Sulfamic Acid in Industrial Cleaning: Applications & Limitations

1. Chemical Properties and Industrial Advantages of Sulfamic Acid

Sulfamic Acid (chemical formula NH₂SO₃H) is a white orthorhombic crystalline solid with a molecular weight of 97.09 g/mol, melting point approximately 205°C (with simultaneous decomposition), and water solubility of 14.7 g/100 mL at 20°C. As a strong monoprotic acid, its 1% aqueous solution has a pH of approximately 1.2, with acid strength between Hydrochloric Acid and Citric Acid. Sulfamic Acid occupies a unique position in industrial cleaning because it combines the high-efficiency descaling capability of a strong acid with the storage and transportation convenience of a solid form, making it one of the most widely used cleaning agents in industrial equipment chemical cleaning.

Sulfamic Acid distinguishes itself from other common cleaning agents in three key aspects. First, solid form — unlike Hydrochloric Acid (liquid, highly volatile, severely corrosive) and Sulfuric Acid (liquid, high density), Sulfamic Acid is supplied as a white crystalline powder with no volatility and no irritating acid fumes, enabling safe and convenient transport, storage, and on-site preparation. Second, relatively mild metal corrosion — the reaction products of Sulfamic Acid with metals are soluble sulfamate salts, which do not cause pitting corrosion or stress corrosion cracking associated with Cl⁻ ions generated by Hydrochloric Acid. Third, highly efficient dissolution of Calcium Carbonate scale — the reaction product of Sulfamic Acid with CaCO₃ is highly soluble Calcium Sulfamate, which does not produce secondary precipitation, a significant advantage over Sulfuric Acid (which produces sparingly soluble Calcium Sulfate secondary scale).

2. Descaling Reaction Mechanisms

Sulfamic Acid dissociates in water to release H⁺ and NH₂SO₃⁻ ions. Its descaling action proceeds through three primary pathways:

Carbonate scale acid dissolution: CaCO₃ + 2NH₂SO₃H → Ca(NH₂SO₃)₂ + H₂O + CO₂↑. The resulting Calcium Sulfamate has extremely high water solubility (approximately 40 g/100 mL at 25°C), far superior to Calcium Sulfate (solubility only 0.21 g/100 mL) and Calcium Citrate (approximately 0.1 g/100 mL), fundamentally avoiding the common secondary precipitation problems encountered during acid cleaning.

Iron Oxide acid dissolution: Fe₂O₃ + 6NH₂SO₃H → 2Fe(NH₂SO₃)₃ + 3H₂O. Although Sulfamic Acid's dissolution rate for iron oxides is lower than Hydrochloric Acid's (due to the absence of Cl⁻ coordination complexation), at temperatures of 50–60°C it still achieves an engineering-acceptable derusting rate. For mixed scale deposits dominated by carbonates with minor iron oxides, a single Sulfamic Acid cleaning cycle can address both types of deposits simultaneously.

Magnesium scale dissolution: Mg(OH)₂ + 2NH₂SO₃H → Mg(NH₂SO₃)₂ + 2H₂O. Both Magnesium Hydroxide and Magnesium Carbonate commonly found in cooling water systems can be effectively dissolved by Sulfamic Acid.

Notably, Sulfamic Acid is essentially ineffective against Silicate Scale — this is one of its most important application limitations. The Si-O-Si bond energy in silicate scale (primarily composed of SiO₂, MgSiO₃, CaSiO₃) is extremely high, and Sulfamic Acid cannot disrupt its network structure. When silicate content in the scale exceeds 10%, the descaling effectiveness of Sulfamic Acid alone drops significantly, requiring the addition of Ammonium Bifluoride or other fluoride-containing cleaning agents for effective removal.

3. Operating Parameter Windows and Limitations

3.1 Concentration Window

The recommended concentration range for Sulfamic Acid is 5%–10% (w/w). Below 5%, the descaling rate is too low, extending cleaning time beyond 12 hours; above 10%, corrosion risk increases and cost-effectiveness declines. For light scaling (scale thickness < 0.5 mm), use 5%–6%; moderate scaling (0.5–1.5 mm), use 7%–8%; heavy scaling (> 1.5 mm), use 8%–10% with appropriately extended cleaning time.

3.2 Temperature Window — The Most Critical Limitation Parameter

The operating temperature for Sulfamic Acid cleaning must be strictly controlled between 50–60°C. This is dictated by two critical factors:

Lower limit: Below 50°C, the descaling reaction rate of Sulfamic Acid drops significantly. According to Arrhenius equation estimation, the reaction rate decreases by approximately 60%–70% when temperature drops from 60°C to 40°C, extending cleaning time by 2–3 times.

Upper limit (critical restriction): Above 60°C, Sulfamic Acid begins to hydrolyze: NH₂SO₃H + H₂O → NH₄HSO₄. The resulting Ammonium Bisulfate not only loses descaling capability but also causes uncontrolled reduction in cleaning solution acidity, while the generated sulfate ions may combine with Ca²⁺ to form Calcium Sulfate secondary precipitation. Above 70°C, the hydrolysis rate accelerates dramatically — over 30% of active ingredient can be lost within 30 minutes. This hydrolysis characteristic restricts Sulfamic Acid to medium-low temperature cleaning scenarios, making it unsuitable for high-temperature online cleaning or boiler boiling-out processes requiring elevated temperatures.

3.3 Material Compatibility Limitations

Sulfamic Acid exhibits significantly different compatibility with various metal materials:

The prohibition on zinc and galvanized materials is the most critical material restriction for Sulfamic Acid. In cleaning scenarios involving galvanized components such as evaporative condensers and cooling towers, using Sulfamic Acid alone causes irreversible damage to the galvanized layer. Such scenarios require a low-concentration Sulfamic Acid + Citric Acid blend, combined with a BTA + Sodium Molybdate multi-layer corrosion inhibition protection system.

4. Comparative Selection vs. Other Cleaning Agents

Selection principle: When the scale layer is primarily carbonate with low silicate content and the equipment contains stainless steel or copper alloy components, Sulfamic Acid is the preferred choice — balancing descaling efficiency, equipment safety, and operational convenience. When iron oxides exceed 30% of the scale composition, a Sulfamic Acid + Citric Acid composite formula is recommended to enhance derusting capability. When equipment contains galvanized components, Sulfamic Acid must not be used alone; it must be blended with Citric Acid under strict pH monitoring.

5. Corrosion Inhibitor System

Sulfamic Acid cleaning solutions must incorporate appropriate corrosion inhibitors; otherwise, corrosion rates on carbon steel and copper alloys will exceed the safety limits specified in GB/T 25146-2010. The following inhibitor formulations are recommended:

Carbon steel protection: Urotropine (Hexamethylenetetramine) 0.15%–0.25%, which hydrolyzes in acidic media to form a formaldehyde adsorption protective layer, reducing carbon steel corrosion rate from 5 g/(m²·h) to below 1.5 g/(m²·h).

Copper alloy protection: BTA (Benzotriazole) 0.15%–0.25%, which self-assembles on copper surfaces to form a [Cu(I)BTA]n polymer protective film, reducing copper alloy corrosion rate from 3 g/(m²·h) to below 0.5 g/(m²·h). Note: BTA thermal stability decreases above 60°C, which in turn constrains Sulfamic Acid cleaning temperature to not exceed 60°C.

Stainless steel protection: Since Sulfamic Acid contains no Cl⁻, its inherent corrosivity toward stainless steel is low (0.3–0.8 g/(m²·h)). Adding 0.05%–0.1% Sodium Molybdate can further reduce corrosion rates to below 0.2 g/(m²·h).

6. Typical Engineering Applications

Shell-and-tube heat exchanger descaling — optimal application scenario: Four shell-and-tube heat exchangers (carbon steel tube side, stainless steel shell side) at a chemical group facility, with 1.2 mm scale on the tube side after 18 months of operation. Scale analysis: Calcium Carbonate 91% + Iron Oxide 7% + Silicate 2%. Cleaning formula: Sulfamic Acid 8% + Urotropine 0.2% + BTA 0.15%, temperature 55°C, circulation cleaning for 6 hours. Post-cleaning results: heat transfer temperature difference restored to 95% of design value, carbon steel corrosion rate 1.3 g/(m²·h), meeting acceptance criteria.

Industrial boiler descaling: A 6 t/h industrial steam boiler at a textile enterprise with 2.0 mm scale on boiler tubes. Formula: Sulfamic Acid 10% + Urotropine 0.25%, temperature 55–58°C, circulation cleaning for 8 hours. Descaling rate 96%, annual natural gas cost savings approximately CNY 150,000.

Copper tube condenser — a boundary case requiring special attention: Sulfamic Acid's corrosion rate on copper tubes is 1–3 g/(m²·h), necessitating strict BTA inhibitor application and temperature control at 50–55°C. During the cleaning of copper tube condensers for a 300 MW power plant unit, a brief temperature excursion to 63°C caused partial BTA failure and a copper tube corrosion rate overshoot to 2.8 g/(m²·h). Normal conditions were restored only after supplementing BTA and rapidly cooling to 52°C. This case fully demonstrates the engineering significance of Sulfamic Acid's upper temperature limit.

7. Common Operating Misconceptions and Corrections

Misconception 1: "Higher temperature means faster cleaning." This is the most common error in Sulfamic Acid application. Above 60°C, hydrolysis occurs — the effective cleaning agent concentration actually decreases, and Calcium Sulfate secondary scale may form. Correct approach: strictly control temperature at 55–58°C, and compensate for the temperature limitation by extending circulation time rather than blindly raising temperature.

Misconception 2: "Higher concentration yields better results." Above 12% Sulfamic Acid concentration, dissolution rate improvement enters a plateau phase (limited by scale mass transfer), while corrosion rate increases linearly. 10% represents the optimal balance between cost-effectiveness and safety.

Misconception 3: "Solid acids don't need corrosion inhibitors." Although Sulfamic Acid is milder than Hydrochloric Acid, at 8% concentration and 55°C temperature, its corrosion rate on carbon steel still exceeds the national standard limit. Corrosion inhibitors are not optional — they are mandatory.

Misconception 4: "Cleaning is complete only when pH stabilizes." The correct endpoint criterion for Sulfamic Acid cleaning is: two consecutive sample measurements showing Ca²⁺ and Fe³⁺ concentration change ≤ 5%, not waiting for pH stabilization. Stable pH only indicates no further H⁺ consumption, but continuously rising calcium and iron ion concentrations indicate residual scale still slowly dissolving.

8. Summary

Sulfamic Acid is one of the most cost-effective cleaning agents in industrial equipment chemical cleaning, particularly suitable for heat exchanger and boiler cleaning scenarios where the scale is predominantly Calcium Carbonate with low silicate content and the equipment contains stainless steel or copper alloy components. Its core advantages lie in the storage and transportation convenience of solid form, the absence of Cl⁻ pitting corrosion risk, and the extremely low tendency for secondary precipitation.

However, its three key limitations must always be remembered: temperature must not exceed 60°C (hydrolysis risk); silicate scale is unaffected (requires fluoride-containing cleaning agent blends); and galvanized and aluminum materials are prohibited (corrosion becomes uncontrolled). With proper understanding of these limitations and corresponding cleaning program design, Sulfamic Acid can successfully complete approximately 80% of routine industrial cleaning tasks. For the remaining 20% of special cases (high temperature, high silica, galvanized equipment), Citric Acid, Ammonium Bifluoride, or other specialized cleaning agents are required.

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