Corrosion Resistance of Exterior Wall Aluminum Baffles: How to Ensure Service Life Over 15 Years

Exterior wall aluminum baffles are exposed to outdoor environments for a long time, affected by various factors such as ultraviolet rays, rainwater, salt spray, acid and alkali gases, temperature changes, etc. If anti-corrosion measures are not in place, problems such as coating falling off, base material corrosion, discoloration and chalking will easily occur, seriously affecting building appearance and structural safety. The exterior wall aluminum baffles of many projects suffer from severe corrosion after 3-5 years of use, mainly due to improper material selection, unqualified surface treatment, unreasonable structural design, and non-standard construction. As long as the quality of each link is strictly controlled, exterior wall aluminum baffles can fully achieve a service life of more than 15 years.

Material selection is the foundation of anti-corrosion. Although aluminum alloy itself forms an oxide film on the surface and has certain anti-corrosion ability, the anti-corrosion performance of aluminum alloys of different grades varies greatly. 1100 series pure aluminum is prohibited for exterior wall aluminum baffles. Pure aluminum has low strength and is easy to corrode in acid and alkali environments; 3003-H24 series manganese aluminum alloy is preferred. The addition of manganese can greatly improve the corrosion resistance of aluminum alloy, while maintaining good forming performance and strength, making it the best base material for exterior wall aluminum baffles; for seaside projects or high-corrosion environments, 5005-H24 or 5052-H24 series aluminum-magnesium alloys can be selected. The addition of magnesium further improves corrosion resistance, especially suitable for coastal high salt spray environments. The thickness of the base material must meet the requirements. Too thin aluminum baffles are easy to deform under wind load, leading to coating cracking, and corrosion starts from the inside after moisture enters. All aluminum alloy materials must comply with national standards and provide material certificates. Inferior aluminum plates rolled from recycled waste aluminum are prohibited. These plates have many impurities, uneven composition, and extremely poor corrosion resistance.

Surface treatment is the key to anti-corrosion. Exterior wall aluminum baffles must be treated with three-coat two-bake fluorocarbon spraying, PVDF content not less than 70%, total coating thickness not less than 40μm, which is the minimum requirement to ensure a service life of more than 15 years. Many low-price projects use polyester spraying or powder coating to pretend to be fluorocarbon spraying. These coatings will chalk, fade and fall off after 3-5 years outdoors, which cannot meet the requirements for exterior wall use at all. Even for fluorocarbon spraying, pretreatment is very important. Before spraying, chromate treatment or chromium-free passivation treatment must be carried out to form a chemical conversion film on the surface of aluminum substrate, which greatly improves the adhesion between coating and substrate and avoids coating blistering and falling off. Fluorocarbon coatings with unqualified pretreatment will fall off in pieces after 2-3 years even if the coating thickness is sufficient. After surface treatment, strict performance tests must be carried out, including adhesion test (cross-cut method level 0), impact test, salt spray test (no change after 3000 hours), artificial accelerated aging test (gloss retention rate above 80% after 2000 hours), and can only leave the factory after passing the tests. For seaside projects or heavy corrosion environments, four-coat two-bake fluorocarbon coating is recommended, increasing the thickness of one primer, with total thickness reaching more than 50μm to further improve anti-corrosion ability.

Structural design is an important guarantee for anti-corrosion. Unreasonable structural design will lead to water accumulation and dust accumulation, accelerating corrosion. First, avoid direct contact between different metals. Insulating gaskets must be set between aluminum baffles and steel keels, stainless steel pendants to avoid electrochemical corrosion. Aluminum has low electrode potential, and when in contact with metals such as steel and stainless steel, it will corrode rapidly as an anode under the action of rainwater; second, avoid water accumulation structures. The cross-section design of aluminum baffles must ensure that rainwater can flow away smoothly, do not design upward grooves, horizontally installed aluminum baffles should be set with 3‰-5‰ drainage slope, and both ends of baffles should be sealed to prevent rainwater from entering the interior of baffles and being unable to discharge, causing corrosion from the inside; third, reserve expansion joints. The linear expansion coefficient of aluminum is 2.3×10^-5/℃. A 6-meter-long aluminum baffle can have a length change of up to 8mm under a temperature difference of 60℃. If no expansion joints are reserved, the baffle will squeeze and deform when temperature changes, coating cracks, leading to corrosion; fourth, ventilation design. Open curtain walls must ensure air circulation in the ventilation layer to avoid long-term humid environment accelerating corrosion.

Anti-corrosion control during construction and installation is also important. First is finished product protection. During transportation and installation of aluminum baffles, avoid scratching the coating, do not tear off the protective film before installation is completed, and repair scratched places with same-color touch-up paint in time, as scratched parts are the starting point of corrosion; second is on-site processing anti-corrosion. On-site cutting and drilling parts should be coated with anti-corrosion paint to avoid bare aluminum substrate being directly exposed to the air; third, avoid contact with corrosive substances. During construction, do not let cement, mortar, strong acid and alkali cleaners contact the surface of aluminum baffles, these substances will corrode the coating and aluminum substrate; fourth, connection node anti-corrosion. All on-site welded steel components must be derusted and coated with two layers of anti-rust paint, and bolt connection parts should be coated with anti-corrosion sealant to avoid rainwater entry.

Post-maintenance is a necessary measure to extend service life. Many people think that fluorocarbon aluminum baffles do not need maintenance, which is a wrong concept. Regular maintenance can greatly extend service life. General projects are inspected once a year, and seaside or heavily polluted areas are inspected every six months. Inspection contents include: whether the coating is damaged, falling off, chalking, whether connection nodes are loose, whether there is water accumulation, whether sealant is cracked. Find small damage to the coating and repair it in time to avoid damage expansion; regularly clean drainage holes to ensure smooth drainage; conduct a comprehensive cleaning every 5-8 years, use neutral detergent to clean dust and pollutants on the surface, do not use strong acid and alkali cleaners; after 10 years of use, a layer of fluorocarbon varnish can be sprayed on the surface to protect the coating, which can extend the service life by 5-10 years.

Avoid several common anti-corrosion misunderstandings: first, think that aluminum will not rust and does not need anti-corrosion. In fact, aluminum alloy will also corrode in specific environments, especially after the surface coating is damaged; second, think that the thicker the coating, the better the anti-corrosion. In fact, the adhesion of coatings exceeding the standard thickness will decrease and it is easier to fall off. Uniform thickness within the standard range is the best; third, only look at the surface color, not the coating quality. Many inferior fluorocarbon coatings have good color when first installed, but will seriously fade and chalk after a few years. Choosing regular manufacturer products with brand and quality assurance, designing and constructing in strict accordance with specifications, and doing a good job in later maintenance, exterior wall aluminum baffles can fully achieve a service life of more than 20 years.