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Chemical Properties of Aluminum and its Alloys
- The standard electrode potential of the (25*C) elemental solution is £-(H) and the molten salt electrode potential in the LiCl-KCl eutectic (45X). (Pt) (see 2-5), reflecting the reactivity or activity of the element, the more negative and lively. The order of £°(H) and £°(Pt) is roughly the same, except that the order of the individual parts S°(Pt) is reversed. It can be seen from the two columns that aluminum is much more active than zinc, and the stability of aluminum is completely protected by oxide film.
- Table 2-5 Standard electrode potential (25$) in the aqueous solution of elemental ion iT (H) and molten salt electrode potential in LiCl-KCl eutectic (450^) £° (Pt)
Table 2-5 Standard electrode potential E°(H)inwater solution (25 )and electromotive force E°(Pt)in molten LiCl-KCI eutectic (450X1 )of some elements
|Element (valence)||£:°(H)/v||Element (valence)||^°(Pt)/V|
|Li(l +/0)||-3.05||Li(l +/0)||-3.304|
|Na(1 +/0)||-2.71||Na(l +/0)||-3.25|
|Sn(2+/0)||– 0.14||Cr(3 +/0)||-1.125|
|Ag(l +/0)||十 0.80||Co(2+/0)||-0.991|
|Au(l +/0)||+ 1.50||Ag(l +/0)||-0.743|
|Au(l +/0)||+ 0.205|
- The two columns of potentials in the table also illustrate an essential problem, that is, the gold field in the front of the sequence, such as the salt solution in the metal after the release sequence (left column) or molten salt 右 (right column), the former will Ion reduction is precipitated in the gold domain, and is itself vaporized into ions into the solution or molten salt. For example, inserting A1 into the ZnCl2 molten salt includes: 2A1 (metal) + 3Zn2t = 2A1, + +3Zni reaction, which is the mother A common reaction between a material and a component in a flux.
- Aluminum is very slowly soluble in moderate concentrations of nitric acid, but is stable in concentrated nitric acid, and the higher the concentration of nitric acid, the more stable it is. Transporting fumes Nitric acid tanks are made of pure aluminum, mainly because nitric acid is a highly vaporizable acid that vaporizes aluminum to form an S thick oxide film. The oxidizing property of concentrated sulfuric acid is also very strong, and it is also difficult to react with aluminum, and the sulfuric acid after the dip is easy to dissolve aluminum. Aluminum bismuth is dissolved in hydrochloric acid and hydrogen acid, but is insoluble in passivation in phosphoric acid. The ability of aluminum to resist alkali is very weak, soluble in aqueous solution of NaOH and KOH to form aluminate NaAIO, or KA102 simultaneously releases hydrogen 2A1 +2NaOH +2H20 = 2NaA102 +3H2 T (2-1)Interestingly, the hawkers selling balloons know how to use this reaction to react the waste aluminum and caustic solution to produce hydrogen to flush the hydrogen balloon. It is also wary to note that it is dangerous to put aluminum and caustic soda together. The oxide film on the aluminum surface can also react with the alkali. A1203 +2NaOH = 2NaA102 + H20 (2-2)
- This reaction is commonly used to remove corrosion products on the aluminum surface and an excessively thick oxide film. When a large hydrogen bubble is found in the reaction liquid, it has been reacted with the base aluminum according to the formula (2-1). The over-reaction will cause crater-like microscopic pits of varying sizes on the surface of the aluminum.
- The electrode potential of all heavy metal ions is positive than that of aluminum (see Table 2-5), so when aluminum is in contact with their solution, aluminum is dissolved due to the displacement reaction and heavy metal ions are deposited. The same is true in anhydrous molten salts. The heavy metals and aluminum that are deposited form galvanic couples, causing numerous microbatteries on the aluminum surface to promote their galvanic corrosion. In heavy metal salts, the damage caused by the salt of strontium to aluminum and aluminum alloy is the greatest, far from the surface corrosion, and the destruction of intergranular fractures inside the base metal. The mercury salt includes HgCl2, Hg(N03)2, HgS04, etc., and after being reduced by aluminum, precipitates gold and forms an amalgam. The Al-Hg alloy is liquid at room temperature, and the oxide film cannot remain intact at this time. Once this process begins, it will continue forever. In the humid environment, A1 in the amalgam is immediately oxidized, and the excess mercury forms a new amalgam with the underlying A1, so that it is repeatedly pushed deeper into the lower base metal until A1 is completely oxidized to A1(0H)3. The reactants in contact with A1 have a visible effect even if the concentration of Hg is less than 10,000. Therefore, in the brazing process of A1, the cleaning solution, the injection, the brazing filler metal, and a 0 may be in contact with the base metal. Mercury and its ions are not allowed in the material. It should be noted that gallium and its salts have similar damage at room temperature of 30T.
- It can be seen from the above description that some aluminum alloys containing elements such as Cu and Zn have poor electrochemical corrosion resistance. In order to improve the turbidity resistance of commercial aluminum alloy materials, it is often used in the outer layer of the sheet or pipe. D-1/10 thickness of the protective alloy layer, the electrode potential of this layer of the juice is lower than the core material by about 0.4 IV, which serves as a cathodic protection. Different alloys have a proprietary protective layer with …289, including cu Hard aluminum, for example, 2A12 (2024) is often coated with a layer of pure A1 protection. When cleaning such alloy workpieces with NaOH solution, it is easy to find that the structure is exposed due to Cu, and the reaction with NaOH has a black CuO formation. The surface is not blackened because of the protection of the pure aluminum layer. This reaction is often used as a simple means to judge the Cu-based alloy. The final rinse of the dilute nitric acid can dissolve the generated CuO to restore the original color of the workpiece. Cleaning the hard aluminum alloy Attention should be paid not to dissolve the bedding layer of surface aluminum by NaOH.
- The passivation zone of aluminum, i.e., the stable zone of the oxide film, has a pH of 4-8.5 at the cup. Al5t is formed in the acidic solution below the lower limit of this value, and A102_ is formed in the alkaline solution above the upper limit of this value.
- Although the pH is stable at 4 to 8. 5, if the salt content in the cup solution is high, pitting corrosion is likely to occur. The corrosion resistance of AI to F-, Cl’ Br_, r is reduced in turn.The brazing seam or solder joint formed after the brazing material is alloyed with the aluminum base material forms a galvanic couple with the base material, and it is inevitable that it is corroded in an electrolyte solution or humid air. The more distinct the boundary between the solder joint and the base metal, the more galvanic corrosion. The brazing material forms a solid solution transition with the base material to have superior corrosion resistance.
Nature of Oxide Film on Aluminum and its Change during Heating
- The structure of the oxide film formed on the surface of pure aluminum is determined by 7-Al203 or A1203. Since “Y- or Y variants are different (1), they are extremely weak, and it is generally considered that the structure of the surface film is “y-AUO” without distinction. There have been many studies on the structure of oxide film on the surface of A1. Hunter and FowW61 have studied the natural oxidation of a series of pure aluminum by electron diffraction method. It is pointed out that the fresh aluminum oxide film at room temperature is about Inm; The oxide film is linearly thickened and has a structure of tj-A120. The film has two layers, the inner layer is in an amorphous state by A1, and the outer layer is in a crystalline state. When heated, the thickness of the layer increases with temperature. When heated for a long time, the whole crystal is crystallized. Pafianel-li (1) studied the surface film of 620 弋 aluminum by electron diffraction, and concluded that the structure of wa is still 丨2()3« (:(><:hrai丨 and Sleppy 8 studied high-purity aluminum and 5052 by electron diffraction method ( Al-Mg alloy oxidation at 450-6401C, the conclusion is that whether high-purity aluminum or 5052 alloy surface film is t; -A120, Pan Ying (4) DTA-TG method used to study high-purity aluminum powder and aluminum foil heating Through the oxidation in the stalk, it is pointed out that the oxidation rate of A1 increases at the soot, and the velocity is the fastest. The oxidation rate of M1 is slowed down after M0弋; the recognition force (the formation of large crystalline oxides in iOOTC [6,8), 6301: Time is an orderly structure that continues to grow. Experimental and calculation results show that the thickness of XnC heated 3min surface oxide film is rapidly increased from lnm to about 20mn. The aluminum box is heated at 600t for 13h, after scratching CH3OH-Br3 % immersion method to obtain the obtained oxide film and 660 heating aluminum foil, aluminum melting and shrinking into the oxide film obtained by electric F diffraction analysis, it is proved that the structure of ?-Al203 (or TAM), is still maintained. [11>1 SEM electron probe and XRD were studied for the change of yttrium oxide during heating of 5A02 (LF2). Table [Quantitative analysis of the composition of fti shows that with the increase of temperature (400 ~ 500^), the surface of Mg, Si, Mn is rapidly enriched; it is multiplied enriched at 580t near the over-burning (see Figure 2) 1) XRD analysis shows that MgO, MgAl204, MnAl6 and M&Si phases are also apparent in the oxide film. This indicates that when the aluminum alloy is heated, the more oxygen-friendly internal alloying elements diffuse into the surface and form new oxides in the film. Phase or composite oxide phase, such as MgO, MgAl204, etc. When the aluminum alloy sheet is heated rapidly, the oxide film attached to the aluminum surface is cracked due to the large coefficient of linear expansion of the base material , such as As shown by the small arrow in Figure 2-2, the aluminum under the crack will also rise upward due to the sudden disappearance of the binding force at the crack (see Figure 2-3) until the new oxide film establishes a new binding force and stops.
Brazing and Soldering
- Jsot is often used as a boundary between brazing and soldering. In the pinning agent, the matrix of the solder is usually organic, and the nailing temperature cannot exceed 350 弋, and the coke will be coked. Therefore, in order to facilitate the discussion, the molten iron used in the wood section is all in the Vacuum brazing furnace range.
- The larger number makes the oxide film attached to the aluminum surface cracking , as shown by the small arrow in Figure 2-2. The aluminum under the crack will also rise upward due to the sudden disappearance of the binding force at the crack (see Figure 2-3) until the new oxide film establishes a new binding force and stops.
Fluxes for Aluminum Brazing
- The chloride flux consists of three functional parts: a matrix, a remover and a surfactant.
- The matrix of the nitride flux is composed of a vapor-mixed molten salt of an alkali gold or an alkaline earth metal, and the substrate is chemically stable and does not substantially react with aluminum. Its function is: the molten matrix is melted on the joint of the solder joint to isolate the air; the composition of the matrix is adjusted so that the melting temperature matches the melting temperature of the solder; the matrix is also the solvent of other components in the flux. In order to match the melting temperature of the A1 solder Al-Zn (3811) and Al-Si (5771C), the melting temperature of the flux matrix should be arbitrarily adjusted. The most commonly used substrate is a mixed molten salt of LiCl containing the u KC1-IJC1 binary system (see Figure 24). The eutectic temperature is only 355T1~, which is one of the low eutectic temperatures in all alkali metal or alkaline earth metal vapor binary systems. Suitable for fluxing fluxes for Al-Zn solders. In the flux of HJ made by other brazing materials, a relatively low melting temperature is required, and a molten salt of a LiCl-KCl-Nalyst ternary system is usually used as a matrix. The phase diagram  of this ternary system is shown in Figure 2-5. It can be seen from Fig. 2-5 that there are two ternary package points 弋 and -• two binary eutectic points E in the system. The temperature and composition of the nonvariant points in the system are shown in Tables 2-6. Due to the different authors of the study, there is a slight difference between the data shown in Figure 24 and Figure 2-5. The molten salt composed of no change points in Fig. 2-5 is most ideal as a matrix because they melt at a temperature range of 0, which does not produce the uncomfortable entanglement from the initial culturing to the full squeezing. When the base culture temperature needs to be higher than 1 P, 406T, it is best to choose P, -m line
- At some point, because the melting interval of the molten salt is relatively smaller than the melting interval of the other liquid phases.
- The ideal matrix salt system is LiQ-SKUj B: system (e point 475″?:) ^LiO-KCl-SiClj system  (£,4021;, £:328, P.340T., p2487r .), ua-KQ-CaQj is called (E, 340X:, £2425, 64751).
Table 2-6 Composition and temperature of nonvariant points in system LiCl-KCl-NaCl
|temperature||Composition (molar fraction, %)||Composition (mass score, %)|
- LiCl-containing matrices have higher activity (see Section 2.6), but the biggest disadvantage is hygroscopicity, which absorbs water in humid air and eventually becomes a solution. When the water-absorbed substrate is heated and dehydrated, LiC丨 will hydrolyze to produce Li,0HCl, which increases the pH and greatly affects the activity of the flux. The range of substrates without I.iCI can be narrow, Caa, -NaCl-KCl [171 series (e506t:, £, 504t;) and SiClj-Na-Ka- series (£, 523^0, E25MX) 🙂 Children are only available in the system, and R melting temperature is biased, only suitable for the preparation of high temperature aluminum flux.
- F in the fluxing flux is the main component of the destruction of the aluminum gasification film. Many researchers have shown that when the concentration in the above matrix is not large, the amount of the oxide film dissolved therein is negligible, but loose, wrinkled, Rupture, shedding or peeling. The rate of peeling increases as the concentration of fluoride increases. It is believed that the ability to remove the film is related to the type of paper, and the ability of the base metal and alkaline earth fluoride to remove is stronger than that of aluminum fluoride and cryolite. IiF has the strongest film removal ability. His experiment was carried out using LiF, NaF, KF, CaF2, A1F3, Na3AlF „ six kinds of fluorides in the LiCl-KCl eutectic with a mass fraction of 5%. Obviously he ignored the same weight of fluoride, The fluorine is different from the concentration. The relative molecular mass of LiF is the smallest. When the various fluorides are the same weight, the amount of LiF molecules is the largest, that is, the maximum concentration of F_, of course, has the best film removal effect. It can be seen that the effect of removing the film is mainly related to the concentration of F. ions, and there is no significant relationship with the type of the compound.
- Fluoride-added matrices generally increase the melting temperature of the matrix, so when determining the melting temperature of the flux, attention should be paid to the combined effect of adding a gasification. In general, in the case of LiF, the added scene rarely exceeds 3% to 4% (mass fraction), which is preferable, and other types of fluoride should be added more.
- The surfactant in the chloride flux is mainly heavy metal ions in Table 2-5. Any metal ion having a potential higher than A1 after A1 can be used as a surfactant in principle. When the flux is reacted, the gold domain ions are reduced and deposited on the surface of the aluminum base material, and A1 is dissolved into the flux to become Al3+. This mass transfer reaction will significantly reduce the interfacial tension between the flux and the parent metal. In order for the solder to wet the base material to the utmost extent, a similar mass transfer reaction should be made between the flux and the molten solder.
- Thus, it can be seen that when an aluminum-based solder is used, Zn2+ can be used as a surfactant. However, when a zinc-based brazing filler metal is used, the flux containing only Zn2′ can cause mass transfer with the
- A-base metal, but it cannot transfer mass with the zinc-based brazing filler metal, so it is impossible to minimize the brazing filler metal. Interfacial tension between the base metal and the base metal. In this case, the flux should contain at least one metal ion after Zn2*, that is, a positive potential than Zn, such as Sn2, Pb2 and the like.
- In the sequence of Table 2-5, the element that is positive than the A丨 potential is 屮, the closer the ion is closer to A1, the slower the rate of reduction by A1, the farther away the recovery is, the faster the reduction is because the battery is formed between them. The farther the electromotive force is, the stronger the power of reduction is. The rate of reduction of the heavy metal ions as the interfacial activator should be matched with the rate at which the precipitated metal is alloyed with the base material, i.e., the metal precipitated and precipitated should be wetted and alloyed with the base material in a timely manner. If the precipitation rate is too fast and the alloying is too late, the precipitated metal will be suspended in the flux in the form of fine particles, and the symptom of the appearance is blackening of the agent. This phenomenon is often seen when using Bi3+ plasma as the active agent.
- In this case, there are three ways to control the heavy metal ions in the surfactant to slow the precipitation rate of the aluminum parent material surface h: -• select heavy metal ions that are slightly closer to A1; second, reduce heavy metal ions in the brazing The content of the agent; the third is to reduce the brazing temperature.
- On the aluminum base metal, heavy metal ions are reduced and precipitated into a metal and alloyed with the base material, and should be in a thin layer of liquid at the brazing temperature, so that it has a more active activity. To achieve this, a salt of a metal having a lower melting point of the metal itself may be selected as a surfactant. For example, a metal which is molten at a brazing temperature (-GOOt) is: Zn (419*t), Cd ( 321t),
- Sn(232t:), Pb (328^), In(1571C), Bi(271<C), T1(30T1C), and the like. Their vapors can be used as a surfactant. In addition, it is also possible to select those salts which form a multi-liquid metal eutectic on the surface after alloying with the base material as a surfactant, for example, K2SiF6 (precipitated Si forms Al-Si eutectic 517 ° C), K2GeF6 (Al- Ge eutectic 4201), CuC12 (A1-Cu co-product 5481C), AgCl (Al-Ag eutectic 567T), and the like.
- There are basically two types of relationship between the metal and the mother tree in which the interface agent is reduced and precipitated: one is a mutual solubility with the base metal, the precipitated metal and the base material are wetted and spread, and the base material is further produced. The reaction, for example, is Zn2+-Zn 和 and the aluminum base material is mutually soluble and penetrates into the base material (see Figure 1-21); the other is extremely difficult to form with the mother tree: the precipitated metal is half on the aluminum surface. Wet droplets. Figure 2-6 shows the reaction of yCl-KCl-NaCl-UF-CdCl2 flux 600弋 with aluminum.
- Although the precipitated Cd is wetted with A1 but cannot be spread, it is not suitable to use a salt of those metals having a very low mutual solubility with AI as an active agent, which includes Cd2′ Pb2*, T12♦ and the like.
- A study of the relationship between the concentration of the interfacial agent in the flux and the spread area of the ft solder (see Figure 2-7) shows that as the surfactant content increases, the activity of the flux increases.
- However, there is a high point, and the activity decreases after this point. Regardless of the ion, the highest point content is different, but the conversion to the mole fraction is almost 3%, indicating that the maximum activity of the flux is related to the number of atoms of the precipitated metal. Therefore, when using different surfactants, the mass fraction used should be different.
- The duration of flux activity varies with the + of the interface active vehicle, 囝2>8 [1 shows the failure curves of several active agents. It can be seen that at 620T, ZnCl2 is a fast-acting but non-permanent active agent. . The quick effect is obviously due to the largest electrode potential difference between Zn and A1, and the reduction speed is fast; it is not durable because Zn and AI have great mutual solubility, and the reduced Zu is extremely infiltrated by the mother. CdCl2 is slow-acting, and the rate of reduction between Cd2.-A1 is slower and the mutual solubility is small. T1C1 has a process of decreasing activity at the beginning stage, which may be related to the process in which TT is first and then briefly gasified to T13′. It can be seen that which active agent has its own disadvantages, and it has a good effect when used together.
- Different from the melting temperature of the fluoride-added flux, the addition of heavy gold derivatives often tends to lower the melting temperature of the flux.Preparation and inspection of nitride flux The test criteria for chloride pesticides are mainly four in the correct formula: one is the melting temperature (liquidus) is standard (ie, with the liquidus of the solder); the second is After melting, the crucible should be completely transparent and clear, and there should be no flocculation or insoluble residue. Third, the flux powder is dry and cannot absorb water. The performance is that the flux powder does not stick to the glass bottle; the fourth is that the pH of the aqueous solution of the flux cannot exceed 6. 5.
- In the components of the injection, LiCl and ZnCl2 absorb the most water, and dehydration inadvertently will cause hydrolysis and produce Li2OHCl and Zn(OH)Cl, thus increasing the pH of the flux and generating flocs, which will be ffl. The flow of solder is impeded. Four dehydration methods are introduced. It is considered that vacuum dehydration is the best, but in the preparation of small scales, the co-heating dehydration with NH4C1 and LiCl or ZnCl2 is a convenient method to effectively inhibit hydrolysis. Among the components of the flux, the most common impurity is Fe3., and their presence is the source of another heavy enthalpy which causes flocculation in the flux.
- The flux is formulated in a melt process and then pulverized. The process is ideal. The product is absolutely uniform, and it is dry and non-hydrolyzed.When the flux is in the liquid state of melting force, it is basically dissociated into a single ion. The cations such as LT, Na\K\Zn2+, Cd2* and CT, F-ming ions, and the type of the original compound have no relationship. For example, 2LiF and ZnClj melt together and then melt together with ZuFj and 2UC1. The result is exactly the same, because the melt is a single ion and 11 is generally more n. In some literatures and manuals, it is often possible to Many different forms of formula, but a closer look, there is no difference in substance.