Corrosion-resistant alloy cast iron embedded parts technology exchange meeting

Selection of Corrosion Resistant Cast Iron Materials for Embedded Parts in Seawater Engineering

Zhou Chuanlu Cui Qingli Jiang Hongjun

Laiwu Tianming Metallurgical Equipment Co., Ltd.

Abstract: This article introduces the basic properties of seawater in the estuary area of ​​rivers, the corrosiveness of this kind of seawater, and the corrosion resistance of metal materials in this kind of seawater; it is proposed that the corrosiveness of seawater under this kind of marine environmental conditions has an impact on seawater hydraulic engineering The requirements for corrosion resistance of gate embedded parts, and then put forward the basic principles of material selection for embedded parts in seawater engineering; according to our design requirements for many years, the types of embedded parts, product production quality, and the actual use of materials actually selected in the project , Proposed alternative alloy corrosion-resistant cast iron materials.

Keywords: seawater engineering; seawater corrosion; corrosion-resistant cast iron; material selection principle; economy; process feasibility;

Preface

　　 The ocean is the cradle of all life, and the hope for the future of mankind is an inexhaustible treasure house of resources on the earth. Ocean development, space development, and atomic energy development are the three major development directions in the world. The 21st century is the century of marine development. Marine development is the development and utilization of marine resources. This requires the construction of various marine engineering. Coastal seawater hydraulic engineering is one of them. This kind of seawater engineering is located in the sea area of ​​the river estuary, and its role is Prevent the erosion of land by sea water and the storage and utilization of fresh water. This is the area where fresh water and sea water are mixed. Its basic properties are different from sea water in ordinary oceans, and it is also different from fresh water. It may also be a kind of polluted sea water. This kind of sea water has certain particularity to the problem of metal material corrosion. In recent years, due to the increasing demand for fresh water due to the development of society, people pay more and more attention to this kind of engineering. The design of seawater engineering and the development and selection of metal materials for embedded parts have become increasingly concerned issues. .

1. The basic nature of seawater at the mouth of a river

　　 Seawater is a complex natural balance system. Generally, seawater is an electrolyte solution with high salt content, conductivity, and biological activity. The pH value is about 8.2, which is a corrosive electrolyte solution. However, some changes have taken place in its basic parameters in the waters near the river's estuary, and these changes will also cause some changes in the corrosiveness of seawater.

The salinity of the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea are respectively about 3.0%, 3.2%, 3.3%, and 3.4% on average. However, for offshore waters, especially in the sea areas where rivers enter the sea, the amount of fresh water injected by rivers changes the seawater. The important reason for salinity is that the salinity of river water is only 0.01% to 0.03%. The diluting of river water reduces the salinity of this sea area. The degree of mixing varies in different places, so the salinity is different. The salinity of the seawater in the region even drops below 1%; moreover, the degree of salinity reduction of seawater at different depths is also different; the amount of freshwater injected in different seasons is different, and the salinity reduction is also different. For example, the salinity of seawater along the coast of Jiangsu and Zhejiang is lower than 3.0%, and the decrease in salinity reduces the corrosiveness of seawater; in general, due to the rich nutrients brought by freshwater, marine organisms are rich, and biological pollution makes the corrosiveness more complicated. It may make O2 and CO2 close to the saturated state, and the increase in oxygen content will increase the corrosiveness; at the same time, it may also cause pollution of domestic and industrial sewage, which will reduce the pH value and increase the corrosiveness. If other corrosive pollutants such as H2S increase , Will make the corrosiveness of seawater more complicated;

　　 Although the basic nature of sea water in this sea area has undergone some changes, the main type of corrosion has not changed. From the perspective of the corrosion process mechanism, it is still electrochemical corrosion, and the form of corrosion damage is still full corrosion and local corrosion. For seawater engineering embedded parts, the type of corrosion that occurs may be very complicated, that is, general corrosion caused by electrochemical corrosion, pitting corrosion, crevice corrosion, stress-induced stress corrosion, fatigue corrosion, and intergranular corrosion may occur. It depends on the engineering design, the specific working conditions and environmental conditions of the seawater engineering embedded parts, and more on the corrosion resistance of the metal materials used.

Comprehensive corrosion is the electrochemical corrosion of metal materials in seawater due to the formation of many micro-cells due to the uneven microstructure. When this corrosion is uniform on the entire surface, it is comprehensive corrosion-corrosion. Comprehensive corrosion can be simulated and predicted. Life, this corrosion is the main factor that determines the design life of the project.

Local corrosion is caused by some reason. Electrochemical corrosion occurs locally on the surface of the material, and it develops deep into the material and suffers damage, that is, local corrosion. It is difficult to simulate and predict the service life of local corrosion, and it should be designed and selected. , The material manufacturing process should be avoided based on experience and failure theory analysis. It includes pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion, fatigue corrosion, etc., and it is difficult to find the degree of corrosion damage. It is often caused by sudden material damage without warning. This is necessary Measures are taken in advance to prevent occurrence when designing and selecting materials.

2. Principles and basis for selection of corrosion-resistant cast iron materials for seawater engineering

　　 As we all know, the failure factors of corrosion-resistant cast iron used in seawater engineering, or the reasons that affect the length of service life, can be summed up in three aspects. One is environmental factors, the other is design factors, and the third is metal materials. The reason for the design is to determine the type of corrosion of the service work piece. Therefore, the design should try to avoid local corrosion of the service work piece. The environmental factor is that when we select the material, the material selected is required to adapt to the environmental conditions of service, and the type, composition, The metallurgical quality and state have become the object of our selection. Of course, the principles of economy and feasibility must also be considered when selecting materials.

2.1 Sea water environmental factors

2.1.1 The working environment factors of seawater engineering embedded parts are important conditions for the selection of hydraulic engineering materials, including:

　　 Salt and its content. The salt of sea water is mainly sodium chloride and magnesium chloride, which account for more than 80% of the total salt. The main ions in seawater are cations such as Na+, Mg＋＋, K＋, Ca＋＋ and negative ions such as Cl－, CO3－－, SO4－－. Of course, the negative ions that corrode metal materials are mainly Cl－. Cl－ occupies all of the seawater. More than 55% of the total content of positive and negative ions play a leading role in the corrosivity of seawater. Chloride ions hinder and destroy the passivation of the metal surface, thereby increasing the corrosion rate. The passivation of different metals is blocked and destroyed differently. The degree of corrosion is also different. For general steel materials, the salinity of seawater, that is, the concentration of chloride ions, is within the concentration range that causes the maximum corrosion rate of general steel materials. Therefore, in engineering design, corrosion-resistant cast iron embedded parts The production quality control should be based on the actual seawater salinity at the project location.

　　 Seawater temperature The higher the seawater temperature, the higher the conductivity of seawater and the higher its corrosiveness. Seawater projects such as sea-blocking dams are the temperature of natural seawater, which varies from region to region. The seawater temperature in the south and the north are different, and the seawater temperature varies throughout the year, so the annual average temperature of this area can be used as the reference temperature. Use it as the basis for engineering design and selection of materials.

The oxygen content of seawater and the oxygen content in the surface layer of marine organisms are about 5-12ppm. The oxygen content of seawater increases due to the photosynthesis of aquatic plants, and the oxygen consumption of marine animals can reduce the oxygen content of seawater. ; The mist-like bubbles formed by the waves will also oversaturate the oxygen content; as the temperature of sea water increases, the oxygen content decreases, and the salt content of the sea water increases. Therefore, the oxygen content in seawater varies with regions, marine organisms, and seasons. As the oxygen content of seawater increases, the amount of oxygen diffused to the cathode increases, and the rate of oxygen depolarization increases, which is the current of the microbattery. As the density increases, the rate of electrochemical corrosion increases, which increases the corrosivity of seawater.

　　 Influence of sand content In the estuary area, the silt brought by rivers increases the sand content in the seawater, resulting in seawater corrosion and sand abrasion. Sediment causes friction on the surface of the workpiece, which causes the protective film, passivation film and corrosion products formed by seawater corrosion on the surface of the workpiece to be worn away. The new surface of the workpiece that has lost its protection is more susceptible to seawater corrosion, or the surface of the workpiece is fundamentally affected by abrasion. The protective film cannot be formed, so the combined effect of corrosion and wear is much higher than the sum of corrosion and wear alone, which intensifies corrosion and greatly shortens the service life of metal materials.

The influence of seawater flow rate Generally, the corrosion rate of metal materials increases with the increase of seawater flow rate. When the seawater flow rate is lower than 1.5m/s, the uniform corrosion rate of copper-nickel alloy and 304 stainless steel is very low, but copper-nickel alloy may produce pitting corrosion. Generally, stainless steel will produce deep pitting corrosion; ordinary steel materials have a higher corrosion rate, but generally do not produce pitting corrosion, as shown in Figure 1. For seawater engineering gate embedded parts, the seawater velocity is usually very low, and the stress is not large in most cases, so the main thing is to consider the various corrosive properties of natural seawater and the corrosion resistance of the selected materials. Claim.

Sea water velocity m/s

←No pitting corrosion of stainless steel above this speed 1.5m/s

The impact of seawater pollution Fresh water in rivers may also bring domestic and industrial sewage, which will pollute the seawater. As a result, the PH value of seawater will decrease and the corrosiveness will increase. If H2S and other corrosive pollutants increase, it will make seawater corrosive Becomes more complicated.

　　 In short, seawater's salinity, temperature, oxygen content, marine organisms, sediment content, flow rate, and pollution levels all change the corrosiveness of seawater, so the influence of various parameters must be considered comprehensively.

2.1.2 Corrosion of metal materials in different zones

For metal materials used in seawater engineering, various parts of different zones above or below sea level have different environmental conditions and different corrosion conditions. They are generally divided into the following zones: marine atmosphere, wave Figure 1 shows the corrosion rate of the same cast iron material in the splash zone, the sea tide zone, the seawater full immersion zone and the sea mud zone. Different metal materials can be selected for different zones, or different coating anti-corrosion measures can be used.

2.2 Metallurgical quality of materials

When selecting materials for seawater engineering embedded parts, the first thing to do is to select the applicable metallurgical product variety and brand. The product of this brand should have corrosion resistance, comprehensive mechanical properties and suitable heat treatment conditions that meet the design requirements; after determining the variety and brand to be used, more The metallurgical quality of the castings must be strictly required, including the accuracy and uniformity of the chemical composition; secondly, the metallurgical process for the production of seawater engineering embedded parts must be considered, because the advanced smelting process is the guarantee for the production of high-quality products, and the cupola is difficult to guarantee The accuracy and uniformity of the composition, only medium and low frequency induction furnaces and other smelting furnaces can smelt high-quality molten iron to ensure that the design requirements are met; furthermore, it depends on the casting process of the product, and the advanced casting process can not only High-quality castings with high appearance quality and good inner quality. These aspects should be paid special attention to when selecting materials.

2.3 The design life of seawater engineering is an important principle for material selection

The service life of engineering materials must meet the requirements of the design life. The design life includes the total life of the project and the life of the maintenance cycle. For the metal materials used in seawater engineering, the possibility of seawater corrosion and the corrosion rate of the non-exposed surface of the embedded parts are very low. The service life of the base metal material used must meet the requirements of the total design life, and the corrosion on the exposed surface is much more serious, including other workpieces, which generally need to be protected, and the effect must meet the life requirements of a maintenance cycle. Of course, there should not be too high remaining life, that is, the life of the material should be compatible with the design requirements of seawater engineering, and the excess performance of the selected metal material is also a waste of resources.

2.4 Economy and process feasibility

Economy is to require materials to have a high performance-to-price ratio, which not only requires materials to meet the performance requirements of design and working environment conditions, but also has a lower price cost, that is, the total life or the average annual cost of each maintenance cycle It is lower. The performance and price of some different alloys and the market supply can refer to Table 1. The number of embedded parts used in each seawater hydraulic engineering is huge, and it is unrealistic to use high-performance rare materials. The market supply of metal materials must be sufficient, and the casting process and machining process of castings are easy to realize, reducing the production cost of the workpiece, thereby reducing the total project cost.

Table 1 Corrosion resistance of some metal materials

2.5 Experience and theoretical analysis Before the selection of metal materials for seawater engineering, it is undoubtedly necessary to conduct theoretical analysis on the corrosion resistance and expected life of the material. However, on the other hand, actual use experience is often more important than theoretical analysis. Therefore, the material selection of seawater engineering should pay more attention to the experience of the type, service life, failure type and mechanism analysis of the materials actually used in the seawater engineering, and the material selection should be combined with theoretical analysis.
2.6 The principle of comprehensive consideration The selection of metal materials for seawater engineering is a complex system engineering, so the above conditions, factors and principles must be considered comprehensively, that is, the corrosiveness of seawater in the sea area where the seawater engineering is located, and the seawater where the castings for the seawater engineering are located. Zone, design requirements, metallurgical quality of materials, theoretical analysis and practical experience, economy, feasibility, and comprehensive consideration and balance of various principles to select the most suitable material, it is not necessary to choose the best performance material. For example, Cr18-Ni9 stainless steel, high-nickel austenitic cast iron, some rare and precious metals and their alloys, or because they are expensive, or the production process is complicated and difficult to achieve, their use is restricted. These are used in seawater hydraulic engineering embedded parts. Alloys are unrealistic, and low-alloy cast iron should be used, which can ensure the design requirements, is economically reasonable, and is feasible in the production process. Therefore, it is very important to comprehensively consider and balance various conditions and principles.
　　 3. Selection of materials for seawater engineering
For metal materials, seawater is a complex corrosion system. The corrosion resistance of materials is related to the salinity, flow rate, oxygen content, temperature, biology, pollution degree and different zones above and below the sea level. The parameters vary greatly. Seawater water conservancy projects are different from seawater desalination, power plant seawater cooling and other projects, so in terms of material selection for seawater projects, the basic properties of seawater are similar. As shown in Table 1, the corrosion rate of different alloys in seawater varies greatly, reaching thousands of times. At the same time, the price difference of these different materials can reach dozens of times, and the amount of resources that the market can supply can reach tens of thousands of times. Therefore, the materials used for any engineering, equipment and facilities must be selected according to the requirements of the actual operating conditions. For seawater hydraulic engineering, the amount of corrosion-resistant metal materials is large, and the amount of one project may reach tens of thousands of tons. The use of advanced alloys is unrealistic and a waste of resources. It is difficult to achieve the use of ordinary carbon steel and cast iron. According to actual use and design requirements, after considering various material selection principles and balancing various factors, it is an inevitable choice to choose low-alloy seawater corrosion-resistant cast iron.
The low-alloy cast iron currently developed and in use is a low-alloy corrosion-resistant cast iron formed by adding elements such as Ni, Cr, Si, Al, Cu, Sb, and RE to the cast iron. The addition of elements such as Ni, Cr, and Si reduces the anode phase in the cast iron. That is, the activity of the matrix moves toward the positive potential; adding elements such as AS and Sb reduces the activity of the cathode phase in the cast iron and moves the cathode phase toward the negative potential. These two types of elements reduce the potential difference between the different phases of the corrosion-resistant cast iron. , Thereby reducing the corrosion rate of the material; adding Cr, Al, Si, etc. to form a protective film on the surface of the cast iron, which is equivalent to increasing the resistance of the corrosion micro battery, reducing the current and reducing the corrosion; adding rare earth elements to degas the cast iron, remove impurities, Improve the structure, especially the impurities on the grain boundary, thereby improving the corrosion resistance of cast iron, especially the intergranular corrosion resistance. In recent years, corrosion-resistant cast iron represented by STNi2CrCuRE has gradually formed, and its chemical composition is shown in Table 2. On the basis of this basic composition, according to the requirements of the working conditions, STNi3CrCuRE, STNiCr2CuRE and other corrosion resistant cast irons with similar corrosion resistance or higher wear resistance are derived, the corrosion rate is about 0.05 ~ 0.2mm/a, and the corrosion resistance The grade is 4~6; the mechanical properties reach the tensile strength σb≥200 MPa; the hardness of the track working surface can reach HB≥280 after the Cr content is increased. It is currently the most widely used corrosion-resistant cast iron material. See Table 3. This kind of corrosion-resistant cast iron has been widely used in seawater hydraulic engineering in Jiangsu, Zhejiang and Shanghai and other provinces and cities in recent years, with good results.

Table 2 Chemical composition of TSTNi3CrCuRE

Note: For the main rail, anti-rail, main anti-rail and other buried parts that need to be in contact with the wheel, the Cr content can be adjusted to more than 1.2%. If the user has no special requirements, the Cr content will not be adjusted for the convenience of machining.

　Table 3 Corrosive cast iron properties for embedded parts in seawater engineering