IL32946A - Process for colouring anodised aluminium by electrolytic deposition - Google Patents

Description

EBOCESS FOB COLOUHING LUMINIUM BY DEPOSITION This invention relates to a process for producing inorganically coloured anodic oxide coatings on aluminium aluminium and in particular to a process in which the anodic oxide coating is coloured by depositions of an oxide or hydroxide of nickel or produced by passing alternating current between an anodised aluminium workpiece and a counter immersed in an acidic bath of a nickel or cobalt salt held at a pH value above pH When operating the known process with a nickel the colour developed can be described as of a shade or tone ranging from a light colour to a very dark bronze or depending on the operating the coloured coating being one that can be sealed and which has With a cobalt the colour is somewhat ranging up to a very dark or indeed nearly black shade The present invention is in relation to the tion of an important aim being to achieve a more readily controlled and reproducible operation permitting the effective easier attainment of an entire range of including very dark in a highly reliable In the known process the is formed of the same metal as the salt in the bath a nickel electrode in a nickel salt or although in some cases certain other relatively inert materials may be employed as the A typical bath tion has comprised an aqueous solution of nickel sulphate boric acid and ammonium sulphate the bath being adjusted to have a pH in the range of about to Advantageously the bath may include a magnesium magnesium sulphate Whilst it has been suggested operate the process at pH values as high as or even in application a pH value of has been It has been found that the known process is somewhat adversely affected by a of aluminium ions in the this has been proven by experience with containing and tests have like adverse effect in the cobalt the bath can be initially constituted to be essentially free of aluminium there is a slow accumulation of aluminium salts in the bath due to solution of amounts the anodic fLlm the piece or aluminium salts being carried over from the anodlslng electrolyte in the pores of the anodic if pieces of aluminium metal accidentally fall I the aluminium salt contamination can especially if any aluminium metal should rest in contact with a thereby being subjected to alternating current In consequence effort made to avoid accumulation of aluminium salts in practical use of nickel either by care in operation or by taking special steps to purify the bath of aluminium In it was found that if more than about 10 per by of aluminium calculated as was present such a that might be called may be encountered especially if the process is operated at the higher voltage or longer time needed to achieve a very bronze that is to minute uncoloured spots or areas tend to whether or not the bath includes magnesium which is added to reduce the Somewhat higher up to may be permitted before spalling occurs when the process is operated to produce less dark but the effect remains a It is very desirable to be able to use the same bath for the entire range of shades or as may be required for different work In consequence procedure for greatly limiting the content of aluminium compounds or for removing aluminium has been recommended and employed in ating the known Investigations conducted to determine the function of the bath throughout a wide range of pH utilising solutions that were free of aluminium ions have shown that it is possible to obtain a wide variety of shades or tones at a pH in the range of about to As the pH of the bath is raised through values above up to about it is found that deep colours can still reached the product is apt to be characterised by an objectionable grey While this grey overtone may be tolerated for some it interferes with the desired richness of especially in the dark bronze This grey overtone seems most serious at pH of about but appears through the entire range up to about While some lighter colours can be produced at higher values of pH that are still the darker shades are not then attainable As it is most desirable to maintain the bath in such condition that it can be employed for the full range of In view of the above the known process has been operated at pH 4 with careful supervision of the bath to maintain aluminium ion contamination at a low value aluminium compounds maintained acidic but at a about pH and with a specific amount of compound of not less than 2 aluminium ion or at least su ficient to maintain the bath saturated with minium ion any value in the pH range which the solubility of aluminium ion is less than 2 very satisfactory results The pH value of the electrolyte is preferably in te of to 6 of the previously indicated range of to It now appears that under these notably where the pH has a value upwards of the solubility of aluminium compounds is so reduced in the acidic solution that the solved aluminium ion concentration is automatically kept at a value of not more than 10 and indeed apparently at not more than about 2 or in the preferred pH range of 5 to while all excess of aluminium compound remains in undissolved At the same time it has been found that with these conditions the grey overtone is permitting effective achievement of the desired richness of colour of the vary dark Whilst it may be thought that the desired level of aluminium ion would rapidly build up in a bath deliberate addition by reason of the causes already it has been found by trial that a period of several weeks elapses before the aluminium ion rises to two parts per million by uptake from the coating of the anodised entire colour yet with the aluminium ions in saturated they are always sufficiently present to obviate the grey overtone heretofore encountered in aluminium Thus an unusually satisfactory mode of operation to make up the bath originally with a deliberate addition of aluminium such as aluminium in sufficient amount to provide about or even of aluminium compound which is capable of liberating dissolved Al ions or remaining as undissolved Al dependent on the pH of the and thereafter simply to operate the bath at the preferred without further attention to aluminium content except preferably to make sure that there is always some undissolved aluminium compound which may be evidenced by turbidity of the Since in practice the effect will be to precipitate some of the aluminium which may have been ally dissolved at a lower pH of bath the solution will have a slightly turbid appearance and at the same time will automatically maintain a sufficient content of solved aluminium ions about 2 or at about pH to avoid any grey overtone or the In if the bath is kept in the optimum pH all that is necessary is to keep it mildly turbid with undissolved or precipitated aluminium compounds then appearing at least in part as by simply introducing at the start sufficient soluble aluminium salt to result in such If the aluminium compound content rises for any the increase will be harmlessly ing the but the dissolved Al ions will sarily remain at the effective low value 10 if the bath for any reason becomes such circumstance can be taken as a warning that the pH is ably not in the correct operating as for example by having somehow dropped to a lower below The improved process thus greatly fies control of the colouring operation with these requiring a minimum of chemical determination or testing and of chemical yet permitting maximum tion of desired The accompanying drawing is an illustrative curve approximately representing the solubility of aluminium ions in an acidic nickel salt bath of the character herein and particularly showing the relationship of such solubility to pH in the operating range of the Satisfactory results of the alternating current colouring using a nickel salt have now been found throughout a pH range from about to about when the total content of aluminium whether undissolved or dissolved depending on is specifically about 10 With this content of aluminium compound and at a pH in the stated the bath may be utilised to obtain any colour within the range of bronze colours without culty by reason of grey overtone or the While like results occur at a somewhat lower content of combined and excellent results are obtainable at an Al ion level of zero to 10 at pH 4 to nevertheless appears for preferred operation anywhere in the range tending downward from about pH that the total aluminium compound content be prevented from rising appreciably above 10 since in the latter pH range the concentration of dissolved aluminium ions can rise to values ranging from about 40 about to much higher amounts at lower pH above at pH and well above as pH is In the above the specific concept of working in the defined pH area of the present above and particularly from about 5 about and indeed most conveniently from to is of unusual value for simplicity of The actual amount of aluminium compound present in the bath need not then be controlled to any maximum but may rise far above 10 the effect of the stated pH being to suppress the dissolved aluminium ions to a value below 10 Even though the bath may contain aluminium compound equivalent to as much as 100 aluminium the dissolved aluminium ion concentration at pH is not more than about 2 or The undissolved aluminium compound remains in the solid state and appears as turbidity in the electrolyte or settles as sediment at the bottom of the Within reasonable limits such accumulation of undissolved aluminium compound does not appear In the pH range of to 6 the automatic control of dissolved aluminium ions to a tration of about 3 has a there cannot be sufficient dissolved aluminium compound to cause or otherwise to interfere with attainment of dark yet there is enough dissolved aluminium ion to vent occurrence of undesirable grey The anodic oxide coating is preferably the porous customarily applied to aluminium for protective While anodisation may be carried out in a wide range of such as aqueous sulphuric chromic or a sulphonic acid such as sulphosalicylic and suitable mixtures of these with other acids or a very suitable anodic oxide coating for ing purposes may be produced by conventional direct current treatment in aqueous sulphuric acid for The conditions of the anodising treatment do not appear to be being selected largely to suit the thickness and other characteristics of anodic coating colouring process to which the present invention is specifically then involves submerging the anodised aluminium article in an acidic nickel or cobalt salt bath and passing alternating current between the anodised aluminium and a for sufficient time to effect the desired colour deposit in the anodic oxide Thus the bath should be a solution of a nickel or cobalt salt and an acidic all in latively low The anionic constituents of the bath are selected to provide the desired solubility of nickel or cobalt ions and suitable acidity of the and supplemental salts or the like may be incorporated for poses of buffering or other function which does not interfere with the formation of the coloured and may include salts or the like that result from initial or subsequent pH Thus acetate and other common anions may be A particularly suitable composition for a nickel bath is made up with nickel sulphate and boric each in amounts of the order of to per usually with some ammonium sulphate 10 to 20 and also very preferably some content of magnesium from 2 to of the conventional hydrated A similar amount of cobalt sulphate may be substituted for nickel Addition of a small amount of aluminium compound to a freshly constituted bath in accordance with the present invention may be in the form of a variety of water soluble aluminium for instance acetate or or conceivably a compound such as aluminium A salt of aluminium with an anion that is already present in the bath is generally The is made of suitable cally conductive preferably an electrode of the same metal as in the metallic nickel for nickel salt baths or cobalt for cobalt salt a graphite may be The colouring treatment involves passage of alternating current at a moderate say from to 20 for a ingly appropriate from a few minutes up to 10 or 15 sometimes with increasing voltage or other propriate electrical In the shade of colour achieved depends on the time of treatment and on electrical for light shades are duced by relatively brief treatment with lower whereas very dark bronze colours require a longer time or greater electrical For further the drawing indicates the manner in which the solubility of aluminium ions in a nickel sulphate electrolyte of the sort here described at varies with pH in the range from to Th solubilit in Al ively high as the pH falls to and correspondingly creases as the pH rises to being of the order of in the region of pH and dropping to and below 10 above pH about at pH Of in alkaline above pH combined aluminium becomes more rising from the low point of the trated but this characteristic is of no significance in the present which is related to acidic It will be readily seen that at the higher pH values of the up to 6 or the solubility of aluminium ions is drastically regardless of the amount of aluminium compound present in the bath as suspended or settled The operating pH range contemplated by the indicated at A in the thus affords the advantage of automatic control of dissolved Al ions to a low By way of specific an aqueous bath is made up to have the following initial composition in grams per litre NiS0 25 20 25 15 To this bath may be in accordance with preferred practice of the an amount of an aluminium pound equivalent to to 10 Al to of aluminium sulphate after the pH is adjusted upwardly to the range of to as for example a value of As a no more than a few of aluminium ions remain in and the excess aluminium compound is precipitated as hydroxide and will impart a somewhat clouded or turbid appearance to the The pH and subsequent maintenance of pH in the desired can be achieved by additions of monium hydroxide or dilute sulphuric acid as With thus constituted and the desired ditions established by standard pH the ventional procedure may then be Anodically dised aluminium articles are immersed in the nickel salt bath with a nickel conveniently in the form of bars or Upon passage of alternating for a selected length of a desired shade of bronze colour is imparted to the anodised surface of the For at voltages of 10 to and with treatment times of 5 minutes or bronze shades from light to dark are For a very dark bronze coating the treatment can be continued for an additional minutes at a higher These permanently coloured anodic aluminium oxide coatings can be sealed in a conventional using water at or near No special chemical testing or control of aluminium ion content is needed over long Even if the aluminium compound content tends to build up somewhat for reasons explained any excess of aluminium compound beyond the original to 10 will not remain in but will appear as an increase of or as a settled deposit at the bottom of the Of the pH of the bath should be checked periodically and corrected as The process is further simplified in that if the bath becomes clear despite the presence of at least the original 5 10 aluminium this is an cation that a test of pH will show it has fallen substantially below the preferred minimum of insufficientOCRQuality

Claims (2)

1. * » 329¾6/2 WHAT IS CLAIMED IS I / 1« A process of colouring anodically oxidised aluminium by passing alternating current, betwee an anodised aluminium workpiece a d a counter-electrode whilejb immersed in an acidic solution of a nickel or cobalt salt characterized in that the pH value f the electrolyte is maintained at a value .above pH ¾ 7 (but below pH 7) and that at to commencement of. the. process an aluminium compound is added to. bring. the; aluminitra ion conten . to a value not less than 2 parts per million or to a value sufficent to saturate the electrolyte with aluminium ions, whichever is the less.

2. , A process according to Claim 1 further characterized in that the electrolyte contains sufficient aluminium compound to maintain it in a turbid condit on* 3· A process according to Claim 1 further characteized in that aluminium compound in an amount of about 10 parts per million calculated as aluminium ion, is added to the electrolyte* , A process aocording to any of Claims 1 to 3, further characterized in that the pH value of the electrolyte is maintained at a value in the range of 5 to 6. 5· A process for colouring anodically oxidised aluminium in an acidic bath containing a nickel or cobalt salt substantially as hereindescribed* 6* A coloured anodised aluminium of any of Claims 1 to 5*