Newsgroup: it.scienza.chimica
Titolo: Cromatura casalinga

Devo cromare una piccola vite in acciaio ma non ho sali di cromo in
casa. Avevo pensato di prendere il cromo (metallico) dalla patina che ricopre
il gambo di una vecchia sedia in acciaio cromato.
Una volta ottenuto qualche mg di cromo metallico potrei scioglierlo in HCl
per poi neutralizzare la soluzione con carbonato di sodio.
La soluzione acquosa risultante sarà composta da CrCl3 + NaCl a pH di circa 7.
Mi stavo chiedendo se tale soluzione potrebbe andare bene per cromare, in via
elettrochimica, la piccola vite, o se è necesario ricorrere, come mi pare di ricordare,
a sali in cui il cromo è presente in uno stato di ossidazione superiore.
Bisogna tener presente che a pH 7 la semireazione di scarica dell'idrogeno
(E = -0.413 V) è termodinamicamente competitiva con la semireazione di scarica
del cromo III che passa a cromo metallico (E=-0.41 V).
Qualcuno sa darmi qualche dritta?
Grazie.

Chicco83 ci ha detto :
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Non funziona ti conviene fare con cromo esavalente previa nickelatura 
del substrato con nickel solfato
IL ph e' fuori comunque perche' hai idrossido di cromo che non placca

ALCUNE SOLUZIONI PRATICHE:
E' SEMPRE BENE NICKELARE PRIMA DI FARE LA CROMATURA CONFERMO. TI DO IL 
TUTTO NELLA PRATICA POI FAI TU
NOTA: E' IMPORTANTISSIMA LA PULIZIA DELLE SUPERIFICI SI FA PRIMA PER 
VIA BASICA POI SI LAVA POI SI ATTACCA CON HCL E POI NUOVAMENTE SI LAVA 
CON ACQUA DEIONIZZATA PRIMA DI PASSARE ALLA PLACCATURA
POI SI PLACCA IL NICKEL SI LAVA BENE
POI SI FA LA CROMATURA
PER I BAGNI
UNDER CURRENT PLATING
NICKEL (WATTS)
8.14. Nickel Deposits
Nickel [7440-02-0] is one of the most important, most widely used, and 
most thoroughly studied of all the electrodeposited metals. Its history 
goes back over 140 years. It is used as a corrosion-resistant 
undercoating for chromium and gold, as a decorative and 
corrosion-resistant finish alone, and for numerous engineering 
purposes. Modern baths are almost all proprietary and deposit the metal 
in varied forms: bright; semi-bright but with excellent leveling 
characteristics; and with varied electrochemical activity so that 
multilayered nickel deposits provide superior corrosion resistance. 
Baths also exist which deposit extremely hard nickel that can be used 
as a machineable substitute for hard chromium. Baths that deposit 
nickel with low internal stress are used for electroforming.
Despite its long history, however, truly highspeed bright plating 
solutions were not developed until the early 1940s when the brightening 
effect of saccharin was discovered. Now numerous bright and/or leveling 
baths are available.
Most of the modern bright baths are based on the Watts type bath:
Watts type bath
Nickel sulfate (NiSO4 · 6 H2O)
300 g/L (40 oz/gal)
Nickel chloride (NiCl2 · 6 H2O)
45 g/L (6 oz/gal)
Boric acid
30 – 37.5 g/L
 (4 – 5 oz/gal)
Control
 Nickel
77.0 g/L (10.3 oz/gal)
pH
2.0 – 5.2
Temperature
32 – 71 °C
Cathode C.D.
1 – 6 A/dm2
 (10 – 60 A/ft2)
Efficiency
95 – 100
The wide range of pH listed is due to the variety of brightener systems 
available. For any specific system, the preferred operating pH usually 
lies in a range ca. 0.5 units broad. The brightener system almost 
always contains a wetting agent to reduce the surface tension of the 
solution and prevent pitting due to the adherence of gas bubbles to the 
surface of the parts. Sodium lauryl sulfate is frequently used, but for 
systems using air agitation nonfoaming types are required. The choice 
of wetter is extremely important since it can interact with the 
brightener components and, if incompatible, cause hazing and/or 
streaking.
Anodes generally are chips, slugs, or electroformed buttons of 
specially modified nickel contained in titanium anode baskets. The 
particular type of anode material is often dictated by the brightener 
system. Anodes should be bagged to prevent roughness.
Agitation varies from the very slight mechanical action obtained by the 
movement of the parts through the solution by automatic conveying 
equipment to relatively violent air agitation. For optimum results, 
continuous filtration is preferred. Since nickel baths are susceptible 
to organic contamination, either from brightener breakdown or 
externally introduced, maintenance of a layer of activated carbon on 
the filter pads is a worthwhile precaution. Even with this treatment, 
periodic thorough treatment with activated carbon and even hydrogen 
peroxide or potassium permanganate to completely remove all organics is 
sometimes required.
In some cases, the all-chloride bath is used to produce bright 
deposits. It is more often used to deposit a dull, hard nickel, which 
is used as a replacement for hard chromium in the repair of worn shafts 
and similar equipment.
All-chloride bath
Nickel chloride
240 g/L (32 oz/gal)
Boric acid
30 g/L (4 oz/gal)
Control
 Nickel
75 g/L (10 oz/gal)
pH, electrometric
0.9 – 1.1
A wetting agent is used to reduce the surface tension to 35 – 45 mN/m 
and prevent gas pitting. For some engineering applications where 
organic additives must be avoided, 5 – 10 ppm of hydrogen peroxide 
achieves a similar effect. Acceptable cathode current densities are 
somewhat higher than in the Watts bath. Other conditions are 
essentially the same.
For electroforming and for building-up worn parts where hardness is not 
required, the fluoborate or sulfamate bath may be used.
Fluoborate bath
Nickel fluoborate
320 g/L (29.3 oz/gal)
Boric acid
30 g/L (4.0 oz/gal)
pH, electrometric
3.0 – 4.5
Control
 Nickel
55 g/L (7.3 oz/gal)
Temperature
32 – 71 °C
Cathode C.D.
5 – 10 A/dm2 (50 – 100 A/ft2)
Sulfamate bath
Nickel sulfamate
450 g/L (60 oz/gal)
Boric acid
30 g/L (4 oz/gal)
Control
 Nickel
105 g/L (14 oz/gal)
pH, electrometric
3 – 5
Temperature
49 °C
Cathode C.D
5 – 30 A/dm2 (50 – 300 A/ft2)
Other conditions are essentially the same as for the Watts bath. 
Non-pitter and proprietary stress-controlling agents may be used as 
required.
Nickel may be stripped in nitric acid if the substrate allows. Organic 
nitroaromatic strippers with cyanide or sulfuric acid may be used, 
depending on the substrate. Numerous electrolytic strippers are also 
available for various substrates.
Black Nickel Deposits. At one time, black nickel, which is a complex 
alloy of nickel, nickel sulfide, and zinc, was widely used for 
producing oxidized and relieved finishes on builder's hardware and 
other items. Since it is a porous, somewhat soft deposit, and has 
little or no intrinsic corrosion resistance, it is usually applied over 
a film of nickel, and it must always be protected by some sort of 
organic film. Its porosity enables it to hold lubricating oil, and it 
was applied to typewriter parts for this purpose. Development of an 
immersion process that could be applied over zinc on typewriter parts, 
and changes in decorator styles, eliminated these major uses, and the 
plating solution fell into disuse. The current trend toward 
nonreflective surfaces on automobiles, and possible use as a solar 
absorber, has rekindled interest in black nickel. For nonreflective 
surfaces, the underlying nickel must be dull or lustrous rather than 
bright, or the nonreflective characteristics are difficult to attain.
Baths fall into two general classes, one more concentrated than the 
other (Table (4)). Control of the current density is probably the most 
important consideration in obtaining good, dull black deposits. Excess 
current produces gray or even pure nickel deposits. Some operators feel 
that small amounts of copper in the solution help to darken the 
deposits even more. Proper activation of the base nickel layer is 
necessary to ensure adhesion.
CHROMIUM
8.4. Chromium Deposits
There are two main uses for the deposits of chromium [7440-47-3]: 
decorative chromium, which is normally restricted to very thin 
deposits, and hard chromium, which is applied at much greater 
thicknesses. The essential differences between the two deposits are the 
thicknesses and the conditions of operation. Hard chromium is most 
often applied at a slightly higher temperature than decorative layers 
and is not as bright. Many shops alternately use the same solutions for 
both purposes merely by adjusting the temperature and the current 
densities.
Chromium is mainly deposited from baths that contain chromic acid 
(chromic anhydride, CrVIO3) and a catalyst. The original catalyst was 
sulfate ion (SO42–) provided by sulfuric acid and was added to the bath 
in amounts that would produce a ratio of 75 : 1 to 125 : 1 
(CrO3/SO42–). There are two general classes of solution, those that 
operate at ca. 250 g/L (33 oz/gal) and those operating in the range of 
375 g/L (50 oz/gal). Other anions can also function as catalysts, 
particularly fluoride. Many proprietary solutions have been developed 
based on mixtures of fluoride salts and sulfuric acid. If the fluoride 
salt has a limited, temperature-dependent solubility, an excess may be 
added to produce a self-regulating solution. By properly choosing the 
catalyst mixture and the operating conditions, the porosity and the 
degree of cracking of the chromium deposit may be controlled. For 
automotive use, two successive layers of differing deposit structure 
are sometimes used to prevent concentration of corrosive effects, 
thereby improving corrosion resistance. Where hard chromium is to be 
used as a lubricated bearing surface, the crack pattern may be 
specified.
The bath is operated at a temperature of 32 – 43 °C for plating 
decorative deposits, and 37 – 65 °C for hard chromium. Cathode current 
density is 10 – 20 A/dm2 (100 – 200 A/ft2) for decorative work and 15 – 
35 A/dm2 (150 –350 A/ft2) for hard chromium. Some trivalent chromium 
must always be present, and optimum results are obtained when the 
Cr(III) concentration is approximately 5 % of the Cr(VI) concentration.
The process is sensitive to chromic acid concentration, catalyst ratio, 
temperature, and current density. In the past, it was reported that 
metallic impurities could be tolerated to a level of 20 g/L calculated 
as the oxides of the contaminants before the bath became unstable and 
difficult to control. The trivalent chromium must be included in this 
calculation. More recent reports have indicated that instability occurs 
at levels as low as 1.5 g/L of contaminant. This is particularly true 
of baths using mixed catalyst systems that include fluoride. Many 
anions are deleterious, but chloride ion is particularly harmful. 
Fortunately, it can be readily removed by dummying at high current 
density, the Cl– being oxidized to Cl2 at the anode and evolved from 
the solution.
For many years, efforts have been made to develop baths based on 
Cr(III) to reduce disposal problems and improve the efficiency of the 
bath. While several such baths were developed, the color of the deposit 
was somewhat dark and did not favorably compare with the blue-white 
color of the deposits obtained from Cr(VI) baths. Public acceptance was 
therefore limited despite the other advantages of the baths. Very 
recently, a new bath was developed and patented which produces deposits 
that closely match the desired color. This bath is now beginning to 
find considerable acceptance in commercial applications. It has high 
efficiency, can be replated without stripping, and has much better 
throwing power than the Cr(VI) baths. It does require anode baskets 
that surround the anodes with a permeable membrane that prevents the 
Cr(VI) formed at the anode from migrating into the solution, where it 
acts as a poison. This bath is available only as a proprietary solution 
(Envirochrome).
Chromium may be stripped by immersion in 22° Bé hydrochloric acid 
diluted 1 : 1 with water, in warm 50 vol % sulfuric acid, or a heated 
solution of sodium hydrogen sulfate. It may also be stripped 
electrolytically in a solution of sodium hydroxide, or a strongly 
alkaline electrocleaner. Properly formulated alkaline electrocleaners 
are to be preferred for heavy deposits since they give somewhat higher 
stripping rates and greater protection against etching of the substrate 
due to localized reduction of pH.
ALTRA SOLUZIONE:
CHROMATING WITHOUT CURRENT
Chromium, vanadium, molybdenum, and tungsten, when they are in their 
high valence states and soluble, have an unusual ability either to 
prevent corrosion or to limit the salt growth that occurs following 
corrosive action. Chromium(VI) compounds are the most widely used 
compounds of the group, and a whole series of chromate conversion 
coatings have been developed for various metals. Specifically, copper, 
brass, cadmium, zinc, and occasionally silver and gold are protected in 
this way. In general terms, the solutions consist of a soluble 
dichromate salt, such as sodium or potassium dichromate, and a catalyst 
similar to those used in chrome plating, such as SO42– , F–, or the 
other effective catalysts including complex fluorides or boric acid. 
Other anions may be added to alter the character of the film formed. 
The film is a colloidal gel that contains soluble chromium(VI) , as 
well as  salts of the base metal. The corrosion protection 
characteristics are dependent on the soluble Cr(VI) leaching out to 
protect the surface. If the parts are dried at a temperature close to 
or above 71 °C, the gel will dehydrate and the Cr(VI) reverts to an 
insoluble salt, and the protective mechanism is lost. Additionally, the 
gel will shrink as a result of the dehydration and exfoliate from the 
surface, becoming powdery and easily removed by abrasion. The original 
chromating solutions were relatively concentrated (120 – 240 g/L 
Na2Cr2O7 , 16 – 32 oz/gal) with catalyst in the ratio of 1 : 100). On 
zinc and cadmium, these produced iridescent yellow coatings that gave 
up to 72-h salt-spray resistance to the development of white corrosion 
salts. On copper, they provided resistance to the development of green 
salts of up to 24 h of salt spray, or up to 1000 h of 100 % humidity. 
Modern chromating solutions are almost all proprietary and operate at 
much lower concentrations. They fall into five categories:
1)	Clear to bluish single dip chromates. Salt spray resistance ca. 24 
h.
2)	Clear leached type chromates. These are iridescent yellow chromates 
that are subsequently leached in a sodium hydroxide – sodium carbonate 
solution to provide a clear coating. Salt spray resistance up to 48 h.
3)	Iridescent yellow chromates, single dip. Salt spray resistance up to 
72 h.
4)	Olive drab chromates, single dip. Salt spray resistance up to 144 h.
5)	Black chromates. These may be single dip chromates in which the 
black color is provided by silver salts, which deposit microscopic 
amounts of silver in the gel. These chromates generally have less 
corrosion resistance than an uncolored chromate since the silver 
particles provide active electrochemical cells for the initiation of 
corrosion. Salt spray resistance 24 – 48 h.
There are also double dip systems in which an iridescent or olive drab 
coating is applied and subsequently dyed black using an organic dye.
Immersion times are from 10 to 30 s. Temperature and agitation affect 
the formation of the coating. Transfer time to the rinsing step can be 
important, and on automatic equipment where transfer times may run 30 – 
45 s, special formulations may be necessary.
BUON LAVORO

Grazie ad entrambi.
Appena riesco ad organizzare il materiale, sperando che non mi
arrestino :-), vi faccio sapere come viene.
Ciao.

Il 05/03/06, Chicco83 ha detto :
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Ci conto

 ha scritto nel messaggio
news:mn.44cb7d631...@gmail.com...

Cavolo, per adesso ho rimediato solo il K2Cr2O7 quando torno all'univ.
vedro' di scippare anche un grammo di NiSO4...