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Chapter 2: Basic Theories of Protection Section 1: The Importance of Protection The protection of metals against corrosion has a long history. China has made significant contributions to the field of corrosion protection; Emperor Qin Shi Huang’s swords, as well as China’s painting industry, are well-known around the world – in particular, Chinese lacquer was already famous back in 1515. Antirust paints were first introduced in the world in 1882, and red lead was recommended for use as a primer in 1885. In Britain, between 1830 and 1850, it was discovered that coal tar, pitch tar, and other materials could be used together to prevent corrosion of steel pipes. In 1919, asphalt was applied inside water supply pipes in the United States. Cathodic protection was first mentioned in 1875. Entering the 20th century, various coating methods, electrochemical protection, and corrosion inhibition techniques developed rapidly, giving rise to a well-established field of metal protection science. I. The scientific basis of protection: Figure 1-2-1 shows the relationship among corrosion loss costs, measures taken to address these losses, and protection efforts. This is a scientific field that is directly related to economic management; by implementing effective protection measures, economic losses can be reduced by 30%-40%, thereby significantly improving the financial performance of factories. II. Regarding the awareness of the importance of protection in management and education, one approach is to enact legislation; for example, the United States has established legal provisions for the protection of buried pipelines, requiring that such pipelines be protected and regularly inspected. Germany and the former Soviet Union also have corresponding regulations; these are mandatory laws. To raise the awareness of all types of personnel, especially managers, a training system should be established to carry out extensive education efforts, thereby enhancing the willingness to take protective measures. By combining this willingness with scientific approaches, the level of protection can be improved, with the aim of reducing economic losses and the occurrence of accidents. The content and scope of corrosion prevention education are shown in Figure 1-2-2. Section 2: Basic Principles of Protection I. Improving the Corrosion Resistance of Metal Materials The principles for enhancing the corrosion resistance of metal materials include: (1) Improving the alloy composition from the perspective of enhancing the thermodynamic stability of the material. Thermodynamic stability depends not only on the metal itself but also on the corrosive environment; the basic principle is to use components that are thermodynamically stable in a particular environment to alloy with metals that are prone to corrosion, thereby achieving protective effects. (II) Enhancing cathode control: on the one hand, reducing the area of the cathodic region in the alloy; on the other hand, adding additives to the alloy that increase the overpotential of the cathodic depolarization reaction. (III) Enhance anode control by adding active cathodic substances to the alloy, such as small amounts of precious metal elements like palladium and platinum to stainless steel. Alloying to improve the passivability of alloy anodes can also achieve anode control. II. Environmental corrosion alteration is the result of the interaction between the environment and the material. Due to differences in environment, corrosion conditions vary; from an environmental perspective, the following points represent important preventive measures. (1) Temperature and flow rate: Generally speaking, reducing the temperature can slow down the corrosion rate ； Flow rate is a very complex factor; increasing the flow rate often accelerates corrosion, but it can also lead to passivation and thereby reduce corrosion. (II) Oxygen content in the medium: Oxygen in the medium acts as a cathodic depolarizer, and its effect on corrosion is also complex. For example, waterline corrosion and oxygen concentration cell corrosion are both caused by differences in oxygen content. (III) Stress: The presence of stress accelerates corrosion and is the main cause of stress corrosion cracking; therefore, it requires sufficient attention. (IV) Corrosion inhibitors: The addition of a small amount of corrosion inhibitors can significantly reduce the rate of metal corrosion, and they have now become one of the main methods used for industrial corrosion prevention. III. Protection from an electrochemical perspective (1) Cathodic protection: It involves artificially preventing cathodic oxidation in order to achieve anti-corrosion goals. (II) Anodic protection: It achieves corrosion prevention by enabling anodic passivation. (III) Metal coatings: These include cathodic coatings, anodic coatings, and multi-layer coatings, which are protection methods aimed at achieving control through cathodes, sacrificial anodes, or a combination of both cathodic and anodic controls. IV. Isolating the material from the corrosive medium – although this is a very old method, it remains one of the main ways to prevent corrosion to this day. The main ones are: (1) non-metallic coatings (paints); (2) linings; (3) rust preventive oils; (4) rust preventive papers. Fifth, to prevent corrosion, design considerations should include the selection of appropriate materials, material compatibility, a rational structure, and proper connections between components to avoid gaps. Section 3: The Role of Corrosion Prevention. Some believe that emphasizing the importance of corrosion prevention solely from the perspective of the negative aspect of corrosion losses is one-sided and does not yield good practical results. In 1993, Professor Bakers, then president of NACE, stated in his presentation at the 12th International Corrosion Conference titled “Transferring Knowledge to Control Corrosion” that making the public and policymakers aware of how much money can be saved through successful corrosion prevention techniques would improve their perception of corrosion scientists more than highlighting the losses caused by corrosion. Following this positive recommendation, the role of corrosion prevention will be explained below from the perspectives of economy, production, new technologies, and resource conservation. First, as is well known in the field of economics, corrosion is a spontaneous process that is not subject to human will, and its occurrence is inevitable. However, like other natural disasters, corrosion can also be predicted, and it can be controlled by adopting certain anti-corrosion measures. Currently, abroad there are many approaches that **classify corrosion losses into two main categories: avoidable and unavoidable. The avoidable corrosion losses refer to those losses that can be prevented by making full use of existing knowledge in corrosion prevention technology, carrying out extensive education on corrosion and its prevention, exchanging experience in related technologies, and implementing strict scientific management practices. For example, after the United States implemented anti-corrosion measures, the increase in corrosion losses over the 10-year period from 1975 to 1985 was reduced by half. Furthermore, the corrosion industry worldwide generally believes that this avoidable portion of losses accounts for about 1/4 of the total losses; in other words, our country could recover at least 25 billion yuan in losses each year. It is evident that the task ahead of tens of thousands of professionals in chemical corrosion prevention is arduous, yet the prospects are also very broad. II. In the modern chemical industry, corrosion prevention has become one of the important indicators reflecting the level of chemical production. This is because in large-scale and continuous modern chemical production, the mutual influence between equipment is a major characteristic of chemical plants. Corrosion damage to a critical piece of equipment can cause the entire installation to come to a halt, and the economic losses resulting from this are much greater than the cost of simply replacing the equipment. For example, in a domestic oil field, more than 100 oil wells were unable to produce oil due to corrosion and perforation of the well casings, which severely affected crude oil production. Another example is the converter in a synthetic ammonia plant with an annual production capacity of 300,000 tons; replacing a single tube due to high-temperature corrosion requires the entire production facility to be shut down for 3 days. If the production is halted 100 times, the losses will equal the total investment in the entire ammonia synthesis plant. It is reported that corrosion in modern chemical plants has become the main cause of equipment damage. American insurance companies have reported that corrosion was responsible for 31.1% of the major chemical accidents that occurred in recent years. It is precisely because of this crucial role that corrosion prevention plays in production that it is essential to pay close attention to the corrosion protection of chemical processing equipment. III. The development of corrosion prevention technologies for new technologies and new industries not only ensures the smooth operation of chemical production but also facilitates the rapid adoption of new processes and technologies, as well as the emergence of new industries. Take the development of corrosion inhibition technology using corrosion inhibitors as an example to illustrate. The industrial production of large-scale urea processes is made possible entirely thanks to the successful development of oxygen supply corrosion prevention technology using austenitic stainless steel. The advanced technological process of oil field acidization to increase oil production could also be put into industrial use only after corrosion inhibitors for oil field acidization were developed. The development of the Lan-5 and Lan-826 acid cleaning inhibitors by the Chemical Machinery Research Institute of China’s Ministry of Chemical Industry led to the emergence of a brand-new, nationwide cleaning industry. IV. By using corrosion-resistant technologies for materials, a large amount of steel and high-grade alloy steel can be saved. We will still use the corrosion inhibition technology with corrosion inhibitors as an example. For example, using a corrosion inhibitor during pickling can reduce metal loss to 1/9. In the extraction of certain oil and gas fields, corrosion inhibitors are used, which can reduce steel degradation to 1/40–1/60 of its original level. In ammonia synthesis plants, where monoethanolamine or Benfey’s solution is used for decarburization, the effective use of corrosion inhibitors in these decarburization solutions reduces the corrosion rate of carbon steel from several millimeters per year to a level where complete corrosion resistance is achieved. As a result, carbon steel can be used for all equipment in the decarburization system, thereby saving a large amount of high-grade stainless steel. In short, extensive chemical production practices in China have shown that the development of the chemical industry is closely linked to advancements in corrosion prevention; with each step forward made by the chemical industry, a series of corrosion-related challenges must be overcome. With each step forward in the chemical industry, the level of corrosion prevention technology also advances accordingly. Since the reform and opening up, to ensure the normal operation of key industries such as fertilizers, pesticides, petrochemicals, and chlor-alkali, the Ministry of Chemical Industry has taken a series of measures to strengthen corrosion prevention in the chemical sector from various aspects, achieving commendable results. First, a professional team of tens of thousands of people in research, design, and construction was established, including several thousand engineering and technical personnel. This team helped solve numerous corrosion-related problems in China’s chemical industry, in the renovation of old factories, and in production maintenance, thereby contributing to the development of the chemical industry. Second, over 100 new anti-corrosion products have been developed, including large-scale chemical equipment made of mechanically wound fiberglass-reinforced plastic and large-scale pre-vulcanized rubber linings. At the same time, various anti-corrosion technologies and testing methods, as well as anti-corrosion coatings, corrosion inhibitors, and water quality stabilizers, have been developed and promoted. Nearly a thousand manufacturers of corrosion-resistant products and companies specializing in corrosion protection work have been established and improved, essentially meeting the needs of China’s industrial sector for such products, spare parts, and corrosion prevention maintenance services. Third, a large number of scientific and technological achievements have been made in recent years, including multiple **Invention Awards. In the field of corrosion and anti-corrosion across the country, there are the most projects awarded in the field of chemical anti-corrosion, and these projects hold the highest rankings. Among these, achievements such as anodic protection for concentrated sulfuric acid coolers, cleaning of insoluble scale in urea processing equipment, and safety monitoring methods for stress corrosion cracking in austenitic stainless steel equipment have been put into practical use, yielding significant results. Fourth, building on the achievements in promoting the Lan-5 and Lan-826 acid cleaning inhibitors, a national cleaning industry led by Lanxing Cleaning Group Corporation was established, making significant contributions to production. Fifth, comprehensive corrosion control engineering has been widely adopted in the chemical industry, and its potential impact is enormous.