Key words: magnesium alloy; NaCl solution; corrosionMagnesium alloys are widely used in automotive, aerospace, transportation and other fields due to their low density, high specific strength, large specific modulus, good heat dissipation, good shock absorption and high impact load capacity compared with aluminum alloys. Especially with the light weight and environmental protection needs of the automotive industry in recent years, as well as the continuous advancement of magnesium alloy research and development technology and recycling technology, magnesium alloys are increasingly used in the automotive industry. As we all know, the core technology of automobile competition is to develop high-performance engines, the so-called "the engine is the best in the world."Magnesium alloys are widely used in automobile engines because they require the engine to be lightweight and high-speed. The working environment of the automobile engine system is relatively complicated, and the temperature variation range is large (-40~150 °C). The requirements for the material properties of the engine are generally high, and it is required to withstand the heat resistance, the flame resistance and the corrosion resistance in different corrosive media.However, there is still a big gap between the domestic application of magnesium alloy materials as automotive engine materials and the theoretical reasons. The main reason is that the corrosion problem of magnesium alloys has not been overcome. It is of great practical significance to study the corrosion behavior and mechanism of magnesium alloys used in automobile engines for guiding the application of magnesium alloy materials to automobile engines.

In recent years, with the year-on-year growth of automobile production and sales, the income and profit scale of China's automobile and parts manufacturing industry have further increased. However, due to factors such as fierce competition in the industry, the overall profit rate of the industry has declined. The competition situation has been divided, and some of the more competitive vehicle and component manufacturers still maintain a good profit margin.Faced with increasingly fierce competition, what are the favorable factors in the development of the auto parts market to help the healthy development of the industry?Favorable factor(1) Support for national policies and industrial policiesIn recent years, the state has successively promulgated the "Automotive Industry Development Policy", "Several Opinions of the State Council on Accelerating the Revitalization of Equipment Manufacturing Industry", "Automotive Industry Adjustment and Revitalization Plan", "Opinions on Promoting the Sustainable and Healthy Development of China's Automobile Products Export", etc. Industrial policy, the above policy proposes: we must vigorously promote the structural adjustment and industrial upgrading of the auto parts industry; improve international competitiveness, promote the sustained and healthy development of China's auto and parts exports; support the auto parts and components to expand the scale through mergers and acquisitions, Improve the market share of domestic and foreign auto accessories; promote automobile consumption, and encourage cars to enter the family. The introduction of the above-mentioned support policies has provided a good macro-policy environment for China's auto parts industry, which will effectively promote the sustainable and healthy development of the industry.(2) The demand for automobiles and passenger vehicles is growing steadilyThe continuous growth of China's auto market demand has effectively promoted the stable development of the auto parts industry. In 2009-2016, China's auto sales volume grew at a compound annual growth rate of 12.75%, and the proportion of auto sales to global auto sales was also 20.86% in 2009. It has rapidly increased to 27.43% in 2015. The growth rate of the passenger vehicle market, which is the main body of China's automobile market, is higher than that of the automobile market. The annual sales growth rate is 15.38%. It is expected that by 2020, China will still be in the stage of simultaneous acceleration of industrialization and urbanization, and the national economy will continue to develop steadily, and automobile consumption will be further upgraded.China's economic growth provides a solid foundation for the sustainable development of the passenger vehicle market.The development of the passenger car market has maintained a strong correlation with the development of the national economy, the income level of residents, the national industry and consumption policy, and the degree of urbanization. As the income of urban and rural residents and the level of disposable income in China continue to increase, the pace of upgrading the consumption structure will accelerate. Passenger cars have increasingly become ordinary consumer goods, and the rigid demand for passenger cars in the domestic consumer market will continue to increase. From the press release of the Traffic Management Bureau of the Ministry of Public Security, as of the end of 2016, the number of motorists in China was 360 million, including more than 310 million motorists, the number of drivers ranked first in the world, the huge number of drivers and the rigid consumer demand. It laid the foundation for the development of China's passenger car market.China's car ownership is still low, and its development momentum is strong.In recent years, China's automobile industry has developed rapidly. In the past ten years, the annual automobile production has increased to about 28 million vehicles. It has rapidly expanded to the world's first scale in terms of scale, but the difference in automobile consumption areas is still large. It is still low, and the Chinese auto market is expected to maintain steady growth in the next 5-10 years. At the end of 2014, the total number of civilian vehicles in the country was 144.75 million (excluding three-wheeled vehicles and low-speed trucks), and the number of 1,000-person vehicles was 105.83 vehicles per thousand, which is a gap compared with the level of more than 500 vehicles/1000 people in developed countries. Larger, so the future development of China's auto market is huge.According to the “Statistical Communiqué on National Economic and Social Development 2015” issued by the National Bureau of Statistics on February 29, 2015, the number of civilian vehicles in the country reached 162.73 million (excluding three-wheeled vehicles and low-speed trucks) at the end of 2015, and the number of thousand people was It reached 118.38 vehicles / thousand people.International experience shows that there is a significant positive correlation between per capita income and car penetration, with per capita GDP reaching 1,000-2,000 US dollars, and entering the era of Volkswagen consumption, while 3,000-6,000 US dollars and 6,000-10,000 US dollars are the fastest rising car ownership rate. In the two periods, China’s per capita GDP has exceeded US$7,500 in 2015, so in the future, China will still be in a period of rapid increase in car ownership.The demand for automobile consumption in second and third-tier cities began to break outSince 2009, driven by a series of encouraging automobile consumption policies, such as the automobile industry revitalization plan, the demand growth rate of passenger car market in second- and third-tier cities in China is higher than that in first-tier cities. This indicates that the automobile market in first-tier cities is gradually saturated in China's automobile market. The speed of transfer to second- and third-tier cities and rural markets will accelerate. The National 13th Five-Year Plan Outline clearly proposes to increase consumption power, improve public consumption expectations, tap rural consumption potential, and focus on expanding household consumption. Steadily promote large-scale consumption of housing, automobiles and health care. At the same time, it is proposed to speed up the pace of new urbanization, accelerate the development of small and medium-sized cities and characteristic towns, improve the level of socialist new rural construction, and strive to narrow the gap between urban and rural development and promote the integration of urban and rural development. With the promotion of the urbanization process in the country, the traffic environment in the second and third tier cities has improved, the population is concentrated, and the income of residents has increased. At the same time, the first-tier cities have gradually entered the stage of upgrading, which will generate huge demand for the automobile market and lead the Chinese automobile industry. A round of growth.At present, China's auto market has entered a second stage of growth, and the market focus extends from first-tier cities to second- and third-tier cities with greater development potential. China's current 1,000-person car ownership has just exceeded 100 vehicles, which is a huge difference from the US's nearly 800 vehicles. This means that China's auto market potential is still huge. With the continuous development of the economy and society and the acceleration of urbanization, China's automobile consumption will Growing.Continuous improvement of highway transportation infrastructureIn recent years, China's highway transportation infrastructure has been continuously improved. From 1998 to 2015, China's highway mileage increased from 1,278,500 kilometers to 4,457,300 kilometers, with an average annual compound growth of about 7.79%; among which the expressway increased from 0.87 million to 12,350 kilometers, which is a large-scale automobile in urban and rural areas. The application laid the foundation. The continuous expansion of the road network, especially the expressway network, has promoted the increase in demand for automobiles and provided a favorable development environment for auto parts manufacturers.(3) The global procurement strategy of vehicle manufacturers also provides new opportunities for domestic auto parts suppliers.Affected by the international financial crisis in 2008, cost control has become the mainstream strategy of vehicle manufacturing enterprises, and global parts procurement has become one of its important measures. The global procurement and centralized procurement strategies implemented by vehicle manufacturers have brought new market opportunities to domestic excellent auto parts manufacturers. At present, China has become a global auto parts production base. With the upgrading of products and the improvement of synchronous development capabilities, the share of China's component manufacturing enterprises in the global auto parts market will further increase.(4) Opportunities brought by the state's policy on maintaining normal competition order in the marketDue to the continuous “high zero ratio” of imported cars, in September 2014, the Ministry of Communications and other 10 ministries and commissions issued the “Guiding Opinions on Promoting the Transformation and Upgrade of the Automobile Maintenance Industry and Improving the Service Quality”, and proposed to break the monopoly of auto repair parts. Specific methods: “Breaking the monopoly of repair parts channels, encouraging original parts manufacturers to provide original parts and independent after-sales parts with independent trademarks to the aftermarket.” “Protect all maintenance companies and owners to enjoy the right to use the same parts to repair cars. ". The impact of the above guidance on international parts companies will promote the international vehicle joint venture to strengthen the localization strategy of auto parts production, bringing opportunities for domestic auto parts enterprises. (Article from reprint!)

For the modified wheel hub, the friend of the performance fan may be scorned, because in most cases, the modification of the large-sized wheel, in addition to slower acceleration, poorer comfort, greater noise, fuel consumption is also rising, and it is becoming more and more slow. Case. But must this be the case? Let's let Netease use the test data to talk about the performance of the first forged magnesium alloy wheel in China.First of all, for the first-time conversion enthusiasts, we need to mention why the modification of large-size wheels will affect acceleration, comfort, noise and fuel consumption. First of all, the increase in the size of the hub will inevitably increase the weight, although for the whole vehicle, it is not a matter of reducing a few kilograms, but for the hub, this is the power output source. For example, if you let you walk with weight, are you willing to tie the sandbag to your back? Still tied to your feet? Obviously, every weight on the hub is far more important than the weight of the car.In addition, the weight of the hub will greatly affect the output of the power, and will greatly affect the driving comfort. On the one hand, a larger wheel hub will inevitably increase the width of the tire, and the larger friction area of the tire will inevitably increase the resistance and noise. In addition, in terms of shock absorption performance, since the hub is connected to the suspension damping system, the weight of the hub also affects the shock absorption performance of the suspension. This is why trucks that are fully loaded are often smaller and more comfortable than empty trucks.But will the modified wheel hub be heavier and heavier? It is necessary to briefly introduce the types and characteristics of the hub.The current mainstream wheel hubs on the market can be divided into steel wheels and alloy wheels according to the material, but in special fields, such as F1 racing, rally racing and Porsche 918Spyder, Pagani wind son Cinque and other super sports cars, there are also some carbons. High-end wheels made of brazing, titanium alloys and wrought magnesium alloys.Steel wheel hub:Advantages: simple manufacturing process, relatively low cost, and strong resistance to metal fatigueDisadvantages: ugly appearance, heavy weight, large inertia resistance, poor heat dissipation, and easy to rustAluminum alloy wheels:Advantages: light weight, low inertial resistance, beautiful appearance, reducing tire rolling resistance, thus reducing fuel consumptionDisadvantages: using casting process, the price is cheap, but it is difficult to ensure uniform density and moderate weight; forging process, the weight is relatively light, but the price is slightly expensiveCarbon brazing: The characteristic is that there is no shelf life! No material fatigue! Will not be corroded! The same intensity is the lightest! . The 2013 Königssee Agera is equipped with the world's first hollow, one-piece, ultra-lightweight carbon fiber hub. Although there are various advantages, because of the anisotropy of the carbon brazing itself (it is difficult to maintain a uniform density of the hub), the processing process is complicated and it is difficult to mass-produce. Germany's wheel manufacturing expert WHEELSANDMORE, the recently launched "Megalight-Forged-Series" two-piece wheel, only the outer ring is made of carbon fiber, the inner hub is a lightweight alloy, it has been lighter than the same size wheel. 40% or so; of course, the price is also considerable. Take 20-inch as an example, a group of wheels costs 20,000 euros, about 218,600 yuan!Titanium alloy: It is mainly used to make aircraft engine compressor parts, followed by structural parts of rockets, missiles and high-speed aircraft. It has the characteristics of high strength, good corrosion resistance and high heat resistance. The advantage is that the working temperature of titanium alloy can reach 500 °C, and the aluminum alloy is below 200 °C. And working in humid atmospheres and seawater media, its corrosion resistance is much better than stainless steel. However, the disadvantages are also obvious. The process performance of the titanium alloy is poor, and the cutting process is difficult. In the hot working, impurities such as hydrogen, nitrogen, nitrogen and the like are easily absorbed. There is also poor wear resistance and complicated production processes. This property forces the titanium alloy to be different from conventional conventional refining, melting and casting techniques, and often causes damage to the mold; as a result, the price of the titanium alloy becomes very expensive.Forged magnesium wheels: Metal materials that have received extensive attention in the automotive and electronics fields in recent years. Due to its light weight (specific gravity 1.74 g/cm3) and good mechanical properties, it has been used in batches in the field of vehicles and electronics. From Corvette's engine mounts to Cadillac's trunk inner panels, the lightweighting effect is also very significant, and the cost is about 10% higher than the traditional lightweight material aluminum alloy.The magnesium-aluminum alloy, which seems to strike a balance between cost and weight, is also very disadvantageous. Because of its high chemical reactivity, it is corroded in acidic, neutral and weakly alkaline solutions. In the past, when everyone was in the middle school chemistry class, they saw that pure magnesium can burn in the air, so there may be some concerns about magnesium alloy. In fact, magnesium is difficult to burn after passing through the alloy, but it is easy to oxidize. Due to the easy oxidation characteristics, the surface of magnesium alloy parts is usually coated, and the processing is also carried out under closed conditions plus inert gas protection.In addition, in the application of the hub, the magnesium alloy is further improved after the forging, and the special coating protection can ensure the use of more than 5 years. Even if the coating is smashed, the metal is exposed, and only the riders who are often used in wet and acidic environments need to repaint the paint as soon as possible (from January to March). If used in a dry environment, it is determined by the peeling of the coating around the wound.

Experts say that the field of magnesium die castings that can be used in automobiles can be roughly divided into two categories: one is to replace the original use of other materials for die casting; the other is to use welded steel or aluminum plates until now. The occasion of the material. For the former, the most common is to replace aluminum die-casting parts with magnesium die-casting parts. Although the two materials have the same casting characteristics, it is of course more reasonable to use magnesium if the weight and cost factors are comprehensively measured.