BTA vs MBT: Corrosion Inhibitor Mechanisms for Copper Alloys

1. Molecular Structure & Electronic Properties

BTA (C₆H₅N₃) features a benzene ring fused with a triazole ring containing three nitrogen atoms. N atoms at positions 2 and 3 carry lone-pair electrons that serve as donors for Cu⁺ d-orbital coordination. The HOMO energy of the triazole N atoms is approximately -8.9 eV, well-matched to the Fermi level of Cu(111) surfaces for stable σ-coordination.

MBT (C₇H₅NS₂) features a benzene ring fused with a thiazole ring, with the critical difference being a thiol (—SH) group at position 2. The thiol sulfur atom has stronger electron-donating capability (electronegativity 2.58 vs N at 3.04), forming stronger Cu—S bonds. In alkaline conditions, —SH ionizes to —S⁻, giving MBT a negative charge that alters its adsorption behavior.

2. Film Formation Mechanisms

2.1 BTA — Coordination Polymer Film

BTA film formation proceeds in two stages: Stage 1 — rapid physisorption (0-30 s), BTA molecules lie flat via π-electron interaction; Stage 2 — chemisorption (1-30 min), deprotonated N⁻ from position 1 coordinates with Cu⁺ forming Cu(I)-BTA polymer chains. Each Cu⁺ is bridged by two BTA molecules via N—Cu—N linkages, producing a highly ordered film 5-50 nm thick. XPS analysis shows Cu:N ≈ 1:3, stable in both air and water.

2.2 MBT — Thiol Bonding Film

MBT film formation is S—Cu bond dominated. The thiol S atom coordinates with Cu⁺ forming Cu—S bonds, while the thiazole N provides auxiliary bidentate chelation. Unlike BTA's chain polymer, MBT forms a monolayer chemisorbed film ~2-10 nm thick. FTIR confirms disappearance of the —SH stretch at 2560 cm⁻¹, indicating chemical bonding rather than physisorption.

3. Performance Comparison & Selection Boundaries

3.1 Acidic Conditions — BTA Dominates

At pH 1.5-4.0 (e.g. 5% Sulfamic Acid), BTA achieves 95-99% inhibition efficiency vs 50-70% for MBT. In acidic media, BTA remains neutral with unimpaired N-donor capability, while MBT's thiol is protonated (—SH form), weakening S-donor strength. BTA is the clear choice for copper condenser Sulfamic Acid cleaning.

3.2 Alkaline Conditions — MBT Overtakes

At pH 9-11 (alkaline degreasing, cooling water treatment), MBT reaches 90-95% vs BTA's 70-85%. The ionized —S⁻ group electrostatically attracts the positively charged copper surface (Cu(OH)⁺), dramatically improving coverage. BTA conversely suffers from increased solubility and film dissolution at high pH.

3.3 Temperature Tolerance

BTA-Cu film is stable below 80°C; MBT-Cu film degrades above 70°C. For high-temperature cleaning (>80°C, e.g. thermal oil boiler decoking), BTA should be paired with Propynyl Alcohol.

3.4 Multi-Metal Compatibility

BTA is highly copper-selective — in Cu-CS bimetal systems, it preferentially adsorbs on copper with near-zero carbon steel protection. MBT provides moderate protection for both, making it suitable for copper-tube/carbon-steel-tubesheet condensers. However, MBT underperforms BTA on brass due to weak Zn-MBT complexes that may promote dezincification.

4. Synergistic Formulations

5. Selection Guidelines

(1) Acidic cleaning → BTA: pH <5 environments including Sulfamic Acid, Citric Acid, and HCl+inhibitor systems — BTA with 95%+ efficiency is the standard. The benchmark formula: 5-8% Sulfamic Acid + 0.1-0.3% BTA + 0.1% Surfactant.

(2) Alkaline/neutral → MBT: pH >8 alkaline cleaning, cooling water treatment, alkaline degreasing — MBT outperforms BTA. Pre-dissolve MBT in NaOH before adding to the system due to poor water solubility.

(3) Complex systems → synergize: Cu-CS bimetal equipment benefits from BTA+MBT or BTA+KI combinations. High-temperature cleaning (>80°C) should incorporate Propynyl Alcohol to reduce single-inhibitor failure risk.

In field practice, inhibitor selection must also consider equipment materials, deposit analysis results, and economics. A 0.1% concentration difference can produce an order-of-magnitude corrosion rate change — detectable within 2 hours in coupon testing. Lanxing Qingxi brings 20 years of copper alloy cleaning inhibitor application data to every project, providing customized formulation verification and on-site technical support.

All acceptance criteria follow GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment.

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