Have you ever wondered why your "aluminum alloy windows" start to turn white, powder, or develop unsightly black spots after a few years of use? Why do aluminum alloy window frames in seaside villas always corrode faster than those in inland cities?
The answer often lies in surface treatment processes that are invisible to the naked eye.As the most classic and widely used anti-corrosion technology in the aluminum window profile industry, anodizing is not just "coating a layer" on aluminum.Its corrosion resistance is directly related to the thickness and quality of the transparent oxide film.
This article explores the basic principles of anodizing, compares the effects of different film thicknesses in various applications, and analyzes in depth the anti-corrosion principles of aluminum window profiles to help you make accurate choices and avoid future corrosion losses.
In fact, pure aluminum has strong chemical reactivity and readily reacts with oxygen to form a natural oxide film.However, this natural oxide film is extremely thin (0.01–0.1 μm) and unevenly distributed, making it unable to resist environmental erosion.To ensure that extruded aluminum window profiles can withstand decades of wind and rain, anodizing—a surface treatment technique—must be used to artificially "grow" a dense and hard protective layer.
The extruded aluminum profile blank is placed in an electrolyte (usually dilute sulfuric acid) and a direct current is passed through it. At this point, the aluminum profile acts as the anode.Oxygen ions in the electrolyte undergo an electrochemical reaction with the aluminum surface to generate a hard and dense aluminum oxide (Al₂O₃) film, which is tightly bonded to the substrate.
Unlike the thin and brittle natural oxide film that forms on the surface of aluminum, artificial anodizing allows for precise control of film thickness, typically between 5 and 30 micrometers.Unlike spraying or electrophoretic deposition (both of which are post-coating), the oxide film is integrally fused with the aluminum substrate, eliminating the risk of delamination or peeling.This is the core technology for achieving long-term corrosion resistance in aluminium profiles for doors and windows.
Chemically stable alumina crystals can completely isolate the active aluminum substrate, protecting it from external moisture, oxygen, salt, acid rain, and other corrosive substances.
The oxide film has a unique bilayer structure: the bottom layer is an extremely dense barrier layer, while the top layer is porous.These pores are sealed through subsequent sealing processes, further preventing the penetration of corrosive media.The resulting oxide film exhibits excellent corrosion resistance and wear resistance.
Anodized films not only have high hardness (up to HV 300-500), but also exhibit excellent corrosion resistance—high-quality oxide films can withstand thousands of hours in a 3% NaCl salt spray environment without corrosion.
The thickness of the anodized film directly determines its resistance to different climatic conditions—a primary factor for buyers when choosing aluminum alloy windows.Oxide films of different thicknesses exhibit significant differences when exposed to strong ultraviolet radiation, high temperature and humidity, salt spray and acid rain.
International standards for the thickness of anodized film on aluminum profiles (e.g., ISO 7599:2018) classify film thickness into several grades.Based on industry practice, recommended applications for different film thicknesses are shown in the table below:
| Film Grade | Min. Avg. Thickness | Min. Local Thickness | Recommended Applications | Environmental Tolerance |
| AA 5 | 5 μm | 4 μm | Furniture, decorative strips, electronic products | In a dry, non-corrosive indoor environment, it provides basic corrosion protection, preventing surface oxidation and discoloration. |
| AA 10 | 10 μm | 8 μm | Interior aluminium alloy doors and windows, household appliances and digital accessories. | Can only withstand minor daily contact corrosion and is not suitable for long-term exposure to outdoor environments. |
| AA 15 | 15 μm | 12 μm | Standard sun-exposed exterior doors, windows, and curtain walls. | Can withstand typical urban climate conditions and moderate rainwater erosion, but it is difficult to withstand prolonged salt spray or strong acid rain erosion. |
| AA 20 | 20 μm | 16 μm | Building curtain walls, outward-opening windows and floor-to-ceiling windows in high-rise buildings, outward-opening doors and windows for balconies, handrails, fences, and terrace railings. | Suitable for cities with frequent outdoor activities, light industrial areas, and coastal areas without strong sea winds. |
| AA 25 | 25 μm | 20 μm | Aluminum profiles for coastal buildings, aluminum railings for ship decks, and exposed decorative profiles for ship cabins. | Resistant to strong light, strong ultraviolet radiation, high salt spray corrosion, and extreme acid and alkaline environments. |
When discussing the corrosion resistance of aluminum alloy window profiles, it is important to clarify one point: anodizing protects the aluminum material itself, not the window's insulation structure or glass.
However, even if standard aluminum alloy windows, thermally broken aluminum alloy windows, and system aluminum alloy windows use the same anodized film thickness (e.g., AA 15), their actual corrosion resistance in use will differ due to differences in structural design.
Standard Aluminum Alloy Windows
System Aluminum Alloy Windows
The corrosion resistance of anodized aluminum window profiles mainly depends on the thickness of the oxide film—the thicker the oxide film, the stronger its ability to prevent corrosive substances from penetrating.By selecting the appropriate coating thickness for specific applications and performing proper maintenance, these profiles can last for decades, making them a durable, sustainable, and aesthetically pleasing green building material.If you’d like to find out more, click here to read on"What Are the Anodizing Options Available for Aluminium Profiles for Windows and Doors?"
Are you still looking for a long-term aluminum profile supplier for your door and window projects?As a reliable supplier, we have advanced equipment and a professional team that can provide flexible customization options and a variety of customized aluminum alloy door and window frame solutions to meet non-standard design requirements and localization needs.
