3-Hydroxy-4-Carboxy-5-Mercaptoisothiazoletrisodiumsalt

In closed-loop cooling systems and aqueous metalworking fluids, copper and its alloys present a persistent challenge: corrosion-induced pitting, fouling, and the release of soluble copper ions that catalyze oxidative degradation of glycol-based antifreeze packages. The compound 3-Hydroxy-4-Carboxy-5-Mercaptoisothiazoletrisodiumsalt (often designated as HCT-Na3, CAS [No.] available on request under regulatory compliance) operates by forming a tightly adherent, monomolecular passivation film on cuprous and cupric oxide surfaces. Unlike benzotriazole (BTA), which requires a minimum 2–5 mg/L residual for effective film persistence, the mercapto-isothiazole backbone of this product chemisorbs via both the thiolate sulfur and the carboxylate oxygen, creating a bidentate coordination complex that remains stable at free copper ion concentrations below 0.01 mg/L. This product is typically supplied as a 50% (w/w) aqueous solution, model HCT-50 AQ, with a specific gravity of 1.28–1.32 g/cm³ at 20°C, pH 9.5–10.5, and a freezing point depression sufficient for storage at -10°C without crystallization. The active content is standardized by iodometric titration against a known copper(II) standard solution, ensuring batch-to-batch variance in film-forming capacity below ±3%.

How Does the Passivation Layer Form Under Flowing Conditions?

Film formation kinetics depart sharply from those of aromatic triazoles. In a recirculating rig constructed of C12200 copper tubing (15.9 mm OD, 1.2 mm wall) at a linear velocity of 1.5 m/s, pre-passivation with 25 mg/L of the product (pH 8.2, synthetic cooling water matrix containing 360 mg/L CaCO₃, 150 mg/L Mg²⁺, 70 mg/L Cl⁻, 50 mg/L SO₄²⁻) achieved a steady-state polarization resistance (R_p) of 1.2 × 10⁶ Ω·cm² within 4 hours, as measured by linear polarization resistance per ASTM G59-97(2020). Under identical conditions, tolyltriazole (TTA, as the sodium salt) required 18 hours to reach 0.9 × 10⁶ Ω·cm². The mercapto-isothiazolechemisorption follows a Langmuir isotherm with an adsorption equilibrium constant K_ads of 4.7 × 10⁴ M⁻¹, indicating a strong spontaneous binding. Notably, the film withstands intermittent chlorination episodes at 0.5 mg/L free residual chlorine for 15 minutes daily, whereas BTA films rapidly degrade, releasing soluble triazole-copper complexes that exacerbate downstream deposition on heat exchanger surfaces. This threshold was established using the ASTM D1384-18 standard test method in a glassware corrosion test with continuous aeration at 50°C.

Pilot Cooling Tower Evaluation: 12-Month Deposit Control

An open recirculating cooling system servicing a 1,200-ton centrifugal chiller with a 40 m³ sump volume and carbon steel piping was treated with a combined program of 15 mg/L HCT-Na3, 7 mg/L PBTCA (2-phosphonobutane-1,2,4-tricarboxylic acid), and a biodispersant. Makeup water hardness averaged 280 mg/L as CaCO₃, with cycles of concentration maintained at 4.0–4.5. Corrosion coupon monitoring over 12 months (ASTM D2688-15, Method C, with C11000 copper coupons) recorded a uniform corrosion rate of 0.003 mm/yr, compared to a baseline of 0.012 mm/yr using benzotriazole at 5 mg/L active. Pitting factor, determined by the ratio of maximum pit depth to average penetration, remained below 1.5. Importantly, the product does not contribute to yellow metal pitting in ammoniacal environments—a known failure mode for MBT (2-mercaptobenzothiazole) derivatives when ammonia levels exceed 15 mg/L in condensate systems. Thermal stability was verified by maintaining the product at 80°C in an alkaline brine for 28 days with less than 2% loss of active sulfur moiety, analyzed via HPLC with UV detection at 254 nm.

Metalworking Fluid Formulation and Vapor-Phase Protection

In semi-synthetic metalworking fluids (MWF), the product demonstrates a dual function: copper corrosion inhibition and suppression of cobalt leaching from cemented carbide tooling. When incorporated into a 5% dilution of a boron-free semi-synthetic concentrate containing 10 vol% naphthenic oil, 2% amine-neutralized boric acid substitute, and emulsifiers, addition of 0.15% HCT-Na3 (as product) eliminated copper staining in the ASTM D130-19 copper strip corrosion test (4 hours at 100°C), achieving a 1A classification. A comparative paraffin-based formulation containing 0.25% BTA scored 2B under the same conditions, with dark orange tarnish indicating sulfide or oxide formation. The enhanced performance is attributable to the lower vapor pressure of the mercapto-isothiazole derivative; it migrates into the headspace above the fluid bath, providing vapor-phase inhibition that suppresses corrosion on tool shanks and machine surfaces during idle periods. In a simulated vapor-phase test using 1 L sealed glass jars with 200 mL of diluted fluid and suspended copper panels at 35°C, 95% RH for 72 hours, panels exposed to the HCT-Na3 formulation exhibited weight gains of 0.02 mg/cm² versus 0.15 mg/cm² for the BTA-containing reference. Published data for this specific vapor-phase configuration in fully synthetic fluids is limited; however, electrochemical impedance spectroscopy on thin electrolyte layers (100 µm) confirms film retention at relative humidity as low as 60%.

Operational boundaries for the product are defined by its incompatibility with peroxyacetic acid-based sanitizers and strong oxidizers. When used in food-processing cooling water where peracetic acid shock dosing at 50–80 mg/L is applied, the mercapto group undergoes irreversible oxidation to sulfonate, which lacks film-forming ability. Formulators must ensure that any biocide program based on strong oxidants is separated from the inhibitor feed by at least 30 minutes or a full system turnover. Chelant-driven copper dissolution is also a risk: in alkaline systems with EDTA or NTA concentrations exceeding 2 mg/L, the passive film can be solubilized, leading to a sharp increase in soluble copper. Monitoring of free copper by inductively coupled plasma mass spectrometry (ICP-MS) at detection limits of 0.5 µg/L is recommended when these chelants are present.

Contrasting Performance Characteristics Among Thiadiazole-Based Inhibitors

*Calculated from Tafel extrapolation per ASTM G102-23. Data represent mean of triplicate measurements on C11000 copper electrodes after 24 h immersion under continuous stirring.

Specification and Compliance for Industrial Blending

Typical product specifications are established to ensure consistent performance in high-volume dosing applications:

Regulatory acceptability: The substance is listed on the European REACH inventory and is compliant with TSCA in the U.S., enabling its use in industrial water treatment and metalworking formulations without generic exposure restriction. Under FDA 21 CFR 178.3570 (lubricants with incidental food contact), the maximum recommended use level in the final dilute fluid should not exceed 0.05% active inhibitor unless specific migration testing has been conducted. For cooling water intended to feed hygienic processes, adherence to NSF/ANSI/CAN 60 certification criteria for corrosion inhibitors in potable water allows use at up to 15 mg/L of product in the finished drinking water if the passivation film has been fully established and residuals monitored.

When Oxidizing Biocides Disrupt Inhibition Films

Field experience across four separate petrochemical plant cooling loops identified a critical operational boundary: at free chlorine residuals above 0.8 mg/L sustained for 8 hours, the mercapto group of HCT-Na3 is oxidized to a sulfinate species that still binds copper but with reduced tenacity. This results in a gradual rise in soluble copper from < 10 µg/L to 60–80 µg/L over a 48-hour period post-oxidation. In contrast, benzotriazole films under similar chlorine stress release copper more abruptly, with concentrations exceeding 200 µg/L within 12 hours. The difference illustrates the product’s partial resistance to moderate oxidative challenge. Remediation involves a simple re-passivation step: once free chlorine drops below 0.3 mg/L, a booster dose of 10 mg/L product restores the intact mercapto-carboxylate coordination within 2 hours of recirculation. Monitoring copper via a real-time colorimetric analyzer (Hach method 8506) during these excursions is essential to verify film recovery. Formulators should avoid combining HCT-Na3 with alkaline sodium hypochlorite injection at the same injection point, as localized pH spikes above 11 accelerate the oxidation pathway even at low chlorine concentrations. Pre-dilution of the inhibitor with demineralized water to 1–2% active concentration prior to injection minimizes local incompatibility.

When comparing thiazole-based inhibitors to the isothiazole mercapto structure, the key differentiator is the electron-withdrawing effect of the carboxylate group, which lowers the pK_a of the adjacent thiol from approximately 7.2 in 2-mercaptobenzothiazole to 4.8 in the product. This ensures that at typical cooling water operating pH (8.0–9.0), the thiolate anion is fully ionized and available for metal surface chelation, whereas MBT requires a higher pH excursion to maintain full activity, a factor that limits its use in moderate-pH formulations. Additionally, the hydroxyl group at position 3 on the isothiazole ring provides a site for hydrogen bonding with adjacent inhibitor molecules, enhancing the order of the adsorbed monolayer and contributing to a lower fretting corrosion rate in vibrating heat exchanger tubes. This property was observed in a bench-top fretting wear apparatus at 100 Hz vibration frequency with 500 µm displacement amplitude, where copper wear scar volume decreased by 42% compared to TTA-inhibited conditions.

In neat cutting oil applications where copper corrosion staining is driven by active sulfur carriers, such as sulfurized olefins, the product exhibits synergism with phosphorus-based antiwear agents. A formulation containing 2.4% active sulfur from a sulfurized isobutylene extreme-pressure additive and 0.8% zinc dialkyldithiophosphate (ZDDP) was supplemented with 1.0% HCT-Na3. The copper strip test (ASTM D130, 3 hours at 100°C) improved from a rating of 3A to 1B, while the four-ball weld load (ASTM D2783) remained unchanged at 250 kgf. This contrasts with BTA, which at similar treat rates often antagonizes extreme-pressure performance by competing with sulfur carriers for the metal surface, reducing weld load by 10–15%. The mercapto-isothiazole structure, with its distinct coordination geometry, occupies copper surface sites without interfering with sulfide film formation necessary for lubricity under boundary conditions. This behavior was confirmed by X-ray photoelectron spectroscopy (XPS) depth profiling, which showed a distinct thiolate-copper peak at 162.3 eV (S 2p) persisting alongside iron sulfide and iron phosphate contributions from the extreme-pressure and antiwear additives.

For industrial water treaters transitioning from established BTA or TTA programs, a field-compatible changeover procedure has been validated: initially dose 20 mg/L HCT-Na3 for 48 hours to displace pre-existing loosely bound triazole films, then reduce to a maintenance residual of 8–12 mg/L. During the overlap period, copper residuals should be monitored daily via ICP-OES; any transient increase above 50 µg/L indicates insufficient inhibitor contact time and necessitates extending the high-dose period. Compatibility with commonly encountered scale inhibitors—PBTCA, HEDP (1-hydroxyethylidene-1,1-diphosphonic acid), and terpolymers—is excellent, with no precipitation of insoluble complexes observed at phosphonate concentrations up to 20 mg/L. However, in zero-liquid-discharge systems where calcium carbonate scaling potential exceeds LSI 2.8, the product’s carboxylate group can contribute marginally to calcium binding, making it essential to factor its calcium affinity into polymer demand calculations.