HCl vs Citric and Phosphoric Acid for Descaling: When Mild Acids Win

Industrial descaling selection is frequently reduced to HCl vs citric and phosphoric acid for descaling: when mild acids win is determined by scale composition, base-metal chloride tolerance, rinse-water quality limits, and post-clean passivation. Hydrochloric acid is a strong monoprotic acid with pK_a -7 at 25 °C; citric acid is a tribasic organic acid with pK_a1 3.13, pK_a2 4.76, and pK_a3 6.40; phosphoric acid is a tribasic mineral acid with pK_a1 2.15, pK_a2 7.20, and pK_a3 12.35. Acid strength alone does not predict descaling performance, because real scale dissolution depends on calcium salt solubility, iron chelation, sludge formation, gas evolution, and chloride-based corrosion.

At working concentration, 5 wt% hydrochloric acid has a pH below 0, while 5 wt% citric acid has a pH near 1.8. Both dissolve calcium carbonate, but the anionic profile is different: hydrochloric acid leaves chloride, citric acid leaves citrate, and phosphoric acid leaves phosphate. The term mild acid is therefore a process-safety and anion-compatibility descriptor rather than a strict pH descriptor. The selection boundary is an engineering threshold: HCl is preferred where chloride-compatible carbon steel dominates and heavy carbonate scale must be removed rapidly; citric or phosphoric acid is preferred where stainless steel passivation, low chloride residual, or iron phosphate film stability is required.

These values do not constitute a direct performance ranking. Citric acid is a chelating acid; its descaling rate on iron oxide can be greater than expected from pK_a alone because the citrate anion complexes Fe(III). Phosphoric acid forms insoluble iron phosphate at higher pH, which can act as a protective layer but also complicates retreatment. Hydrochloric acid generates chloride; therefore the material specification, rather than scale type alone, is often the first screening test.

How Far Does HCl Go Before Chloride Metallurgy Becomes the Constraint?

Hydrochloric acid attacks calcium carbonate by the stoichiometric reaction CaCO₃ + 2 HCl → CaCl₂ + H₂O + CO₂. At 20 °C, calcium chloride solubility is 74.5 g/100 mL, so the reaction product remains largely dissolved and redeposition is low. For carbon steel boilers, descaling with inhibited HCl at 5–10 wt% and 60–82 °C is common where the tube circuit is fully flooded and vented for CO₂. The base-metal corrosion rate is verified on pre-weighed carbon steel coupons per ASTM G31. The same cycle is not transferable to austenitic stainless steel, aluminum, zinc, galvanized steel, copper, or nickel alloys. Residual chloride in crevices and under gasket seating can initiate pitting; with tensile stress and temperature above 50 °C, chloride stress corrosion cracking becomes a credible failure mode in AISI 304 and AISI 316L. Hydrochloric acid therefore wins in heavy carbonate scale only where the circuit is carbon steel, where gas evolution can be vented, and where chloride is tolerable in the bulk fluid.

In a thin-wall AISI 316L plate heat exchanger with 0.5 mm corrugated plates and EPDM gaskets, the descaling programme typically avoids HCl after the first chemical cleaning. The narrow flow gap and gasket groove produce crevice regions in which chloride concentration can rise during shutdown by evaporation. A citric acid cycle at 3–5 wt% and pH 1.9–2.3 is circulated at 60 °C for 2–4 h, followed by demineralized-water rinse until chloride is below the plant limit. Corrosion coupons of the same plate material are placed in a side-stream rack per ASTM G31. Published comparative field data for identical plate packs is limited; selection therefore relies on coupon mass loss and post-clean surface roughness measured per ISO 4287.

Phosphoric Acid Passivation and Sludge Control in AISI 304 Evaporator Service

Phosphoric acid functions as both a scale dissolver and a passivating agent. In AISI 304 falling-film evaporators and dairy plate units, formulations of 5–10 wt% phosphoric acid at 50–70 °C remove milkstone and calcium phosphate/carbonate scale while forming an iron phosphate layer on steel. The iron phosphate film reduces flash rusting after rinse and is evaluated by ASTM A967 passivation tests. However, phosphate chemistry has an operational boundary: if the spent acid pH rises above 3.0 during neutralization, calcium phosphate can precipitate as a secondary scale and block narrow distribution holes in spray nozzles. The cleaning circuit must therefore be flushed with demineralized water before alkali neutralization. Phosphoric acid is also not suitable for zinc or galvanized surfaces; it is less aggressive to stainless steel than HCl but can still corrode carbon steel if inhibitor dosing is inadequate. Corrosion rate is measured by ASTM G31, and the passivating film is checked by humidity-cabinet rust testing.

On a copper-brazed plate heat exchanger used for potable water heating, inhibited HCl is excluded because the brazing alloy and copper base metal are attacked by chloride-containing solutions. Citric acid at 3–5 wt% is circulated at 45–60 °C with a flow velocity above 1.0 m/s to remove calcium carbonate while minimizing gas blanketing. The pH is held below 2.5 to keep calcium citrate in solution. Because published corrosion rate data for this particular brazed assembly is limited, a side-stream coupon of identical braze material is exposed per ASTM G31 during the cleaning campaign.

When Citric Acid Wins in Austenitic Stainless Steel Descale Cycles

Citric acid wins where chloride-induced localized corrosion is the dominant risk and where the scale contains iron oxide. The acid dissolves iron oxide by a combination of acid attack and ferric citrate chelation; the chelation effect is significant at pH 2.0–3.0 and temperatures of 60–80 °C. Unlike HCl, citric acid does not contribute chloride to rinse water, so post-clean chloride loading on stainless steel surfaces remains controlled by the water source alone. The descaling rate on calcium carbonate is slower than HCl, but for thin deposits on plate surfaces the reduced corrosion risk is often the controlling variable. Calcium citrate has limited solubility; the cleaning bath must be maintained below pH 3.0 and rinsed immediately after the acid is discharged. Citric acid is also specified for passivation of stainless steel under ASTM A967, where a solution of 4–10 wt% citric acid at 60–82 °C removes free iron and improves chromium oxide enrichment. The process window is narrower than phosphoric acid, but the absence of phosphate sludge makes citric acid suitable for spray balls and low-flow zones where precipitation would block apertures.

Hydrochloric Acid Requires Chloride-Compatible Metallurgy

Hydrochloric acid remains effective for carbon steel boiler descaling when the deposit is predominantly CaCO₃, Mg(OH)₂, and Fe₂O₃. The reaction with iron oxide is Fe₂O₃ + 6 HCl → 2 FeCl₃ + 3 H₂O. Ferric chloride is highly soluble, which prevents iron redeposition. Industrial descaling formulations typically use 5–15 wt% inhibited HCl; the inhibitor package is formulated for carbon steel and may contain quaternary amine or acetylenic alcohol derivatives. The evolution of CO₂ from carbonate scale creates foam and gas pockets in low-flow channels, so circulation pumps are sized to maintain velocity above 0.6 m/s in shell-side circuits. HCl should not be used on austenitic stainless steel unless a chloride-free formulation and suitable inhibitor are specified, and even then the service is not recommended above 50 °C under tensile stress. Aluminum, galvanized steel, zinc, copper, and brass are incompatible. Because the acid can release hydrogen chloride vapour, local exhaust ventilation and acid-resistant equipment are required. The acceptance criterion is corrosion coupon mass loss according to ASTM G31, with the maximum allowable loss set by the plant engineering standard.

For a gas-fired cast iron boiler with heavy water-side scale, inhibited HCl at 8 wt% and 70 °C is introduced only after mechanical brushing of loose scale and isolation of copper-containing trim. The acid is circulated through the lower headers with vent ports opened to prevent CO₂ pockets. After descaling, the circuit is drained, flushed, neutralized with sodium carbonate, and rinsed again. Coupon mass loss is converted to a corrosion rate in millimetres per year. If the rate exceeds the corrosion allowance in the boiler pressure-part standard, the cleaning is terminated and alternative mild acid chemistry is evaluated.

On a water-side shell-and-tube oil cooler with admiralty brass tubes, descaling with any of the three acids is constrained by copper alloy corrosion. HCl is incompatible because chloride attacks the brass; phosphoric acid can passivate steel but can attack copper under acidic conditions. Citric acid is used at 2–4 wt%, pH 2.0–2.5, and temperature below 50 °C for short contact, with copper alloy corrosion coupons per ASTM G31. If scale thickness is above 2 mm, mechanical cleaning must be completed before acid circulation.