『Original by HaiChuan Translation Team』Introduction to Corrosion Inhibitors (Part 2)

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English title: An Introduction To Corrosion Inhibitors. Original article link: Click here. Original author: ankur2061. Translator’s ID: @kid_ptd. Proofreader’s ID: @MyChemicalRomance. Those who wish to subscribe to the articles published by the “Haichuan Translation Team” are invited to leave a message below saying “I want to subscribe” or send me a message; we will notify everyone as soon as new articles are available. Classification of corrosion inhibitors: The chemical components of corrosion inhibitors can react with the metal surface or with substances in its surrounding environment, thereby forming a protective layer on the metal surface. Corrosion inhibitors usually adsorb onto the metal surface, providing protection by forming a thin film. Corrosion inhibitors are usually used in the form of solutions or suspensions; in some cases, they are also incorporated directly into the formulation of coatings. The steps by which corrosion inhibitors reduce corrosion are as follows: – Increasing the polarization behavior of the anode or cathode (Tafel slope) – Reducing the movement or diffusion of ions toward the metal surface – Increasing the resistance of the metal surface. Different authors classify corrosion inhibitors in various ways; some prefer to classify them based on their chemical functions, as detailed below: Inorganic types: These are usually mineral salts, such as sodium chromate, phosphates, molybdates, etc. Only their anions among these substances are involved in the process of reducing metal corrosion. When zinc is used to replace sodium, zinc cations can provide some beneficial effects. Such zinc-containing additives are known as mixed charge corrosion inhibitors. Organic anionic types: sodium sulfonates, phosphonates, or thiobenzothiocarbene (MBT) derivatives are commonly used in cooling water or antifreeze solutions. Organic anionic type: In concentrated form, these substances appear as liquids or waxy solids. Their active groups are usually large fatty chains or aromatic rings, along with positively charged organic amine groups. However, the most popular organizational classification of corrosion inhibitors at present is still based on their functional mechanisms, as detailed below: Passivation corrosion inhibition (anodic corrosion inhibition). These inhibitors cause a widespread anodic shift in the corrosion potential, forcing the metal surface to enter a passivated state. There are two common types of passivation inhibitors: one is the oxidative ionic type, such as chromates, nitrites, and nitrates, which can passivate steel in an oxygen-free environment ； Another category is the non-oxidizing ionic types, such as phosphates, tungstates, and molybdates, which form passivation only under aerobic conditions. This is the most effective type of corrosion inhibitor, and therefore its applications are the most widespread. Chromate-based corrosion inhibitors are the cheapest among them, and have been used in various applications until recent years (such as the circulating cooling systems of internal combustion engines, distillation towers, refrigeration units, and cooling towers). Typically, sodium chromate at concentrations of 0.04% to 0.1% is used in such applications; higher concentrations of sodium chromate are required when the temperature is higher or when the chlorine content in fresh water exceeds 10 ppm. If necessary, add sodium hydroxide to the system and adjust the pH between 7.5 and 9.5. And when the concentration of chromate is below 0.016%, the corrosion of the system accelerates. Therefore, it is essential to conduct colorimetric analysis regularly to avoid such situations. In short, when the concentration is below the minimum limit, passivation corrosion inhibitors do cause pitting and exacerbate corrosion; therefore, it is crucial to analyze and monitor the concentration of these inhibitors. Cathodic corrosion inhibition: Cathodic inhibitors either directly slow down the cathodic reactions themselves, or deposit selectively on the cathode surface to increase the surface impedance and restrict the diffusion of reducing substances toward those surfaces. Cathodic corrosion inhibitors can provide the following three different corrosion inhibition mechanisms: (1) as cathodic toxicants ； (2) As a cathodic deposit ； (3) As a deoxidizer. Some cathodic corrosion inhibitors work by making the recombination and release of hydrogen more difficult, such as compounds containing arsenic or antimony. Others form a protective layer on the metal surface through oxide deposition, such as calcium ions, zinc ions, or magnesium ions. Deoxidizers work by preventing corrosion by suppressing the cathodic depolarization caused by oxygen. At room temperature, the most commonly used deoxidizer is sodium sulfite (Na2SO3). In the presence of organic corrosion inhibitors, both anodic and cathodic effects may occur simultaneously. However, as a general rule, organic corrosion inhibitors only become effective on the entire surface of the corroding metal when they reach a sufficient concentration. Organic corrosion inhibitors usually refer specifically to film-forming agents that create a hydrophobic film on the metal surface to protect it. The protective effect depends on its chemical composition, molecular structure, and the attraction to the metal surface. Since film formation is an adsorption process, temperature and pressure in the system are very important influencing factors. The adsorption of organic corrosion inhibitors is directly related to their own ionic charge as well as the charge on the metal surface. Anodic corrosion inhibitors such as organic amines, or cathodic corrosion inhibitors such as sulfonates, will be preferentially adsorbed due to the negative or positive charge on the metal surface. The strength of the adsorption bond is a decisive factor in characterizing soluble organic corrosion inhibitors. The molecules of such materials adsorbed on the metal surface form a protective layer that prevents the metal from dissolving in the electrolyte. The degree of coverage on the metal surface is directly proportional to the concentration of the corrosion inhibitor; therefore, the concentration of the corrosion inhibitor in the medium is crucial. For any given corrosion inhibitor, there is an optimal concentration for use in a specific type of medium. For example, in water at pH 7.5, when used in combination with 17 ppm of sodium chloride or 0.5% by mass of ethyl octanol, sodium benzoate at a concentration of 0.05% or sodium cinnamate at a concentration of 0.2% are very effective. Corrosion in the ethylene glycol cooling water system can be controlled by using ethanolamine as a corrosion inhibitor. Precipitation corrosion inhibition: Corrosion inhibitors that induce precipitation generally act on the metal surface to form a film, thereby directly blocking cations and anions. Such corrosion inhibitors facilitate the deposition of substances on the metal surface, thereby forming a protective layer. Hard water, which contains high concentrations of calcium and magnesium ions, is less corrosive than soft water; the salts in hard water tend to deposit on metal surfaces and form a protective layer. The most common representatives of such corrosion inhibitors are silicates and phosphates. For example, adding sodium silicate to various water softeners can prevent the formation of rusty water. In pressurized hot water systems, sodium silicate can also protect steel, red copper, and brass materials. However, this protection is not always reliable; it depends largely on pH, as well as a saturation index influenced by the composition and temperature of the water. Phosphates also require oxygen to provide effective corrosion inhibition. Although silicates and phosphates provide less protection than chromates and nitrites, they are very useful in situations where non-toxic additives are required. Volatile corrosion inhibitors Volatile corrosion inhibitors (VCIs), also known as vapor-phase corrosion inhibitors (VPIs), are substances that can move to the site of corrosion through volatilization in enclosed environments. In boilers, some volatile substances such as morpholine and hydrazine vaporize along with the steam; by neutralizing acidic carbon dioxide or reducing the pH of metal surfaces to a more acidic and corrosive level, corrosion in the tubes of the condenser can be prevented. In enclosed gas-phase spaces such as ship cargo holds, volatile solids such as dicyclohexylamine and cyclohexylamine can also be used. Upon contact with metal surfaces, these salt gases condense and hydrolyze, releasing protective ions. For an effective volatile corrosion inhibitor, both rapid corrosion inhibition and long-term protective effects are required; however, these two properties depend on the volatility of the inhibitor – rapid action requires high volatility, while long-term protection does the opposite.