In chemical treatment programs for industrial circulating cooling water systems, corrosion control of copper and its alloys (such as brass, naval brass, and cupronickel) is a critical aspect of maintaining heat exchange efficiency and extending equipment lifespan. 2-Mercaptobenzothiazole (MBT), as a classic organic corrosion inhibitor, has long dominated water treatment formulations due to its excellent film-forming efficiency and cost advantages.
1. Core Advantages: MBT’s Chemical Adsorption Mechanism and Copper Protection
MBT is considered the preferred copper corrosion inhibitor primarily due to its unique molecular structure and interaction with metal surfaces. Unlike conventional inorganic corrosion inhibitors that rely on physical coverage, MBT is a typical adsorption-type film-forming corrosion inhibitor.
From a chemical structure perspective, the sulfur (S) and nitrogen (N) atoms in the MBT molecule have strong coordination capabilities. When MBT is added to circulating water, it quickly diffuses to the copper surface. The thiol sulfur atom and the nitrogen atom in the ring of the molecule provide lone pair electrons, which undergo a chemical coordination reaction with metal ions on the copper surface (mainly Cu+ or Cu2+), forming a complex precipitate film called [Cu(MBT)]n.
♣ This protective film has the following three significant technical characteristics:
- Hydrophobicity and Density: The resulting coordination film is highly hydrophobic, effectively preventing water molecules, dissolved oxygen, and chloride ions (Cl-) from contacting the metal substrate, thus interrupting the cathodic and anodic processes of electrochemical corrosion.
- Rapid Film Formation: Compared to Tolyltriazole (TTA), MBT has a faster film formation kinetic rate. In the early stages of system startup or when the film layer is damaged, MBT can repair the exposed metal surface in a short time.
- Accompanying Bactericidal Effect: MBT itself has a certain degree of biotoxicity, which can interfere with the metabolic processes of microorganisms. This allows it to assist in inhibiting the growth of anaerobic bacteria such as sulfate-reducing bacteria in the circulating water system while performing its corrosion inhibition function, thereby reducing the risk of microbiologically induced corrosion (MIC).
2. Application Challenges: Degradation and Failure Analysis in Oxidizing Environments
Although MBT exhibits excellent film-forming speed, its biggest challenge in practical applications comes from interference from “oxidizing biocides.” Modern industrial circulating water systems typically add liquid chlorine, sodium hypochlorite, or bromine-based biocides regularly to control Legionella and biological slime.
The thiol group (-SH) in the MBT molecule has strong reducing properties. When the residual chlorine content in the circulating water is high (usually exceeding 0.5 – 1.0 mg/L), MBT will react with the oxidant preferentially over bacteria. This reaction has two negative consequences:
- Reagent consumption and precipitation: Chlorine will oxidize MBT into disulfide (MBTS), and even further oxidize it into sulfonic acid substances that lose their corrosion inhibition ability. The oxidation product MBTS is insoluble in water and will form suspended particles or precipitates, which not only causes water turbidity but may also deposit on the surface of heat exchange tubes, forming scale and causing under-deposit corrosion.
- Unnecessary consumption of biocides: The reaction between MBT and chlorine significantly consumes the effective chlorine in the system, leading to a decrease in sterilization efficiency. To maintain the residual chlorine standard, excessive amounts of biocides are often added on-site, which in turn accelerates the decomposition of MBT, forming a vicious cycle.
Therefore, in some harsh water quality environments with high concentration ratios and high residual chlorine, using MBT alone is often difficult to maintain long-term corrosion inhibition.
3. Optimization Strategies: Blending Technology and Precise Dosing Control
To overcome the shortcomings of MBT’s poor oxidation resistance while retaining its advantages of fast film formation and low cost, a strategy combining “blending for synergistic effect” and “process control” is usually adopted in industrial applications.
♣ Constructing a multi-azole blending system
The most mature solution currently is to utilize synergistic effects by blending MBT with Benzotriazole (BTA) or Tolyltriazole (TTA).
- Complementary advantages: MBT is responsible for quickly establishing an initial protective film on the copper surface, while BTA / TTA, with its stronger chemical stability, resists the attack of oxidants in the main water flow and repairs the micropores in the film layer.
- Cost Control: This blended formulation can reduce the amount of expensive TTA while ensuring high performance. In common high-efficiency formulations, the ratio of MBT to other azoles is usually controlled at 1:1 or 2:1, depending on the chlorine content level of the system.
♣ Addition Sequence and Timing Management
Optimization at the field operation level is also crucial. It is recommended to use “shock dosing” rather than “continuous dripping” to establish the base concentration, and to strictly manage the timing of biocide addition.
- Staggered Dosing: Avoid adding MBT and oxidizing biocides simultaneously. The best practice is to allow the system 4-6 hours of circulation time after adding MBT to allow the protective film to stabilize before performing oxidative biocide treatment.
- Non-oxidizing Biocide Alternatives: During maintenance periods with high MBT concentrations, it is recommended to prioritize the use of non-oxidizing biocides such as isothiazolinones or quaternary ammonium salts to eliminate chemical interference at the source.
♣ Alkalinity and pH Synergy
The solubility of MBT is significantly affected by pH (usually added in the form of sodium salt). Maintaining the circulating water pH above 7.0 helps maintain the dissolved state of MBT, preventing it from precipitating and becoming ineffective due to local acidic environments. At the same time, appropriate alkalinity helps enhance the mechanical strength of the protective film on the copper surface.
