Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal coating
Reexamination Certificate
1998-12-18
2001-05-08
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal coating
C205S284000
Reexamination Certificate
active
06228244
ABSTRACT:
TECHNICAL FIELD
The present invention relates to chromium plating baths with organic additives, resistant in solutions of chromium, to obtain electrodeposition of penetrating and covering chromium while avoiding anodic corrosion.
BACKGROUND OF THE INVENTION
Alkane sulfonic and disulfonic acids were first used as additives for electrolytic baths in 1930, at the Politecnico of Milan. After the Second World War American, French, German, Polish and Soviet researchers reported and claimed disulfonic acids and their salts as improvers of cathode efficiency in chromium plating baths. However, application of these types of baths on a large scale over a period of time revealed inferior properties compared to traditional baths, in that they cause accelerated corrosion of the anode (an alloy of lead).
The mechanism that leads to these drawbacks is described as follows:
Acidic dissolution of PbO
2
due to the polarization of acid concentration:
PbO
2
+2H
+
=PbO
2+
+H
2
O
Reaction of the lead oxide favored by the excess of acidity with H
2
O
2
formed at the anode
PbO
2
+H
2
O
2
+2H
+
=Pb
2+
+O
2
+2H
2
O (the reformation and the stabirization of the PbO
2
is, on the contrary, favored by a deficit of free acid: Pb
2+
+O
2
+H
2
O=PbO
2
+H
2
O
2
+2H
+
).
The anode degradation rate is further increased by the fact that the Pb
2+
ions formed are removed from the equilibrium by the formation of stable complexes with ions in solution—for instance traces of halides and degradation products of the organic acids. Many proposals have been suggested to eliminate the drawbacks described above, by chemical and electrical means, but with unsatisfactory results.
DISCLOSURE OF THE INVENTION
This patent claims the use of certain additives in specific concentrations, to improve the covering and penetration power of the chromium plating baths while avoiding anodic corrosion.
Anodic corrosion can be drastically reduced or eliminated by adding appropriate concentrations of aminoalkanesulfonic compounds or heterocyclic nitrogen containing bases to the chromium plating baths containing Alkanedisulfonic or Alkanesulfonic acids or salts.
These substances in elevated concentrations can lead to a cathode efficiency below that of a traditional chromium plating both.
a) The aminoalkanesulfonic and the heterocyclic bases are added to the chromium plating baths containing Alkanedisulfonic and Alkanesulfonic acids and salts, in such concentrations as to give a Faraday output of 15-16% constant (not of interest in this patent which claims other parameters).
b) The corrosion inhibitors, chemical compounds, added to the chromic solutions containing Alkanesulfonic and Alkanedisulfonic acids and salts, drastically reduce the corrosion rate of anodes immersed in them, shifting the corrosion potential to values nobler than the primary potential, or increasing the overload of the anodic or cathode process or of both simultaneously according to their chemical nature.
Such purpose is achieved by the present invention, which relates to chromium plating baths comprising one or more compounds selected from compounds having general formula:
X—(CH
2
)
n
—SO
3
H [1]
where:
n=integer from 1 to 12
X=NH
2
and salts thereof,
and nitrogen containing heterocyclic bases and/or their complexes with CrO
3
.
Preferred compounds of formula [1] are aminoalkanesulfonic acids and salts C
2
-C
6
and most preferably C
2
and C
3
compounds. Preferably, nitrogen containing heterocyclic bases are provided as complexes with chromium, namely with CrO
3
. An example of such complexes is the complex between pyridine and CrO
3
, as shown by the following formula:
Further preferred complexes are those of pyridine homologues, optionally with ring substituents, such as e.g. nicotinic acid, picolinic acid, 4-pyridinethanesulfonic acid, etc.
In presence of these compounds the anodic corrosion is drastically reduced even in the presence of high concentrations of compounds of general formula:
Y—(CH
2
)
n
—SO
3
H [2]
where:
n=integer from 1 to 12
Y=H or SO
3
H;
and salts thereof.
These additives are employed in chromium plating baths, in combination with the previously disclosed compounds in order to give penetrating and covering chromium deposits without corrosion of the lead alloy anode.
The additives object of the invention are provided within the range of 0.1-40 g/l, preferably within 1-20 g/l, more preferably 1-10 g/l, and most preferably within the range of 2-3 g/l.
Another object of the invention is a concentrated formulation containing CrO
3
and one or more additives of formula [1], and/or one or more nitrogen containing heterocyclic bases and their complexes with chromium, and/or compounds of formula [2] for the preparation of chromium plating baths.
Further objects of the invention are the uses of the compounds of formula [1] and [2], including nitrogen containing heterocyclic bases and chromium complexes thereof.
A further advantage of the present invention is given by the fact that the addition to a chromium plating bath of compounds of general formula [1] and [2] with 6-12 atoms of carbon, leads to a reduction of the surface tension of the bath with the advantage of eliminating splashing, reducing the losses to transport with notable saving of chromic acid, so much so that their employment is cost-reducing and improves the work environment (TLV-TWA values).
Penetrating power is a grading of the metal as a function of the electric current, where chromium plating baths have scarce penetrating power.
Various methods for the measurement of the penetrating power of the electrolytic baths exist as for instance:
a) the technique of E. Haring and W. Blum;
b) Method of C. Pam.
REFERENCES:
patent: 4093522 (1978-06-01), Dillenberg
patent: 4588481 (1986-05-01), Chessin et al.
patent: 4810336 (1989-03-01), Martyak
patent: 523 968 (1972-07-01), None
patent: WO 98/36108 (1998-08-01), None
Lido Frediani
Merello Giovanni
Cobrin & Gittes
Gorgos Kathryn
Leader William T.
Luigi Stoppani S.p.A.
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