US3069333A - Chromium plating - Google Patents

Chromium plating Download PDF

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US3069333A
US3069333A US126623A US12662361A US3069333A US 3069333 A US3069333 A US 3069333A US 126623 A US126623 A US 126623A US 12662361 A US12662361 A US 12662361A US 3069333 A US3069333 A US 3069333A
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chromium
bath
plating
gram
acid
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Alden J Deyrup
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EIDP Inc
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EI Du Pont de Nemours and Co
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Priority to NL268828D priority Critical patent/NL268828A/xx
Priority to NL132271D priority patent/NL132271C/xx
Priority to BE625233D priority patent/BE625233A/xx
Application filed by EI Du Pont de Nemours and Co filed Critical EI Du Pont de Nemours and Co
Priority to US126623A priority patent/US3069333A/en
Priority to GB30990/61A priority patent/GB981481A/en
Priority to FR872068A priority patent/FR1309239A/fr
Priority to DEP27799A priority patent/DE1247801B/de
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    • 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
    • C25D3/04—Electroplating: Baths therefor from solutions of chromium
    • C25D3/10—Electroplating: Baths therefor from solutions of chromium characterised by the organic bath constituents used

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  • the present commercial chromium plating processes are based on the electrolysis of chromium trioxide, CrO (chromic acid), solutions containing small amounts of a catalyst, e.g., sulfates, fluorides or the like.
  • CrO chromium trioxide
  • a catalyst e.g., sulfates, fluorides or the like.
  • the current density and temperature during plating must be closely controlled.
  • the throwing power of the plating bath is very low as compared to other metal plating processes. Because of poor throwing power, it is necessary to provide anodes conforming to the shape of the object to be plated.
  • the current efliciency of the commercial plating baths are usually no greater than 8-12% and under the most optimum conditions only 15-20%.
  • the objects of this invention may be accomplished, in general, by preparing an electrolytic aqueous chromium plating bath comprising a chromium salt in which the chromium has the valence of 2 (he einafter referred to as a divalent chromium salt). at least one carboxylic acid taken from the roup consisting of formic and glycolic acids, and an alkali metal salt taken from the group consisting of sodium and potassium formates and glycolates,
  • the dissociation constant K is defined as where (H (A) and (HA) are respectively the molar concentrations of hydrogen ion, acid anion and undissociated acid.
  • H (A) and (HA) are respectively the molar concentrations of hydrogen ion, acid anion and undissociated acid.
  • Many of the strong acids such as HCl and H have dissociation constants too large to measure, much greater than 10*, so they ordinarily do not appear in published tables of dissociation constants.
  • the bath may also advantageously contain boric acid, and, if boric acid is present, it may also advantageously contain sodium fluoride. Boric acid and/or sodium fluoride are not however essential ingredients of the chromium plating bath of this invention.
  • This electrolytic plating bath may be operated electrolytically at room temperature, or at elevated temperatures up to C., to deposit bright continuous, highly corrosion-resistant chromium plate.
  • the metallic surface to be plated is first thoroughly cleaned in accordance with cleaning procedures well established in the art.
  • the metallic surface to be plated e.g., a copper or nickel surface
  • the metallic object to be plated is suspended as the cathode in the aforesaid electrolytic bath and spaced fairly evenly from an inert anode, e.g., a carbon or graphite anode.
  • plating may be carried out by passing an electric current of 30 to amps/sq. ft. at room temperature up to 200-450 amps/sq. ft. at elevated temperatures above 60 C. between said cathode and anode.
  • the present divalent-chromium plating baths have higher current efficiencies, higher maximum plating speeds and produce a whiter chromium deposit.
  • I It has, moreover, been found, in accordance with this invention. that under certain specific conditions as hereinafter set forth, it is possible to produce a new and previously unknown form of chromium having an exceedingly high tensile strength, being completely self-supporting even in the form of a thin electroplated film, having very different electrical resistivity and temperature co etficient of resistivity than previously known chromium and being very useful in the form of films or foils.
  • Such unique form of chromium may be obtained by the electrolytic deposition from the above-described chromium plating bath when the sole carboxylic acid present is formic acid, the sole carboxylate is sodium or potassium formate, the plating temperature is above 50 C. and the bath contains less than 10 parts per million of either sulfur or selenium, impurities normally found in chro-' mium.
  • the accompanying illustrations are drawings of X-ray diffraction patterns of chromium electrodeposits, in which: FIGURE 1 illustrated the X-ray diffraction pattern of chromium produced by the conventional commercial hexavalent chromium plating process. and FIGURE 2 illustrates the X-ray diffraction pattern of chromium produced by the divalent chromium plating process of this invention.
  • FIGURE 1 the sharp distinction between successive X-ray diffraction bands in FIGURE 1 clearly indicates that the chromium deposited from the prior art hexavalent process has a distinct crystallin ty of large well-defined crystals.
  • the wash d-out indistinct X-ray diffraction bands shown in FIGURE 2 designate a lack of d stinct, well-defined crystallinity or crystals of an exceedingly small size.
  • the plating baths of this invention are greatly superior to commercial hexavalent chromium plating baths'inaoeasse current efficiency, throwing power and plating range. These terms are, for convenience, defined as follows:
  • Current efficiency is generally defined as the ratio of weight of metal actually deposited by a given current for a given time to that which would have been deposited if electro-reduction of'the desired metal were 100% eflicient. The latter is calculated from Faradays law, which say that 96,500 coulombs (ampere-seconds) is required to electrodeposit one gram equivalent of metal, if no sidereactions occur. such as liberation of hydrogen. Current efliciency is commonly expressed as percent. It is important to note that current efiiciencies as thus calculated are not directly comparable between VI, III, and II-valent chromium.
  • Throwing power is a term used in, the plating industry to signify the degree to which plating occurs in recesses or on surfaces which are remote from the anode as compared with plating on flat surfaces or those near to the anode. Every one in the plating industry knows what good throwing power is, and that commercial chromium plating baths have very poor throwing power. However, there does not appear to be a generally accepted measure of throwing power as such. To a principal degree, the differences in plating on the variously disposed. surfaces of an article in a plating bath are governedby the response of the plating system to the local current densities. Therefore, we can secure good information about throwing power by measuring plating range.
  • the divalent chromium salt may be composed of any chromous compound which will dissolve in the plating bath and which will not introduce into the said bath nonoxidizing anions.
  • chromous chloride CrCl chromous bromide, CrBr chromous iodide, Crl chromius fluoride, CrF chromous for-mate,
  • chromous glycolate Cr(CH OH-CO -H O; or chromous sulfamate, Cr(SO NH may be used for this purpose.
  • chromous salts of acids having an ionization constant greater than 10- are preferred, however, a few chromous salt of acids having an ionization constant less than 10- such as chromous carbonate may also be used. Except for chromous formate and/or chromous glycolate, chromous salts of acids having ionization constants between 10 and 10- are much less desirable because of lowrplating efiiciency.
  • the carboxylic acid present in the plating bath must be formic acid or glycoiic acid or a mixture thereof. It may be added either as the acid itself or as an alkali metal formate or glycolate together with a strong acid such as hydrochloric acid (an acid having a dissociation constant of at least 10* which will react to produce the desired formic or glycolic acid in situ, care being taken to avoid an excess of the strong acid which might reduce the pH below about 1.7.
  • a strong acid such as hydrochloric acid (an acid having a dissociation constant of at least 10* which will react to produce the desired formic or glycolic acid in situ, care being taken to avoid an excess of the strong acid which might reduce the pH below about 1.7.
  • an alkali metal formate, glycolate, or mixture thereof may be present as the sodium or potassium salt of either of these acids.
  • the alkali metal carboxylate need not be derived from the same carboxylic acid as the carboxylic acid used in making the bath.
  • the carboxylic acid may be formic acid, and the alkali metal carboxylate may be added as sodium glycolate. It is not necessary in all cases to have an alkali metal formate or glycolate present in the oath, although sometimes it is preferable. If the bath contains no alkali metal carboxylate, then it is essential to have present in the bath an alkali salt of a strong acid (dissociation constant l0- as, for example, sodium chloride.
  • fluoride In cases where fluoride is used in the bath, it should be sodium fluoride. In the absence of fluoride, alkali salts" may be present, and are usually desirable to improve conductivity, and these may be either sodium or potassium or, in some cases, ammonium salts. However, when fluorides are used in the bath, potassium and ammonium salts from any source are undesirable because they may remove both fluorine and boron as insoluble fluoborates.
  • Boric acid when used, may be added as borax, boron oxide, boric acid, or sodium oxyfluoborate, a complex compound having the formula 4NaF-5B O (see US. Patent No. 2,823,095). All of these materials are equivalent to boric acid in aqueous solution and react toform the desired BF OH ion further discussed below.
  • fluoborates i.e., the well-known compounds containing the BF; ion, may be used as the source of boron, they are not recommended since the baths are not satisfactory for a period of days until the fluoborates have had time to hydrolyze to make the desired complexes.
  • the essential components of the plating bath are (1) a divalent chromium salt, (2) formic acid or glycolic acid or mixtures thereof, and (3) either an alkali formate or glycolate or mixture thereof, or an alkali salt of a strong acid (dissociation constant 10-
  • desirable components for some purposes are boric acid or boric acid and sodium fluoride.
  • the above-described essential and nonessential but desirable components of the plating bath may be constituted of compounds which may comprise one or more of these components.
  • sodium oxyfluoborate may be the source of both the fluoride and the boric acid; or the sodium fiuochromate may be the source of both the essential divalent chromium salt and'the fluoride; or chromium formate may be the source of both the divalent chromium and the formate radical.
  • the es sential constituents of the electroplating bath of this invention are preferentially present in the plating bath in certain proportions. This is also true of the desirable but nonessential constituents. In 1000 grams of plating solution, it is preferred that the several constituents be Divalent chromium salt 0.1 to 2 Free formic and/or glycolic acid 0.3 to 1 Alkali formate and/ or glycolate, or
  • compositions given in Table 1, below may precipitate crystalline material at room temperature. This does not ordinarily interfere with their useful plating. However, the precipitated material should be left in if one chooses to plate at an elevated temperature, when it will redissolve.
  • the function of the boric acid in a plating bath of this invention not containing a fluoride is to act directly as a buffer to control pH at the cathode.
  • the presence of boric acid usually broadens the conditions of temperature and current density at which plating is satisfactory.
  • the presence of boric acid increases the current density at which bright chromium can be plated without decreasing the threshold current density at which chromium first deposits.
  • the pH should be kept within the range of 1.7 to 3.5.
  • fluoride should be excluded from the bath because fluoride in the presence of boric acid at pHs lower than 2.7 increasingly reacts to form lluoborate ion, 3P4", which I find to be useless or even harmful in the system.
  • the pH may be used in the range 2.7 to 4.5.
  • brittleness and/or weakness is not apparent when chromium is applied as a coating on other metals.
  • the cromium layer is removed, as by dissolving away from it a temporary base metal, the chromium layer is always found to disintegrate into tiny flakes because of inherent brittleness and weakness.
  • narrowed conditions comprise the following essentials; (1) formic acid with or without formate as the sole carboxylates; (2) a deposition temperature above about 50 C.; (3) substantially complete freedom from sulfur and selenium compounds (less than 10 ppm).
  • formic acid with or without formate as the sole carboxylates comprising the following essentials; (1) formic acid with or without formate as the sole carboxylates; (2) a deposition temperature above about 50 C.; (3) substantially complete freedom from sulfur and selenium compounds (less than 10 ppm).
  • plated hardware becomes more resistant to corrosion, after deformation thereof, thanwhen previously-known chromium plating is used.
  • a temporary metal base such as brass, it is possible to release sheets of chromium foil when the base metal is dissolved away, as by use ofnitric acid.
  • Such thin, ductile and/ or high-strength chromium metal films should be of utility where high film strength and exceptional corrosion resistance are desired in metal foils. For example, they may be used advantageously for the metal transfer films of my copending patent application, Serial No. 33,951, filed June 6, 1960. While the ductility and strength of such foils as deposited and released are exceptional for chromium, it is found that these properties can be enhanced by baking the chromium films either before or after release at temperatures from 200 to 350 C. for periods of a few seconds up to one hour. The high tensile strength chromium as produced within the aforesaid narrow ranges of conditions also dififers from previously known chromium in its electrical properties.
  • prior art chromium has an electrical resistivity of 13x10 ohm cm.
  • this chromium has a resistivity of l49i9 10" ohm cm.
  • this chromium has a much lower temperature coeflicient of resistivity than prior art chromium, i.e., :5 l0 per degree C. versus about 2,000 to 3,000 l0 per degree C.
  • Divalent chromium compounds in solution are oxidized by exposure to air. Hence it is desirable to pad the electrolytic cell with an inert gas such as nitrogen. Another workable process is to keep the bath substantially covered with an inert foam. Such a foam wil often form on the bath during use. It may also be desirable to store the previously prepared baths under an inert gas, e.g., nitrogen to minimize air-oxidation of the divalent chromium. A little chromium oxidation does no great harm.
  • an inert gas e.g., nitrogen
  • trivalent chromium salts formed by oxidation do not appear to poison or harm the bath. However, if a large proportion of the divalent chromium becomes oxidized to trivalent chromium, the bath stops plating. The amount of chromous ion in the bath must be sufficient that the chromium metal plated out is from the chromous ionrather than the chromic ion.
  • alkali salts of st'ong acids dissociation constant l0 such as sodium chloride, bromide or iodide.
  • the foreign anions such as chloride ion may cause some problems at the anode. This may be avoided by placing the anode in a separate compartment with a porous diaphragm to limit mixing of the solution from the anode and cathode regions, or they may be completely avoided by use of a cation permeable membrane. Cloths coated with cation exch nge resins are commercially used in preventing transfer of anions while permitting free passage of cations.
  • Salts of weak acids should not be added except those of essential formic and/ or g ycolic acids. Such sats of weak acids react with the formic and/or glycolic acids, removing these essential materials. For example, acetic acid (dissociation constant about 10 and acetates are detrimental, incre:singly so with increasing concentration. It is also to be noted that alkali salts should not be added of those acids whose anions might be reduced at the cathode, such as nitrates.
  • Surf. ce-active agents may be added to the plating baths of this invention.
  • Such agents have several useful purposes.
  • Petowet R a satura ed hydrocarbon sodium sulfonate
  • Normal octyl alcohol in amounts of the order of 0.01% may be added to counteract the excessive foaming tendency of Petrowet R and also to reduce any tenden:y of pitting.
  • Chromium (ll) chloride CrCl t 0.4 Formic acid, I-I CO 0.5
  • EXAMPLE H One gram-mole of chromium (ll) chloride in solution was prepared by reacting at -l00 C. an excess of commercial pure chromium with two gram-moles of hydrogen chloride in a total volume of 510 ml. of water, the acid being slowly added to avoid excessive reaction speed. The solution was filtered to remove excess chromium metal and evaporated in vacuo to dryness, in order to free it of traces of volatile selenium and sulfur compounds. The chromium (ll) chloride was dissolved in water; 0.2 gram-mole of sodium formate and 0.6 gram-mole of formic acid were added; and the solution was made up with water to a weight of 1,000 gm.
  • Example 11 was prepared as in Example 11. To this was added 1.17 Cr(CH OH-CO 'H 0, (c) CrBr and (d) CrI In gram -moles sodium formate, 0.57 gram mole formic each case plating results with these modified baths were acid, 1.33 gram-moles boric acid, 2.90 gram-moles of 50- substantially the same as the results set forth in Example dium chloride, and water to a total weight of 1,000 gm. ill.
  • the electrolytic plating baths of this invention may Petrowet R, a saturated hydrocarbon sodium sulfocontain stabilization agents, wetting agents, oxidation innate and 0.1% by weight of normal octyl alcohol, these hibitors or other additive materials commonly used in additions being convenient for improving uniformity of the plating art, inasmuch as the plating baths herein distribution of electrodeposits.
  • This solution when elec- 15 disclosed have a tendency toward oxidation, both from trodeposited, yielded very bright electrodeposits over a the air and at the anode as a result of oxygen formawide range of temperatures from 7 to 90 C.
  • the plating baths of this invention and the process n th p ysical prope ties f the r sultant Chr mium, of plating therewith has produced bright chromium electhere are described below electrodeposits A, B and C. trodeposits at current densities of 20 to 450 amps. per sq. All of these were deposited to the same thickness on ft.
  • the throwing power of the baths is generally greater brass panels from a bath of the above composition, but than that of commercial hexavalent chromium plating respectively at 6 C., 25 C., and 55 C. All the l baths and current efficiencies of 15 to 40%, based on trodeposited panels were baked at 300 C.
  • the pH value 1 t 2 gram-moles of a di al nt chromium alt, 0.3 n the maXimum rate of bright Plating in microinchfis to 1 gram-mole of at least one carboxylic acid taken from P minute Th6 baths of Examples IX and X contain no the group consisting of formic and glycolic acids and fluOfine- Bath of Example IV contains glycolate ions 0.3 to 4 gram-moles of at least one alkali metal salt but no formate ions whereas the bath of Example V conf an i t k f o th group consisting of formic, thins formate but 110 glycolate ions, and bath of EXamglycolic and acids having a dissociation constant of at ple VI contains both formate and glycolate ions.
  • An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of at least one carboxylic acid taken from the group consisting of formic and glycolic acids and 0.3 to 4 gram-moles of an alkali metal formate.
  • An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of formic acid and 0.3 to 4 gram-moles of an alkali metal formate.
  • An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to l gram-mole of glycolic acid and 0.3 to 4 gram-moles of an alkali metal glycolate.
  • An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, Oil to 2 gram-moles of a divalent chromium salt, 0.3 to 1 grammole of formic acid'and 0.3 to 4 gram-moles of an alkali metal salt of an acid having a dissociation constant of at least 1 l0 7.
  • An aqueouselectrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g.
  • bath 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of formic acid as the sole carboxylic acid in the bath and 0.3 to 4 gram-moles of an alkali metal formats as the sole carboxylafe in the bath, said bath containing less than 10 ppm. of a material taken from the group consisting of sulfur and selenium.
  • Chromium electroplate having a tensile strength exceeding 200,000 lbs. per sq. in. and having an electri-cial resistivity of 149i9 l0 ohm cm.
  • the process of electroplating a bright chrmium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g.
  • the process of electrop'ating a bright chrom'um electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathcde in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g.
  • bath 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of at least one carboxylic acid taken from the group consisting of formic and glycolic acids and 0.3 to 4 gram-moles of an alkali metal formatc.
  • the process of electroplating a bright chromium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of ta h,
  • the process of electroplating a bright chromium electroplate which comprises passing a direct electric current with a current density of 20 to 450, amps /sq. ft. between an inert anode and ametallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and.
  • the process of electrcplating a bright chromium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g.

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  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
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US126623A 1961-07-25 1961-07-25 Chromium plating Expired - Lifetime US3069333A (en)

Priority Applications (7)

Application Number Priority Date Filing Date Title
NL268828D NL268828A (de) 1961-07-25
NL132271D NL132271C (de) 1961-07-25
BE625233D BE625233A (de) 1961-07-25
US126623A US3069333A (en) 1961-07-25 1961-07-25 Chromium plating
GB30990/61A GB981481A (en) 1961-07-25 1961-08-28 Improvements in or relating to electroplating
FR872068A FR1309239A (fr) 1961-07-25 1961-08-31 Procédé de chromage
DEP27799A DE1247801B (de) 1961-07-25 1961-08-31 Bad und Verfahren zur galvanischen Herstellung von Glanzchromueberzuegen und -folien

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BE (1) BE625233A (de)
DE (1) DE1247801B (de)
FR (1) FR1309239A (de)
GB (1) GB981481A (de)
NL (2) NL132271C (de)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3203876A (en) * 1959-10-07 1965-08-31 Du Pont Process for preparing chromium film products
US3713999A (en) * 1969-10-10 1973-01-30 Permalite Chem Ltd Electrodeposition of chromium
US4690735A (en) * 1986-02-04 1987-09-01 University Of Florida Electrolytic bath compositions and method for electrodeposition of amorphous chromium

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB292094A (en) * 1928-03-26 1929-08-08 Ternstedt Mfg Co Improvements in chromium plating
US1922853A (en) * 1927-12-01 1933-08-15 United Chromium Inc Process for the electrolytic deposition of chromium
US1975239A (en) * 1929-10-16 1934-10-02 Siemens Ag Method of chromium plating

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1922853A (en) * 1927-12-01 1933-08-15 United Chromium Inc Process for the electrolytic deposition of chromium
GB292094A (en) * 1928-03-26 1929-08-08 Ternstedt Mfg Co Improvements in chromium plating
US1975239A (en) * 1929-10-16 1934-10-02 Siemens Ag Method of chromium plating

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3203876A (en) * 1959-10-07 1965-08-31 Du Pont Process for preparing chromium film products
US3713999A (en) * 1969-10-10 1973-01-30 Permalite Chem Ltd Electrodeposition of chromium
US4690735A (en) * 1986-02-04 1987-09-01 University Of Florida Electrolytic bath compositions and method for electrodeposition of amorphous chromium

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DE1247801B (de) 1967-08-17
FR1309239A (fr) 1962-11-16
GB981481A (en) 1965-01-27
BE625233A (de)
NL132271C (de)
NL268828A (de)

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