What are the risks of non-phosphorus cooling tower treatment?

This comparison covers open recirculating cooling towers with carbon steel and galvanized steel. It does not cover closed loops, once-through cooling or the permit limit itself, which the discharge permit for each site sets.

How do phosphate and phosphorus-free cooling tower programs differ?

Phosphate and phosphorus-free cooling tower programs differ in how they protect metal: phosphate programs rely on deposits that form on the surface, while film-forming phosphorus-free inhibitors attach to the metal itself. Phosphate-phosphonate programs protect through deposition of calcium phosphate, iron phosphate and zinc hydroxide (Chemical Processing, Buecker and Post, 2019). A reactive polyhydroxy starch inhibitor instead forms a surface barrier through the functional groups on its molecules, according to its developer (Chemical Processing, Buecker and Post, 2019). The difference matters for a switch because the two families fail in different ways. Phosphate-phosphonate programs are far from simple, and under- or overfeed can cause either corrosion or scale (Water Tech Online, Buecker, 2021). Film-forming programs fail mainly through underfeed and through water conditions that the film does not tolerate.

The table below decides which program family fits a tower and its permit.

The key row is discharge fit: Buecker counts phosphonates as organic phosphates and names phosphate-phosphonate chemistry as the source of the phosphorus discharge in blowdown, so an all-organic phosphonate program still puts phosphorus in the blowdown (Water Tech Online, Buecker, 2021).

How does removing phosphate and zinc affect galvanized steel in a cooling tower?

Removing phosphate and zinc from a cooling tower takes away chemistry that helps galvanized steel keep its passive layer, so the main risk of the switch is white rust. White rust is a porous mix of zinc carbonate and zinc hydroxide; corrosion of the steel underneath can concentrate under it and develop into pitting, and leaks can form in a basin in as little as two months in severe cases (Kurita America, 2021). Without conditioning, or when later operation raises the pH, zinc forms a non-protective deposit that can become very destructive if left alone (Chemical Processing, Buecker, 2025).

Two cases need different handling. A new or re-coated galvanized tower needs a passivation period first: Kurita America's guide calls for pH 6.5 to 8.0, at least 50 to 100 mg/L calcium hardness as CaCO3 and free chlorine below 1.0 mg/L for generally 12 to 14 weeks, and names stabilized phosphate as very effective in promoting zinc passivation (Kurita America, 2021). If the permit forbids phosphate even during startup, the passivation plan itself has to be phosphorus-free and proven on that water.

An already passivated tower faces a different risk: overfeed of phosphonates and chelating polymers can remove the passivation from the zinc and force a new passivation procedure (Kurita America, 2021). Chem-Aqua also lists pH and oxidizer excursions and disinfections as reasons re-passivation may be needed (Chem-Aqua, n.d.).

Does phosphorus-free treatment protect carbon steel as well as phosphate?

Phosphorus-free treatment can protect carbon steel as well as phosphate or better, but the published evidence is chemistry-specific, often vendor-reported, and holds only at the right dose and pH. The developer of a starch-based inhibitor reports that at one industrial site in the southeastern United States, replacing polyphosphate and zinc cut carbon steel corrosion from 0.2 to 0.25 mm/yr to 0.0025 to 0.0075 mm/yr (Water Tech Online, Buecker, 2021). That is one vendor-authored case, not an independent trial.

Laboratory work shows how much the result depends on conditions. Polyepoxysuccinic acid used alone gave limited corrosion inhibition on carbon steel at low concentration, low efficiency in acidic conditions and about 80% in alkaline conditions (Zhang, Tian and Li, 2015). A phosphorus-free composite reached 99.2% corrosion inhibition in the laboratory and worked between pH 6 and 8, but its formula included zinc chloride (Htet and Zeng, Pol. J. Environ. Stud., 2022).

Field reports add two dosing risks. Some users of the starch inhibitor trimmed feed too far and localized corrosion sites developed, and systems that already suffered corrosion and rusting likely need a significantly higher dosage than clean systems (Chemical Processing, Buecker and Post, 2019). Ask the supplier for coupon data from water like yours, not only the best case.

Which makeup water conditions make a phosphorus-free cooling program fail?

High sulfate, high iron and soft makeup water are the conditions most often named as making a phosphorus-free cooling program fail, and high chloride adds risk for galvanized steel. The starch inhibitor's developer states that the product is sensitive to excessive sulfate, protects well against chlorides, and has had difficulties in cooling waters with high iron (Chemical Processing, Buecker and Post, 2019). Chem-Aqua lists high sulfate or chloride, low hardness, high alkalinity or pH and changing makeup water quality among the factors behind white rust (Chem-Aqua, n.d.).

Check these makeup water items before a trial:

1. Sulfate, including sulfate added by sulfuric acid feed for alkalinity control.

2. Total and dissolved iron, and whether iron bacteria have appeared before.

3. Calcium hardness: soft water prevents passivation of galvanizing, and Kurita America's startup guide requires a minimum of 50 to 100 mg/L as CaCO3 (Kurita America, 2021).

4. Chloride and pH at the planned cycles of concentration.

5. Nutrients and organics if the makeup is reclaimed water, which can bring nitrogen, phosphorus and organic compounds (Chemical Processing, Buecker and Post, 2019).

6. Seasonal variation: a single grab sample can miss it.

Comprehensive, ideally historical, makeup water analyses are needed to choose a polymer blend, because the right polymer depends on the cooling water chemistry (Water Tech Online, Buecker, 2021).

How does a phosphorus-free program change microbial control in a cooling tower?

A phosphorus-free program removes a nutrient that feeds algae in a cooling tower, but it does not replace the biocide program, and weak biocide feed shows up faster on some phosphorus-free chemistries. Phosphorus is often the limiting nutrient for algae blooms (Water Tech Online, Buecker, 2021). At one site described by the starch inhibitor's developer, removing phosphate stopped severe algae formation in a clarifier and recycle pond (Water Tech Online, Buecker, 2021).

The opposite risk sits in iron-bearing water. With the starch inhibitor in high-iron cooling water, iron bacteria formed black colonies, particularly when biocide feed was insufficient (Chemical Processing, Buecker and Post, 2019). Slime layers also shelter anaerobic bacteria such as sulfate reducers, whose byproducts can cause severe corrosion (Chemical Processing, Buecker and Post, 2019).

Galvanized towers add a limit on the oxidizer itself. During passivation, Kurita America's guide keeps free chlorine below 1.0 mg/L and restarts passivation if a spike above that lasts more than 4 hours (Kurita America, 2021). Chem-Aqua lists high chlorine or bromine levels among white rust factors (Chem-Aqua, n.d.). If the switch also changes the biocide or its feed point, plan both changes together, and keep oxidant residual records through the trial so a corrosion change can be told apart from a biology change.

How do you test a phosphorus-free cooling program before a full switch?

A phosphorus-free cooling program should be tested on one tower or loop with a measured baseline, fixed pass criteria and rollback triggers written before the trial starts. ASTM D2688-23 is the active test method for corrosion rate by coupon weight loss and pitting in open recirculating cooling water, in the absence of heat transfer (ASTM D2688-23). Coupons in cool locations may not reflect heat exchanger tube surfaces (Chemical Processing, Buecker and Post, 2019), so add an exchanger inspection or heat transfer data.

The table below decides when to continue, adjust or roll back; the trigger values are illustrative and belong to your site.

The key row is galvanized surfaces, because white rust can turn into leaks faster than a coupon cycle reports it. The same discipline applies to weighing the risks of moving a customer to a new formulation. A feasibility review Task in Lubinpla fixes the assumptions and options for a switch like this, then sets early warning signals and rollback conditions for each scenario, in a cited document for your review.