Performance Evaluation of 1,3-Diaminopropane (DAP) in Water Treatment Corrosion Inhibitor Formulations
Overview
1,3-Diaminopropane (DAP), also known as trimethylenediamine, is an organic compound with the chemical formula C₃H₁₀N₂. It belongs to the class of aliphatic diamines and features two primary amine groups located at terminal positions on a three-carbon chain. This structural configuration imparts DAP with high reactivity, strong chelating potential, and excellent solubility in water—properties that make it a promising candidate for use in corrosion inhibition formulations within industrial water treatment systems.
Corrosion remains one of the most significant challenges in industries such as power generation, oil and gas, desalination, and cooling water systems. Uncontrolled metal degradation leads to equipment failure, safety hazards, and economic losses estimated globally at hundreds of billions of dollars annually. To mitigate these effects, corrosion inhibitors are widely employed. Among various classes of inhibitors, nitrogen-containing compounds—particularly amines and polyamines—have demonstrated exceptional performance due to their ability to adsorb onto metal surfaces and form protective films.
This article provides a comprehensive evaluation of 1,3-diaminopropane’s performance in water treatment corrosion inhibitor formulations. The discussion encompasses its molecular characteristics, mechanisms of action, compatibility with common water chemistries, synergistic effects with other inhibitors, and practical application data derived from laboratory and field studies. Comparative analyses with conventional inhibitors such as zinc-based compounds, phosphonates, and imidazolines are included, supported by experimental tables and referenced findings from both domestic (Chinese) and international research institutions.
Chemical and Physical Properties of 1,3-Diaminopropane
Understanding the physicochemical properties of DAP is essential for assessing its suitability in aqueous environments. The following table summarizes key parameters:
| Property | Value / Description |
|---|---|
| Chemical Formula | C₃H₁₀N₂ |
| Molecular Weight | 74.13 g/mol |
| IUPAC Name | Propane-1,3-diamine |
| Appearance | Colorless to pale yellow liquid |
| Odor | Strong ammonia-like |
| Boiling Point | 146–148 °C |
| Melting Point | ~17 °C |
| Density (20 °C) | 0.885 g/cm³ |
| Solubility in Water | Miscible |
| pKa₁ (protonated amine) | ~10.5 |
| pKa₂ | ~8.9 |
| Vapor Pressure (25 °C) | ~0.1 mmHg |
| Flash Point | 43 °C (closed cup) |
| Viscosity (25 °C) | ~1.2 cP |
Source: CRC Handbook of Chemistry and Physics, 102nd Edition; NIST Chemistry WebBook
The dual amine functionality allows DAP to act as a bidentate ligand, capable of forming stable complexes with transition metals such as iron, copper, and zinc. Its high basicity enables protonation under typical pH conditions encountered in cooling water (pH 6.5–9.0), resulting in positively charged species that readily interact with negatively charged metal oxide surfaces.
Mechanism of Corrosion Inhibition
Corrosion in aqueous systems primarily occurs through electrochemical reactions involving oxidation of the metal (anodic reaction) and reduction of oxygen or hydrogen ions (cathodic reaction). For carbon steel—a common material in pipelines and heat exchangers—the overall process can be summarized as:
Anode:
Fe → Fe²⁺ + 2e⁻
Cathode (neutral to alkaline conditions):
O₂ + 2H₂O + 4e⁻ → 4OH⁻
Inhibitors like DAP function by interfering with one or both of these processes. The mechanism of DAP involves several interrelated pathways:
1. Adsorption and Film Formation
DAP molecules adsorb onto the metal surface via donor-acceptor interactions between the lone electron pairs on nitrogen atoms and vacant d-orbitals of iron. This physisorption is followed by chemisorption, leading to the formation of a compact monolayer that blocks active corrosion sites.
According to Zhang et al. (2021) from Tsinghua University, DAP exhibits Langmuir-type adsorption behavior on mild steel in simulated cooling water, with a maximum surface coverage of 92% at 50 mg/L concentration.
2. Chelation and Precipitation
DAP forms insoluble complexes with dissolved metal ions (e.g., Fe²⁺), which precipitate as protective layers on the metal surface. These films reduce ion diffusion and suppress localized corrosion such as pitting.
Research conducted at the University of Manchester (Smith & Patel, 2020) demonstrated that DAP significantly reduces the rate of Fe²⁺ release in flowing systems, indicating effective passivation.
3. pH Buffering Effect
Due to its dual amine groups, DAP contributes to local pH stabilization near the metal-solution interface. Maintaining a slightly alkaline environment favors the formation of passive iron oxides (e.g., magnetite, Fe₃O₄), enhancing natural protection.
Performance in Different Water Chemistries
The efficacy of DAP varies depending on water composition, temperature, flow regime, and system metallurgy. Below is a comparative analysis across different water types.
Table 1: Inhibition Efficiency of DAP in Various Water Systems
| Water Type | pH | Temperature (°C) | [DAP] (mg/L) | Inhibition Efficiency (%) | Test Method | Reference |
|---|---|---|---|---|---|---|
| Simulated Cooling Water | 8.2 | 40 | 25 | 85 | Weight Loss / EIS | Liu et al. (2019), Harbin Inst. Tech. |
| High-Chloride Brackish Water | 7.5 | 60 | 50 | 78 | Potentiodynamic Polarization | Chen & Wang (2020), Zhejiang Univ. |
| Soft Freshwater | 7.0 | 30 | 20 | 90 | Linear Polarization Resistance | Gupta et al. (2018), Imperial College |
| Hard Water (300 ppm Ca²⁺) | 8.5 | 50 | 40 | 88 | Electrochemical Impedance Spectroscopy | Zhao et al. (2022), Sichuan Univ. |
| Seawater Simulant | 8.1 | 25 | 100 | 70 | Salt Spray Test (ASTM B117) | Tanaka et al. (2017), Tokyo Univ. |
These results indicate that DAP performs optimally in soft to moderately hard freshwater systems but requires higher dosages in aggressive environments such as seawater or high-chloride brines. However, even in challenging conditions, it maintains moderate protection levels.
Synergistic Effects with Other Inhibitors
While DAP shows good standalone performance, its effectiveness is significantly enhanced when combined with co-inhibitors. Synergy arises from complementary mechanisms—for example, DAP providing film-forming capability while another component stabilizes the oxide layer or scavenges corrosive agents.
Table 2: Synergistic Performance of DAP-Based Blends
| Formulation | Components | Concentration (mg/L) | Efficiency (%) | Synergy Index | Application |
|---|---|---|---|---|---|
| DAP + Sodium Molybdate | DAP: 20, MoO₄²⁻: 30 | Total: 50 | 94 | 1.32 | Closed Recirculating Loop |
| DAP + Hydroxyethylidene Diphosphonic Acid (HEDP) | DAP: 25, HEDP: 25 | Total: 50 | 96 | 1.41 | Cooling Tower System |
| DAP + Benzotriazole (BTA) | DAP: 30, BTA: 10 | Total: 40 | 91 (Cu alloys) | 1.28 | Heat Exchanger (Brass Tubes) |
| DAP + Zinc Sulfate | DAP: 35, Zn²⁺: 5 | Total: 40 | 93 | 1.15 | Once-Through Cooling System |
| DAP + Polycarboxylate Dispersant | DAP: 30, Polymer: 20 | Total: 50 | 89 | 1.20 | Scale-Corrosion Control |
Note: Synergy Index = (Efficiency of blend) / (Sum of individual efficiencies normalized to total dose)
Studies by Li et al. (2023) at Tianjin University revealed that DAP-HEDP blends form ternary complexes with Ca²⁺ and Fe²⁺, creating dense, cross-linked protective films resistant to shear stress and microbial attack. Meanwhile, DAP-zinc systems benefit from the cathodic inhibition provided by Zn(OH)₂ precipitation, augmented by DAP’s anodic suppression.
Compatibility with Common Treatment Regimens
Industrial water treatment often involves multiple chemical additives—including biocides, scale inhibitors, and dispersants. Compatibility testing ensures no adverse interactions occur.
Table 3: Compatibility Assessment of DAP with Common Additives
| Additive Class | Example Compound | Observed Interaction | Recommendation |
|---|---|---|---|
| Oxidizing Biocide | Sodium Hypochlorite | Partial oxidation of DAP to nitriles; reduced efficacy | Avoid simultaneous dosing |
| Non-Oxidizing Biocide | DBNPA (2,2-Dibromo-3-nitrilopropionamide) | No visible reaction; full activity retained | Compatible |
| Phosphonate Scale Inhibitor | ATMP (Aminotris(methylenephosphonic acid)) | Enhanced dispersion and stability | Recommended combination |
| Polyacrylate Dispersant | Acrylic-Maleic Copolymer | No precipitation; improved particle suspension | Fully compatible |
| Silicate-Based Inhibitor | Sodium Metasilicate | Gel formation at >40 °C; viscosity increase | Use with caution above 35 °C |
| Morpholine pH Modifier | Morpholine | No interaction; stable pH control | Safe to co-administer |
Field trials at Shenhua Group’s thermal power plant (2021–2022) confirmed that DAP-based formulations maintained over 90% corrosion inhibition efficiency when used alongside non-oxidizing biocides and phosphonates, with no fouling or dosage instability observed over a 12-month period.
Environmental and Safety Considerations
Despite its performance advantages, the environmental profile of DAP must be evaluated. It is biodegradable under aerobic conditions, though the degradation rate depends on microbial community composition.
According to OECD 301B tests, DAP achieves 72% biodegradation within 28 days, classifying it as "readily biodegradable." However, acute toxicity studies show moderate ecotoxicity:
- LC₅₀ (Rainbow Trout, 96 hr): 48 mg/L
- EC₅₀ (Daphnia magna, 48 hr): 32 mg/L
- Algal Growth Inhibition (72 hr): IC₅₀ = 18 mg/L
These values suggest that DAP should be handled carefully in once-through discharge systems. Nevertheless, in closed-loop cooling systems with minimal blowdown, environmental impact is negligible.
From an occupational health standpoint, DAP is corrosive to skin and eyes and releases toxic fumes when heated. Proper PPE and ventilation are required during handling. The permissible exposure limit (PEL) recommended by ACGIH is 2 ppm (time-weighted average).
Field Application Case Studies
Case Study 1: PetroChina Refinery – Cooling Water System Upgrade
At the Daqing refinery, traditional chromate-based inhibitors were phased out due to environmental regulations. A pilot program introduced a DAP-HEDP-zinc formulation at 45 mg/L total dosage.
- System: Carbon steel piping, stainless steel heat exchangers
- Flow Rate: 1.8 m/s
- pH: 8.3 ± 0.2
- Monitoring Duration: 18 months
Results showed:
- Average corrosion rate reduced from 8.2 mpy to 1.3 mpy
- No pitting observed via ultrasonic testing
- Microbiological counts remained below 1×10⁴ CFU/mL
Case Study 2: Shanghai Desalination Plant
In a reverse osmosis pretreatment loop with high chloride content (up to 2,200 mg/L Cl⁻), a DAP-polycarboxylate blend was tested against conventional imidazoline.
| Parameter | Imidazoline-Based | DAP-Based |
|---|---|---|
| Initial Corrosion Rate (mpy) | 6.5 | 5.8 |
| After 6 Months (mpy) | 9.1 | 3.2 |
| Biofilm Accumulation | Moderate | Low |
| Chemical Cost ($/m³ water) | 0.42 | 0.38 |
The DAP formulation not only outperformed the benchmark but also reduced operational costs.
Comparison with Conventional Inhibitors
To contextualize DAP’s role, a direct comparison with widely used inhibitors is presented below.
Table 4: Comparative Analysis of Corrosion Inhibitors
| Inhibitor | Typical Dosage (mg/L) | Efficiency (%) | Environmental Impact | Cost (USD/kg) | Stability at High T | Main Drawbacks |
|---|---|---|---|---|---|---|
| 1,3-Diaminopropane (DAP) | 20–50 | 85–96 | Moderate (toxic to aquatic life) | 4.50 | Good (up to 70 °C) | Requires pH control; odor issue |
| Zinc Sulfate | 5–10 | 70–80 | High (zinc accumulation) | 1.20 | Poor (>50 °C) | Environmental discharge restrictions |
| Sodium Nitrite | 200–500 | 80–90 | High (nitrosamine risk) | 0.80 | Excellent | Carcinogenic potential |
| Imidazoline Derivatives | 10–30 | 85–92 | Low to moderate | 12.00 | Excellent | High cost; foaming issues |
| Phosphonates (e.g., HEDP) | 10–25 | 75–88 | Moderate (eutrophication) | 6.00 | Good | Limited anodic protection |
Data compiled from industry reports (NACE International, 2022) and Chinese National Water Treatment Standards (GB/T 18175-2018).
DAP emerges as a cost-effective, high-efficiency alternative, particularly suitable for systems where zinc and nitrite use is restricted.
Optimal Formulation Guidelines
Based on extensive testing, the following guidelines are recommended for designing DAP-based corrosion inhibitor packages:
- Dosage Range: 20–50 mg/L for carbon steel; up to 100 mg/L in high-chloride or elevated-temperature systems.
- pH Range: 7.5–9.0 for optimal protonation and adsorption.
- Co-Inhibitors: Combine with HEDP or molybdate for enhanced film stability.
- Dosing Strategy: Continuous feed preferred; avoid slug dosing to maintain consistent surface coverage.
- Monitoring: Use corrosion coupons, ER probes, and LPR sensors for real-time assessment.
Additionally, pre-filming procedures involving higher initial doses (e.g., 100 mg/L for 48 hours) are advised during system startup to establish a robust protective layer.
Conclusion
1,3-Diaminopropane demonstrates strong potential as a next-generation corrosion inhibitor in industrial water treatment. Its molecular architecture enables multifunctional inhibition through adsorption, chelation, and interfacial pH modulation. When formulated synergistically with phosphonates or molybdates, DAP achieves inhibition efficiencies exceeding 95%, rivaling or surpassing conventional technologies. Field applications confirm its reliability across diverse water chemistries, from soft freshwater circuits to aggressive brackish environments.
While environmental and handling concerns exist, they are manageable within engineered systems. With increasing regulatory pressure on heavy metals and nitrites, DAP represents a viable, sustainable alternative aligned with green chemistry principles. Continued research into hybrid formulations, biodegradability enhancement, and smart delivery systems will further solidify its role in modern corrosion management strategies.
