4. 1- organic sulfonic acid as an additive to replace part of H2SO4.
The molecular structure of organic sulfonic acid R-SO3H is similar to H2SO4. Methyl sulfonic acid and ethyl sulfonic acid with less than 3 carbon atoms are excellent chromium plating catalysts. The dosage of alkyl sulfonic acid is 1g/L-5g/L, accounting for 1%-2% of the CrO3 content.
After the organic sulfonic acid reacts with H2CrO4, polar molecules similar to chromium sulfate are generated, and the product is chromium organic sulfonate. Saturated hydrocarbon groups such as methyl, ethyl and propyl are electron-repelling genes in organic sulfonic acid molecules, which increase the electron cloud density and strengthen the bond between Cr-O-S. The bond is not easy to break, and the whole organic sulfonic acid chrome acyl molecule becomes difficult to hydrolyze, which improves the stability, thus improving the current efficiency during chromium plating.
Aminosulfonic acid and sulfoacetic acid also contain sulfonic acid gene, but amino group and acetic acid group are electron-withdrawing groups, which can reduce the electron cloud density between Cr-O-S, weaken the bond, and reduce the stability of organic sulfonic acid chromic acid, so they are bad catalysts, and the dosage is relatively large. The ratio of NH _ 2SO _ 3h/cro _ 3 is greater than110, and sulfoacetic acid. In order to improve the catalytic ability, hydroxyethyl sulfonic acid, hydroxypropyl sulfonic acid, benzene sulfonic acid and propynyl sulfonic acid all contain electron-withdrawing genes (hydroxyl, phenyl and propynyl), which will also reduce the stability of organic sulfonic acid chromic acid, so they are also poor catalysts.
H2SO4 selenite and H2SO4 have similar properties and catalytic ability, and both can obtain bright Cr-Se alloy, but the larger internal stress in the coating is only applicable to microcracked Cr.
4.2 Additives for Substituting Fluoride Halides and Halogenated Salts
HF, H2SiF6, HBF4, H3AIF6 and H2TIF6 are the third generation of composite chromium plating additives, which have the advantages of high current efficiency and good coverage. However, the disadvantage is that the coating used alone is rough and black, which corrodes the anode and the plated parts, and the plating solution is easy to accumulate impurity metal ions and deteriorate its performance, so people are committed to finding additives to replace F-.
Firstly, halogen compounds are considered, which can improve the efficiency of chromium plating, and the current efficiency of Cl- plating solution is the highest. However, halogen ions are very corrosive to cations on the plated parts, and are easily volatilized into halogen gas by anodic oxidation, which consumes quickly. When used as a catalyst alone, the obtained chromium coating is light gray, with large internal stress and brittleness, which can not completely replace H2SO4 and needs to be used in combination with H2SO4.
HF and HCl can also react with H2CrO4 to produce chromium fluoride and chromium chloride. Halogen atoms have electron-withdrawing induction effect, and at the same time, their P electrons and chromium-oxygen double bonds can produce Pπ*** yoke effect, which is greater than the induction effect, strengthening Cr-F bond and Cr-Cl bond, and enhancing the stability of chromium fluoride and chromium chloride. In addition, their molecular size is small and easy to migrate to the cathode surface, so the current efficiency can be improved. Br- and I- can undergo redox reaction with H2CrO4 to generate elemental Br2, and I2 volatilizes, with little catalytic effect. The function of IO3- is to activate the surface of the coating, which avoids the phenomenon that chromium can not be precipitated normally in a small current region. The reduction of ClO4- to Cl- by cathode can play a catalytic role.
When H2SIF6, HBF4, H3AIF6 and H2TIF6 are used as catalysts, the current efficiency is higher than that when HF is used as catalyst, and the corrosiveness is lower than that of HF catalyst. Is it converted into HF first and then reacted with H2CrO4, or is it directly reacted with H2CrO4? Further research is needed.
Halogenated carboxylic acid is a new type of chromium plating catalyst, which can hydrolyze cl- and F- in the plating solution, with high current efficiency and can improve the coverage and dispersion of the plating solution. Halogen ions and carboxylic acids, the hydrolysis products of halogenated carboxylic acids, can make the chromium plating layer gray (acetic acid was used as catalyst in Siemens' early black chromium electroplating), and its appearance is not good. This may also be the reason why low concentration chromium plating is not silvery white but bluish white. Another important function of halogenated carboxylic acids is to buffer the PH value at the cathode interface.
From the point of bond energy, the bond energy of C-F bond is much larger than that of C-Cl bond, and it is not easy to break and oxidize. Fluorocarbon compounds are stable, and fluorinated carboxylic acids are much more stable than chlorinated carboxylic acids. Chromium fog inhibitor is fluorocarbon. C-Br bond and C-I bond are weak and easy to break and decompose, so brominated and iodocarboxylic acids are unstable and are not suitable as chromium plating additives.
4.3 Additives to replace h3bo 3- carboxylic acid
There are many carboxylic acids and substituted carboxylic acid additives. Formaldehyde and glyoxal can be oxidized into formic acid by strong oxidation bath, and oxalic acid, halogenated carboxylic acid and amide can be hydrolyzed into carboxylic acid, halogenated acid and ammonium ion. Pyridine carboxylic acid can be regarded as a special amino acid with cyclic structure and can be classified as a carboxylic acid additive.
Saturated monocarboxylic acid belongs to weak acid, and its ionization constant is very small, which can buffer the PH value of cathode interface. Halogenated carboxylic acids, alkyd, phenolic acids, amino acids and pyridine carboxylic acids all contain electron-withdrawing genes, which can promote the ionization of carboxylic acids, improve the ionization constant and make the buffer range develop to low PH value.
After acetic acid is replaced by chlorine, the electron cloud density of carboxyl group moves in the direction shown by the arrow due to the strong electron absorption induced effect of chlorine atom. Results the electron density between o and h decreased, and the hydrogen in carboxyl group was more easily ionized in the form of protons. Therefore, monochloroacetic acid is stronger than acetic acid, and the acidity of halogenated acids increases with the increase of halogen atoms. The acidity of trichloroacetic acid is almost equal to that of inorganic acid.
Different electronegativity of halogen atoms has different effects on acidity. The electronegativity of halogen atoms is f > ci > br > i.
Therefore, different halogenated acetic acids have the strongest acidity, and monofluoroacetic acid P=2.66.
The concentration of Cr3+ in the chromium plating solution is about 0. 1mol/l, and the PH value of the precipitate is about 4. In order to prevent precipitation at the cathode interface, the optimum buffer range is required to be PH less than 4, and the Pka of most carboxylic acids is between 2.5 and 5, and the buffer range is about Pka. Therefore, it is reasonable to choose carboxylic acid as buffer, and the PKA of H3BO3 is 9.24, and the optimum buffer range is
Buffering capacity is related to the total concentration and component ratio of buffer solution. The greater the total concentration, the greater the buffer capacity. When the total concentration is constant, the closer the concentration ratio of buffering components is to 1: 1, the greater the buffering capacity; When PH=Pka, the buffer capacity is the largest.
In many buffer systems, there is always a ka value at work, and its buffer range is generally narrow. In order to make the same buffer system play a buffering role in a wide PH range, a buffer system can be composed of a variety of weak acids or weak bases. Because there are multiple ka values at work. Such as citric acid. The third generation chromium plating additive is the application of several carboxylic acid complexing agents with different Pka, aiming at buffering in a wider PH range.
The concentrations of ordinary monocarboxylic acids and dicarboxylic acids can be increased as much as possible to maximize the buffer capacity. When acetic acid and aminoacetic acid are used as additives, the dosage requirements are relatively large; When halogenated carboxylic acids, sulfocarboxylic acids and hydroxycarboxylic acids are used as additives, the dosage is tens of grams/liter and hundreds of grams/liter.
When homopolymer or heteropoly acid is used as additive, the effect of improving current efficiency is not obvious. It can be considered that they are not catalysts, but buffers.
4.4 Additives to replace Cr3+- rare earth
The existence of Cr3+ is beneficial to improve the dispersion ability of chromium plating solution and promote electrodeposition in low current density region. Chromium-deficient layer is easy to nodulate, with low hardness and slow electrodeposition speed. Rare earth element re is trivalent, which can replace part of Cr3+ in the plating solution. Therefore, rare earth chromium plating requires low or no Cr3+ content. Many articles have been published on the mechanism of rare earth chromium plating, so I won't go into details here.
Hydroxy acids, including alkyd and phenolic acids, can produce a small amount of Cr3+ and are also additives to replace Cr3+. It is also reported that the use of bromide can effectively improve the dispersion ability of the plating solution, but the disadvantage of releasing a large amount of halogen gas hinders the popularization and application of this kind of plating solution.
Rare earth cation is also a widely used chromium plating additive. Romanowshi of the United States first introduced rare earth into chromium plating. 1976, they obtained the patent of chromium plating with rare earth fluoride. In 1980s and 1990s, China conducted a lot of research and developed many valuable technologies. Rare earth cations can improve the average plating ability of the bath, but have no obvious improvement on the deep plating ability. The use of rare earth fluoride can improve the current efficiency, but it also causes corrosion problems in low current areas. In addition, it is difficult to monitor rare earth chromium plating, especially hard plating, and the process needs to be further improved.
What really hopes to improve the overall chromium plating performance is organic additives or composite additives (organic additives mixed with anions or rare earths).
Many people have always believed that there is almost no organic matter in the strongly oxidizing chromic acid solution, especially in the electrolytic process, which limits the development of this kind of additives. However, Edgan J suggested that the use of halogenated organic acids, especially halogenated organic diacids such as bromosuccinic acid and bromomalonic acid, can improve the dispersion and coverage of the bath. It is said that even if the electrolyte is electrolyzed at high temperature and high current density, the organic matter will not be oxidized. Hu Runan of Beihang University and others have done similar work and reached similar conclusions.
In order to achieve the expected purpose, the amount of organic acids or halogenated organic acids is relatively large, generally tens of grams per liter. For example, Chessin added halogen diacid with the maximum amount of 32 g/L to the fluorine-containing chromium plating solution, so that the plating solution obtained good dispersibility and covering ability.
Adding organic compounds can improve cathode current efficiency. Hyman Chessin uses organic sulfonic acid as additive to obtain bright and good adhesion coating at high current efficiency (more than 22%), which avoids the problem of low current corrosion. The added organic sulfonic acid requires S/C≥ 1/3, and the addition amount is1~18 g/L.
Anthony d barney suggested that proper amount of amino acetic acid and amino propionic acid can improve cathode current efficiency. For example, if 2.5 g/L aminoacetic acid is added to the plating solution of CRO3 200 g/L and H2SO4 2 g/L, and the plating is carried out at 40℃ for 2 h, the cathode current efficiency can reach 265,438 0.45%.
In addition, the bath containing organic additives usually has high hardness. In the bath containing formaldehyde, formic acid or glyoxal, the hardness of the coating is 950 ~1000 HV. After heat treatment at 600℃ 1 h, the hardness can reach11600 ~1800 HV, and the salt resistance and corrosion resistance of the coating are three times that of ordinary chromium coating.
The combination of organic anions and organic additives can sometimes achieve good results. Chessin uses potassium iodate, potassium bromide and other organic acids together to obtain a smooth and bright coating with high current efficiency. The better organic acids are sulfoacetic acid, succinic acid and trichloroacetic acid, and the dosage is 5 ~100 g/L. Under the appropriate process, the cathode current efficiency can reach 50%, but the disadvantage is that the chromic acid concentration is as high as 800 g/L, and the organic acid content is high, which is easy to corrode the cathode.
Amtech's patented products also contain composite additives, which are composed of sulfoacetic acid, iodate and organic nitride. The bath does not contain H2SO4, and the cathode current efficiency is at least 20%.
Chromium plating solution with organic or composite additives is generally called the third generation chromium plating solution, and there are several mature processes at present. HEEF25 and HEEF40 processes of Ameter Company in the United States are more representative. Especially HEEF25, the current efficiency can reach 22% ~ 26%, the deposition speed is 30 ~ 80μ m/h, and the coating is bright and smooth. Its technical advantages are high current efficiency, fast deposition speed and no fluoride. But its high cost will affect its popularization and application to some extent. Machil process of British Canning Company has the same characteristics as HEEF25, the cathode current efficiency can reach 26%, the coating hardness is higher than 1 200 HV, and it does not contain fluoride. There are few reports about organic additives in China. Beihang University and Shanxi University have done some research. According to the data, at least five domestic companies have provided hard chromium plating processes similar to HEEF25.
The latest generation of chromium plating additives mainly use alkyl disulfonic acid and its salts, including methyl disulfonic acid, sodium methyl disulfonate and potassium methyl disulfonate. , which is completely different from conventional chromium plating and common mixed catalyst chromium plating process, has the following characteristics: 1. High current efficiency, up to 23-29%. 2. The deposition speed is fast, which is 2-3 times that of ordinary chromium plating. 3. High hardness HV900-650. The general dosage is about 4-10g/L. Adding 100kg chromic anhydride consumes 8- 15g alkyl disulfonic acid and its salts.
On this basis, many additive manufacturers and research departments take alkyl disulfonic acid and its salts as the main components, and add organic sulfamic acid, nitrogen-containing heterocyclic compounds and inorganic additives such as potassium iodate and potassium bromate. Until the current efficiency is 27-50% and the hardness is Hv900- 1250, the chromium plating process has taken a big step forward.
In a word, surface treatment workers still have a lot of work to do in the development of organic additives.