CA1057231A - Chromium plating process - Google Patents

Abstract

NOVEL CHROMIUM PLATING PROCESS

Abstract of the Disclosure - In accordance with one of its aspects, this invention relates to novel compositions and to a process for electroplating chromium plate onto a basis metal which comprises passing current from an anode to a cathode at least a portion of which contains a conductive metal layer through an aqueous acidic chromium plating solution, at least one chromium compound providing hexavalent chromium ions for electroplating chromium and aluminum ions in combination with fluoride ions and sulfate ions and maintaining a molar ratio of total fluoride to total aluminum of from 0.5 to 6, for a time sufficient to deposit a chromium electroplate.
In accordance with another of its aspects, this invention relates to novel compositions and to a process for electroplating chromium plate onto a basis metal which comprises passing current from an anode to a cathode at least a portion of which contains a conductive metal layer through an aqueous acidic chromium plating solution containing at least one chromium compound providing hexavalent chromium ions for electroplating chromium, sulfate ions, fluoride ions, complex fluoride ions, wherein at least one member selected from the group consisting of AlF3 Al2F6 AlF3XH2O Na3AlF6 and Al2F6XH2O, where X is a lower integer, is maintained in the solid phase for a time sufficient to deposit a chromium electroplate.

- i -

Description

K~IW(102ll)J~

~ 23~

NOVEL CHROMIUM PLATING PROCESS

Thls invention relates to novel compositlons and to novel processes for the electrodeposition of chromium ~rom an aqueous acidic chromium plating bath.

BACKGROUND OF THE INVENTION

In the present art of plating there is great hesitakion -in chromium plating any basis metal containing aluminum~
~luminum alloys or even alumlnum-bearing alloys such as zinc-base ~ die castings with high-speed (fluoride containing) baths. In ,~ the plating of these metals, it ils usual to use chromium plating baths containing only the sulfate ion as a catalyst because fluoride or complex fluorlde ions cause more rapid dissolution of the aluminum-containing metal. Increased etching from the presence o~ fluorides or complex ~luorides in the chromium plating bath has two notable e~fects~ (1) exposed metal may show an etched or dull appearahce and (2) the chromium bath may actually manifest in passive deposits, reduced plating efficiency ~and bad composition balance. The desirabllity of having fluorideæ or complex fluorides present for high-speed plating ~in~hard and decorBtive plating and for good activation stainless steels and some metals is well known.
Heretofore inclusion of ~luminum ln chromium plating ~baths~has been~considered undesirable because of the debilitating ~e~fects characterized as white streaks~ blotches in the high ~and intermediate ourrent density ranges and brown or iridescent ; 25 films in the low current density ranges as well as l!OSS in ~ :

, ' :
i', , ~
.,.' . ' . . , ., .. ,. . .
". , ,,,.............. _, _ .

~57Z3~

cathode efficiency. The concentration of aluminum in the chromium plating bath continually varies during operation due to the addition of aluminum impurities arising from the catalyst and basis metal decomposition products.
It is an object of this invention to provide improved electro-plating compositions and processes. A further object of the invention is to provide stabilized chromium plating bath compositions. Other objects of the invention will be apparent to those skilled in the art upon inspection of the following detailed description of the invention.
In accordance with one of its aspects, this invention relates to a process for electroplating chromium plate onto a basis metal which com-prises passing current from an anode to a cathode, at least a portion of which contains a conductive metal layer through an aqueous acidic chromium plating solution, at least one chromium compound providing hexavalent chromium ions for electroplating chromium, and aluminum ions in combination with fluoride ions and sulfate ions, maintaining a molar ratio of total 1uoride to total aluminum of from 0.5 to 6, for a time sufficient to deposit :~ .
~. . ~ : ' .. . . . .
.. ~ .
} ! . ~
'`' ~ . ': ' `~.. : : '. ' . :
: .: ~ . -,'' : : ' ' ,`'`: :
```, '', '; ~ ' ' :
~` ` ' ' '~ ' :
`,' ' ' :
. ': ' .
.. ~ .~ .. ...
, '~ ` ''" ' ' ' 1~57~3~1;

a chromium electroplate.
According to another aspect of the invention, there is provided a composition suitable for electroplating chromium plate onto a basis metal, the composition being an aqueous acidic chromium plating solution, contain~
ing at least one chromium compound providing hexavalent chromium ions for electroplating chromium, and aluminum ions in combination with fluoride ions and sulfate ions maintaining a molar ratio of total fluoride to total aluminum of from 0.5 to less than 6.

;
' ~

" ' .

~ :, . .
~ ~' ','.":

, . -3-' : ,.

.
' ~, ~ 1057231 It has now been found that the addition of aluminum ions in combination with fluoride and sulfate catalyst ions in an aqueous acidic bath containing chromic acid results in a chromium electro-~ plating composition which is stable over the life of the bath and -~ maintains the activi~y of the bath over its useful life by regulating the concentrations of the aluminum ions and fluoride ions as the ehromic acid eoncentration is depleted and in the presence of typical impurities ` and decomposition products which occur during the chromium plating ` operation.
Preferred basis metals are stainless steels, aluminum alloys and aluminum. Other metal articles which may be plated in accordance with this invention include niekel, nieklsl-eobalt, and eopper plated `` ~ine allo~ articles.
~. . .
~; ,., ' ' . , .
;~ . .
. ' '' .
. ~;
.. , ~ .
, ~ ..
, .
....
. i ,~

. . . . : : . .
"' ' ' ~, ,`f '~ "'"

."' ' ' .
:``~' ." ',. .
' :' ,, _ ' ,.' :

:, ' ' .
. . .

1057Z3~

The fluoride to be added to nullify excess aluminum may be a simple fluorlde such as is obtained from hydrofluoric acid, alkali or alkaline earth fluorides and other fluoride salts or may be a complex fluoride such as silicofluoride and other complex fluorides which will furnish fluoride ion in solution.
Other complex fluoride ions which may be utilized in the operation of the bath may include ions of BF4-l, AlF6-3, TiF6-2, ZrF6~2, etc.
The baths are characterized by the presence of aluminum and fluoride in a ratio such that there are fewer than six~atoms of fluoride present per atom of aluminum. Fluoride means total soluble ~luoride, including that found in complexes.
Aluminum is total soluble aluminum. These can be determined by usual analytical methods, e.gO, aluminum by atomic absorption and fluoride by distlllation and titration. ~hus, where M
i~ the moles~litre Or aluminum and MF is the moles/litre of fluorlde, it is required thats ~ : ~ MF :' ; ~ 0.5 ~ 5-5 Al ~ -and prefer:

; MF
~ 20 1.0 ~ 4.0 ¦¦ M~l ¦
`"~',; : ~ , , ~ :
.~ .
~., ....
` i .: ~ - 5 -.. . . . . ,. . .... .. , . - - . .. , .. .' ; . . . . . : . . .
. . : . . . , ...... . ~ , ... . . ,. ~ . . , j , . " . .. . .

~IL05~;~3~

In addition the chromium plating baths must conform to the usual specifications on ratio of chromic acid to catalyst to be operative.
(CrO3) (S0-4 ~ (F-) Where y is a number depending on the complex e.g. 2 for SiF-6 and l/4 for simple F-.
The dissolution of aluminum causes a decatalyzing effect, i~e., khe presence of aluminum in solution effectively reduces the available catalyst, especially the available fluoride.
Thus, dissolving aluminum in the bath causes the deposits to be blotchyj gray, and even form ~ilms characteristic o~ a bath of too high ratio, i.e., low or reduced catalyst content. (F-) is defined as available fluoride. ~[F/MAl is less than six (6), : r as used for oomplex fluorides is appropriate, i.e., r =2.
The operable ratio may be defined as:

- CrO3 R = ~
(S0~ + (FAv3/2 ~ - .
Useful and operative baths of this invention ~all within the approximate 11mits of:
; 50 ~ R ~ 160 and pre~erably:
;~ ~ ~ 70 ~ r - 130 ~ ~ .
'.' : , ' ,.

,, ~, .~" , . .

~ 5723~ -:

The available fluoride may be determlned in either of two ways.
A series of standards are prepared containing known quantities of hydrofluoric acid or its simple non-complexing salts such as sodium fluoride or potassium fluoride. These standards may be prepared in solutions of chromic acid and at appropriate temperatures. The effect of this available- fluoride is measured in either of two ways:
(1) A selective ion electrode reversible to fluoride ion may be inserted into the baths and the emf produced recorded.
A graph of available fluoride ion concentration vs. emf is thus prepared with di~erent curves ~or dif~erent chromic acid concentrations and different temperatures. Subsequent emf measurements with the electrode in chromium plating baths then uniquely determine the available fluoride ion concentration in the bath.
~2) Standard curves can also be produced by measuring weight loss of a known area o~ aluminum immersed in a given amount o~ chromic acid solution for a given period at a given r ~ I temperature.
The determinatlon o~ the active fluoride content of an .
~acid solution containing fluoride may be effected by immersing aluminum metal in said acid solution, maintaining said aluminum metal in said solution for a predetermined time during which a ,~ ; portion o~ said aluminum metal dissolves in proportion to the ~ 25 active fluoride content of said solution, withdrawing said , - 7 -, : . ' ,.
:~" . ' ' ;~ " f'~ ~.
. " . 5 - . ' , . ' " . . ' . : . . : :

~ 723~

aluminum metal from said solution, determining the weight loss from said aluminum metal during said immersion, and converting said weight loss into values showing the concentration of active fluoride.
The determination of the active fluoride content of such a chromium plating bath may be effected by preferably isolating an aliquot test portion of the bath. Typically the aliquot portion may be lO0-lO00 ml.~ say 500 ml. Preferably the analytical test may be effected at 43C.~lC.; and accordingly the aliquot may be placed within a constant temperature water reservoir malntained at this temperature for at least one hour until water reservoir temperature is reached.
` In practice a piece of aluminum may be employed. This piece may be pure aluminum or aluminum alloy in the form of a strip or sheet, typically 0.2 to 2, say 1 mm. thick, kypically 5 mm. to 25 mm., say lO mm. wide~ and 50 mm. to 150 mm., say 75 mm~ long. The strlp may be cleaned typically by wiping lt wlth a cloth and bent into convenient U-shape which facilitates placlng the strip in the aliquot and maintaining it in exposed relation to the allquot~with minimum area of contact with the ~container. The so-treated, cleaned strlp may then be weighed to the nea~rest tenth of a milllgram. Satis~actory results may normally be obtained by use of a str-ip which typically may w-18ht ~ro 1 g. to 5 g., typloal1y about 2 g, ,'~ ` : ._ ,.. ' , 8 '': .
:,';,~ '.. ' ~57Z3~

The so-treated aluminum strlp may then be placed in central portion of the container in which the aliquot solution is maintained, so that the strip is standing up in the center of the container. The strip may be maintained therein for an exact predetermined period of time, typically 10 minutes to 60 minutes, and preferably for exactly 30 minutes. During this time, the solution is maintained preferably at the hereinbefore noted temperature of about 43C. i.e. 43C.+1C., and no agitation is provided other than that generated by the reaction of the aluminum with the solution. ~
~ The chromium plating bath which may be employed in practice of this invention may be an aqueous solution containing 10 g./1.-500 g./l., typically 15t) g./1.-300 g./l., say 225 g.il.
~chromic acid CrO3. The bath may also contain catalysts, as herein defined, the ratio of chromic acid to catalyst typically being from 10:1 to 150:1, pre~erably 50:1 to 100:1, say 80:1.
The ratio as the term is used in this appllcation, refers to the ratio of ~-CrO

~ ~ - ( SO I, ) + ~i7 i~ 20~ wherein each of the quantities i6 specified in g./l. ~he symbol ~F- may refer to the fluorlde-con~ainlng catalyst which may be, ;~ e.g. fluoride se or silioofluoride SiF6=, or other ~luoride-- contalning ions as set forth infra. ~ is an integer which may be ~ two ror fluoride complexes,~e.g. SiF6= and 0.25 for fluoride ion :! 25 ~e. Typically the concentration of catalyst ln the bath may be :,~ . , . . , : .
'., ,.. , , g _ ~57231 0.7-50.0 g./l., preferably 1.5-6.0 g./l. The catalyst may include sulfate S04-~ typically provided as sulfuric acid, strontium sulfate, etc. The sulfate ion may typically comprise 5%-85%, preferably 10%-50%, say 30% of the total catalyst concentration. It is a particular feature of this invention that the chromium plating bath contain fluoride 3 typically supplied as fluoride ion F- or as a complex fluoride. Typically complex fluorides may include fluoroaluminates, fluorozirconates~
fluorotitanates, fluoroborates, etc. The preferred fluoride ion may be silicofluoride SiF6-, preferably provided as potassium silicofluoride. The preferred catalyst may comprise fluoride in amount of 15%-95%, typically 50%-90%, say 70% o~
the total catalyst.
In the practice of this invention, the temperature o~ plating may be 27C.-90C., typically 50C.-70C., say 60C.
; The chromium electroplating bath compositions o~
the invention may contain about lon-500 ~ . of chromlc acid (expressed as CrO3) and preferably about 200-350 e-/l f ;
~20 ~ chromic acid. The fluoride ions may be added to the chromium ~electroplating bath in the form of compounds su~h as sodium sllicofluoride (Na2SiF6), fluosilicic acid (H2SiF6), strontlum silicofluoride (SrSiF6), ammonium silicofluoride [(NH4)~SiF6], magneslum silicofluoride~(MgSiF6), calcium ailicofluoride (CaSiF6) ¦¦ cto. Obhe luoride compound~ whioh may be y~ed in the bath ''` , : i ' ' ' :'' ;.; - - 10 - ' `
:'' ',''` . .' . ~

~057Z3~ ~

according to the invention include sodium flu~ride~ potassium fluo.ride, calcium fluoride, hydrofluoric acid, ceric fluoride, cerous fluoride, etc. Other fluoride lons (including complex fluoride ions) which may be employed include, for exampleg fluoalum,nates, fluoborates, fluotitanates, and fluozirconates.
The sulfate ions may be added to the chromium -~
electroplating bath in the form of suitable sulfate compounds such as strontium sulfate (SrSO4); sulfuric acid (H2SO4);
lithium sulfate (Li2SO4); ammonium sulfate [(NH4) 2S4 ]; .
calcium sulfate (CaS04); etc.
The chromium electroplating process may use temperatures of 30-70C. with a chromic acid bath containing 100-600 g./l. of chromic acid (as CrO3). The ratio of chromic acid to sul*ate ion (CrO9:SO~~) may be malntained at 100-550:1, typically ;~ 15 150-300;1 and preferably about 200:1. :
The:Pollowing examples are submitted for the purpose o~ illustration only and are not to be ~onstrued as limiting the ~scope of the invention in any way. - ~ .
~: : ~ The unexpectedly advantageous properties obtained by 20~ ;using the aluminum ion in combination with fluorldes (including ~complex Pluorldes) may be shown by reference to the examples ~;: summarised in Table ~I. In~each experiment, a sparingly soluble ;~ ~fluorlde salt was Pormed and enough oP the compound was used ~n the bath to provide an excess of undissoIved compound. Thus, : 25 ~ an~alumlnum ~luorlde-oontalning precipi~ate remained undissolved ~ in each solution and acted as a reservoir to control the :
:~ concentratlon of:~the catalytic 10n7 . In each o~ the bath compositions described in Table I, excellent chromium deposlts were obkalned, ~. , , ~ ' . ~ ~
~ ~ ~ o~ L~ ~ ~o co co l ~ ~
__ ~: ~ X ~ ~:: ~: H o ::~
~d O O O O O O O O u~ ~ :~
~ ~5 /
_ _ _ _ __ ~ t- L~ ~ CC) ~_ P: O O ~ Ct) O~ O ~ ~r ~D
. __. ~ ~ ~ ~ ~1 ~1 I ¢ o, ~ ~ o, ~ ~ o ~ ~
~, ~ ~, ~, ,, o o _ _ _ {~ a~ o~ o~ ~ a~ o~ cs~ o~ o~
~ o ~ ~ :~ 3 ~ 3 ~ ~ =t _ _ __ _ _ _ _~
~ . L~ L~ O O O¢ rl 00 ~D ~1 ~1 1S~ ~1 ~ N co H 1~ ~1 C~l N ~1 c~.i ~i ~--i .
: ~ , ___ __ __ ~¢ O
Ç~ ~ Lt~ O L~
,. . ~: a: ~ U~ ~ O L~ ~ O~
l u~ ~ ~r L~
1~4 . . . . . . .
~` ~ _ O o O __ O o O o . ...

~1 ~1 O o ~1 N N N o ~
;~ : ¢ O O O O C- O O O O
, ~ . . _ . _ _ _ _ . _ :: o~ L~ L~ 15~ CO 0~ CO CO 0~ OD , ` : U~ ~ ri ~ r-i O O C:) O O O
. _ - _ - . . _ ,_ . :
~,- ~1 ~ o o O O O O O O o . ~ O ~ O O O O O o o o o C~ W ~ ~) ~) N N N N N N
: ~_ . .
O ___ _ _ _ i l ¦ l~-- L L l r ~ A ¦ ~O ~ ¦ ~ ~
, ,' ' - '~, ,~ .
. ~ , , .

. , ,,, : ,. . ,, , . .......... . :. .. - .

.
Test panels were plated in a plating cell and increments o~ fluosiliclc acid were added to a bath originally containing 200 g/l CrO3, o.8 g/l S04= and 7.5 g/l A1~3 (0.278 Moles).
Up to 6 g/l of added SiF6= the test panels all showed haze, blotch and brown films, the usual symptoms o~ a bath contaminated with aluminum. At 7.2 g/l fluosilicate the panels were acceptable with some high current density haze which was lost at 11 13 g/l. Excellent deposits were thenceforth plated with some loss in coverage first being noted at about 17 g/l and no untoward high current density effects were obtained until about 24 g/l atarted to show serious over-catalyzation.
Thus under these conditions 7 g/l (0.049 Moles) to 24 g/l (0.169 Moles) fluosilicate was acceptable with 11 g/l (0.077 Moles) to 15 g/1 (Ox0~105 Moles) being preferred. Note that the operative range o~ mole~ular ratios of ~luosilicate to aluminum have a range o~ 1.1 to 2.3. ~his is in contrast ~wlth usual fluoride-containing baths when as little as 1-2 g/l ~of~ excess alumlnum cause the b~th to be 1noperative.
In;a bath containing 200 g/1 CrO3~ o.s g/l S042 ab 20~ ~ 490C~ A satis~actory deposits were obtained on stainless ~teel~
~when the available~fluoride concentration was from about .9 3,7 g/1. ~

: ~ ~ ~ : :
; ~
' : . .' ~ ~ ~ ~ 13 !
'' . ' ' .
: .' .," ~ 1 . . . , . .

~L~57Z3~

EXAMPLE ll.
A bath containing 600 g/l CrO3, 2.84 g/l Na2 S04, 4.20 g/l Na3 Al F6 and 3.12 g/l Al (OH)3 by first making a dry mix of the above then dissolving in distilled water and diluting to one liter; MF 0.12 Moles, MAl o.o6 Moles, 3.2 g/l estimated (F-Av), 2.0 g/l MF/MAl, R 170, resulted in a deposit with passivlty patches.

EXAMPLE 12 (Control) Hull Cell panels were also plated using the following chromium plating bath composition:
ComponentConcentration Chromic acid ~ 600 g/1 tcrO3) Sulfate 1.92 g/1 (SO.,=). , ` Fluoride 3.2 g/l (F-) MF/MAl 2.0 g/1 . After a plating time of 3 minutes at 5 amperes the resultine deposit had passivity patches.

EXAMPLE 13.
The prooess of Example 12 was followed except that the ratio of fluoride to aluminum was adjusted to 2.9. A total plating time of 5 minutes was required to obtain an acceptable .~ ¦¦ deposlt wi a Fhrom~um co~erage of 75 mm.

. ~ ''' . ~ ' . : ' ~ 14 ., ~ _ _ ' . , .. : ,. . . .. . . . ... . ", , . ;: . .

EXAMPLE 14.
The process of Example 12 was again followed except that a bath with the following composition was used:
600 g/l CrO3, 1.92 g/l S04, MF 0.17 Moles, MAl 0.06 Moles, 4.0 g~l tF-AV) (estimated), 2.9 g/l MF/MAl~ R. 153, which resulted in a deposit with very good chromium coverage of 75 mm. ;~
600 g/l CrO3, 1.92 g/l SO", MF 0.22 Moles, MAl 0.06 Moles, 4-8 g/1 (F AV)~ 3-7 g/1 MF/MAl, R- 95, resulted in an acceptable deposlt w1th chromium coverage of 48 mm.

Make a dry mix of 150 g/l CrO3, 1,48 g/l Na2 S04 ~
1.27 g/l NaBF4. Add distilled water, dissolve, dilute to 1 liter.
Test panels - 4 amps. 3 min. 35C.
A. Original test panel, no aluminum, 51 mm chromium coverage.
B. Dissolved 4 grams of Al therein ~by adding Al (OH) 3] .
Test panel showed extensive passivation.
150 g/l CrO3, 1.0 g/l so4-2, .148 MF/MAl, 0.80 g/l (F-AV) (estimated), MF/MAl = 0.31, R. 107, plate passive.
`~ 20 CO Added 4 g/l TiF6.
Test panel good bright chromium.
150 g/l CrO3 ~ 1. O g/l S04 2, .146 MF, .148 MAl~ 2-1 g/l (F-A~T) (estimated~, ME ll~l - I.O, R. 71, chron1um coverage to 61 n ' ~ - 15 _ .' ~ . , .
' ~
,'' . . ' ,, , ,;, -. . . , . ~

~a357;231 EXAMPLE 16.
. . .. ..
Although chromic acid concentrations of less than 100-600 g/l may be used, the preferred range is 100-600 g/l~ and the most preferred range is 200-400 g/l.
In general, as the aluminum concentration increases the equilibrium solubility of the alumino fluoride remains relatively constant. There are at least two results from this fact. The bath parameters may conveniently be changed in two ways: the addition of fluoride would eventually cause precipitatlon of cryolite or the addition of aluminum could directly precipitate aluminum fluoride (likely hydrated).
Effective regulations in aluminum fluoride ratios can be obtained by changes in f'luoride concentration with or withou corresponding modifications of aluminum concentration. The -concentration of the fluoride ion may thus be suppressed by the use of an excess of aluminum ion to directly precipitate an alumino fluoride. This was verified by a solubility study on aluminum fluoride under various conditions of chromic acid oonoentration and temperature. The data is summarized in Table II.

` i~
~, : ~' "~ ' . ..
.~ : : :: ' :i, ~ ~ :
'`~ ' . '. ~
~'' ~', - 16 -! ` .
','~1" ,, . . :"

ABLE II

Concentration Temperature Solubility of Chromic Acid Aluminum Trifluoride gi~l C . ' g/l . ' ' 200. 38 12.ll 200. 49 14.7 200. 60 17.5 400. 49 14.7 100 4~ 12.l~

Thus it has been shown that aluminum fluoride has limited solubility in chromic acid solutions and in a useful range for chromium plating.

EXAMPL~ 17.
Hull Cell was used with a bath of the following composition:
ComponentCon -ntration . .
Chromic acid 200 g/l :' (CrO3) Sulfate:0.8 g/l Fluorideo.8 g/l - 3.25 g/l (F-) Aluminum0 g/l _ 4.5 g/l (Al) ~ 9.2 grams per liter of hydrofluoric acid and 4.5 grams per liter o~ aluminum were added incrementally to the bath to plate 304 stainless steel Hull Cell panels. The coverage on 304 stainless ; ~ steel psne1s lmproved from 55 to 83 mm. as the added aluminum ,, .
-. ~
.; - 17 ' -iO5'7Z;~l ~

content was increased from zero to .35 g/l. The fluoride concentration test showed a concomitant decrease in fluoride concentration from 3.25 g/l to 1.2 g/l. At 4.5 g/l excess added aluminum the fluoride decreased to o.8 g/l and the test panel showed passivity. ~hus it has been shown, unexpectedly, that a self-regulating bath can be based on the limited solubllity of AlF3 and that Al+3 can be used as a suppressant.

Further examples of the use of aluminum as a suppressant were conducted where chromium coverages of 90 mm.
on stainless were obtained from an Al~3-saturated bath with an excess aluminum addition of only 0. o8 Moles. Neutralization is not significant at this level.
, According to this invention, the aqueous acid chromium plating bath compositions of the invention may be employed at temperatures of about 30-70C., to prepare improved chromium plated art~cles by passing current ~rom an anode to a basis metal cathode through said aqueous acidic chromium plating solution at a temperature of 30-70C., for a time sufficient to deposit a chromium electroplate. Current densities which may be employed may be l.0-90 amperes per square decimeter (asd) and preferably about 3-50 asd. The bath compositions of the invention may be used with lead anodes and/or lead alloy anodes.
Although this invention has been illustrated by reference to speoific embodiments, modifications thereo~ which 7~ 25 are clearly wlthin the scope of the invention will be apparent to those skilled in the art.
., . : :

_ 1~ ~ ...
' ,~

Claims (8)

CLAIMS:

1. A process for electroplating chromium plate onto a basis metal which comprises passing current from an anode to a cathode, at least a portion of which contains a conductive metal layer through an aqueous acidic chromium plating solution, at least one chromium compound providing hexavalent chromium ions for electroplating chromium, and aluminum ions in combination with fluoride ions and sulfate ions, maintaining a molar ratio of total fluoride to total aluminum of from 0.5 to less than 6, for a time sufficient to deposit a chromium electroplate.

2. A process for electroplating chromium plate onto a basis metal as claimed in Clalm 1 wherein the aqueous acidic chromium plating solution contains about 100-600 g/l of chromic acid.

3. A process for electroplating chromium plate onto a basis metal as claimed in Claim 1 wherein the electroplating process is carried out at 30-70°C.

4. The process of Claim 1 wherein said basis metal ie stainless steel.

5. The process of Claim 1 wherein said basis metal is an aluminum alloy.

6. A composition suitable for electroplating chromium plate onto a basis metal, the composition being an aqueous acidic chromium plating solution, containing at least one chromium compound providing hexavalent chromium ions for electroplating chromium, and aluminum ions in combination with fluoride ions and sulfate ions maintaining a molar ratio of total fluoride to total aluminum of from 0.5 to less than 6.

7. A composition for electroplating chromium plate onto a basis metal as claimed in claim 6 including chromic acid in a concentration of about 100-600 g/l.

8. A composition for electroplating chromium plate onto a basis metal as claimed in claim 6 including a chromic acid to sulfate ion ratio of 100-550:1.