Corrosion and protection of the inner wall of pressure vessels

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Container corrosion: Due to varying operating conditions, pressure vessels can suffer from various types of metal corrosion such as general corrosion and local corrosion, which significantly affect their safety in actual operation. Metal corrosion can lead to the failure and destruction of pressure vessels. It can even lead to explosions, resulting in casualties and property damage. Therefore, it is particularly important to study the corrosion and protection of pressure vessels in petrochemical production. Types and characteristics of corrosion: Since the phenomena and mechanisms of metal corrosion are quite complex, there are various methods for classifying metal corrosion. Generally, it can be divided into three major classification systems based on the course of the metal corrosion process, temperature, and the form of damage. But they are often interconnected. Classification based on the course of corrosion: According to the characteristics of the corrosion process, metal corrosion can be divided into chemical corrosion and electrochemical corrosion. The specific mechanism by which a metal material corrodes depends mainly on the type of medium it is in contact with. Chemical corrosion refers to the damage caused by a pure chemical reaction between the metal surface and a non-electrolytic medium. A characteristic of this type of corrosion is that no electric current is generated during the reaction process. Chemical corrosion is further divided into: a) Gas corrosion: the corrosion of metals in dry air (without moisture condensation on the surface) or under the action of high-temperature gases. b Corrosion of metals in non-electrolyte solutions: The corrosion that occurs when metals are in non-conductive liquids. Electrochemical corrosion refers to the degradation that occurs when a metal surface undergoes an electrochemical reaction with an electrolyte solution; a characteristic of this type of corrosion is the generation of electric current during the reaction process. Electrochemical corrosion follows the basic laws of electrochemical kinetics. Electrochemical corrosion is the most common and widespread form of corrosion. For example, the corrosion of metals in various electrolyte solutions, as well as the corrosion of metal structures in most types of media in chemical and metallurgical production, all fall under electrochemical corrosion. According to the temperature classification of corrosion, under the same environmental conditions, metals exhibit significantly different types of corrosion reactions and corrosion rates due to variations in ambient temperature; generally, high temperatures accelerate the corrosion process. Based on the temperature of the corrosive environment, metal corrosion can be divided into two main categories: corrosion at room temperature and corrosion at high temperatures. Corrosion at room temperature refers to the degradation of metals caused by chemical or electrochemical reactions between the metal and the environmental medium under normal temperature conditions. Corrosion at room temperature is widespread; the corrosion of metals in a dry atmosphere is a chemical reaction, whereas the corrosion of metals in a humid atmosphere or in chemical substances such as acids, bases, and salts at room temperature is a type of electrochemical degradation (also known as wet corrosion). High-temperature corrosion refers to the degradation of metals caused by chemical or electrochemical reactions between the metal and the environmental medium under high-temperature conditions. Generally, corrosion that occurs at temperatures above 100°C is classified as high-temperature corrosion during the corrosion process. Corrosion of metals in mixed gases at high temperatures (also known as dry corrosion), corrosion of liquid metals, and molten salt corrosion all fall under the category of high-temperature corrosion. According to the destructive classification of corrosion, metal corrosion usually starts at the metal surface and then gradually spreads inward, causing damage to the metal’s shape or internal structure. Therefore, based on the basic characteristics of the failure modes of metal corrosion, metal corrosion can be classified into general corrosion, local corrosion, and corrosion resulting from the combined action of stress and environmental media. General corrosion refers to corrosive damage that spreads over the entire surface of the metal in a continuous manner. Based on whether the corrosion rate of different parts of the metal surface is the same, general corrosion can be further divided into uniform corrosion and non-uniform corrosion. Local corrosion refers to a condition in which the corrosive process is confined to a specific area on the metal surface, with little damage occurring in other parts of the surface. In terms of the characteristics of corrosion patterns, local corrosion is more harmful than general corrosion, and it is also more difficult to predict. This is because it is impossible or difficult to estimate its corrosion rate, which often leads to accidental or premature damage to equipment, machinery, tools, etc., and can even result in catastrophic accidents. Corrosion prevention: Depending on the various uses of pressure vessels, as well as the medium, temperature, and pressure conditions in which they operate, different materials can be selected to form corrosion-resistant alloys during the manufacturing process of these vessels. Alternatively, alloying elements can be added to metals to enhance their corrosion resistance, thereby preventing or slowing down metal corrosion. Coating protection: The simplest and most effective way to protect metal materials from corrosion is to isolate the materials from the corrosive environment. For example, organic coatings, enamel made of inorganic materials, and the like are applied to metal surfaces to isolate the material from corrosive environments. There are roughly three main types of barrier layers or composite barrier layers: inert or essentially inert, corrosion-inhibiting, and sacrificial. Commonly used coatings at present include oxide coatings, chromate coatings, inorganic coatings, organic coatings, metal platings, composite coatings, etc. The advantages of coating for corrosion protection include simple processing, wide applicability, low cost, abundant available resources, ease of operation, and low metal consumption; it is suitable for various types of structures. The disadvantages are a short maintenance cycle and high maintenance effort. Electrochemical protection: The advantages of electrochemical protection for corrosion prevention are a moderate protection duration, the use of few devices to cover a large area, and excellent protection effects. The disadvantages are poor performance in areas where water flows, as well as energy consumption (in the form of metal). It is suitable for use in underwater environments. Metal plating involves applying a layer of another metal or alloy to the surface of a metal using electroplating as a protective layer. Electroplating is a method that uses electrolysis to deposit a thin layer of another metal on the surface of metal parts. It includes processes such as pre-plating treatment (oil removal, rust removal), applying the metal layer, and post-plating treatment (passivation, deoxygenation). During electrolysis, the metal part is used as the cathode, while the metal to be plated serves as the anode. Both are immersed in an electrolyte containing the components of the coating layer, and direct current is applied; after a certain period of time, a deposited coating is formed. The advantages of electroplating for corrosion protection are a long protection period and low maintenance requirements; it is suitable for structures with limited area or those in harsh environments where maintenance is difficult. The disadvantages are a complex manufacturing process, high costs, and the need for specialized equipment. Anodic protection refers to the process of passivating metals using anodic polarization, and maintaining this passivated state with a weak current in order to protect the metals. In practice, the metal component to be protected can be used as the anode, with graphite serving as the cathode. A certain current density is applied, and the anode potential is maintained within the passivation range, thereby protecting the metal component. The chemical industry primarily uses metals or various alloys to manufacture reactors and oil tanks. Therefore, the anodic protection method is widely used in chemical production. Cathodic protection is a method that involves cathodically polarizing a metal body to protect it from corrosion in an electrolyte. If the cathode potential is negative enough, the metal can remain unoxidized (un-dissolved), thus achieving complete protection. Cathodic polarization can be achieved in two ways: a. External current method: An auxiliary electrode is added to the electrolyte; the positive pole of an external power supply is connected to this auxiliary electrode, while the metal substrate to be protected is connected to the negative pole of the external power supply. The applied current is then adjusted so that the metal reaches the cathodic potential required for protection. More often, a high-power potentiostat is used to control the potential of the metal being protected. b. Sacrificial anode method: A more reactive metal is attached to the metal substrate, forming a galvanic cell that creates a short circuit in the electrolyte, with the metal substrate acting as the cathode. The active metal, on the other hand, acts as the anode and is continuously oxidized or dissolved. Corrosion inhibitors: Small amounts of substances that, when added to a certain medium, can significantly suppress metal corrosion are known as corrosion inhibitors. The corrosion inhibition mechanisms of corrosion inhibitors can be divided into promoting passivation, forming a precipitate film, and forming an adsorption film, etc. Passivation-promoting corrosion inhibitors include chromates and nitrites, which, due to their strong oxidizing capacity, promote the passivation of steel materials. Typical corrosion inhibitors that form a precipitative film include polyphosphates, polysilicates, and organic phosphates; these substances form a precipitative film together with corrosion products or ions such as Ca2+ and Mg2+ present in the environment, thereby suppressing corrosion. Most of the corrosion inhibitors that form adsorption films are organic substances; they form single-layer or multi-layer adsorption films on the metal surface through physical or chemical adsorption, thereby separating the metal surface from the corrosive environment. The advantage of corrosion inhibitors for anti-corrosion purposes is that they can effectively reduce or even eliminate corrosion, prevent brittle fracture accidents, avoid product contamination, and prevent heat transfer losses. The disadvantages are high technical requirements and immaturity, along with a lack of standardization and comparability.