N,N-Diethylhydroxylamine (DEHA) functions primarily as an oxygen scavenger and metal passivator in boiler water systems. Its working principle involves two key mechanisms: chemical reduction of dissolved oxygen and formation of a protective oxide film on metal surfaces.
Oxygen Scavenging Mechanism
DEHA reacts directly with dissolved oxygen in water, reducing it to harmless products. The overall reaction is:
2(C₂H₅)₂NOH + O₂ → 2 C₂H₅NOC₂H₄ + 2H₂O
The primary organic product is a nitrone (CH₃CH=NO₃-C₂H₅), formed on an equimolar basis with the DEHA consumed. The reaction proceeds via a radical chain mechanism involving HO₂ and its conjugate base O₂⁻ as chain carriers, which propagate by hydrogen atom abstraction from DEHA. The rate of oxygen uptake is strongly pH-dependent, with higher pH accelerating the reaction.
The stoichiometric requirement is approximately 1.24 ppm DEHA per 1 ppm O₂ theoretically, though 3 ppm DEHA per 1 ppm O₂ is recommended in practice due to competing reactions and system losses.
Key Limitation and Catalysis
DEHA’s reaction with oxygen is slow at low temperatures, which is a particular problem when it is dosed into feed lines (typically 5–120°C). This limitation is overcome through:
Tannin catalysis: Small amounts of tannin (e.g., Mimosa tannin) catalyze the oxygen scavenging activity, enabling effective performance at significantly lower temperatures. The optimal DEHA-to-tannin ratio is about 4:1 by weight.
Hydroquinone catalysis: Hydroquinone also accelerates the reaction, making the formulation suitable for wet-conservation during shutdowns.
Molybdate synergy: Sodium molybdate exhibits a synergistic catalytic effect with DEHA.
Metal Passivation Mechanism
Beyond oxygen removal, DEHA acts as a strong metal passivator. As a reducing agent, it converts porous hematite (Fe₂O₃) to magnetite (Fe₃O₄), a dense, protective oxide layer that reduces further corrosion. Polarization measurements confirm that DEHA enhances passive film formation on mild steel, and this effect is independent of solution pH between 8.0 and 9.0. The passivation effect is attributed to adsorption of the inhibitor on the metal surface.
Volatility and System-Wide Protection
DEHA is volatile, with a vapor-liquid distribution ratio comparable to neutralizing amines. This volatility allows DEHA to vaporize in the boiler and travel with steam into condensate and return lines, providing corrosion protection throughout the entire steam-condensate circuit by scavenging residual oxygen and neutralizing carbonic acid. This distinguishes it from non-volatile scavengers like hydrazine or sodium sulfite.
Decomposition Products
At elevated temperatures, DEHA decomposes into acetaldehyde, acetic acid, dimethylamine, and ethylamine. The formation of acidic species (acetic acid) during the transient period can temporarily increase acidic conductivity and decrease pH. The final decomposition products are ultimately acetate, nitrogen, and water.
Summary of Working Principle
Function Mechanism Key Detail
Oxygen Scavenging Direct reduction of O₂ via radical chain mechanism Products: nitrone + water; rate pH-dependent
Catalysis Tannin/hydroquinone/molybdate accelerate low-T reaction Optimal DEHA:tannin ≈ 4:1
Metal Passivation Conversion of hematite to protective magnetite Enhances passive film via adsorption
System Protection Volatility distributes DEHA through steam-condensate circuit Protects feed, boiler, and condensate lines
DEHA is particularly recommended for low and medium pressure boilers and is considered one of the safest oxygen scavengers (oral LD₅₀ in rats = 2190 mg/kg), with no toxic or solid by-products upon decomposition.