depends on its SiO2:Na2O ratio, so the supplier SDS of the actual product applies.