Research and Application of Corrosion Inhibitors for Shutdown Protection of Thermal Equipment

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During the downtime of thermal equipment, if no effective protective measures are taken, severe oxygen corrosion can occur on the inner surfaces of their steam-water systems. This type of corrosion is known as “downtime corrosion” of thermal equipment. Downtime corrosion can cause extensive damage to the equipment in a short period of time and generate large amounts of corrosion products within the steam-water system. This not only prolongs the time required to restart the equipment, but also exacerbates corrosion and scaling, ultimately reducing the efficiency of the equipment or even forcing it to be shut down. Clearly, downtime corrosion poses a great threat to the safe and efficient operation of thermal equipment; therefore, effective protection measures must be implemented for such equipment during its downtime. The use of corrosion inhibitors in downtime protection offers numerous advantages: they provide excellent protection, have a long-lasting effect, and are easy and cost-effective to apply. As a result, research and application of corrosion inhibitors for downtime protection have attracted increasing attention. This paper provides an overview of the characteristics of commonly used corrosion inhibitors for downtime protection both domestically and internationally, along with methods for evaluating their effectiveness, influencing factors, and practical applications. It also offers suggestions regarding future research directions in this field.

1 Characteristics of Corrosion Inhibitors for Downtime Protection
Corrosion inhibition for thermal equipment during downtime can be categorized into two main types: wet protection and dry protection. The corrosion inhibitors used in each method possess distinct properties and mechanisms of action.

1.1 Wet Protection Using Corrosion Inhibitors
Wet protection involves filling the equipment with a protective solution containing a certain concentration of corrosion inhibitor after it has been taken out of service. This ensures that the metal surfaces come into contact with the solution and are thus protected. Common wet protection inhibitors include hydrazine, dimethyl oxime, acetaldoxime, diethylhydroxylamine, isopropylhydroxylamine, and sodium ascorbate. Their molecular structures and LD50 values (the dose at which half of test subjects die after oral administration) are listed in Table 1. As shown in the table, hydrazine has an LD50 value of just 59 mg/kg, classifying it as a highly toxic chemical. Additionally, hydrazine vapor can harm human skin and respiratory systems. In November 1985, the U.S. Occupational Safety and Health Administration (OSHA) officially designated hydrazine as a “hazardous substance” and identified it as a potential carcinogen. It was stipulated that the permissible concentration of hydrazine in workplace air should not exceed 1 mg/L. Consequently, since the mid-1970s, researchers have been developing new, non-toxic or low-toxicity deoxidizing and passivating inhibitors to replace hydrazine. These inhibitors exhibit strong reducing properties; they can reduce dissolved oxygen in water, thereby serving as deoxidizers. Moreover, they can convert high-valence metal oxides into lower-valence forms (such as Fe₃O₄ or Cu₂O), promoting the formation of a complete and dense protective film on metal surfaces—hence their passivating effects as well. Results from potentiodynamic polarization tests indicate that at room temperature, low-carbon steel immersed in aqueous solutions containing small quantities (≤200 mg/L) of HDZ, AO, DEHA, NIPHA, or ERA exhibits a clear passivation region on its anodic polarization curve. Therefore, these inhibitors primarily inhibit metal corrosion through deoxidation and passivation. However, since the temperature of the protective solution closely matches ambient temperature, their reaction rates with oxygen—particularly that of HDZ—are quite slow under such low temperatures. Thus, they cannot solely rely on deoxidation to prevent oxygen-induced corrosion, especially during the initial phase of downtime. In this context, their passivating effect plays a crucial role in preventing metal corrosion. Wet protection using corrosion inhibitors is mainly employed for boilers, superheaters, high-pressure heaters, and deaerators kept in cold standby or long-term storage; however, it is unsuitable for protecting equipment undergoing maintenance. Furthermore, these protective solutions typically contain relatively high concentrations of ammonia, making them inappropriate for equipment containing copper alloys.

1.2 Dry Protection Using Corrosion Inhibitors
Corrosion inhibitors suitable for dry protection mainly fall into two categories: volatile corrosion inhibitors and film-forming inhibitors.

1.2.1 Volatile Corrosion Inhibitors
Volatile corrosion inhibitors (VPIs) emerged in the 1940s as effective anti-corrosion materials. Due to their high efficiency, long-lasting effects, ease of application, and independence from equipment geometry, they are widely utilized in the machinery, electronics, and defense industries as key means for rust prevention and long-term storage of mechanical products and military hardware. VPIs possess appropriate vapor pressures (generally ranging between 0.00013 Pa and 0.013 Pa), allowing them to fill the entire protected space and adhere to metal surfaces, thereby inhibiting corrosion via both corrosion inhibition and alkalization mechanisms. For thermal equipment, VPIs are primarily applied to boilers, high-pressure heaters, turbines, etc., during periods of cold standby or extended storage. Commonly used VPIs include cyclohexylamine carbonate and dicyclohexylamine nitrite, typically applied at concentrations of 80–100 g/m³ within the protected area. In practice, VPIs can be vaporized via hot-air inflation systems and introduced into the steam-water systems of nearly dry equipment, ensuring their concentration exceeds 30 g/m³. Alternatively, they may be dissolved in water and subsequently sprayed onto metal surfaces. While VPIs offer the aforementioned benefits in downtime protection, they also present certain limitations. In many cases, it is necessary to remove VPIs from metal surfaces prior to equipment startup. Additionally, given their corrosive nature toward copper alloys, they are unsuitable for protecting equipment containing such components. Finally, due to their inherent toxicity, VPIs should not be used for protecting equipment undergoing maintenance.

1.2.2 Film-Forming Inhibitors
Film-forming protection entails introducing corrosion inhibitors into the steam-water systems of thermal equipment during shutdown procedures, thereby forming a protective film on metal surfaces that prevents corrosion. Initially, straight-chain amine compounds containing 10–20 carbon atoms were predominantly employed as film-forming inhibitors; hence, they are also referred to as “film-forming amines.” Octadecylamine (ODA) is the most widely used film-forming amine, though decylamine, hexadecylamine, and hydroxyethyl sojaamine (CH₃(CH₂)₄CH=CH(CH₂)₁₁N(C₂H₄OH)₂) are also utilized. Besides these, China has recently developed imidazoline-based film-forming inhibitors such as ODW and SM-ODM. The molecules constituting these inhibitors comprise polar hydrophilic groups centered around highly electronegative nitrogen atoms, alongside non-polar hydrophobic hydrocarbon chains. Upon exposure to corrosive environments, the polar groups attach themselves to metal surfaces, while the non-polar groups orient themselves away from the surface, ultimately forming a hydrophobic protective layer across the entire surface. Thus, they primarily inhibit corrosion caused by dissolved oxygen and CO₂ in water via the shielding effect exerted by their non-polar groups. This protective technique enables long-term dry protection of steam-water systems in thermal equipment; it is highly effective and simple to implement, making it particularly suited for protecting units undergoing major overhauls. Nevertheless, it does have certain drawbacks: its efficacy diminishes significantly in low-temperature condensate systems, and it might adversely affect ion-exchange resins within condensate purification systems.

2 Evaluation Methods for Corrosion Inhibition Effectiveness
In both research and application of corrosion inhibitors for downtime protection, it is essential to assess their effectiveness. Common evaluation techniques include the CuSO₄ drop test, humidity-heat testing, and electrochemical analysis.

2.1 CuSO₄ Drop Test
This test utilizes an acidic CuSO₄ solution prepared by mixing 40 mL of 0.4 mol/L CuSO₄, 20 mL of 10% NaCl, and 15 mL of 0.1 mol/L HCl. When this solution is dropped onto a film-coated test specimen, its corrosive nature gradually erodes the protective film, exposing the underlying metal substrate. Subsequently, Cu²⁺ ions from the solution displace Fe atoms in the substrate, resulting in the precipitation of red metallic copper; consequently, the film’s color shifts from blue to red. A longer duration before this color transition occurs indicates stronger protective capabilities; conversely, a shorter interval between initial reddening and full coloration suggests greater uniformity of the film. This method is straightforward, comprehensive, and highly practical for assessing film quality.

2.2 Humidity-Heat Testing
Humidity-heat testing involves suspending film-coated specimens inside a chamber maintained at constant temperature (49 ± 1°C) and relative humidity exceeding 95%. Researchers then monitor the timing at which rust spots and widespread corrosion manifest on the specimens. Longer intervals before rust formation imply superior film performance, whereas shorter durations between rust initiation and extensive corrosion denote more uniform film distribution. This approach allows for batch testing of multiple specimens, albeit requiring considerable observation time.

2.3 Electrochemical Analysis
Electrochemical analysis evaluates film quality by measuring specific electrochemical parameters of film-coated electrodes. Typically, a three-electrode setup is employed, wherein the working electrode consists of the film-coated specimen itself. The testing solution generally comprises 0.025 g/L NaCl, 0.057 g/L Na₂SO₄, and 0.164 g/L Na₂CO₃. Two primary techniques exist: polarization curve analysis and alternating current impedance spectroscopy. Polarization curve analysis involves determining the anode polarization curve of the coated electrode and inferring inhibition effectiveness based on breakdown potential—higher positive values signify better protection. Alternating current impedance spectroscopy, meanwhile, gauges electrode impedance; the Nyquist plot corresponding to such measurements usually appears as an arc situated below the real axis. Both the diameter of this arc (representing resistance Rf) and the impedance magnitude measured at the lowest frequency in the testing spectrum serve as indicators of protective efficacy—greater values denote superior performance. Although electrochemical analysis demands more time than the CuSO₄ drop test, its results are considerably more objective and precise.

3 Influencing Factors Affecting Corrosion Inhibition Effectiveness
Numerous studies have explored factors influencing volatile corrosion inhibitors; herein, we focus on elements affecting wet protection inhibitors and film-forming inhibitors.

3.1 Wet Protection Inhibitors
For wet protection inhibitors, pH levels of the protective solution and inhibitor concentration represent critical determinants of overall effectiveness.

3.1.1 Impact of pH Levels
When employing wet protection inhibitors, optimal results are achieved only when the pH level of the protective solution surpasses 10. Accordingly, industry guidelines recommend adjusting the pH of ammonia-hydrazine solutions to 10–10.5 using ammonia water. Zhao Fengjuan et al. conducted experiments wherein 20A carbon steel specimens were immersed in varying pH-level solutions containing 400 mg/L DMKO (pH adjusted via ammonia water). Findings revealed that higher pH levels correlated with prolonged protection durations