CA1061956A - Aluminium polishing compositions - Google Patents

Abstract

S P E C I F I C A T I O N

ALUMINIUM POLISHING COMPOSITIONS

A B S T R A C T

Aluminium polishing solutions containing phosphoric, nitric and sulphuric acids provide etched finishes If the proportion of sulphuric acid is increased.
The invention inhibits such etching by addition to the bath of an aromatic ring compound, in which at least 2 hetero atoms are conjugated with the ring such as benztriazole.

Description

9S~i The present invention relates to aluminium polishing compositions of the type which comprise a mixture of phosphoric and nitric acid and in particular those which additionally contain sulphuric acid.
S The use of compositions of the above type is well known.
Typically the essential ingredients are phosphoric and nitric acids, but because of the high cost of phosphoric acid it has often been found comMercially advantageous to substitute cheaper sulphuric acid for a part of the phosphoric acid.
Typical polishing compositions of this type comprise about 70 - 76% by weight of phosphoric acid (specific gravity = 1.75) about 15 ~ 20% concentrated sulphuric acid, about 5% concentrated nitric acid (specific gravity 1.50). Polishing baths also contain a small amount e.g~ about 0.1% of heavy metal, which has been found necessary to provide a bright~ attractive finish. Generally the heavy metal may be copper, nickel or iron, however in baths containing a substantial amount of sulphuric acid, the use of copper has been found essential. The compositions may also conveniently contain a s~all amount of boric acid and a wetting agent. Ammonium and substituted ammonium have also been included in aluminium - polishing solutions to inhibit fuming.
It would be economically advantageous to lncrease the proportion of sulphur~c acid, but a particular problem, common to phosphoric/
sulphuric/nitric acid polishing compositions has prevented the commercial introduction of any composition containing more than about 24% by weight of sulphuric acid i.e. one part by weight of commercial concentrated (s.g. 1.84) sulphuric acid to three parts concentrated (s.g. 1.75) phosphoric acid, This problem is called transfer etchll.

-2-~ ~ 3 ~ ~o6~56 Transfer etch occurs when the pollshed work is removed from the polishlng bath and dralned preparatory to betng transferred to the next treatment stage ~usually a rtnstng stage). If the work 15 allowed to dratn for too long, an unstghtly, whtte, etched effect mars the surface of the work. In baths contatnlng a hlgh proportton of phosphortc acld the onset of transfer etch ts generally sufftctently slow for tt to be practlcal to transfer work before slgntflcant etchlng can occur.
Howevsr, if the proportion of sulphurlc acid Is increased, the onset of transfer etch becomes more raptd, shortentng the permisstble time avaitable for transferring the work unttl eventually lt ts Impossible 7n practtce to polish the work wlthout a qulte unacceptable degree of etching. Gen~rally transfer etch becomes a serlous problem when the proportton of sulphuric acld to phosphorlc acid In the bath exceeds about I : 3 (measured as parts by wetght of +he commerclal, concentrated actds).
We have now dtscovered that certaln aromatlc organic compounds have a beneficlal effect Tn reducing the occurrence of transfer etch In alumlnium polish7ng solutlons. The presence of such etch inhlbttors therefore permlts the proportlon o~ sulphurlc acld In an aluminlum pollshing solution to be substanttally Increased.
Our Inventlon therefore provldes an alumlnlum pollshlng solutlon comprlsing phosphorlc acld, nltrlc acid, sulphuric acld and dlssolved copper, whtch addltionally comprises as an etch Inhlbltor, an organlc compound comprtstng an aromatlc rlng havlng at least two hetero atoms conjugated therewlth.
The etch inhlbltor may be any aromal-lc rlng compound (tncludtng heteroaromattc rtngs~ whtch has at least two hetero atoms tn or conjugated wlth the aromattc rlng. The aromattc rtng Is preferably a benzene rlng but may alternattvely be a naphthalene rlng or a ~ 4 ~ 1~61~56 pyridine, pyrazine or other heteroaromatlc r7ng. The hetero-atoms are preferably nitrogen, oxygen or sulphur atoms havlng electron palrs conJugated wlth the aromatlc rTng.

i~ecause oF the aggresstve nature of the pollshing/wY~h Its strongly acldic and nltrattng characterJ ihe etch Inhibitor effect-ively present in the ~iution must be sufficlently stable to wlth~
stand such a hostile medlum. Thls we have found Is suffTctently achleved by the presence of an aromattc r7ng system wlth its resonance stabillsed structure. The resonance must extend to cover at least two hetero atoms whtch are capable of formTng complexes and which are conjugated (or conJugable~ ln or wtth the rlng. However~
in vlew of the chemically actlve nature of solutlon, the etch inhibltor effectlvely present In the compos7tion wlll In many instances differ apprectably from the compound orlginally added.
Any compound which is converted by the medlum to provlde an etch tnhibitor as herelnbefore deftned may be used as a precursor. For example compounds whlch possess the essential ring nucleus togeher with oxldisable, or simllarly vulnerable substltuen-i groups may be used, If In practice the unstable substituents are destroyed to leave the effective nucleus with its conjugated hetero atoms. The hetero atoms may form part of any group whtch does not preclude them from conJugattng with, or as part oF, the aromattc ring. The nltration of the aromatic nucleus by the medium has not been found to have an adverse effect on the performance of the etch Inhlbttor.
For exampie, when benztrtazole, one of our preferred etch tnhlbltors, ts added to the composttlon, the Inittal reddish colour Is gradually replaced by a green colouratlon assoclated with the nitration of the benztrTazol, but the performance of the etch InhlbTtor Is not impaTred.

- 5 ~ ~ ~61956 The hetero atom may be part oF an amlno or Imino group, hydroxyl group, the keto group of a quin ne, or a het~rocycllc ring, such as trlazole, thlazole or thladlazola rlng.
Preferably the etch Inh1bitor has a benzene or benzo ring fused to a hetarocycltc rTng~ e.g. a flve membered heterocycltc rlng, for example, benztrtazole ~' Is partlcularly effective.

Substituted banztriazoles In which the benzene nucleus Is substTtuted wtth for example, hydroxy~ alkoxy, amino, nitro, or alkyl groups are also operative as are halo-substltuted benztrlazoles. Other trlazole compounds whlch may be used Include napthalene trlazole and naphthalene blstrlazole.
~0 Benzofuroxan ~ ~ and substltuted benzofuroxans such as ~ ~N~

nltro9 hydroxy alkoxy, amlno , alkyl or halo benzofuroxans are 21s0 hlghly effectlve~ Other compounds whlch are partlcularly effectlve Include benzthiadlazole ~ ~ substituted benzthtadlazoles, ~`~

benzthlazole and substltuted benzthlazoles Includlng benzthlazoles of the formula ~ ~ C R where R is hydrogen or an alkyl, hydroxy, alkoxy, amlno, mercapto) alkyl sulphlde or other group, e.g.
2-mercapto benzthlazole. Benzlmldazole and substltuted benzlmldazoles of the formula ~ ~ C Rwhere R has the same signlflcance as H
prevlously9 are effectlve as are benzimidazoles havlng substltuents on the benzene rlng. Ben~oxazole, substltuted benzoxazoles of the formula ~ ~C Rwhere R has the same slgnlflcance as before and nuclear substltuted benzoxazoles are slmllarly effective as etch inhlbltors.

. ,~

Di and poly substituted benzenes in which at least two substltuent groups are selected from amlno, nltroJ hydroxy and atkoxy groups are effective, such as o. phenylene dlamlne, o. amlno phenol, m. phenylene dtamine, catechol, dinitrobenzene.
Simllarly dl- and poly-substl-iuted naphthalenesJ such as teira amino naphthalenes are effective. Diaminonaphthalenes would doubtless be effective but have been avoided because c~f the rlsks of carc~Dgan!c actlon o. and p. benzoqulnone and ~eir mono and d1 Imines are effective, and so are heteroaromatic compounds contalnlng one mare or/hetero atom in an aromatic ring system, such as, for example, hetero-substituted pyrtdlnes, pyrazine, substltuted pyraz7nes and melamlne. `
From the foregoing it wlll be apparent that a very great varlety of compounds wlll be effectlve as etch Inhlbitors according to our inventton. In many instances the foregoing compounds wlll undergo chemical changes In the so!ution e.g. nltratton, oxldation or coupling to form azo compounds, so that the effectlve etch Inhlbitor present In the solutlon may dtffer from the compound added to the compositlon, For thts reason it is often posslbie to Inhtbit transfer etch by add7ng to the composition a compound which ts not itself an etch Inhibltor as deflned above, but whlch Is a precursor, converted to an etch Inhlbltor in sltu by the acTdTc medlum.
EffictTve etch tnhTbitors are readily TdentTfted by the presence of an aromatic ring system (usually, but no-i essentiatly~ a slx carbon rTng) which ~s stable ln the hTghly acldic medlum, and at least two hetero atoms conJugated or conJugable ~ith the rTng.
An aromatTc system Ts essentTal for stablllty Tn the a~gressTve polTshlng solutTon. Compounds lacking an aromatTc rTng system, such ~ 7 ~ 1 ~ 6 ~ 9 5 6 as thiazole, thtadiazole, dlmercaptothiadiazole or triazole are lneffective, probably due to instabillty In the medium. At least two hetero atoms~ preferably nitrogen, oxygen or sulphur, especially nltrogen, stablllsed by conJugatlon wlth rlng, are necessary, probably to provlde chela-i-lng power.
The etch inhibTtor Is preferably present In a proportion of from 0.05~ by welght up to 0.7~ or hlgher. Proportlons above 0.5%, although not harmful, are usually unnecessary and therefore undestrable on commerclal grounds. Proportions less than 0.05~
usually glve Insufficlent Inhibitton of transfer etch. Generally It Is deslrable to use hTgher proportlons of the etch Inhlbltor in baths which have been used for some tlme, than are necessary In freshly prepared baths. For example, baths contaln1ng less than about 30 gm per litre dissolved aluminium work satlsfactorily with from 2 to 4 gm per litre of etch Inhibitor, while baths contalnlng more than 30 gm per lltre aluminlum may conveniently contaTn from 4 to 6 gm per litre ~ the etch inhibitr.
The proportlon of nltric acid in the baths of our Invention may typlcally be the same as in conventlonal aiuminlum pollshing baths, e.g. 3 to 10% by volume as concentrated (s.g. - 1.42) nitrTc acid, or from 1.2 to 4.2% by weight of 100~ nitric acTd.
It Is preferred to adjust the proportlon of nitrlc acid in accord-ance with the aluminium content of the solutton. Typically a freshly prepared bath Ts In the upper part and fully aged bath (at equilTbrium) is in the lower part,of a preferred range of from - 4 to 8% v/v concentrated acid. Preferably the proportlon of 100%nltric acTd Ts 1.6 to 3.5$ by weTght and most preferably between 2.4 and 3.1%. The proportion of phosphoric to sulphurlc acid in the baths of our invention may be as low as I : 2 by weight, measured as the commercTal concentrated acTds T.e. 40~ P205 . j ..

1~6~956 phosphorlc acld ~s.g. = 1.75) and 98~ sulphurlc acld (s.g. 1.8~).
Lower propor-tions are preferably avotded due to the rtsk of reductng the sulphurtc actd9 gtvtng rese to fumes of S02 and H2SJ the deposttton of sulphur on the work, and the preclpltatlon of copper sulphtde.

The maxtmum Is not crttlcal and may for example be up to

3 : I or even htgher. However such hlgh proporttons are undesirable on econlmlc grounds. Moreover, transfer etch ts not such a sertous problem at htgh phosphortc actd levels. We therefore prefer to employ proportlons of phosphortc to sulphurtc iess than 3 : 1, e.g. 1.5 : to I : 1.5, preferably 1.2 : I to I : 1.2, typically I : 1. Sulphurtc and phosphoric actd together usually constltute at least 90~, preferably at least 93% e.g. at least 95% of the welght of the composttton.

k5 The proportton of water Is not crttlcal and is usually below 5~ by wetght. It the composltlon is formulated In the usual way, ustng the ordinary technlcal, concentrated phosphoric, nltrtc and sulphuric acids, whtch contatn small amounts of water, tt Is no~
normally necessary to add any further water. However if the proportton of phosphoric acld Is hlgh and/or the alumintum content rtses to a htgh level~ It may be necessary to add water to prevent the precipitatlon of alumlnlum phosphate. If the proportton of water ts too high, there ts a decltne tn specularlty. Accordtngly tt ts praferred to add the mlnTmum amount of water required to prevent preclpltatlon of alumtnlum phosphate, whtIst malntalntng good speculartty.

- 9 - 1~16~L956 `~ The baths of our Invention contain copper as an essentlal ingredtebt9 e.g. In a proportion o~ up to 0.2~ by wei~h-l, preferably 0.01$ to 0.16%. most preferably Ool to 0.15/o. The copper may conveniently be Inlroduced by adding a copper salt~
preferably of one of the acid anTons of the system, for example from I to 10 9 per Ittre~ preferably 4 to 5 9 per litre of hydrated copper sulphate.
Polishtng baths of the present invention may optlonally contain ammonlum or substttuted ammonium, In order to reduce fuming.
For example9 the bath may contain between 0.05 and 0.75 molar of ammontum or substituted ammonium~ preferably 0.2 to 0.~ molar.
The concentration may conveniently be Increased to excess of 0.75 molar9 or 100 gpl expressed as (N~14)2S0~, in replenlshtng solutions in order to matntain th~ concentratlon of ammonium or substltuted ammoniurn In the bath at l-rs optimum working level.
The ammoniur, or substitoted ammonium is preferably added as fhe ammonium salt if one o-f the acid components of the bath~ e;g.
ammoniunl sulphate or diammonium phosphate. Compositions of our invention may also optionally contain some boric acld.
In additlon to the foregoing components, polishtng baths conven+lonally contain wetting a9~nts9 and these are also preferably presebt In our novel bath. Any of the wetting agents used hitherto In polishTng baths may be employed for example~
non-iontc surfactants, such as al~yl polyethers. The wetting agent is normally present in trace quantittes of for example up to 0.01 although higher proportions may be used.
After a perlod of use the bath also accumulates dissolved aluminium9 which typlcally rises to an equTlibrium value9 when fresh dissolution of aluminium in the bath is balanced by dragout losses. The equilibrium value depends to some extent upon the condttions of the operation of the bath but under normal condlttons `~ is about 30 gms - 50 gms aluminium per litre of solution.

- io- ~L~6~9S6 The normal operatlng temperature of our novet baths Is about 90C to 115C.
Our novel baths may be used to polish alumlnlum and a wlde varlety of alumlnlum contalntng alloys.
The work Is typlcally Immersed for from 0.5 to 5 mlnutes, depending on the alloy~ most usually about 3 mlnut0s.
It Ts also possible to use our novel solu~ons for electro-polIshlng. The work, usually after a prelImlnary perlod of immerslon in the bath, is made anodic wlth respect to the tank contalning the bath or a separate electrode Immersed In the bath.
The bath may be maintalned by periodlc topping up with fresh solutlon to replace drag out losses. Occaslonal addltlons of nltrlc acld or water to make good losses due to evaporation may be required.
The inventlon will be illustrated by the following examples:-EXAMPLE I
A chemlcal polTshing solution was prepared containing 45i~
w/w H3P04~1.75 s.g.), 50~ w/w H2S04 (1.~4 s.g.), 1.5~ w/w diammonium phosphate, 0.25~ w/w copper sulphate, 2~ nltric acTd (1.50 s.g.), the rest being water The bath was aged to 30 gpl Al by dissolving aluminlum and the nltric acid content readjusted to 2% w/w.
Components of HE9 alloy and BA 211 brlght trim alloy were pollshed in this bath for 3 minuteS at 100 C and subiected to various drainage times before rTnsing In hot water. It was found that at draTnage ~imes greater than ten seconds a grey 'transfer etch1 appeared on the upper surface of components and could not be removed In 50' nTtric acid desmutting solutton.

I, lV~9S6 To the above polishlng solution 3 9 per lltre benztriazole was added and the tests carried out again. Transfer etch appeared only after a dralnage time of 25 to 30 seconds, In contrast to the above solut70n wlthout benztrlazole. The solutton was used for pollshing until the aluminium content rose to 35 gpl and a further 2 gpl benztriazole was added. This solutlon continued to give good resuJts and no loss of benztriazole coutd be cietected. The solution was maintained in the usual way by add7ng fresh polishing solution and nltrlc acid as requlred. The replenlshlng solutlon contalned 5 gpl benztrlazole.
EXAMPLE 2.

-Composltion of polishlng solution employed:

Constituent w/w %

H3P04 s.g. 1.75 56.0 H2S04 s.g. 1.84 38.5 HN03 s.g. 1.50 3.4 CuS04 SH20 0.25 ~2 1.85 s.g. after agelng 1.80 Samples of thls composltlon were aged) I.e. theTr alumTnTumcontents were raTsed to 30 9/1 Al~ a typTcal concentratlon found Tn workTng aluminTum chemTcal polTshlng solutTonsO

~ir)6~ 6 A sample of the aged polishing solutiorl was heated to 105C and adjusted to the optimum/nitric acid content of 3ZO ~/W SG 1.50 acid.
Test pieces of an aluminium alloy suitable for chemical polishing ~BA 211) were treated for 2 minutes by immersion in the s~lution whilst gently agitated. These test pieces were drained in air for (i) ~ I second and (ii) 30 seconds before rinsing. The short draining time was too short for the transfer etch to manifest itself and was taken as a standard that the particular solution sampl~
was performing satisfactorilyO A transfer time of 30 seconds is the longest used in commercial practice and in solutions of the above composition produced a complete coating of light grey transfer etch over the whole surface of the test piece.

The compound to be tested was added t~ the sample in incremen~s of 1 gpl and between such addition, after complete dissolution~
test pieces were treated as above and drained in air for 30 seconds before rinsing in water~ The efficiency of the compound at each concentration was estinated by visual estimation of the proportion of the area of the test piece covered with transfer etch to the nearest 10h. Additions were carried on until:-(i) 100~/o removal of transfer etch was obtained;
20 (ii) The transfer etch reached a ~inimum which was not reduced by subsequent additions;
(iii) ~o effect was observed in reducing transfer etch and additions totalled 10 gpl.
1~ 2, 3 Benztriazole itself has been tested up to 50 gpl without any further effect upon performance being observed after complete suppression of transfer etch at 5 gpl~

- 13 - ~ ~ 6 ~ ~ 5 6 The compounds are llsted In decreaslng order of Image clarlty tspecular brlghtness) of the flnish and increaslng or~r of transfer etch.
... _ .. . . _ , __ _ ~ _ . __ . _.. _._ .. ___ _ . __ _.. ...
COMPOUND FORMULA CONCENTRATION
% REDUCTION IN

TRANSFER ETCH
. . ... .. _ . . _ _ ._......... .. _ ..
BEI~ZT~IAZOLE ~ ~ N~N 5 9PI 100 N
H _ _ _ _ ,o N
~2 1 ~
BENZOTHIADIAZOLE I
2 9PI 100%
~ J ;~N
.. .. . _ ..

so~
f NH2 `~
~H2 M-PHENYLENEDIAMINE ~ NH
I 9PI 90%

~,/
CATECHOL ~ ~y~OH I 9PI 90~

~ J~ OH
, _~ , _ _ _ _ _ . . . . .
O-AMIN~PHENOL ~ f NH2 901L, I~
__ . ~

~ 14 1~6~95~
COMPOUND FORMULA CONCENTRATION '~ REDUCTION IN
' l _N_r-r~ ~,T, 2-MERCAPTOBENZTHIAZOLE ~ / N
~MBT~ ~ S ~ C-SH I 9PI 90 H

_ _ , , , ,, _ ,, ,, _ , .. ..

(MBT) ~ H ~ -SH I gpl 80 2-MERCAPTOBENZOXAZOLE ~ I gpl 70d ~ j C SH

MELAMINE 4 gpl 60%
NH2~ff NH2 N~
NH
CO~IPAR~TIVE EXAMPIF , . _ _ TRIAZOLE 10 9pl 01,~
HN~ -N

~ _ .
10 gpl 0 2, 5 - DIMERCAPTO
7 37 4 - THIADIAZQLE ~ N
HS S - SH
PENTAMETHYLENE TE`rRAZ~L~ 10 gpl 0`%
N

N -~N

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An aluminium polishing bath consisting essentially of:-(a) phosphoric and sulphuric acid in a relative proportion of from 1:2 to 3:1, measured as 40 percent P2O5 phosphoric acid and 98 percent sulphuric acid and together constitutingat least 90 percent of the total composition;

(b) nitric acid in a proportion by weight of from 1.2 to 4.2 percent as 100 percent nitric acid;

(c) dissolved copper in a concentration of from 0.01 to 0.2 percent by weight;

(d) dissolved aluminium in a concentration of between zero and saturation;

(e) from 0.05 percent to 0.7 percent by weight of an organic etch inhibitor which is a bath soluble organic compound having an aromatic ring system and at least 2 hetero atoms selected from the group constituting nitrogen, oxygen and sulphur atoms conjugated with said eromatic ring system;

(f) the balance substantially of water.

2. An aluminium polishing solution as claimed in claim 1 wherein the etch inhibitor is an aromatic organic compound having an aromatic 6-carbon ring and at least 2 hetero atoms selected from nitrogen, oxygen and sulphur conjugated with said ring.

3. An aluminium polishing solution as claimed in claim 2 wherein the hetero atoms are present in any member of the group selected from amino-, imino-, hydroxy-, alkoxy- and quinone groups and a heterocyclic ring.

4. An aluminium polishing solution as claimed in claim 3 wherein the etch inhibitor is selected from the group consisting of hydroxy-, amino-, alkoxy-, nitro-, alkyl-, and halo- substituted benztriazoles.

5. An aluminium polishing solution as claimed in claim 3 wherein the etch inhibitor is benztriazole.

6. An aluminium polishing solution as claimed ln claim 3 wherein the etch imhibitor is selected from unsubstituted and nitro-, hydroxy-, alkoxy-, amino-, mercapto-, alkyl- and halo, substituted members of the group consisting of benzofuroxan, benzthiadiazole, benzthiazole, benzoxazole, and benzimidazole.

7. An aluminium polishing solution as claimed in claim l wherein the etch inhibitor is a substituted benzene having at least 2 substituent groups selected from amino-, nitro-, hydroxy-, and alkoxy groups.

8. An aluminium polishing solution as claimed in claim 1 wherein the etch inhibitor is a benzoquinone, or imine thereof.

9. An aluminium polishing solution as claimed in claim 1 containing from 0.05 to 0.75 molar ammonia.