Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc

Definitions

• the present invention relates to a method of zincing as well as an acidic aqueous bath used in this process.
• zinc is often used (when it is simply about protection) for its relatively reduced price by compared to copper, nickel and cadmium.
• Electrolytic galvanization is strongly challenged by thermal galvanization, by the spraying and Sherard methods. Electrolytic galvanizing is certainly preferred to thermal galvanizing (more commonly used) for carefully hardened steel parts, or for precision parts for which significant increases in temperature would be harmful. For many semi-finished products, in particular also for steel wires and blades, electrolytic galvanization is progressing, especially since today it is able to galvanize wires and blades with solutions similar to those used in the electrolytic extraction of zinc from ores with relatively high current densities (22,000 A / m 2 ).
• the types of galvanization follow one another in this order: thermal galvanizing, galvanic zincing, sherardization, spray galvanization and micronized zinc galvanization, 92% suspended in organic or inorganic medium.
• the main advantage of galvanic zinc plating is in the huge economy achievable thanks to a reduced zinc consumption compared to the process thermal.
• the protective appearance and power of electrolytic zinc coatings can be improved by a final "passivation" treatment.
• acid sulfate solutions are preferable to those with cyanide, insofar as the tendency of metallic impurities to pass from the anode to deposits is less in sulphate baths than in cyanide ones, in which the potentials discharge of many of the foreign metals are relatives.
• Brass plating illustrates this phenomenon, zinc and copper are deposited simultaneously, exclusively at from their cyanide solutions while it's almost impossible to deposit them from solutions acids. Therefore, it is more important to use pure zinc anodes for galvanizing in a bath cyanide as for that in sulphate bath.
• Cyano-alkaline baths by virtue of their power penetrating, are particularly used for the galvanizing of profiled structures. Sometimes a pre-galvanization in cyano-alkaline baths is performed on these parts, then reinforced by a galvanizing in acid bath.
• the very solubility anodes is generally better in solutions acids than in alkaline ones.
• the baths acids have higher conductivity, which leads to reduced energy consumption electric.
• the color of the transformation layer can be pearly, iridescent, yellow-bronze, olive green, black next the type of chemical conversion.
• the basic product which is used in such baths and which serves to give the content of zinc ions is crystallized zinc sulphate (ZnSO 4 7H 2 O). This must have a beautiful white appearance and be practically or almost deprived of foreign metals in particular such as lead, copper, arsenic or iron. The presence of such impurities can be the cause of spongy deposits.
• the concentration of zinc sulfate which varies from 240 g / l to 410 g / l, is generally 360 g / l.
• the galvanizing tanks vary according to the type of bath, they are made of resistant materials either acidic or alkaline.
• the tanks coated with VIPLA (trade designation) hardened army are excellent.
• the anodes usually consist of zinc (99.99%) which are suspended from the anode bars at using adequate hooks of pure zinc.
• the baths strongly sulphate acids, lead anodes, lead and silver, zinc alloy with mercury, aluminum, magnesium or calcium can also be used.
• Zinc-mercury alloy anodes, zinc-aluminum or better zinc-magnesium are also used good for baths that are less acidic than alkaline.
• Acid baths almost always contain a certain amount of free sulfuric acid, having for objective of increasing the conductivity of the electrolyte and improve its penetrating power, which is rather low.
• the penetrating power can be greatly increased with the addition of sodium sulfate.
• the acidity of conventional galvanizing baths is between pH 3.5 and 4.6 for executions in common tanks; such acidity corresponds to a acid content from 0.01 to 0.02 N. If the baths become alkaline, deposits become spongy and unusable. If the acidity is too high, the coatings obtained are spongy and hailed. Being given that zinc dissolves chemically in acid solutions (releasing hydrogen), in acid galvanizing baths, unlike example of those of nickel plating, there is a dissolution anodic electrochemical (caused by current) and chemical (caused by free acid); it follows that the acidity gradually decreases, so that it is necessary from time to time to make additions acid. It also follows that in such baths, the anodes must not be left submerged when the current does not pass.
• Aluminum sulfate, aluminum chloride, boric acid and sodium acetate have actions pads; that is to say that they control the variations the pH of the solution. This phenomenon deserves a particular consideration it is due to the hydrolysis of aluminum sulfate or chloride, which occurs at a determined pH of the solution.
• the pH of the bath must be determined. Indeed, if the solution is too acidic, there is a risk of a strong development of hydrogen at the cathode during the passage of the current, while the attack of the anodes by chemical means is considerable with open circuit. However, if the solution is not acidic enough, the spongy form appears (due to the formation of basic compounds by hydrolysis of zinc salts). Repeated experiments have shown that the pH of the zinc salts, which undergo hydraulic splitting, is fairly close to 5. Also, to work properly, the pH of the solution must be just under 5. However, both during the use of the bath and during the hours of inactivity, it is constantly changing; which represents continuous and expensive corrections.
• colloidal aluminum hydrate that forms does not no harmful action on the zinc deposit, on the contrary it promotes the formation of more structured crystals fine.
• Boric acid has a modest buffering action in these solutions, because it opposes variations in pH only when it expects values of 5.5.
• the use of boric acid in place of sulfate aluminum is used in some cases for baths employed in a rotary device for machining mass of small parts in which it is necessary to maintain a slightly pH value greater than 5. The results obtained in this case are technically perfect.
• chlorides are used and especially those ammonium. Chlorides are good for increase the conductivity, as well as the sulfates of sodium, ammonium and magnesium. The latter, with the aluminum sulfate, silicate, thiosulfate and sodium bisulfate, as well as substances colloidal, contribute to the improvement of crystal structure of the deposit.
• the galvanization of iron wires requires equipment suitable for continuous operation.
• the wires must be coated on the basis of their diameter, for example wires of diameter 0.25-1.5 mm or 1.6-3.5 mm or 3.6 -5 mm must respectively have approximately 30-35 g / m 2 , 50-75 g / m 2 , 75-120 g / m 2 of electrolytic zinc.
• the sulphate baths for the galvanization of steel wires and ribbons have the following composition:
• the current density is a function of the speed of the material. For example, a density of 25 A / dm 2 can be used for a wire speed of 30 m / min. Higher performances can still be achieved with fluoroborate baths.
• the main, and more common, addition agents are: dextrin (1 g / l), extract (or powder) of licorice root (1 g / l), glucose (30 g / l), gelatin, carpenter's glue, maltose, glycerin, pyridine, quinoline, sodium naphthalindisulfonate (5 g / l), citric acid or alkaline citrates (30-60 g / l), mercury chloride (0.25-1 g / l).
• the best acid-based electrolyte on which to possibly act an adequate polish is as follows: Zinc sulfate 180 g / l (1 M) Zinc chloride 14 g / l (0.1 M) Boric acid 12 g / l (0.2 M) Temperature 30 ° C pH 4 Current density 0.5-7.5 A / dm 2 Zinc charge deposition rate 10-11 ⁇ / hour
• the addition agents are mobilized (in weak part) in the deposits, moreover they coagulate and are therefore from time to time to be substituted. You should not exceed in their use, because otherwise the yield cathodic decreases a lot and the penetrating power in suffers.
• the anode area must be equal or slightly higher than cathodic. Small additions organic substances such as glucose, gelatin, nicotine, furfurol, licorice and glycerin make them smoother deposits.
• a special application of galvanizing is the Tainton process in which insoluble lead-silver alloy anodes are used, more particularly for the coating of wires, high free acidity (75 g / l) and high current density (160 A / dm 2 ); suitable equipment quickly passes the wires through the electrolyte.
• All sulphate acid baths are at most contained in tanks coated with lead, vilpa or gum.
• the heaters, insufflators, etc. are made of lead.
• the anodes of the laminated electrolytic type, must have a composition whose contents (in%) are about: Zn 99.81; Pb 0.08; Sn 0.002; Cd 0.08; Cu 0.01; Fe 0.01; Mg 0.005; Al traces. They can be bagged with vinyl or propylene fiber fabric or Perlon. Their area must generally be equal to the cathodic one.
• filtration weekly periodical is recommended after transfer. If the bath is used intensively it is preferable that filtration is continuous.
• Zinc content analysis can be performed weekly or monthly; the same for the aluminum sulfate and boric acid.
• a bath for these objects can be composed as follows: Zinc sulfate 400 g / l Sodium sulfate 59 g / l Sodium chloride 15 g / l Boric acid 20 g / l Sodium acetate 15 g / l Licorice 0.5-1 g / l Temperature 25-30 ° C pH 5.0-5.3 Current density 5-10 A / dm 2 Voltage 6-12 V Zinc charge deposition rate 8-10 ⁇ / hour
• Dextrin can replace licorice. be used (0.5-1 g / l), or chloride mercuric (0.25-0.5 g / l).
• the anodes must be of electrolytic zinc rolled and with a surface area as wide as possible. Continuous filtration is advised.
• the conditions of deposition are comparable to those of sulphate or fluoroborate baths.
• the anodic polarization is practically negligible; the anodes dissolve normally and no phenomenon of passivity hinders its functioning.
• Caffeine and EDTA strongly influence cathodic polarization.
• Colloids such as dextrin and gelatin
• certain metals nickel, cobalt, iron
• a zinc chloride bath which added small amounts of dextrin, glucose or glycerin, would lead to shiny deposits is as follows: Zinc chloride 135 g / l Sodium chloride 230 g / l Aluminum chloride 22.5 g / l Temperature 18-30 ° C pH 3.0-4.0 Current density 1.5-4.5 A / dm 2 Zinc charge deposition rate 7-9 ⁇ / hour
• the density of current can be raised to 10 A / dm2.
• PH should be regulated with boric acid.
• Anthraquinone sulfonate can be used as an addition agent.
• the standard composition of fluoroborate baths which can also be used for small objects is: Zinc fluoroborate 200 g / l Ammonium chloride 54 g / l Ammonium fluoroborate 35 g / l Licorice extract 1 g / l Temperature 25-40 ° C pH 3.5-4.0 Current density 2-10 A / dm 2 Zinc charge deposition rate 9-10 ⁇ / hour
• the current density can be raised to 15 A / dm 2 .
• the anodes must be of pure molten zinc (99.99%) with an area equal to or twice that of the cathode.
• the composition of the bath when stirred is as follows: Zinc fluoroborate 300 g / l Ammonium chloride 27 g / l Ammonium fluoroborate 35 g / l Licorice extract 1 g / l Temperature 20-50 ° C pH 3.5-4.0 Current density 2.5-75 A / dm 2 Zinc charge deposition rate 10-11 ⁇ / hour
• the current density can be increased to 100 A / dm 2 . This is useful for galvanizing steel wire or tape.
• the anode area must be equal to or greater than the cathode area.
• the preparation of the two aforementioned baths proceeds as follows: zinc fluoroborate, which generally is in 48% aqueous solution (p: p) approximately, is diluted in the tank plating filled with 75% water; then the ammonium chloride and ammonium fluoroborate are added by shaking carefully with a piece of wood or an ebonite stick. Then zinc carbonate or zinc oxide or fluoroboric acid is added to bring the pH to its exact value. Finally, the volume is supplemented with water.
• galvanizing baths with low acidity asserted themselves. These can compete with the best alkaline baths, as well as a penetrating power, as a quality of deposit, understood the shiny appearance. They generally act a pH of around 5.5. According to some, zinc is deposited with a BR structure (it reproduces that of the metal of base), but FT structures (field orientation) can also appear, or UD (not oriented). The coating hardness is around 55-60 HV. The cathodic yield is rather high (90-95%) and is independent of current density. In many cases, lucid coatings can be obtained by using the same polishing agents, active for cyano-alkaline electrolytes.
• base polishers such as polyalcohols, fatty amines or oxyethylate, fatty alcohols, ether or ester polyglycolic and nitrogen containing compounds quaternary with oxyethylated chains.
• the polishers "primaries” are aromatic aliphatic compounds and heterocyclic having carboxylic groups. According to some researchers, the “combinations” well specific aldehydes, ketones, gelatin, surfactants and other organic substances under patent cover, are of great importance. Always according to these researchers, a weakly acidic solution of chloride zinc + ammonium chloride, or zinc sulfate + ammonium chloride is used as the electrolyte of based. Chloride-free baths (less conductive however) are also possible.
• the coatings can be subsequently chromed in blue, yellow or olive green coloring depending on the requirements.
• Cyanide galvanizing baths have a optimum penetrating power and give deposits compact, fine-grained and beautiful looking if the electrolyte is free of impurities.
• cyanide baths are quite sensitive to impurities if these consist of lead (fairly harmful), tin, copper or cadmium. Through Therefore, it is necessary that the components of baths are as pure as possible and that all that comes into contact with the liquid (tanks, heaters, etc.) either of steel or better of vipled materials or any unassailable way.
• cyanide baths have the property, apart from their penetrating power, to be easily able to transform into bright galvanizing baths, so compete with the same cadmium plating baths.
• the main component of cyanide baths is formed by double sodium cyanide (Na 2 Zn (CN) 4 ) or potassium. In the baths, there are also sodium zinc, free cyanide, sodium hydrate and sodium carbonate.
• double cyanide arises from the following considerations. By dissolving the zinc cyanide Zn (CN) 2 , in sodium cyanide, NaCN, the double cyanide is formed: Zn (CN) 2 + 2 NaCN ⁇ Na 2 Zn (CN) 4
• cyano-alkaline baths are double cyanide, cyanide, hydrate and let's also say carbonate.
• this last substance which occurs spontaneously in the bath by decomposition of cyanide and by absorption of carbon dioxide from the air at using free alkali it hasn't been well yet established whether its effect is favorable or not to plating. The fact is that its content should not not exceed 60 g / l and if this happens you must remove excess by crystallization at -5 ° C, or by addition of calcium sulphate, calcium hydrate or barium hydrate.
• M Total cyanide (in g / l NaCN) g / l zinc
• PH also has a certain importance in galvanizing baths. It seems that its value more cost-effective, especially for anodic corrosion and quality of the deposits, ie between 13 and 13.5.
• the anodes When not in use, the anodes must be removed from the bath. Adding a small amount (1-5 g / l) of sodium fluoride, sodium (1-2 g / l), aluminum sulfate, gum arabic, lignin, glucose and sodium stannate may improve deposits.
• sodium cyanide The function of sodium cyanide is to complex with zinc to form complex salts soluble, allow obtaining a grain deposit very fine, and to increase the penetrating power. His increase is manifested by current efficiency reduced and by a strong increase in polarization cathodic. A lack of sodium cyanide causes rough deposits and reduced penetrating power, while that the structure of the deposit assumes a coarse-grained appearance and spongy.
• caustic soda and sodium cyanide are essential elements for the good conduct of the bath of alkaline galvanization. These complement each other the other and reciprocally cancel their faults.
• the electrolyte is kept as pure as possible.
• 1-2 g / l of sodium sulfide and 0.5-1 g / l of very pure zinc powder is added to the bath.
• the sodium sulfide removes foreign metals (e.g. lead, tin, etc.) in the form of insoluble sulphides and zinc powder particularly removes, by displacement, copper, cadmium, lead and nickel.
• the anode area must be equal to the cathodic for low current densities; the double the cathodic area for densities of high current.
• the protective anode bags are not recommended: they would cause polarization excessive anodic and therefore a lowering of the anodic current efficiency.
• Continuous or intermittent filtration is necessary. It allows obtaining more deposits smooth and compact whose protective value is naturally higher.
• hard rubber filter elements with filters carbon fiber board are required; as co-adjuvant it is useful to use Meraklon fiber.
• the anodes must be of electrolytic zinc, laminated to 5-10 mm thick; among these must be put on iron anodes. Instead of rolled zinc zinc spheres are recommended, arranged in a stack inside an iron spiral, all connected to the anode bar.
• the laminated iron anodes and the iron spiral have intended to depolarize the anode. Indeed, during electrolysis, zinc anodes, by effect of decrease in ionic concentration in the film anodic, cover themselves with a black patina and become practically insoluble.
• the film noir on the surface of the zinc anodes does not take alarming proportions, insofar as the anodes of zinc are in action with the area of the blades or iron spirals which, at some point, are the only ones to act as electrodes. Meanwhile, the anodes of zinc depolarize and black film dissolves, so well that they become effective.
• calcium and aluminum, alloyed with zinc confer qualities similar to those of the zinc-magnesium alloy.
• the electrolyte tends to get rich in metal. To this enrichment also contributes the different current efficiency between the anode and the cathode (which in the first case can reach 100% and can go down to 60% in the second case).
• composition of the baths for galvanizing small objects in rotary immersion or bell apparatus is approximately as follows: Total sodium cyanide 115 g / l Total sodium hydrate 60-70 g / l Zinc oxide 50 g / l (Zinc as metal) 40 g / l Sodium sulfide 1-2 g / l Pure zinc powder 0.5-1 g / l Temperature 25-30 ° C Report M 2.8 Voltage 12-16 V Zinc charge deposition rate 5-6 ⁇ / hour
• Zincing can be carried out in an alkaline bath by even lack of cyanide or small doses of it, so as not to cause cyanide formation complexed with zinc.
• the bath has the advantage of being economical and can also be used for cast iron.
• Tin is introduced as sodium stannate, or as tetrachloride.
• the operation is carried out with 1-1 / 2 A / dm 2 at 20-25 ° C and with 1.5-2 A / dm 2 at 50 ° C.
• gentle agitation using compressed air or by reciprocating the cathode bar, is recommended.
• organic substances such as polishes, mercury salts, cadmium, molybdenum, chromium, manganese, selenium and tellurium
• organic substances those which act favorably are: vanillin, resorcin, acid phenylglycolic, gelatin with benzoic aldehyde, glycerin, rosin, anisidized aldehyde, glue fish, piperonal, cabinetmaker's glue, dodecyl sodium diphenyl oxidisulphonate, glycolic acid, beta-naphthol, licorice extract, aldehyde salicylic, dextrin, formaldehyde and other compounds also, for the most part, patents.
• Some patents are based on combinations of high molecular weight substances with polishes already used for cyano-alkaline baths.
• polishes already used for cyano-alkaline baths Through example: gelatin, polyvinyl alcohol, methylcellulose, fish glue, casein with aldehyde anisida, piperonal, vanillin or other aldehydes aromatic.
• the anodes must be of the electrolytic, laminated or titrated sphere type 99.99%, i.e. very pure, taking into account that for these baths the alkali sulfides cannot be employed as depurifiers.
• depuration in effect, you have to resort to low electrolysis current density on selective cathodes, followed or no treatment with zinc powder, very effective and able to fully push the depuration itself.
• the successive area conversion treatments zinc plated do not present any particular difficulties. Passivants already known, with low chromium content, are used.
• the major part belongs to the category of cyano-alkaline baths.
• M must be between the "optimum" values, depending on the operating temperature and current density, for which the best electrodeposits are obtained.
• the caustic soda content is generally around 60-150 g / l.
• the polishes used can be inorganic or organic in nature. In the first case, they can always be chemically dosed, which is not without benefit for the electroplating; this is the case, for example, of the most frequent polishing agent, molybdic anhydride (MoO 3 ).
• MoO 3 molybdic anhydride
• the concentration of polish varies according to these are resting baths or for small objects.
• mercury could also be employed, but its action is not as energetic than those of other substances; mercury is by against advantageous for the zincing of the cast iron.
• the anodes Adding mercury leads to better handling of the anodes, insofar as during the period of inactivity these overlap, by chemical displacement, mercury and surrender unassailable by the electrolyte. Of course this involves the addition of a small amount of mercury each time you return to work. However, it is desirable that the anodes be 98.25% zinc more 0.5-1% mercury. The anode area must be double the cathodic.
• Soluble aluminum salts are also used, manganese, chromium, tungsten, rhenium, iron, nickel and cobalt, the latter in the form of diethyldiamine complex (0.2 g / l).
• These metals, tungsten and manganese, with certain aromatic aldehydes (piperonal, aldehyde aniside, vanillin), or with alcohol polyvinyl and other high-weight substances molecular give shiny coatings.
• the following compounds can also be used benzaldehyde (0.25-0.5 cm 3 / l in union with 1-2 g / l of gelatin), salicylic aldehyde, condensation products of phenols , thiourea, glycerin, furfurol, coumarin, triethanolamine, diphenyldiamine, safrol, veratric aldehyde, condensation products of aldehyde derivatives with urea and various heterocyclic compounds. Some common products are even used: dextrin, gum arabic, glucose, gelatin, thiosulfate etc.
• This operation is quite important and is carried out on zinc deposits to improve the appearance and make them more resistant to atmospheric agents (passivation). It consists of a brief treatment chemical or electrochemical (anodization) in solutions adequate. After the chemical process the appearance turns out brilliant, if the electrodeposit is already by yourself shiny, semi-shiny or light gray; more hardly deposit will improve its aesthetic quality if it is dark gray from the start.
• the colors obtained can be iridescent, yellow, azure, green olives, bronzes, blacks, etc.
• the most important fact, in any case consists of a "conversion" surface to inhibit zinc corrosion (formation of the so-called “white rust”).
• Nitric acid 1.42) 0.3-0.5% by volume
• Oxygenated water at 100 vol. 2-5% by mass Sulfuric acid conc 2-5% by mass
• Chromic anhydride 100-350 g / l Sulfuric acid 2-3.5 g / l
• Galvanized objects immediately after extraction of the bath and after careful washing in running water, are immersed for 5-10 seconds in these solutions, under agitation. Solutions often need to be because they are quickly contaminated zinc, copper, iron, etc., and lose their effectiveness also producing spotted areas. Both first solutions are rarely used alone, even if nitric acid treatment gives some improved gloss (it does, however, increase sensitivity to fingerprints and deflowering); if they are used, they are followed by immersion in solution for chromization.
• One of the first conversion processes uses the following immersion bath: Sodium dichromate 200 g / l Sulfuric acid conc. 5-9 cm 3 / l Temperature 20-45 ° C
• Iron or vipla-covered tanks are used. Depending on the temperature the effects obtained are varied.
• the operating technique is the following: the objects, immediately after zincing and a adequate washing in running water, are immersed in this solution for 10-20 seconds and kept under moderate movement. After which, they are removed from the solution and are left in the air for 30 seconds, then washed thoroughly under water and finally dried under warm air (maximum 50 ° C).
• An iridescent film of chromium chromate forms on surfaces zinc plated. Such a film gives zinc a extraordinary resistance to atmospheric corrosion. This film, however, is not very resistant to abrasion, while its thickness is of the order of 0.001 mm. he Note that a treatment in very hot water and the hot air drying at a temperature above 50 ° C affect the quality of the protective film.
• the solution can be controlled analytically, by determining chromic acid, sulfuric acid, trivalent chromium and zinc. In addition, it is possible to follow this criterion: one liter of solution is taken to which small amounts of sulfuric acid are added until, on successive tests, the iridescent film of normal color is obtained.
• a solution with 20 g / l sodium hydrate, 20 g / l sodium carbonate and 5 g / l sodium hydrosulfite is used; hot (40-80 ° C), on the contrary, an alkaline solution containing 10-20 g / l sodium metasilicate and 2-5 g / l hydrosulfite sodium is used.
• the operation is carried out as follows: the parts are immersed, immediately after zincing and two successive washes in running water, in the chromic solution keeping them for 5-10 seconds under rapid movement. So zinc undergoes the chemical buffing and a beautiful golden yellow color appears. After washing in running water, the passivation is supplemented with alkaline treatment for 10-20 seconds in the second solution. Follow two washes in running water and drying.
• One of these solutions consists of 25 cm 3 / l of concentrated nitric acid and 10 g / l of a mixture of 25 units (by mass) of potassium dichromate, 5 units of chromic alum, 70 units of bifluoride sodium or ammonium, and the pH should be 1-2.
• yellow protective passivations can be achieved using 120 g / l of chromic anhydride, 20 g / l of sodium sulfate, 2 g / l of sodium sulfate. ammonium, 3 g / l of sodium acetate and approximately 20 cm 3 / l of nitric acid so as to have a pH of 1.5. The operation is carried out at 30 ° C. If the addition of nitric acid is too weak, opaque layers of olive green color will form. For heavier shades, it is sufficient to add to the solution 2 g / l of a mixture formed by 2 units of magnesium sulfate, one unit of sodium sulfate and one unit of ammonium sulfate.
• compositions in which, in changing the relationships between components, layers chromic conversion can be performed, with a intense and high value iridescent coloring protective.
• Many of these layers being gelatinous, can absorb already known dyes in the context of the anodic oxidation of aluminum and take various colors, which do not require of "fixing”. It is advisable to increase the wear resistance, protect the colored layer with transparent varnishes and operate on thicknesses zinc of at least 15 microns.
• the characteristics of the coatings have a big role on the efficiency of chromization operations. Coatings uniformly shiny and fine-grained require in all cases lower chromization times compared to those with a more crystal structure abundant and irregularly opaque, to obtain the same final results from an aesthetic point of view.
• the same electrolyte conditions like its formulation perfect, the exact relationship between the metal content and the other constituents, the possible presence of inorganic contaminants, such as copper, can largely influence negatively too well the aspect that the property of resistance to corrosion of chromic films.
• a good rule is to precede the chrome treatment by good washing, followed brief immersion in 0.5% acid solution nitric; this solution is used to neutralize the excess of alkalinity derived from cyalcaline solutions of zincing (or cadmium plating) and acts as an activator shallow in all cases, even if the coatings come from sulphate, fluoroborate or zincate.
• Chromium plating or phosphating are also adopted as excellent bases for varnishing and all organic finishes, which would not adhere to galvanized surfaces not conditioned.
• German patent DE 722921 uses an anodic treatment of zinc by means of the bath indicated below: Sodium dichromate 40-50% Sodium nitrite 2-6% Temperature 20 ° C Current density 0.4-0.8 A / dm 2 Voltage 16 V approx.
• anodizing could indicate that zinc anodized is similar to anodized aluminum.
• the coating of anodized zinc is a complex "sintered" compound (or semi-molten) formed at high voltage, dissimilar from simple aluminum oxide, which is created at a voltage inferior.
• the anodized zinc-plated parts increase the thickness of zinc by 25-40 microns dimensions of the base metal and cannot be colored.
• chromium and phosphatic conversion films are quite different from the zinc layers “anodized", which are distinguished by hardness and high abrasion and corrosion resistance (due to the effect thickness barrier and the presence of ions chromatic maintained in the coating).
• Such anodic zinc coatings are formed by anodic electrical discharge which begins on the parts at 70V and increases until the end of the process.
• the temperatures produced by electric shocks are sufficient to melt the zinc deposit and make a sintered structure (similar to sintering or simultaneous coating of zinc) containing oxide zinc, chromates, phosphates and other compounds in depending on the type of bath.
• Alternating current is preferred, it produces a better effect compared to direct current.
• the objects are simultaneously and alternately anodes and cathodes and precisely on two electrode bars when the alternating current is single-phase and on three bars when it is three-phase.
• the current density is 4.5 A / dm 2 in the case of three-phase current; 7.5 A / dm 2 for the single phase. Since the anodized deposit is dielectric, as the process progresses, the electrical resistance increases and it is therefore necessary to increase the voltage to keep the current density constant.
• the temperature is regulated at 65-82 ° C.
• the baths are made up of chromates, phosphates, silicates and fluorides in various combinations in depending on the desired colors: green is obtained neutral solution (pH 6.8-7.0) with 200 V and 5-7 minutes treatment; gray at pH 12.6-13.0 with 90-105 V and 9 minutes; brown at pH 12.4-12.8 with 95-110 V and 8 minutes; charcoal color at pH 12.6-13.0 with 85-100 V and 10 minutes. For the brown and charcoal colors the parts undergo a second treatment in another 75-90 V solution.
• the anodized deposits are slightly porous and that the electrolytes are based on hexavalent chromium, which is retained by zinc, the application of the process is not recommended for food containers.
• the parts to be welded or to be used for electrical contacts require that they are previously protected by insulating tape or varnishes. If the anodized parts must be noticeably bent, the covering can crease.
• the acidic aqueous bath according to the invention contains potassium chloride which promotes electrical conductivity, zinc chloride which promotes the deposition of metallic zinc, boric acid which stabilizes the pH, a surfactant which prepares and makes the surface of the parts receptive to the deposit of zincing, methyl alcohol which allows the shine of the application in case of low current density, acetic acid which allows the shine of application in case of high current density, copper which stabilizes the application of zinc, iron metallic which acts as a chemical filter so to eliminate the "burn" effect in case of high density of current.
• This bath and method according to the modes of preferred embodiments allow the zinc to be deposited at a speed from 18 to 22 ⁇ thick / hour i.e. superior to that of the state of the art, as described previously.

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• 1999
  • 1999-07-23 EP EP99630061A patent/EP1070771A1/de not_active Withdrawn

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