1, the deposition potential of tin is positive, and the conductivity of the plating solution is high.
Standard electrode potential: Sn2++2e? =Snφ？ =-0.140v
And under alkaline conditions: HSnO? +H2O+2e？ =Sn+3OH？ φ? =-0.79 volts
The mass ratio of stannous sulfate to sulfuric acid in the plating solution is controlled between 1:3~4, and the plating solution has good conductivity. In production, the battery voltage will not be greater than 4V, thus saving electric energy.
2, the electrode reaction is extremely fast.
In sulfuric acid solution, the exchange current density io of tin reaches 1 100 Ma/cm? , 2~3 orders of magnitude larger than Cu2+ and Zn2+. That is, in the absence of additives, its own cathodic polarization is very small, which belongs to a metal ion with extremely fast electrode reaction. When bright coating can be obtained by adding additives, the plating speed is still very fast.
3. Tin ion must be in divalent state.
Metal tin ion has two valence states: +2 and +4, and Sn2+ is called stannous ion; Sn4+ is called stannous ion. However, Sn4+ generally exists not in the form of simple ions, but in the form of compounds, sometimes called SNIV.
In the heated alkaline tin plating solution, tin ions are required to be tetravalent; Sn2+ makes the tin coating dull but not white. Therefore, proper amount of hydrogen peroxide is often added from the vicinity of tin anode solution in production, so that the anode is in a golden semi-passivation state to reduce the generation of Sn2+. On the contrary, in acid tin plating, tin is required to be divalent. Sn2+ is easily oxidized to Sn4+. Once Sn4+ becomes β? When stannic acid is used, the tin solution will be turbid and white, and it will be milky white in severe cases. The viscosity of tin solution increases, the dispersion ability and deep plating ability decrease, the conductivity decreases and the cell voltage increases. The coating is gray and dull, the brittleness increases and the welding performance decreases. How to prevent the transformation of Sn2+ into stable tetravalent tin compounds is the key and difficult point of this process.
Acid tin plating mainly adopts sulfate system and methanesulfonate system. The former is cheaper and more widely used.
2 tin anode
2. The stabilizing effect of1tin on Sn2+
If 1L bright acidic sulfuric acid zinc plating solution is newly prepared, it will be divided into two parts in a 500ml beaker, one for pure tin bars and the other for no tin bars; When placed naturally and observed, it will be found that the electroplating solution containing tin bars remains clear and turbid for much longer than the electroplating solution without tin bars. The reason is that metal tin can reduce tetravalent tin to bivalent tin, which can stabilize the plating solution and reduce β? Production of stannic acid.
For example, the literature [1] lists the reactions: SnCl4+Sn=2SnCl2.
2.2 Should the tin anode be equipped with anode package?
First of all, it should be clear that titanium will be corroded quickly in sulfate tin plating solution without oxidizing medium, so titanium anode basket cannot be used.
Whether the tin anode plate is covered with acid-resistant anode package has advantages and disadvantages. The advantages are that anode mud can be prevented from directly falling into the plating solution, and the burr and roughness of the plating layer can be reduced; The disadvantage is that the stabilizing effect of tin anode is almost lost: the anode ladle hinders the convection-diffusion mass transfer process, and the concentration of Sn2+ in the bath in the ladle is high, so tin can play a stabilizing role; Sn2+ in the plating solution outside the bag is easy to be oxidized to Sn4+, which is easy to form harmful β? Tin acid. In production, most of the hanging plating (cathode can only move, but can't be stirred with air) makes the tank deep, without adding anode package; Roller plating will turn the plating solution over badly, and sometimes an anode package will be added.
2.3 Black sludge produced by anode
2.3. 1 What is anode mud?
Metal tin is a soft silvery white metal, which is not oxidized in air at room temperature. Among stannous compounds, stannous oxide is black, stannous sulfide is dark brown, and the rest are almost colorless. The oxide of tetravalent tin is white. Neither SnO nor SnO2 is soluble in water. It can be judged that the black anode mud is not metal tin slag, but stannous oxide SnO.
2.3.2 production of black SnO sludge
In the process of acidic tin plating, gas, namely oxygen, will also be generated on the anode:
4OH4e？ →2H2O+O2↑
Active oxygen atoms have stronger oxidation ability than oxygen molecules, which makes tin atoms oxidized: Sn+O→SnO.
2.3.3 High chemical stability of anode mud
It's a pity to filter and discard anode mud. According to the literature [1], zinc can completely reduce Sn2+ and Sn4+ to metallic tin:
2Zn+Sn4+=Sn+2Zn2+
Imagine dissolving the black anode mud of SnO with reducing hydrochloric acid, reducing it with zinc powder, filtering to get tin powder and recovering high-priced tin. Remove some anode mud from the bottom of the acidic tin plating tank in the factory, clean and filter it, and put it into a 250ml beaker. First, add hydrochloric acid with the volume ratio of 1: 1, and then stir with a glass rod for a while. Three days and nights passed, and SnO didn't dissolve at all. After filtration, analytically pure concentrated hydrochloric acid is used, which is also insoluble for several days. It can be seen that its chemical stability is very high, and it is impossible to change SnO into SnCl2, let alone reduce it with zinc powder. The experiment failed. (If hydrochloric acid is heated and boiled, the cost is high and it is not environmentally friendly. )
2.3.4 Reduce the production of anode mud.
It is difficult to recover anode mud, so it can only be reduced as much as possible.
The greater the anode current density JA, the lower the anode current efficiency, the more serious the oxygen evolution, and the more SnO sludge produced by oxidation. In order to reduce JA (0.5~ 1.5A/dm? Preferably), the anode area should be larger. People who don't understand this truth think that less anode consumption actually wastes more. Moreover, the sparse anode will make the cathode current distribution uneven and the electroplating ability decrease. There are fewer tin anodes, and as mentioned above, the stability of the plating solution decreases. There is no need to worry that the concentration of tin ions in the plating solution will increase after the anode area is too large, because the autolysis of tin is very small in the range of sulfuric acid.
There is too little sulfuric acid in the bath, the anode activation is poor, the conductivity of the bath decreases, and the anode current efficiency decreases. If there is too much sulfuric acid, anode mud will fall off from the electrode plate quickly, and the surface of new tin will be oxidized quickly to produce sludge.
3β? Formation and properties of stannic acid
The hydroxide corresponding to SnO is Sn(OH)2, which is amphoteric: it reacts with acid to form stannous salt, and reacts with OH? Generate stannous salts, such as:
Sn(OH)2+KOH=H2O+KHSnO2
White tin dioxide is insoluble in neither water nor acid. There are two corresponding hydroxides: Sn(OH)4, namely stannic acid, and H2SnO3 stannic acid, or SnO(OH)2. The former loses a water molecule and becomes the latter: Sn(OH)4? H2？ O = tin oxide.
Sn(OH)4 is amphoteric and mainly alkaline. H2SnO3 is also amphoteric, but mainly acidic. They all react with alkali to form stannate;
H2SnO3+2KOH=2H2O+K2SnO3 reacts with acid: H2SnO3+4HCl=SnCl4+3H2O.
sn(OH)4+2 KOH = 3H2O+k2s NO3 sn(OH)4+4 HCl = sncl 4+H2O
Tetravalent tin is dissolved in alkali and acid to form stannic acid called α? Tin acid.
α? Tin acid is easy to polymerize into tin acid with larger molecular weight, such as [H2NO3] 5 or [(SnO)5(OH) 10], that is, β? Tin acid. Generally, 5 molecules α? Tin acid is polymerized into 1 molecular β? Tin acid: 5H2SnO3→(SnO)5(OH) 10
The higher the liquid temperature, the faster the polymerization rate. What about alpha? If stannic acid is continuously heated and boiled, it will all polymerize into β? Tin acid. α? Tin acid is soluble in dilute inorganic acid, β? Tin acid is insoluble.
In acidic tin plating solution, once the divalent tin ion Sn2+ is oxidized to tetravalent Sn4+, it will become α? Tin acid. Partial alpha? Tin acid is dissolved by sulfuric acid in the plating solution, and the other part is polymerized into β? Tin acid. β? Tin acid is insoluble in neither water nor dilute sulfuric acid, and does not form solid precipitation. It is suspended in the plating solution in a white colloid state, which is difficult to be directly filtered by a solution filter and can only be removed after flocculation and precipitation. Its harm to plating solution and coating has been mentioned before. The higher the liquid temperature, the polymerization will produce β? The faster the stannic acid is.
At room temperature, β? Tin acid gradually increased. This can be shown from the gradual change of liquid color: clear and transparent → turbid and not white → light milky white → deep milky white → milky white.
The purpose of this study is to recover metallic tin from β -stannic acid by flocculation and precipitation. After checking the information, I feel that there are too many steps and it is not cost-effective. I put it forward for the reference of interested people. The steps are as follows:
a、β？ Boiling stannic acid with concentrated hydrochloric acid: generating sncl4: (SnO) 5 (OH)10+20hcl = 5sncl4+15h2o;
B, diluting the solution;
C, adding zinc powder for reduction: sncl4+2zn = sn+2zncl2;
And d, filtering and washing to obtain metal tin powder.
4 The destructive effect of chloride ion on the stability of plating solution
Anyone who has done acid tin plating knows that chloride ion is very harmful to the plating solution, but it is not clear how it works badly. Briefly as follows:
Sn2++2ClSnCl2( 1)
Stannous chloride +H2O = hydrochloric acid+stannous chloride (2)
In the presence of oxygen: 4SNCl2+O2+2H2O = 4Sn (OH) Cl+2Cl2 (3)
Chlorine produced in reaction 3 is not dissociated, but oxidized to sn4+:Sn2++Cl2 = sn4++2cl? (4)
In this way, the cycle of reaction 1~4 goes on viciously, which has two bad effects: First, Sn2+ is continuously transformed into Sn4+, thus eventually forming β? Tin acid provides the conditions. Secondly, the alkaline stannous chloride generated by the reaction is insoluble in water, which makes the plating solution turbid. Stannic acid has similar side effects. Cl？ It will also significantly reduce the overpotential that has been too small, making the coating rough and easy to dendrite.
5 Measures to reduce the formation of tetravalent tin
It is difficult to find a complexing agent with strong coordination effect on metal ions under acidic conditions, and acidic tin plating is no exception. In sulfate bath, divalent tin ions exist as simple Sn2+ and are easily oxidized to tetravalent. And once Sn4+ is converted into β? Tin acid can't be reduced to useful Sn2+, so it can only be removed (generally treated as industrial waste after flocculation, precipitation and filtration). Methanesulfonic acid has certain coordination ability to Sn2+ and Pb2+, so the plating solution is more stable, and it can be plated with pure tin or tin-lead alloy. However, its production cost is high, and sulfate bright acid tin plating is still dominant for products with low unit price in electroplating processing.
The formation of basic stannous chloride, a bivalent tin compound, caused by the discharge of chloride ions is also harmful to this factor. The biggest liquid cost of acidic tin plating is that Sn2+ is oxidized to Sn4+ and eventually becomes useless and harmful β? Tin acid was discarded in vain. Therefore, how to prevent or reduce the oxidation of Sn2+ to Sn4+ and turn it into white β at Sn4+? The key of this process is that Sn4+ can be reduced to Sn2+ before stannic acid. Almost all methods are to add so-called "stabilizers" to additives. Among the additives sold, many stabilizers are added to the opener, and the main brighteners (generally benzylidene acetone and formaldehyde) are added with a large amount of opener, while few or no stabilizers are added to the brighteners.
5. 1 Overview of stabilizer
Literature [2] points out that many substances have a stabilizing effect on sulfate bright acidic bath. For example:
1, phenols
There are many patents about acid tin plating, but the substances proposed in the patents are difficult to buy from the spot. As far as the stabilizers in bottle openers currently sold are concerned, almost all of them contain phenolic substances, such as phloroglucinol, catechol, cresol sulfonic acid and so on.
Phenols have three functions in the bath: a, they are reducible, which can reduce simple Sn4 ++ to SN2+; B, it has a certain brightness, which can improve the brightness and leveling of the coating; C, the allowable cathode current density can be enlarged, and the coating burning loss can be reduced.
Among phenols, hydroquinone is widely used in China, and resorcinol is widely used abroad. The reason may be that compared with hydroquinone, resorcinol has shorter distance between two hydroxyl groups on benzene ring, higher electron cloud density, and it is easier to lose electrons, that is, it has stronger reducing ability to reduce Sn4 ++ to Sn2++.
2. Reducing inorganic compounds
For example, in the periodic table of elements, VB group compounds include vanadium pentoxide V2O5, vanadyl sulfate VOSO4, sodium vanadate NaVO3, niobium pentoxide Nb2O5 and tantalum chloride. And titanium trichloride TiCl3, zirconium oxysulfate ZrOSO4 and sodium tungstate Na2WO4. However, these substances are expensive and difficult to buy, so it is not appropriate to choose inorganic compounds with strong reducibility but chlorine ions. This substance is not used much.
Sodium hypophosphite, the reductant for electroless nickel plating, can also reduce Sn4 ++ to Sn2++, which can be considered.
3. Organic acids
Organic acids with reducibility can be used as stabilizers. Such as ascorbic acid, phenol sulfonic acid, cresol sulfonic acid, naphthol sulfonic acid, sodium sorbate, thiobenzoic acid, tartaric acid and its salts, gluconic acid, etc.
4. Hydrazine substances
Hydrazine substances, such as hydrazine hydrate and hydrazine sulfate, have strong reducibility and low dosage. But it is corrosive, toxic and not widely used.
5. Others
Some people think that ferrous sulfate is a good stabilizer (2g/L). 1998 reported that 2 ~ 2.5g/L potassium fluoborate or 10 ~ 13ml/L garlic extract had been used in production for five years, which proved to be equivalent to the special stabilizer FS- 1. Many "antioxidant" products can be found in "China Chemical Products". Whether any of them can be used as a particularly good stabilizer for acidic tin plating needs a lot of experimental screening.
Among the above stabilizers, ascorbic acid, also known as vitamin C, should be mentioned in particular. In the process of metabolism, the human body will constantly produce harmful oxidative free radicals. After ascorbic acid is supplemented, it can neutralize oxidative free radicals because of its reducibility, which is beneficial to human health. In acidic tin plating solution, the following reversible reactions will occur:
reversible reaction
When two hydroxyl groups on ascorbic acid molecule are oxidized, two hydrogen atoms lose one electron each, which plays two roles: a, reducing SN4+ to SN2+:SN4++2e? →Sn2+； B, the hydrogen atom in the new ecology has strong reducing ability, which can consume oxygen in liquid: 4h+O2 = 2h2o. Its deoxidation reduces the oxidation of Sn2+. The "dehydroascorbic acid" produced by the reaction obtains two negative electrons at the cathode under acidic conditions, and then it is recovered as ascorbic acid. That is, recycling, mainly for consumption. Therefore, almost all existing stabilizers have added ascorbic acid. It is also commonly used for electroplating nickel-iron alloy and zinc-iron alloy to reduce trivalent iron Fe3 ++ to divalent iron Fe2 ++ to prevent Fe(OH)3 from precipitating. But it has no coordination effect on Fe2+, and its effect is not as good as that of RC stabilizer. The simplest stabilizer is the product "SS820", and only one kind is added, namely methacrylic acid.
5.2 Measures to keep the molten pool clean
5.2. 1 Avoid introducing chloride ions.
In acidic tin plating bath, the chemical reaction of chloride ions not only oxidizes SN2+ to Sn4+, which counteracts the function of stabilizer, but also directly reacts with SN2+ to generate basic stannous chloride Sn(OH)Cl, which is suspended in the plating bath and makes the plating bath turbid, and can only be removed by flocculation and filtration.
In production, it is generally impossible to clean with flowing pure water before tin plating. But at least silver nitrate should be used to detect whether the groundwater used is polluted by Cl-. If the pollution is serious, deep groundwater should be extracted for tin plating and cleaning bright acidic copper before bathing. If the content of Cl- in the cleaned workpiece is not heavy, it should be stored in pure water containing about 10g/L sulfuric acid. First of all, it can make the workpiece in an activated state, which is beneficial to improve the adhesion of the coating. Secondly, dilute sulfuric acid is directly brought into the plating solution without cleaning, which can not only supplement the sulfuric acid brought out of the plating solution, but also reduce the Cl- brought in.
5.2.2 Ensure that there is enough stabilizer in the electroplating solution.
Most of the stabilizer components in acidic tin plating bath are consumed by oxidation when tetravalent tin is reduced to bivalent tin, except for anti-blood acid (it is reported that V2O5 has a similar effect). In order to ensure a sufficient amount of stabilizer in the electroplating solution, the following measures can be taken:
1, due to the high content of stabilizer in bottle opener, it is usually necessary to add not only brightener but also an appropriate amount of bottle opener. I met an experienced electroplating boss who used sulfate bright tin plating process to roll tin plating. Usually, he only adds bottle openers when adding additives, and never adds "brighteners". The barrel plating solution has been used for 2 years, and it has been clear and not turbid. That's the reason.
During the Hall element test, 1A was stirred and electroplated for 3 minutes. If the high-end charred area of the specimen is wide and the addition of 4g/L stannous sulfate has no obvious improvement, the addition of a proper amount of high-quality cylinder opener should obviously improve it. This is because phenolic substances that can expand the allowable current density are added to the stabilizer.
2. Add a separate stabilizer product.
There are special composite acidic tin plating stabilizers in the market, which can be selected and added according to the instructions or experiments. It can also be combined and prepared by itself according to the introduction of stabilizer. Provide a reference formula:
Potassium sodium tartrate 120g/L
Resorcinol 8g/L
Ascorbic acid 5g/l
Cresol sulfonic acid 3g/L (optional)
Sodium hypophosphite 6g/L
Polyethylene glycol (molecular weight 6000~ 12000)2g/L (it can be replaced by 0.0 1g/L high-quality polyacrylamide suitable for tin plating).
note:
① Use all analytical pure materials;
(2) When preparing, except resorcinol and ascorbic acid, it is dissolved and stirred with hot water. If the solution is not clear (tartaric acid and its salts are not very soluble), try adding analytically pure sulfuric acid and stirring until the solution is clear.
(3) Resorcinol and ascorbic acid are easy to deteriorate when exposed to light. Black bags should be used as the outer packaging, and stirred and dissolved below 60℃. If the liquid is not used up, put it in a brown glass bottle and an opaque bag.
(4) If cresol sulfonic acid is hard to buy, the mixed cresol can be sulfonated with sulfuric acid for production: 1 volume of mixed cresol is heated to 50~60℃, and an equal volume of concentrated sulfuric acid is added under constant stirring; Keep the temperature at 100~ 1 10℃ for 2h.
⑤ Its dosage is determined by experiments.
5.2.3 Remove the suspended sediment in the plating solution in time.
Timely means that when the plating solution is turbid, it should be flocculated and precipitated; When the plating solution is obviously whitened, β? There is too much stannic acid to flocculate: there are too many products, which must be turned over and filtered after flocculation and precipitation. If the flocculant is added less and more times, and the plating bath is deep, it is not necessary to turn over the bath for filtration every time the flocculant is added; Moreover, the sediment in the lower part is relatively dense and the water content is relatively low.
When discussing the influence of Cl-, it is mentioned that the basic stannous chloride produced in reaction 2 is also a harmful substance suspended in the plating solution. If it can be flocculated and settled at the bottom of the tank in time, chlorine will be removed to some extent, and the occurrence of reactions 3 and 4 can be reduced.
Song, Zuo and others [4] think that adding polyethylene glycol can play a flocculation role in their research on acidic tin plating. The stabilizer components they determined are (confidential): polyethylene glycol, vanadate, sodium hypophosphite and ascorbic acid. When no stabilizer was added, the plating solution became turbid for 5~7 days, and then stood for 3 weeks until it became slightly turbid. (Obviously, pure water is used in the experiment, which does not contain chlorine. In my humble opinion, the stabilizer only considers the reduction of Sn4+, but does not consider that many organic acids have a certain coordination effect on Sn2+. Phenols can expand the allowable cathode current density in mass production, but they are not added.
If a small amount of polyacrylamide with good effect is added to the brightener, the β -suspension of stannic acid and basic stannous chloride will keep the plating solution clear and turbid. Its cost is slightly increased.
By the way, a practical experience in production is provided: when the acidic tin plating solution has turned white seriously, it needs to be flocculated and precipitated with 8mg/L treatment agent for a long time, and there are a lot of precipitates in the lower part of the plating solution, but the upper part is still unclear, it should be changed to the following "two-step treatment method": firstly, add the treatment agent at the amount of 4ml/L, and filter the precipitate; Add 4ml/L treatment agent into the filtered suspension, stir, precipitate and filter. Finally, although the total amount of treatment agent added is the same, the clarity and transparency of the obtained plating solution are much higher than that of the "one-step method", that is, the treatment effect is much better. When filtering, because the solution filter is easy to block the filter element, siphon method is often used to suck out the supernatant in actual production and scoop up the turbid plating solution containing sludge in the lower part. This is a great waste. The correct way is to make a small and deep plastic tank, scoop in the muddy liquid and sludge at the lower part and let it stand for more than 24 hours, then siphon out the clear liquid at the upper part and put it back into the electroplating tank; The rest is dipped into several pieces of filter cloth similar to anode bag, suspended for "bag filtration", and the filtrate is reused as plating solution. Generally, after more than 10 hour, the sludge in the bag has dried, so it can be taken out and treated with wastewater.
6 conclusion
Through the above analysis, in order to reduce the waste of tin in acidic tin plating, in addition to strengthening the recovery of plating solution after plating, we should also pay attention to:
1, the tin anode plate has no anode pocket, so as to play its stabilizing role in reducing harmful tetravalent tin into useful bivalent tin.
2. The area of tin anode should be large to ensure that the anode current density is not greater than1.5a/dm2. In this way, the anode has higher current efficiency, less oxygen evolved on it, high chemical stability (insoluble in concentrated hydrochloric acid greenhouse) and less stannous oxide SnO which is difficult to reuse.
3. Try to avoid chloride ions from polluting the plating solution, otherwise, harmful basic stannous chloride Sn(OH)Cl suspended in the plating solution will be produced, and useful Sn2+ will also be oxidized into harmful Sn4+.
4. Take all feasible measures to prevent Sn2+ from being oxidized into Sn4+. When Sn4+ is H2SnO3, that is, α? When stannic acid exists, the harm is not great; But when α? Tin acid gradually polymerized into white suspension β? When stannic acid (SnO)5(OH) 10, it is very harmful to the bath and coating. To this end,
1) proposed many stabilizers for acidic tin plating solution. They are either organic acids that coordinate with Sn2+ to some extent, or "antioxidants" that can slow down the oxidation of Sn2+ to Sn4+, or Sn4+ has not been converted into β? Before stannic acid, it is reduced to a reducing agent of Sn2+, or it has several functions.
2) Generally, the content of stabilizer in bottle opener is high, and the content of main brightener (benzylidene acetone) is low; However, in the brighteners usually added, the stabilizer content is very low, or even completely absent. Therefore, brightener should be supplemented with cylinder opener at the same time in production.
3) There are also special stabilizers for sale. When the overall stability of the additives used is not enough, they can be purchased first.
4) The stabilizer can also be prepared by itself with reference to this paper and reference [2]. Its quality and its addition amount can be determined by Hall cell test.
5) Whether Sn(OH)Cl or (SnO)5(OH) 10 suspended in the plating solution, it is almost ineffective to remove it with activated carbon. Flocculants are usually used for flocculation and precipitation before solid-liquid separation. The best flocculant is a mixture of organic and inorganic substances; Among them, polyacrylamide PAM is mostly used as organic matter, and the dosage is very small. PAM has three categories: anion, cation and nonionic, and each category has a variety of commodities with molecular weights ranging from hundreds of thousands to tens of millions. Practice has proved that some PAM with good wastewater treatment effect has no flocculation when used as acid plating solution, but it increases the viscosity of the plating solution and reduces the conductivity, which makes it difficult to clean after plating. Therefore, it is best to test and optimize the finished product of water-based "treatment agent". The author advocates adding less treatment agent and adding it frequently: when adding brightener, add a small amount at the same time.