Modern building doors and windows can be roughly divided into plastic windows, composite windows, and aluminum alloy windows. Among them, aluminum alloy windows are the main building door and window products in China today, accounting for a considerable proportion in today's buildings.
In fact, aluminum alloy windows have their advantages in terms of resistance to wind pressure, water tightness, air tightness and environmental protection. This is because the inertia moment of the aluminum alloy window is 3-4 times that of the plastic window compared with the plastic window of the same cross section; with the use of the heat insulation broken bridge in the aluminum alloy profile, the thermal insulation and thermal insulation properties of the aluminum alloy window are greatly improved. Improve, very adapt to the energy-saving requirements that are strongly advocated today.
Aluminum alloy door and window hardware is an important part of it, and its performance and quality have an important impact on the function of doors and windows. Due to the continuous development of modern technology, door and window hardware accessories (handles, handrails, hinges, rails, connectors, etc.) are also increasingly entering the market. The shapes and colors of these products are varied and colorful, which enhances the practicality and appearance of doors and windows. However, due to the difference between the material of the parts and the materials of the surface decoration layer, their service life (mainly referred to as corrosion resistance, durability and strength) is very different. Even if the color is the same, even the aluminum alloy window fittings with similar hand feel can reach the gap. Several times or even dozens of times.
In addition to having sufficient mechanical strength, aluminum alloy door and window hardware must also pay attention to whether the selected material can maintain good corrosion resistance when used in contact with aluminum alloy. Whether the aluminum alloy door and window hardware can meet the design life requirement is very important for the safety and durability of the aluminum alloy door and window during use. Only when the two are combined can the aluminum alloy door and window be beautiful and durable.
Corrosion characteristics and mechanism of aluminum alloy door and window hardware
Accessory material
The structure and materials of different window hardware accessories are not the same. Generally, the aluminum alloy door and window hardware accessories materials are matched with the profiles.
The manufacturer first selects the profile and then selects the hardware accessory. This sometimes leads to the lack of standard hardware selection and the availability of hardware. Most manufacturers now consider the combination of profiles and hardware accessories before giving them a choice. Materials are the key material basis for ensuring the performance, quality and safety of doors and windows.
At present, most of the materials used in aluminum alloy doors and windows, including various accessories, have national or industry standards, but some materials are produced without national or industry standards. Generally speaking, among the materials of aluminum alloy door and window fittings, the handle and transmission parts are made of die-cast zinc alloy; the hinges are made of aluminum alloy; the hinge shaft and the lock point are made of stainless steel, and the lock seat is made of zinc alloy or stainless steel; Sliding bracing, wind bracing, etc. are made of stainless steel; plastic parts are made of modified plastic of nylon 66. For metal parts, effective surface treatment is carried out according to the different environments used, mainly considering atmospheric corrosion resistance and corrosion of materials contacted and electrochemical corrosion between different metal materials.
Corrosion characteristics
Aluminum and aluminum alloy are high-conductivity and heat-conducting metal materials. Under natural conditions, the surface is easily passivated. Under humid acidic conditions, the surface is oxidized to form Al2O3·3H2O. The hydrated alumina itself is stable, and aluminum and aluminum alloys are stable. The finished door and window profiles are generally treated with high performance, and the corrosion resistance is mainly on the hardware. The galvanic corrosion of the hardware is not only in itself, but also affected by the electrodes of the aluminum profile. Corrosion will continue uninterrupted under certain conditions.
For aluminum alloy doors and windows, carbon steel hardware is a combination of large cathode area and small anode area. When corrosion starts, the corrosion products always appear at the edge of the contact between the hardware and the door and window profiles, and initially surround the contact points. Surrounded by, then connected. This is because the potential difference between the aluminum profile and the carbon steel hardware forms a galvanic cell by contact, and an oxidation reaction of iron [F+2] occurs at the contact portion under the intervention of the condensed water film, and an oxygen reduction reaction occurs at the non-contact portion. When [OH-1] appears, the two move toward the middle under the driving of the electric field, and the corrosion products formed by the people are formed at the edge. In fact, the most severe corrosion zone is at the contact surface, but only the corrosion products are not seen.
Due to the addition of chlorine, sulfur dioxide and other media in the atmosphere, it will promote the dissolution of carbon steel, thereby speeding up the damage of hardware.
Corrosion mechanism
Generally speaking, the corrosion of aluminum alloy door and window hardware is a chemical reaction and electrochemical reaction process. It is also a multi-form reaction of many kinds of substances. It is difficult to explain with one or two reaction modes, but it causes aluminum alloy door and window hardware. The cause of corrosion damage is mainly electrochemical corrosion under natural conditions.
The aluminum alloy doors and windows are generally composed of aluminum (profile), stainless steel (component), zinc alloy (component), carbon steel (component), etc., and the metals whose different electrode potentials are in contact with each other form a primary battery in the corrosive medium, thereby occurring Electrochemical corrosion. Especially in the marine atmosphere and industrial atmosphere, the material structure of aluminum alloy doors and windows determines the inevitability of electrochemical corrosion. Among them, the aluminum alloy profile is easy to passivate by itself, and the potential is negatively changed relative to other materials to slow or stop the corrosion, and the steel parts with negative potential may aggravate the corrosion and cause damage to the hardware.
Anti-corrosion measures for aluminum alloy door and window hardware
Use the right materials
At this stage, the energy-saving doors and windows circulating in the market have certain deficiencies. The plastic material will be deformed due to the temperature difference, which will adversely affect the sealing performance. The aluminum alloy needs to increase the heat insulation strip to be able to The insulation performance is maximized, but the combination of the above two materials will affect the strength of the composite door and window due to the difference in the coefficient of linear expansion. Through investigation and analysis, it can be found that the energy-saving doors and windows with excellent comprehensive performance are both FRP windows and solid wood doors and windows, but the price of solid wood doors and windows is relatively expensive, and it is difficult to circulate in the market. Therefore, the article uses glass steel windows as the main research direction.
(1) Use materials similar to the potential difference of aluminum profiles
It is one of the best choices to use materials that are close to the potential difference of the aluminum profile or to use materials with better corrosion resistance, such as aluminum alloy handles, hinges and various connectors. This combination of the same metal is an ideal design to avoid the generation of self-coupled cells. However, there are many parts that are not easy to use aluminum and aluminum alloys because of strength or difficulty in production. Therefore, stainless steel with good corrosion resistance should also be one of the preferred materials.
Stainless steel generally has an anti-rust effect as long as the Cr in the steel is greater than 12. However, in steel, which contains only a single element of Cr, the corrosion resistance is not very good and it is easy to rust. Only steel contains titanium, which promotes the stability of carbides, and has high corrosion resistance, such as 1Cr18Ni9Ti. Stainless steel with good corrosion resistance is generally composed of chromium, nickel, titanium and other components in steel. It is a complex galvanic cell structure and therefore has good chemical stability. In many acid, alkali, and salt solutions, corrosion resistance is high in organic acids, steam, and humid air. The use of stainless steel reduces or avoids the susceptibility of galvanic corrosion of the profile.
(2) Using zinc alloy and appropriate surface treatment
Zinc alloy is widely used in door and window hardware due to its simple manufacturing process and low cost. Zinc alloys are susceptible to corrosion in acid, alkali, and salt solutions, but are easily passivated in the atmosphere and have good oxidation resistance. The surface protection of zinc alloy parts generally adopts metal coating or non-metal coating. Typical treatment processes include zinc alloy passivation treatment, galvanization passivation, painting, spray powder, etc., the former has good wear resistance and passivation. The film has certain acid and alkali resistance, which can prolong the corrosion resistance time of the parts; the latter is a non-metal film layer with good insulation and can avoid galvanic corrosion. There is currently a surface treatment method on the market - electroplated pearl chrome.
The pearl chrome film has high hardness, good wear resistance, and has stainless steel anti-corrosion performance. The appearance is elegant, generous and beautiful. Regardless of the anti-corrosion angle or the choice of appearance, pearl chrome is one of the best surface treatment methods for aluminum alloy doors and windows.
(3) More choice of stainless steel
Exposed ordinary carbon steel hardware is generally not suitable as an aluminum alloy door and window hardware material, because carbon steel itself does not have surface passivation performance, and is a sacrificial anode in aluminum alloy doors and windows during electrochemical corrosion.
A better measure is to add an insulating material or a non-metallic coating to the contact portion of the aluminum profile after the surface is coated with the metal film layer to avoid or reduce the damage of the galvanic corrosion to the hardware. Stainless steel has excellent corrosion resistance, and stainless steel should be promoted to replace carbon steel to make door and window fittings. However, in the current hardware product standards, only products such as sliding supports are clearly specified to be made of stainless steel.
Stainless steel hardware is not optimistic in domestic use. Although it was also made of carbon steel and plastic coated in the early days, the only thing the user accepts is the stainless steel sliding support. Under the same conditions of use and requirements, such as hinges, drive locks, and supports are mostly made of carbon steel. At present, the domestic hinges are also beginning to be made of stainless steel, and the transmission locks are also made of stainless steel abroad. Therefore, it is necessary to publicize and advocate the production of stainless steel hardware, such as hinges, hinges, transmission locks, etc., especially for the accessories of the outer casement window, carbon steel should be discarded with stainless steel.
Corresponding surface treatment
(1) Metallized protective layer
Plating
It is necessary to prepare a conductive aqueous solution as a raw material of the plating layer, and then place the hardware in an electroplating bath containing the above solution (the portion not to be plated can be sealed with an insulating varnish), and the surface of the hardware is applied by an applied electric field. A film of a material contained in the solution is deposited, which is called electroplating. The film deposited on the surface of the hardware is called a plating layer. The material of the plating layer may be metal or non-metal.
Practice has shown that the electroplated layer formed on the surface of the hardware is not only beautiful and smooth, but also has a high bonding strength with the base metal under the plating layer, and the plating layer is not easy to fall off. In addition, this decorative layer is also resistant to corrosion, oxidation and long life. The price is several times to several tens of times higher than unplated hardware (related to the price of the plating material and the difficulty of the plating process).
Electroless plating
The raw material as the plating material is made into a solution, and then the hardware to be plated is immersed in the above solution, and the raw material in the solution is deposited on the surface of the hardware to form a plating layer by using a suitable reducing agent. This method of using a chemical reaction method to obtain a coating for a hardware is called electroless plating. Compared with electroplating, electroless plating does not require power supply and electroplating equipment. It can also be plated with both metal and non-metal plating. The cost is lower than electroplating, but electroless plating forms a coating on the surface of the hardware, regardless of its corrosion resistance and oxidation resistance. The durability, durability and bonding strength with the matrix material are both worse than those of the plating.
In addition, the thickness of the electroless plating layer is generally thinner than that of electroplating, and the hardness of the plating layer is also lower than that of electroplating. This kind of electroless plating is difficult to distinguish between the electroplated layer and the electroplated layer. Only professionals can check it with special equipment. However, the simple test method on the spot is to grind the surface of the two coatings with sandpaper. Since the hardness of the electroless plating is low, the wear marks are wider and deeper than the plating layer.
Brush plating
Brush plating is a special type of plating. Different from ordinary electroplating, the plating does not require a plating bath, but a plating solution is supplied to the surface to be plated while being rubbed on the surface of the part by a charged plating brush (also called a plating pen) to obtain a plating layer. Since there is no need for a plating bath, it is also called slotless plating. Brush plating is a particularly economical and convenient simple plating method. The power supply and equipment are also very simple, and can be constructed by one person in any simple environment, which is convenient for individual operators.
Brush plating can not only plate various shapes of hardware, but also the plating speed is much higher than electroplating and electroless plating. At the same time, it can repair (complement) the damaged metal surface coating with a minimum thickness of 1μm and a maximum thickness of 2mm. -3mm, as far as the current technology is concerned, brush plating also allows the metal to be plated with various metal and non-metal layers that can be plated by electroplating and electroless plating.
It is worth noting that in addition to the appearance of the brush plating, other properties (such as wear resistance, corrosion resistance, oxidation resistance, durability and bonding strength with the base metal) can not keep up with the plating layer, and can not keep up with the electroless plating. In addition, the brush plating and the plating layer can only be identified by special equipment. Usually, with a magnifying glass, the surface of the brush plating layer is not as smooth and shiny as the plating layer, and the thickness of the plating layer is not as uniform as that of the plating.
(2) non-metallic protective layer
Surface oxidation treatment
The surfaces of steel and aluminum hardware are very susceptible to oxidation in air. The former will produce a layer of loose rust. This layer of rust is not only unsightly, but the air will continue to be loaded with the underlying base steel through this layer of rust until the entire steel plate becomes rust. A slightly grayish white oxide film is formed on the surface of the aluminum. Although the oxide film on the surface of the aluminum material is relatively dense, the underlying aluminum matrix can be substantially prevented from being further loaded, but the original silver-white gloss is more severely damaged.
In order to prevent the occurrence of the above two phenomena, in addition to the protective decorative layer, an artificial oxide film can be artificially fabricated on the surface of the newly fabricated hardware. The oxide film on the surface of steel hardware is generally made in lye. After oxidation treatment, a blue-black dense Fe2O oxide film (known as bake blue) is formed on the surface, and a uniform texture of Al2O3 oxide film is formed on the surface of the aluminum hardware.
The artificially formed oxide film on the surface of steel and aluminum hardware is not only beautiful, flat and dense, but also has a much higher bonding strength with the substrate and is difficult to grind. This oxide film also prevents the underlying metal from further corroding by the action of air or other harmful gases. Since such an oxide film on the surface of steel and aluminum hardware has lost its original metal property to become non-metal, it is also called a non-metallic protective decorative layer.
Surface coating
The non-metallic protective decorative layer sprayed on the surface of metal hardware is currently mainly made of plastic and paint. The color of plastics and paints can be said to be all-encompassing, which is usually beyond the reach of metal decorative layers. However, the main disadvantages of plastics and paint layers are their poor resistance to oxidation and UV rays, which gradually age and become brittle over time, losing the original bright color, beginning to darken and cracking and falling off. In addition, they The bonding strength with metal is not as high as that of metal plating, and it is easy to be worn away. Therefore, it is not suitable for protective decorative coating as a moving part. In order to improve the performance of the paint on the metal surface, the paint layer of the hardware is mostly made of amino baking varnish (paint). For special performance requirements such as oil, insulation, and fluorescence, these special paints can also be applied. For the hardware that sprays plastic and paint protective decorative layers, it is mostly used for furniture below mid-range.
(3) Double-layer anti-corrosion
At present, the materials used in aluminum alloy door and window hardware fittings are carbon steel, such as transmission lockers and lining steel. For carbon steel, the main cause of corrosion is insufficient thickness of the galvanized layer. For carbon steel plating parts, it is required to increase the thickness of the galvanized layer. At the same time, the post-treatment process should be strengthened, and the inorganic salt such as silicate or organic film should be used for covering treatment, which can greatly improve the corrosion resistance of the galvanized layer. After the closure, the original rainbow colored passivation film becomes golden yellow.
Some parts in the hardware parts are painted or painted with steel after stamping. Such products can be tested for up to 1000 hours in a neutral salt spray test, but in practical use these products have poor corrosion resistance, because such products rely mainly on the coverage of organic film to achieve rust-proof performance, once the coating is destroyed The base metal quickly corrodes. In practical use, especially in the installation process, it is inevitable to damage the surface of the cover layer, especially at the edges and corners, and these parts are particularly thin due to the edge effect. It is recommended to use a galvanized layer as a pre-treatment of the spray so that even if the organic cover is broken, there is a galvanized protective layer inside. Practice has proved that the corrosion resistance of the product after this process will be greatly improved.
Both of the above forms have one thing in common, that is, a double-layer anti-corrosion method is adopted. The former is mainly galvanized, and is covered with a transparent organic protective film. The appearance is galvanized parts, the latter is mainly made of organic coating, supplemented by a galvanized layer, and the appearance is a plastic coated part.
(4) Adopting new technologies, new processes, and new materials
The adoption of new technologies, new processes and new materials is the only way to improve the corrosion resistance of door and window hardware accessories. Foreign hardware manufacturers use physical vapor deposition (PVD, also known as vacuum plating) to produce hardware parts with excellent appearance and corrosion resistance. Vacuum plating has strong adhesion, and the coating is not easy to fall off; the diffraction is good, the thickness of the coating is uniform; the coating is dense, the pinholes are less; the treatment is easy before plating, the process is simple; the materials are used, and the pollution is harmless. A simple substance, alloy or compound can be plated on a metal or non-metal surface. It has been widely used in the industrial sectors of aviation, aerospace, electronics and optics to prepare coatings with special properties.
With the continuous improvement, large-scale and continuous vacuum coating equipment, and the development of coating technology, decorative coatings with good bonding strength, high abrasion resistance and high brightness can be obtained. In foreign countries, the surface of some accessories such as handles and hinges is vacuum-plated with Au, Ag, Al, Zr, TiN, 1Cr18Ni9, etc., and the outer surface with excellent comprehensive performance is obtained, so that the single functional accessories such as handles and hinges have Use functional and decorative dual performance accessories.
Through the PVD technology, the concept of manufacturing a single metal for handles can also be changed. Many foreign accessories, especially the handles, are manufactured using engineering plastics. By using the injection molding process, labor productivity and product cost can be greatly improved. PVD technology metallizes its appearance. Secondly, new technologies and new processes such as fluorocarbon spraying and cathodic electrophoresis have been applied in aluminum alloy doors and windows, and aluminum alloy doors and windows with excellent aging resistance and corrosion resistance are produced, so that door and window fittings are also used. With the continuous improvement of profile performance, the surface treatment process of single galvanizing and spraying should be changed, and new technologies, new processes and new materials should be adopted as soon as possible.
Aluminum alloy doors and windows are used by more and more buildings because of their beautiful appearance, flexible opening, convenient use, light weight and large lighting area. Doors and windows are often improved with the technical progress of hardware accessories to improve their functions. of.
Due to the characteristics of aluminum alloy which is easily passivated under natural conditions, when the hardware is made of general metal materials, the structure of aluminum alloy doors and windows will be a combination of large cathode and small anode. How to avoid or reduce galvanic corrosion is the best choice for aluminum or aluminum alloy. There are many stainless steel grades, and the varieties with better corrosion resistance should be selected. Carbon steel should be added with an insulating layer or a non-metallic coating to effectively prevent electrochemical corrosion. Especially in today's industrial rise in the world, the dangers of industrial atmosphere to doors, windows and hardware have become quite common. The choice of materials for hardware and proper surface protection have been valued by the industry.
In recent years, aluminum alloy profiles have been improved in many aspects, such as the use of surface electrostatic spraying, so that the surface color of aluminum alloys is colorful, which also makes aluminum alloy doors and windows play a key role in the rich facade effect of new residential communities, expanding Use space.
