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. sulphamic acid pickling), 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.

Note on figures: the inhibition-efficiency values in this section and below are laboratory test data, given for mechanism comparison and inhibitor selection only; they are not acceptance criteria. Corrosion rate and corrosion mass are accepted per GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment, Table 2 — copper alloy corrosion rate K ≤1 g/(m²·h) (laboratory verification), ≤1.5 g/(m²·h) (on-site measurement), corrosion mass A ≤20 g/m²; the two quantities differ and must not be interchanged. Inhibition efficiency is rated per clause 3.3 of the same standard, based on the actual acid-cleaning inhibition test result.

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. For copper-tube condensers the working system is a sulphamic-acid pickling agent with a dedicated copper inhibitor, verified by same-material coupons; ratio and concentration are set per case from the deposit and site conditions.

(2) Alkaline/neutral → MBT: pH >8 alkaline cleaning, cooling water treatment, alkaline degreasing — MBT outperforms BTA. Pre-dissolve MBT in alkali solution 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 25 years of copper alloy cleaning inhibitor application data to every project, providing customized formulation verification and on-site technical support.

6. Frequently Asked Questions

Q1: During copper tube acid cleaning, should I choose BTA or MBT?

Choose BTA for acidic conditions (pH 1.5-4.0, for example Sulfamic Acid cleaning), where laboratory-measured inhibition efficiency reaches 95-99%; choose MBT for alkaline conditions (pH 9-11), at 90-95%. The decision follows the pH window of the cleaning solution — the two cannot be swapped by habit.

Q2: Can BTA and MBT be used together?

Yes. Combining them pairs the dense film of BTA with the strong thiol bonding of MBT: in acid-dominant cleaning solutions BTA is the main inhibitor with a small MBT assist, and the actual ratio is fixed by the cleaning plan according to deposit type and materials of construction.

Q3: How much inhibitor should be added?

The dosage follows the calculated amount in the cleaning plan, normally dosed by cleaning-solution volume and metal surface area; the controlling indicator is that the coupon corrosion rate meets the standard (copper alloy ≤1.0 g/(m²·h)). The system is pre-filmed by circulation before the main cleaning step.

Q4: How do I judge that the inhibitor has failed?

Discoloration of the cleaning solution, coupon corrosion rate exceeding the limit, and localized pitting on the copper surface are all failure signals. Exceeding the decomposition temperature (BTA approximately 350°C, MBT approximately 280°C) or drifting outside the effective pH range also causes failure.

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

© 2026 Danyang Blue Star Anti-corrosion Cleaning Co., Ltd. All rights reserved.