In January of this year, China Aviation Industry Corporation's Shenyang Aircraft Corporation released a video showing a new domestically produced test aircraft wearing green primer soaring into the sky, successfully completing its first flight of the new year. In the video, this unique 'green leather machine' has attracted widespread attention. Why is the "skin" of the test plane green? This kind of 'skin' cannot do without a material - chromate. It is reported that this green primer rich in chromate has extremely strong anti-corrosion and rust prevention functions, providing the most basic and important protection for metal bodies, just like the "moisture-proof putty" applied to rough walls during house decoration, which can effectively prevent internal damage. Since the 1960s, China has been using yellow primer for test flights of J-8, J-10 series, J-15/16 prototypes, and even early J-20 prototypes. This yellow primer has undergone three generations of material upgrades and has strong anti-corrosion properties. Currently, the design of green primer is more driven by tactical needs and testing purposes. One is to protect the aircraft body. The body of modern fighter jets is made up of aluminum alloy, titanium alloy, composite materials, and other splicing and combination materials. Although these materials are lightweight and sturdy, they are extremely sensitive to temperature changes and humid air at high altitudes, and rust may appear in just a few days. As a barrier to protect the body structure, chromate primer can form a passivation film on the surface of the body metal and composite materials, resisting corrosion in high salt and high humidity marine environments. This coating can provide stronger basic protection for the long-term deployment of fighter jets on aircraft carriers in the future. The second is to assist in research and development design. During the first and initial flight stages, green painted fighter jets are beneficial for engineers to comprehensively evaluate the aircraft's flight stability, maneuverability, and structural tolerance. This time, the test aircraft did not have complex coatings except for the green primer, which can save unnecessary painting steps, facilitate the inspection of cracks, stress deformation and other issues on the body skin, and enable rapid marking, positioning and repair. The third is to improve the performance of fighter jets. For stealth aircraft, chromate primer has superior adhesion properties, which can tightly bond with the metal surface, forming an extremely uniform and smooth transition layer, providing a good adhesion base for subsequent stealth coatings, and avoiding the impact of coating peeling on the stealth performance and appearance protection of the aircraft. Many stealth coatings are corrosive, and the primer as an adhesive layer can also separate the metal material from the stealth coating, reducing the corrosion of the aircraft's metal materials. For carrier based aircraft exposed to high salt and high humidity environments for a long time, this green primer with extremely strong resistance to salt spray corrosion is crucial. For example, US carrier based aircraft also use green primer during factory testing, which requires resistance to salt spray for 2000 hours without scratches or corrosion, and resistance to filamentous corrosion for 1000 hours, with relevant standards far higher than land-based fighter jets. It can be said that "green skin" is currently the most reliable protector of carrier based aircraft, which to some extent determines the service life and structural safety of carrier based aircraft. In addition, spraying green primer on the test aircraft also reflects the practical consideration of cost savings. Spraying expensive formal coatings is too wasteful when the design and condition are not fully cured. Once the body structure needs to be modified, these coatings need to be ground off and re sprayed. So, spraying only green primer can greatly reduce the cost of factory testing. As the test flight work progresses, the test aircraft will also enter different stages of painting, and will only be painted with formal combat paint after being finalized and mass-produced. From simple anti-corrosion to the need to consider complex requirements such as stealth, high temperature resistance, and anti-corrosion in marine environments, the development of aircraft primer is a microcosm of the development of aviation materials and process technology. In the future, the "green skin" of fighter jets will focus more on environmental protection, pollution resistance, and lightweight, while better protecting frontline operators, reducing drag and weight, and enhancing anti-corrosion and stealth capabilities for fighter jets. (New Society)