Go Back   Science Forums Biology Forum Molecular Biology Forum Physics Chemistry Forum > Regional Molecular Biology Discussion > Forum Chimica
Register Search Today's Posts Mark Forums Read

Forum Chimica Forum Scienza Chimica. Italian Chemistry Forum.


Cromatura casalinga

Cromatura casalinga - Forum Chimica

Cromatura casalinga - Forum Scienza Chimica. Italian Chemistry Forum.


Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 03-04-2006, 10:36 AM
Chicco83
Guest
 
Posts: n/a
Default 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 sar 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 necesario ricorrere, come mi pare di ricordare,
a sali in cui il cromo presente in uno stato di ossidazione superiore.

Bisogna tener presente che a pH 7 la semireazione di scarica dell'idrogeno
(E = -0.413 V) 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.



Reply With Quote
  #2  
Old 03-04-2006, 02:13 PM
revenge
Guest
 
Posts: n/a
Default Cromatura casalinga

Chicco83 ci ha detto :

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


Reply With Quote
  #3  
Old 03-04-2006, 02:52 PM
Soviet_Mario
Guest
 
Posts: n/a
Default Cromatura casalinga

Chicco83 ha scritto:


su questo ti ha gi risposto Revenge : se vuoi tenere il cromo
disciolto serve un pH decisamente acido.
Secondariamente ho qualche dubbio che tu riesca a sciogliere il
cromo della cromatura con HCl, fosse pure l'azeotropo.
Converrebbe acido nitrico.
In ogni caso, potresti provare a fare la dissoluzione anodica
(elettrolita magari bisolfato di potassio+solfato di potassio,
tanto per fare un primo tentativo). Il pezzo sotto protezione
catodica non si altera molto per l'acidit.

Per quanto riguarda la scarica dell'idrogeno, beh, considera che
appena il catodo si placca di cromo magari per uno straterello
di un decimo di micron, gi l'idrogeno stesso trova una certa
sovratensione (ammesso che non ne avesse gi sull'acciaio
placcando).

Revenge ti ha consigliato uno strato intermedio di nichelatura
(io non ricordo mai se si usa il cromo per legare il nichel o
viceversa, quindi prendi per buono il consiglio uso).

Considera anche che solitamente basta una scarica di idrogeno
molto modesta per rovinare parecchio i depositi (porosi,
fragili). Ora le concentrazioni di cromo che si producono con la
esclusiva dissoluzione anodica inizialmente sono basse e
rimangono cmq basse, per cui la competizione con il protone non
ottimale. Bisognerebbe comunque creare un bagno ben
concentrato di sali di creomo anche quando usi l'anodo cromato.
Se come anodo usi acciaio cromato, poi, cura che la dissoluzione
proceda poco, perch se si espone il ferro sottostante (e il
processo tende a essere divergente e disomogeneo), passa in
soluzione pure il ferro, e magari deposita in lega.

I bagni professionali spesso contengono addittivi, gelatine,
colla, tamponi a pH corretto, anche complessanti, quasi sempre
anche tensioattivi (per favorire il desorbimento di gas, forse,
non ricordo bene). Quelli garantiscono depositi solidi, il fai
da te parecchio erratico come risultati.
Persino il bagno a nitrato di argento con aggiunta di soda e
formalina o acetaldeide placca alcune volte bene, altre produce
polvere nerastra.
Bisognerebbe anche usare un anodo a reticella che ingabbia la
tua vite, tra l'altro, o ruotare costantemente la vite. LA
faccia "in ombra" di solito si placca poco o punto, con normali
elettrodi cilindrici o a piastra e tenuti fermi (naturalmente la
corrente segue il percorso di minima resistenza, e quindi il pi
breve .... ah, e le linee di campo sono fitte in corrispondenza
dei punti frastagliati e appuntiti, a elevata curvatura, tutti
fattori che contribuiscono a dare strati non omogenei)

Se si trova, potresti cercare di usare un cromato complesso e
riducenti chimici, come boridruro, o ipofosfito, o
idrossilammina e soda. Improvvisando le condizioni floppano pi
spesso, ma se il deposito riesce spesso migliore
(nell'industria le vie chimiche sono di rado economiche in
rapporto al prezzo dell'elettricit)


CrCl3 potrebbe avere un pH tra 4 e 5, probabilmente. Oltre
comincia a idrolizzare il cromo III


purtroppo nulla di direttamente applicabile. Considera cmq la
termodinamica solo in senso negativo (nel senso di scartare
quello che proibisce), e manco tanto neppure quella. La
deposizione elettrodica quanto di pi cinetico e meno
termodinamico si trovi, come processo, ed molto sensibile a
variabili non ben prevedibili (come l'effetto di ioni e/o
sostanze che manco partecipano a bilancio)
ciao
Soviet_Mario
Reply With Quote
  #4  
Old 03-05-2006, 10:50 AM
revenge
Guest
 
Posts: n/a
Default Cromatura casalinga

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


Reply With Quote
  #5  
Old 03-05-2006, 10:39 PM
Chicco83
Guest
 
Posts: n/a
Default Cromatura casalinga

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



Reply With Quote
  #6  
Old 03-08-2006, 07:27 PM
revenge
Guest
 
Posts: n/a
Default Cromatura casalinga

Il 05/03/06, Chicco83 ha detto :

Ci conto


Reply With Quote
  #7  
Old 03-12-2006, 09:20 PM
Chicco83
Guest
 
Posts: n/a
Default Cromatura casalinga


"revenge" <[Only registered users see links. ]> ha scritto nel messaggio
news:[Only registered users see links. ]...

Cavolo, per adesso ho rimediato solo il K2Cr2O7 quando torno all'univ.
vedro' di scippare anche un grammo di NiSO4...



Reply With Quote
Reply

Tags
casalinga , cromatura


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
sintesi casalinga carbonato d'ammonio Michele Forum Chimica 0 06-10-2008 06:32 PM
Cromatura plastica Fallenius Forum Chimica 16 05-02-2006 05:43 PM
Idrogenazione casalinga della plastica BAGS Forum Chimica 6 01-08-2006 01:05 AM


All times are GMT. The time now is 05:24 PM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright 2005 - 2012 Molecular Station | All Rights Reserved
Page generated in 0.20296 seconds with 16 queries