Corrosion
Corrosion is a common occurrence when performing chemical process operations involving surface preparation, primarily in preparation for paint application. The protective oxide film of aluminum is only stable in the pH range of 4.5-8.5. However, many process solutions intentionally exceed this pH range for cleaning, metal removal, and subsequent fouling removal. These process schemes are formulated to avoid detrimental pitting or preferential etching.
The susceptibility of CNC machined parts to pitting corrosion depends on external factors such as chloride ion concentration, pH and initial surface conditions. Electrochemical measurements by potentiodynamic scanning have proven to be an effective tool in analyzing the propensity of certain process solutions to facilitate the observed pitting conditions.
This article reviews several process solutions for CNC machined parts, examines coolants, solvent cleaning, alkaline cleaning/etching, and deoxidation/desliming, and lists both intentional and unintentional chemical reactions, as well as possible mechanisms that favor corrosion formation. The role of incoming water is further explained, which is used for process solution replenishment and numerous flush tanks. Advice is provided for electrolysis processes that may easily affect stray currents from auxiliary equipment, thereby introducing harmful contaminants into the process solution due to corrosion products of piping, fittings and fasteners in heating and cooling units.
Strict adherence to process specification controls, regular monitoring of suspected contaminants, sound housekeeping, and best practices for the handling of CNC machined parts can reduce the occurrence of surface corrosion on many aluminum CNC machined parts.
Oxidation
Oxidation treatment mainly includes anodic oxidation, chemical oxidation and micro-arc oxidation. Xu Lingyun et al. The mechanical properties and corrosion resistance of A 356 aluminum alloy were investigated by three different methods. Surface treatment of CNC machined parts by chemical oxidation, anodization and micro-arc oxidation. Through scanning electron microscopy (SEM), wear test and corrosion resistance test, the surface morphology, oxide layer thickness, wear resistance and corrosion resistance of aluminum CNC machined parts were studied sex. Surface treatments were analyzed and compared in detail.
The results show that different thicknesses of oxide films can be formed on the surface of surface-treated aluminum CNC machining parts, the surface hardness and wear resistance are significantly improved, and the corrosion resistance is also improved to varying degrees. In terms of comprehensive performance, micro-arc oxidation is superior to anodizing, and anodizing is superior to chemical oxidation.
Chemical oxidation
Chemical oxidation refers to a coating method in which a dense oxide film is formed by chemical action on the surface of clean aluminum CNC machined parts and oxygen in an oxidizing solution under certain temperature conditions. Depending on the properties of aluminum and aluminum alloy solutions, there are various chemical oxidation methods.
It can be divided into alkaline and acidic. According to the properties of the film, it can be divided into oxide film, phosphate film, chromate film and chromium phosphate film. The thickness of the oxide film obtained by chemical oxidation of aluminum and aluminum alloy CNC machining parts is about 0.5~4μm, the wear resistance is poor, and the corrosion resistance is lower than that of the anodic oxide film. It is not suitable for use alone, but has certain corrosion resistance and good physical properties. Absorbency is a great primer. The chemically oxidized coating of aluminum and aluminum alloys can greatly improve the bonding force between the substrate and the coating, and improve the corrosion resistance of aluminum CNC machining parts.
Plating
Electroplating is to deposit a layer of other metal coatings on the surface of aluminum CNC machined parts by chemical or electrochemical methods, thereby changing the physical and chemical properties of the surface of aluminum CNC machined parts.
Surface conductivity; copper, nickel or tin plating can improve the solderability of aluminum alloys; hot dip tin or aluminum-tin alloys can improve the lubricity of aluminum alloys; chrome or nickel plating can generally improve the surface hardness of aluminum CNC machined parts and wear resistance; chrome or nickel plating can also improve its decorative properties. Aluminum can be electrolyzed in an electrolyte to form a coating, but the coating peels off easily. To solve this problem, aluminum can be deposited and coated in an aqueous solution of zinc-containing compounds. The zinc immersion layer is the connection between the aluminum and its alloy substrate and the subsequent coating. Important Bridge, Feng Shaobin et al.
The application and mechanism of aluminum-based zinc immersion layer were studied, and the latest technology and application of zinc immersion process were introduced. Electroplating after immersion in zinc can also form a porous film on the surface of aluminum CNC machined parts before electroplating.
Electroless plating
Electroless plating refers to a film-forming technology that deposits a metal coating on a metal surface through an autocatalytic chemical reaction in a solution coexisting with a metal salt and a reducing agent. Among them, the most widely used is electroless Ni-P alloy plating.
Nickel-phosphorus alloy is a low-polluting chrome-plating process and a good chrome-plating material. However, chemical plating process equipment is numerous, material consumption is large, operation time is long, the process is cumbersome, and the quality of plated parts is difficult to guarantee. For example, Feng Liming et al. The process specification for electroless nickel-phosphorus alloy plating based on the composition of 6063 aluminum alloy was studied, and the process only included pretreatment steps such as degreasing, zinc immersion and water washing. The experimental results show that the process is simple, the electroless nickel plating layer has high gloss, strong bonding force, stable color, dense coating, phosphorus content between 10% and 12%, and the hardness of the coating can reach more than 500 HV, which is much higher than that of the anode. and oxide layer. In addition to electroless Ni-P alloys, there are other alloys, such as the Ni-Co-P alloys studied by Yang Erbing. The film has the characteristics of high coercivity, small remanence, and good electromagnetic conversion performance. It can be used in fields such as high-density disks, and can be electroless plated.
The Ni-Co-P method can obtain magnetic alloy films of uniform thickness on substrates of any complex shape, and has the advantages of economy, low energy consumption, and convenient operation.
The method of ion implantation is to bombard the target with a high-energy ion beam in a vacuum state. Almost any ion implantation can be achieved. The implanted ions are neutralized and left in substitutional or interstitial sites in the solid solution, forming an unbalanced surface layer.
Improve the surface hardness, wear resistance and corrosion resistance of CNC machined parts. Magnetron sputtering pure titanium and then injecting PB 11 nitrogen/carbon can greatly improve the microhardness of the surface of the modified CNC machined parts. Magnetron sputtering combined with nitrogen injection can increase the substrate hardness from 180 HV to 281.4HV. Magnetron sputtering combined with carbon implantation can be increased to 342 HV. Magnetron sputtering pure titanium and then injecting PB 11 nitrogen/carbon can greatly improve the microhardness of the surface of the modified CNC machined parts. Liao Jiaxuan et al., on the basis of plasma ion implantation of LY 12 aluminum alloy, carried out composite implantation of titanium, nitrogen and carbon, and achieved remarkable modification effect. Zhang Shengtao and Huang Zongqing of Chongqing University performed titanium ion implantation on the surface of aluminum CNC machining parts.
The results show that the implantation of titanium ions into the surface of CNC machined aluminum parts is an effective way to improve its resistance to chloride ion corrosion, and it can improve the ability of the surface of CNC machined parts to resist chloride ion corrosion. The passivation potential range of aluminum alloy in NaCl and other solutions is widened, and the density and size of corrosion holes corroded by chloride ions are reduced.