Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal coating
Reexamination Certificate
2000-07-24
2001-12-18
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal coating
Reexamination Certificate
active
06331241
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a method of thin plating steel with chromium; more particularly, it relates to the use of an electrolyte of a particular ionic form to make “tin-free steel.”
2. Description of the Related Art
In the American steel industry, low carbon sheet steel thinly plated with chromium is generally referred to as tin free steel (“TFS”). In Europe and Asia, the same type of product is more commonly called electrolytically chromium coated steel (“ECCS”). In either case, the typical practice is to prepare an electrolyte containing 70 to 120 grams per liter of CrO
3
, (chromic acid) together with small amounts of sulfate ions (about 0.2-0.8 grams per liter) and fluoride ions (about 1.0 to 5.0 grams per liter). This invention does not concern the making of decorative and hard chromium plates, which utilizes baths containing generally 200 to 400 g/L.
In the manufacture of TFS, the sheet steel is utilized as the cathode in an electrolytic cell for the solution, power is applied, and a coating of chromium metal is formed on the sheet steel. Typically, the finished product will have a layer of 3 to 13 milligrams per square foot of metallic chromium (a common target is 5 mg/ft
2
) and will have a layer of about 0.5 to 1.5 mg/ft
2
chromium oxide on top of the metallic chromium. The strip must be rinsed quickly and properly to avoid significant staining. Large quantities of this product are made by continuous strip plating.
The conventional electrolytic chrome treatment as described above presents formidable environmental problems and expense. One response to these problems and expense is to search for a way to reduce the amount of waste electrolyte generated. The literature, however, discourses an approach of reducing the quantity of chromic acid in the electrolyte. See, for example, lines 2-32, of column 4 of Allen et al U.S. Pat. No. 3,642,587: “Forty g/l of CrO
3
has been found to be a minimum amount useful in the baths contemplated herein because below that amount bright chromium plate cannot be obtained by the process of the invention, and, further, a heavy, dark-colored coating of hydrated chromium oxide is produced.” The baths used by Allen et al included sulfate and fluoride catalysts.
A study by Marcel Pourbaix, “Atlas of Electrochemical in Equilibria in Aqueous Solutions,” National Association of Corrosion Engineers, Houston (Library of Congress 65-11670), Second English Edition 1974, pages 256-271, illustrates the complexity of the chromate ion and its various states in aqueous solution. Numerous forms of chromate ions are shown. But, although the term “hexavalent chromium” is commonly used, a simple Cr
+6
ion has never been identified. When chromium trioxide (CrO
3
) dissolves in water, it forms chromic acid: CrO
3
+H
2
O→H
2
CrO
4
(Equation 1). The chromic acid is at equilibrium with HCrO
4
−
(acid chromate ion) and Cr
2
O
7
=
(dichromate ion) as shown in H
2
CrO
4
<—> HCrO
4
−
+H
+
(Equation 2) and 2H
2
CrO
4
=
<- - > Cr
2
O
7
=
+H
2
O+2H
+
(Equation 3). Three types of hexavalent anion are generated: HCrO
4
−
, CrO
4
=
, and Cr
2
O
7
=
; their concentrations in the solution depend on the pH and the initial CrO
3
concentration. It is known, as pointed out in “Industrial Electrochemical Processes” edited by A. T. Kuhn, Elsevier Publishing Company (1971) page 354, that “(T)he [HCrO
4
]
−
ion predominates in dilute CrO
3
solutions, whereas the [HCr
2
O
7
]
−
ion is formed preferentially in concentrated solutions, such as those that are used for plating baths.” The [HCr
2
O
7
]
−
ion may also exist as Cr
2
O
7
=
. This publication goes on to discuss the importance of the presence of trivalent chromium to the deposition of the chromium layer, particularly focusing on the role of the sulfuric acid catalyst in preventing the formation of an impermeable film primarily of Cr(OH)CrO
4
.
It is desirable to reduce the environmental consequences in the manufacture of tin free steel in spite of the complexities presented by chromate electrochemistry.
SUMMARY OF THE INVENTION
Contrary to the practice of the prior art, which employs primarily [HCr
2
O
7
]
−
ions, my process involves the use of acid dichromate ions—HCrO
4
−
—in the electrolyte bath. My electrolyte baths include sulfate and fluoride ions as well, preferably in concentrations about 1/100 of chromium ion present. I may make up the solution using CrO
3
, but permit primarily HCrO
4
−
to exist in the bath as it is used. Workers in the art may prefer to express the concentration in more familiar terms as CrO
3
, in which case it may be said that the CrO
3
is dissolved and optionally diluted to provide a concentration from 5 to 35 grams per liter, preferably 20 to 30 g/l. The pH may vary from 1.0 to 1.5, which will cause the HCrO
4
−
concentration to change accordingly. My invention utilizes an electrolytic bath including CrO
3
at 5 to 35 g/l at a pH between 1.0 and 1.5, yielding HCrO
4
−
at concentrations of 5.8 to 40.6 g/L. The solutions include sulfuric acid in amounts to provide sulfate ions, and a source of fluoride ions, sufficient for effective enhancement of chromium deposition, preferably in concentrations from 50 to 200 ppm sulfate ions, and fluoride ions in concentration from 500 to 2000 ppm. Generally, the process may employ 100-1000 Amperes/square foot, (with a voltage of 3-17 depending on the current density) and provide a residence time of, preferably, 5-15 seconds. Temperature of the bath is not critical, but 100-30° F. is preferred.
My process provides several benefits in addition to the amelioration of the disposal problems associated with chrome plating. For example, while providing good lacquer adhesion, the process exhibits increased current efficiency, it reduces the cost of chromic acid additions to the bath, it minimizes surface staining problems, since diluted drag-out film is easier to rinse, it minimizes maintenance cost by reducing the corrosion to the process equipment, and it stabilizes the Cr(III) concentrations. Instability of Cr(III) concentration is known to decrease current efficiency in chrome plating.
REFERENCES:
patent: 3642587 (1972-02-01), Allen
patent: 3903237 (1975-09-01), Smith et al.
patent: 4432842 (1984-02-01), Inui et al.
patent: 4519879 (1985-05-01), Ichida et al.
patent: 4579633 (1986-04-01), Kobayashi et al.
patent: 4746414 (1988-05-01), Carpenter et al.
patent: 4875983 (1989-10-01), Alota et al.
patent: 5168015 (1992-12-01), Shimizu et al.
patent: 6004448 (1999-12-01), Martyak
Marcel Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, 2nd English Edition, 1974, pp. 256-271: Chromium, No month available.
Fukuda et al, “Process for Coating Tin-Free Steel with Layers of Metallic Chromium . . . ” J. Electrochem. Soc., Mar. 1969, 398-402.
Edwin J. Smith “Chromium Coated Steel for Container Applications” Technical Seventy-Fifth General Meeting, 265-286, No month available.
A. T. Kuhn Industrial Electrochemical Processes Elsevier Publishing Company 1971, 354-360: “Electrodeposition of Chromium,” No month available.
Gorgos Kathryn
Krayer William L.
Parsons Thomas H
USX Corporation
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