Some hard oxidized parts in the market have white spots, but I don’t know why. Next, perform anatomical analysis on the parts of the leukoplakia phenomenon. The analysis process and results are as follows:
Use a magnifying glass and a stereo microscope to observe the macroscopic appearance of the connector. The surface of the connecting seat showed a white spot area, and a white spot area was also observed near the deep hole at the bottom. Careful observation revealed that the white spot area had obvious trace features similar to the flow of liquid. In order to perform a three-dimensional inspection on the connecting seat, a deep threaded hole in the part is dissected. There is whitening in the hole. There is a certain color difference with the base material, and there is a small amount of suspected corrosion products at the bottom of the hole.
2. Film Morphology And Composition Analysis
Using scanning electron microscope and X-ray energy spectrometer to conduct in-depth analysis of the hard chromate oxide film on the surface of the aluminum alloy connector. Observing the surface in the area where no white spots appear, it is found that the macroscopic surface of the film in the normal area of the chromic acid hard oxide film is very dense, and there is no abnormal macroscopic hole defect. The SEM morphology and the EDAX composition analysis result. After observation, the white spot area has obvious corrosion morphology, not only turtle cracks. And there are typical corrosion pits. The results show that: the composition of the damaged film layer contains abnormal chlorine element, the mass fraction of which is as high as 5.49%.
3. Corrosion Product Morphology And Composition Analysis
Further observation and analysis of the inner surface of the bottom end of the threaded deep hole showed that the chromic acid hard oxide film inside the deep hole has obvious corrosion phenomena. The main corrosion mode is pitting corrosion, and the diameter of the corrosion pits is usually less than 10 um. A large number of corrosion products are found in and around the pitting pit: at the same time, there are flaky corrosion products falling off and typical cracked “mud pattern” on the inner surface of the bottom end of the threaded deep hole. The EDAX test results of the corrosion products show that the chemical elements include a small amount of foreign elements chlorine and sulfur.
In order to verify the stability and reliability of the chromic acid hard oxidation process on the surface of the connecting seat, a spot experiment was carried out on the normal hard oxidized surface and the surface of the white spot area of the connecting seat. The detection temperature is 25℃, and the qualified time is ≥3 min. . The test results of this connector are: normal hard oxidized surface> 10 min, white spot area surface <2 min. The results show that the white spot area is severely non-corrosive.
5. Experimental Summary
In the surface treatment methods of aluminum and aluminum alloys, hard oxidation is the most common industrial application. Commonly used methods include hard oxidation of sulfuric acid, hard oxidation of chromic acid, hard oxidation of oxalate and hard oxidation of phosphoric acid. The hard oxidation process of aluminum uses the principle of metal oxidation to generate colored hard aluminum oxide film by electrolytic oxidation.” This film not only has good wear resistance. It also has corrosion resistance and adsorption of paint and color. The materials are very strong and can be used as the bottom layer on the surface of the parts.
After analyzing the reasons, it is believed that after the chromic acid hard oxidation process is completed, the residual liquid in the threaded deep hole of the connector cannot be discharged in time, and it remains at the bottom of the hole. Although the inspection party had a gasoline cleaning link on the surface of the connecting seat in the entire process, it still failed to clean the residual acidic liquid in the threaded deep hole. During the packaging and sealing process of the connecting seat, the residual liquid flows out from the deep hole. Adsorbed on the contact part of the plastic film and the parts, after a period of storage or storage, the chromic acid hard oxide film will be corroded and damaged. Thereby forming a white spot area. This is also consistent with the liquid flowing pattern in the white spot area on the surface of the macroscopic examination. Compared with the hard oxide film, it has very strong penetrating ability. Especially in the process of moving with the migration of the flowing liquid, it is easy to react with the aluminum matrix and the oxide film to form soluble metal chlorides. The electrode potential is generated to form a galvanic cell in a small local area, which in turn leads to the generation of corrosion holes.
The quality of aluminum alloy chromic acid hard oxide film and its anti-corrosion protection performance mainly depend on the composition of aluminum alloy, film thickness, hard oxidation treatment process conditions, such as temperature, current density, water quality and hardness. Filling and sealing process after oxidation and so on. Therefore, in order to avoid the failure of the hard oxide film similar to this article, it is necessary to check the details, strictly standardize the production operation procedures, and prevent the acidic liquid residue generated after the aluminum alloy hard oxidation process, so as to prevent the anodized film from being damaged by corrosion. .
Link to this article：The Causes Of White Spots On The Surface Of Parts After Hard Oxidation
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