What Is Corrosion Inhibition?
What Is Corrosion Inhibition?
Corrosion inhibition means suppressing or slowing the corrosion reaction between a metal and its environmental medium by chemical agents. Its mechanisms include adsorption film formation and alteration of electrode reaction processes [1] [4-5]. According to the Shanghai engineering construction standard, corrosion inhibition is defined as "a treatment process that suppresses or slows the corrosion of metal", which explicitly requires corrosion inhibition treatment and corrosion-rate monitoring for air-conditioning water systems [3]. In metal cultural-relic protection, corrosion inhibitors protect artifacts by forming passivation films (e.g., benzotriazole for copper objects) or adsorption layers that block corrosive media [4].
The application of corrosion inhibitors began with metal pickling in the mid-19th century. In 1845, American steel companies used additives to prevent corrosion in acid-immersion processes. In 1860, Baldwin of the United Kingdom obtained the world's first corrosion-inhibitor patent, a mixture of syrup and vegetable oil. In 1872, Marangoni et al. published rosin- and lavender-oil inhibitors for the pickling of copper and zinc, as well as animal-glue inhibitors for iron in acidic media; early inhibitors mostly used animal and plant raw materials [8].
From the early 20th century to the 1930s, research shifted toward isolating nitrogen-, sulfur- and oxygen-containing organic compounds from coal tar. In the mid-1930s, synthetic organic inhibitors were successfully developed, and research on inorganic inhibitors for neutral media also advanced. From the 1930s onward, debates over inhibitor mechanisms drove theoretical progress; in 1961, Hackerman proposed applying the Hard-Soft Acid-Base (HSAB) principle to inhibitor design [8].
Since the beginning of this century, research on polymers as corrosion inhibitors has received growing attention. Today, inhibitor applications have expanded from traditional industries to high-tech fields such as new energy, electronic devices and aerospace; the protected objects have expanded from conventional metals to light alloys and multi-metal assemblies; and the service environments have expanded from aqueous media to oil, gas phases, coatings and concrete [8]. Notable milestones: in 1943, Shell Development Co. of the USA developed dicyclohexylamine nitrite; in 1981, the Corrosion Inhibitor Specialty Committee of the Chinese Society of Corrosion and Protection was founded; in the late 1970s, China developed the 7701 composite inhibitor, which was successfully applied in ultra-deep wells [14].
In the 1950s, inorganic inhibitors came into wide use; the 1960s and 1970s were the golden age of organic inhibitor development [9]. In the 1980s, efforts began to extract active inhibitor ingredients from natural plants; in 1993, Aramaki systematically classified the active centers of inhibitors according to elemental properties [8]. After the 1970s, the number of inhibitor varieties grew slowly, and work turned to theoretical and evaluation studies [9].
By composition, corrosion inhibitors are classified into inorganic types (chromates, silicates), organic types (amino-acid types, benzotriazole) and composite types [1-2] [4]. In addition, by the characteristics of the protective film they form, they can be divided into oxide-film, precipitated-film and adsorbed-film inhibitors; by application, into pickling, seawater, oil/gas-well acidizing, cooling-water and vapor-phase inhibitors, etc. [8] [14].
Anodic inhibitors (such as calcium oxide) block the anodic reaction by forming a protective film; cathodic inhibitors (such as molybdates) suppress the cathodic process; mixed types combine both characteristics [6]. The main performance indicators of a corrosion inhibitor are its inhibition efficiency and its persistent (post-effect) performance [8] [14]. Corrosion inhibitors are highly selective: their effectiveness is strictly affected by environmental conditions such as concentration, medium pH, temperature and flow velocity [10-12].
Using two or more inhibitors together may produce a synergistic effect that enhances inhibition; formulated composite inhibitors are usually more efficient than single ones. Combining a cathodic inhibitor with an anodic inhibitor is a common formulation principle [11]. Formulation technology can raise inhibition efficiency: for example, blending polyethylene glycol diacid with zinc oxide reduces the corrosion rate of aluminum alloys by an order of magnitude [2].
In industry, corrosion inhibitors are applied to circulating cooling water (neutral media), scale-removal hydrochloric acid (acidic environments) and oil and gas exploration [1] [5]. In heritage conservation, the "Nanhai No. 1" ancient shipwreck uses corrosion-inhibition technology to control seawater corrosion [4]. According to the World Corrosion Organization, global annual corrosion losses amount to USD 1.8 trillion (2023 statistics); corrosion-inhibition measures can reduce these economic losses and extend equipment service life [1] [6]. China's Design Code for Chemical Treatment of Air-Conditioning Water Systems sets a technical limit requiring the corrosion rate of carbon-steel equipment to be below 0.075 mm/a [3].
Current research focuses on the development of environmentally friendly inhibitors and mechanism analysis, with emphasis on:
Research trends toward replacing traditional toxic agents with natural-product extracts [1] [4]
Optimization of synergistic effects in formulation systems (e.g., combinations of calcium hydroxide with amino acids) [1-2]
Matching the corrosion-inhibition requirements of metal-air batteries [1] [3]
Green inhibitors mainly include natural plant extracts (e.g., extracts of bark, leaves, fruits and seeds) and biodegradable organic compounds (e.g., amino acids, imidazolines and Mannich bases). Among them, polyaspartic acid (PASP) is a successful representative of green, phosphorus-free corrosion-and-scale inhibitors. [7] [13] [16-17] Frontier research areas include the action mechanisms of inhibitors on existing corrosion-product films, the study of mechanisms at the molecular scale using in-situ analytical techniques, the improvement of rapid, accurate and in-situ field evaluation methods for inhibitors, and the study of synergistic mechanisms and quantum-chemistry-based structure-activity relationships. [15]
China is one of the most active countries in corrosion-inhibitor research worldwide, contributing about 17% of the related academic papers. Future directions for green inhibitors include: extracting and separating active ingredients from natural products; developing new inhibitors from agricultural and industrial by-products; applying quantum chemistry and artificial neural networks to design highly efficient, low-toxicity products; conducting in-depth research on inorganic green inhibitors such as molybdates, tungstates and rare-earth compounds; and developing multi-functional inhibitors (e.g., combining biocidal and scale-inhibition functions) as well as inhibitors tailored to specific environments such as reinforced concrete, the vapor phase, and aluminum/magnesium alloys. [15-16]
