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Interaction of Oxide Film on Alumi-num with Molten Salt Flux

  • Early scholars believed that the aluminum deuterated film during brazing was removed by dissolving it in a fluoride containing flux. Experiments have shown that • y-Al203 is almost insoluble in the molten flux, but is simply peeled off and suspended in the agent. The work of Sully et al. supported West’s experiment. They pointed out that the insoluble matter of the flux after flux dissolving and brazing is mainly sheet-like aluminum telluride. ■lofdan11-1 uses a single-color light under Jft micro-mirror to see the interference pattern and image of the wrinkle when the vaporized film is peeled off. 60% of the oxide film was peeled off in the flux of 7% (NaF), which was peeled off in the first 30 seconds, and 20 minutes in the total peeling. This observed phenomenon was further confirmed by the electrochemical experiments he designed. The reference electrode is “no-film aluminum sheet (pre-emptive of the oxide film in the flux for a long time), and the other electrode test piece is heated in air to cover the oxide film. The two electrodes are inserted after the flux The potential difference of 600 ~ 650mV is then attenuated to zero in about 40ms. The filmed aluminum sheet is the positive electrode, and the filmless is the negative electrode. This experiment shows that the oxide film is peeled off and the window appears very fast, and all peeling takes 1~ 2min? One of his most interesting findings is that during the brazing process, aluminum is afraid of oxidation, but must also have dissolved oxygen to participate. In the case of high vacuum completely anaerobic, despite the flux fluoride, The brazing process does not occur. Therefore, he proposed a membrane removal mechanism: an electrochemical process involving oxygen, aluminum is the anode, the reaction is A1 = A154 +3e, and the oxide film on the aluminum surface is the cathode 20+4e = 202 -.
  • The experimental history reported by TerriH M1 fully demonstrates that during the brazing process, the oxide film is a stripping process rather than a dissolution process. An aluminum test piece with a phosphorus-labeled aluminum oxide film of a radiation member is immersed in a flux, and when the flux is not fluoride-containing, the test piece does not lose radioactivity, and after impregnating the fluoride-containing flux, the flux is used. The water dissolves the soluble fraction, and the remaining insoluble residue contains up to 95% of the original radioactivity. Terrill also found that the braze must have a certain degree of humidity during the brazing process; after the high vacuum completely removes the moisture, the brazing process cannot proceed despite the high fluoride content of the flux. However, after half an hour of air release, the experiment began to see the process; after lh, the brazing process occurred normally. He therefore proposed a membrane removal mechanism: moisture and fluoride react to produce HF, while HF loosens the oxide membrane. He also cites an experiment in which the brazing process cannot occur in the brazing of the vapor-free NaCl-KC1-丨iCl molten salt, and when the HF gas is passed, the brazing can be satisfactorily verified. His theory.
  • There are also many scholars who have reported the mechanism of membrane rupture: Miller [M1 reported that after the introduction of HCI or Cl2 in the alkali gold sulphide molten salt bath, the brazing process can be carried out; Oropiatt 1861 indicates that KCl-I.iCl-NaCl Anode polarization was carried out in the salt bath, and it was observed that there was no F? It was also possible to break the membrane; a patent report [>7] contacted the Ni block with A1, and the Ni-Al couple could activate the salt bath to remove the film. A summary article report examines the role of nine oxidants (that is, substances or methods that can make A1 into A13♦) in the process of stencil welding: Anyone who can effectively remove the film to allow the brazing process to proceed must be intentional. Or innocently involved in the formation of an oxidizing agent, which reacts on the boundary between the oxide film and the matrix aluminum, and raises the valence state of the aluminum in the matrix to loosen the gasification film. The amount of this oxidant is very small, and more It causes excessive oxidation or corrosion of the aluminum of the substrate; if the oxidant is completely absent, the oxide film cannot be loosened, and the brazing process cannot occur.
  • Regarding the cause of the wrinkling and loosening process of the oxide film in the fluorine-containing flux, reference  reported that 7-Al20 was used as the simulated oxide film and the flux to find:1) Regardless of the fluoride added to the flux of the IiCl-KCl matrix, 7-Al203 produces a strong UF peak on the X-ray diffraction line, which cannot be removed by thorough washing, indicating LiF and rAi2o3 In a combined state.-y-AI^Oj (440) The diffraction angle 261 is reduced from 67. 2° to 66.8. , indicating that the unit cell is inflated.
  • The diffraction peaks of -A1203 are sharpened after the action of the agent, indicating that the crystallinity is increased after the action with the flux.
  • After analyzing, calculating and comparing the size of the lattice gap and the size of the ionic radius, it is concluded that the aluminum oxide film produces swelling and wrinkling under the action of the flux, which is because the ionic ft of the flux is small and the LT is squeezed into the Y. – The octahedral or tetrahedral voids of the oxygen skeleton in the structure of A1203, thereby accumulating the oxygen skeleton. Li♦ is attractive again?
  • Form LP-F+ “ion pair” and stabilize. After the action of the flux, the crystallinity is lowered by the increase in crystallinity of Y-A1203. The concept of IT crowding not only explains the wrinkling and susceptibility of the f oxide film, but also explains the reason why the flux containing U + is more active.
  • Regarding the behavior of aluminum oxide films in Isocolok aluminosilicate fluxes, Field and Stewanl 1891 were studied using a hot stage microscope. The experimental conditions were exactly the same as those of Jordan1181, but he did not see the interference image of the wrinkling of the monochromatic light diffraction oxide film described by Jordan after the Nocolok flux was melted. There was no indication that the oxide film was penetrated and lifted. Up and loose. Therefore, he tends to think that the aluminum oxide film is dissolved and removed in the Nocolok flux. But in the end he added another word: unless Xuan agent quickly broke the membrane into very fine particles, it was not observed by his microscope.
  • The reason for the strong membrane removal ability and activity of Nocolnk potassium fluoroaluminate flux is reported in reference [37]. In addition to the high concentration of vapor, it is mainly caused by SiFf in the flux: 4A1 +3Sip6_ = 4Al3 ^ + 3Si i + 18F”
  • SiFj- not only raises the valence state of the aluminum under the bismuth film, but also precipitates Si to produce mass transfer. When the brazing temperature is eocrx:, a liquid Al-Si eutectic layer is formed, thereby improving the activity of the flux. Nocolok flux prepared in platinum with high purity A1F, or A1203 and potassium salt has almost no activity [37]0 containing Si) 0.01% flux, which can be seen shining on the aluminum surface. Al-Si alloy layer with high activity. Si is the most difficult impurity to remove in aluminum raw materials, and industrial A1203 contains Si in the order of several thousandths or more. The exact content of Si is not indicated at all on the specification sheet of the purification reagent, which is why the Nooololc potassium fluoroaluminate flux is generally highly active and unknown. When a company produces Nocolok flux, it intentionally increases the Si content according to reference [37].
  • In the vaporized molten salt containing no F?, the action of the oxide film and the molten salt is extremely weak. In the presence of only heavy metal ions, heavy metal ions can penetrate into the film along the weak point of the grain boundary, but the film cannot be broken [u], and Figure 2-19 shows Lia-Ka-ZnCl2 (0.5) 600^: An image of Zn was precipitated on the aluminum along the grain boundary after 60 s of the aluminum oxide film. Office (:) 2 infiltrated into the film along the weak point of the grain boundary film, and the Zn metal is reduced and precipitated on the base material matrix. Since it is deposited on a part of the grain boundary, it exhibits an antler-like pattern; With the extension of time, the deposition spreads to the grain boundary and expands to the inside (see Figure 2-20). In the undeposited area, the oxide film is still intact and there is no wrinkle damage, indicating that the process is under the film. In progress, work 11 (: 12 does not have a membrane-breaking effect. The report in [11] also shows that when the base metal is heated, the thermal crack of the oxide film (see Section 2.4) is immediately closed for the new membrane, which does not help the penetration of the flux. The weak point of the aluminum oxide film first appears on the grain boundary.