Laboratory scale investigation on the use of sulphuric-oxalic acid anodismg bath for plating purposes

are presented and the results of this bath are compared with other standard baths after plating in respect

of coverage, adhesion etc. The experimental results indicate that direct plating of copper isquite possible

on oxide films formed from sulphuric-oxalic acid bath with good coverage, adhesion and fairly higher thickness from an acid copper bath

Kanakaraj, D.

Narasimhan, V.L.

Subramanian, R.

Venkatachalam, R.

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Bulletin of Electrochem~stry 2 (3) May -June, 1986, pp. 273-275 PLATING ON ANODISED ALUMINIUM D KANAKARN, R VENKATACHALAM. V L  NARASIMHAN and R SUBRAMANIAN Central Electrochemical Research Institute, Karaikudi - 623006 ABSTRACT Laboratory scale invest~gations on the use of sulphuric-oxalic acid anodismg bath for plating purposes are presented and the results of this bath are compared with other standard baths after plating in respect of coverage, adhesion etc. The experimental results indicate that direct platmg of copper isquite possible on ox~de f~lms formed from sulphuric-oxalic acid bath with good coverage, adhesion and fairly higher thckness from an a c ~ d  copper bath. Key words: Anodised aluminium, copper electroplating. H2S04- Oxalic acid bath INTRODUCTION A number of methods have been described [ I ,  21 for plating on anodised aluminium, making use of anodic coatings produced from electrolytes such as phosphoric acid, oxalic acid, chromic acid, phosphoric- sulphuric acid, sodium carbonate etc. It is generally concluded that anodising in phosphoric acid or phosphoric acid based electrolytes is the only one to provide a.  substrate for good adhesion [3] without any conditioning treatment. Another wellknown procedure for plating on anodised aluminium is known as Krome-Alume process [4, .5] which is based on anodising in oxalic acid. This anodic film requires a surface conditioning treatment in a cyanide or a fluoride solution to ensure good adhesion of the electroplate. The interest on plating on anodised aluminium has been revived recently as is evident from a score of papers appearing on this subject (6 - 91. It has been concluded (6, Y ]  that phosphoric acid anodisingis the best suited for subsequent plating because of the large pore diameter, optimum pore density and structure of phosphoric anodised films. However, differences have been expressed on the theory of adhesive bonding of the electroplate to the porous oxide film. It is reported [6] that adherent plating on sulphuric acid anodised aluminium is possible only when the thickness of the anodic film is less than 3 ~ .  However, in view of their high hardness and strength the coating from sulphuric acid should function well as a pretreatment for electrodeposition if the pore diameter is sufficient to allow substantial pore filling by the electrodeposit [Y]. It was felt that by using a mixture of oxalic acid and sulphuric acid, the pore structure of sulphuric acid film could be modified to have large pore diameters without loss of strength of the oxide film - a requisite for good bonding strength. A printed circuit patterri of 3 5 ~ m  thick copper on anodised aluminium with no electrical contact between the two metals was required for an electronic instrument. It was therefore decided to have anodized film of thickness 2 .5~  on aluminium and then plate with copper. In this paper the preliminary investigations of copper plating on anodized aluminium (2s  alloy) are reported. EXPERIMENTAL Aluminium panels (2s alloy) [.5 cm ( I )  x 8 cm ( b ) ]  were mechanically polished to a bright finish, degeased in trichloroethylene and soak cleaned in a dilute solution of sodium carbonate-trisodium phosphate at 60' -6.i°C for 2 minutes, rinsed. desmutted in I : I nitric acid, washed in running water. rinsed in deionised water and anodised in the electrolytes, the compositions of which are given in Table I. Table- I: Composition and operating conditions of anodising. electrolytes No. Bath Compcr Temp. Volt Current Time sition (1 "C) age  density (V) ( ~ . d m ?  ( m i d  I. Phosphoric acid 1.5'% v /v 40 ' 30 6.0 5 (85%) 2. Phosphoric acid 1.5'%, v/v (85%) + Sulphuric acid 1 5 ' f l  v /v 38 13 3.3 1 5 (sp.gr. 1.84) 3. Sulphuric acid 7% v /v (sp.gr. 1.84) + 3 2.5 40 1.2 40 Oxalic acid 3% v/v  Anodic film was conditioned, whenever desired, in the electrolyte5 shown in Table 11. All the electrolytes were prepared from LR v i d e  chemicals. Table- II  : Anodic film conditioning treatments by immersion No. Bath type Composition Temp. Time "C (mid I, Cyanide 6% w / v NaCN 3.5 10 2. Fluoride 6%) w / v NHIHF2 3.5 5 A thin film of metal was deposited on the anodized films with and without preconditioning treatment from the electroplating baths (vide Table 111). This was followed by plating in acid copper bath at 2..5 ~ . d m - ~  for 60 mins. with the temperature of the bath maintained at 35OC. High purity copper anodes bagged in terylene cloth were used. ~anakaraj et al-Plating-on anodized aluminium T a b -  Ill : Pldlng bath compodtlonr and operatlnq condklonr for lnkid dopmltlon of a thln layer of copper on anodlwd alumlnlurn panel8 No. Bath type- Cornpolition pH Temp Current Time ($dl) (OC) density (mins.) (~.dm-') 1. Acid copper 2. Pyrophos- phate COPP 3. Watt's nickel (low pH) CuSQ.5H20 200 H2SQ 10 ml l Clue 0.5 1 - 1 . 5 3 5  1 10 Copper 30 +phosphate 200 Oxdate 15 Nitrate 5 Ammonia 3 8.5 50 2 10 NiSO4.7 H 2 0  240 NiC12.6 H 2 0  45 Boric acid 30 2.5 50 5 5 Thickness of anodic films fonned on aluminium were determined separately by stripping in phosphoric acid-chromic acid bath at  90°C. The surface coverage of the oxide film by initial thin layer deporitr were visually examined. The copper deposit on the edges of the panels were ground off before the adhesion test was carried out. Adhesion of the electroplated copper to aluminium was tested by repeated bending of the plated specimen through 180" over a mandrel of 3 mm diameter steel rod held in a vice and then hammering it flat [ 10). In caseof very poor adhesion, the deposit lifted off from the panels during the preliminary grinding operation itself and could be peeled off easily. RESULTS The experimental observations on coverage and adhesion are given in Table IV. On phosphoric-sulphuric acid anodised aluminiui, the coverage by nickel and adhesion of copper were very good with or without film conditioning treatments. Coverage by initial acid copper layer was good with film conditioning. Table- IV: Surface coverage of anodic films by initial thin layerdeposits and adhesion pf copper deposit from an acid bath on anodlsed aluminium Anodising Anodic Initialt Coverage Adhesion bath' film" electro- ' condi deposit tioning 1 Nil 1 No Very poor 2 No Very poor 3 Tiny specks Very poor Poor coverage 1 1 .I No Very poor 2 Poor . Poor 3 Gdod Poor 2 Nil 1 Poor Poor 2 Good Poor 3 Good Very good 2 1 1 Good Very good 2 No Very poor 3 Good Very good 2 ' 2 1 2 Good good Good Poor 3 Good Very good 3 Nil 1 Good Poor 2 Good Poor 3 Good Very good 3 1 1 2 Good Very@ Good Poor 3 Good Very good 3 2 1 Good very good Please See Table I fbr details of anodising baths and operating pardm.eten ** Please see Table I1 for details of film conditioning t Please see Table III for initid electrodeposit's ndure and operating parameten. To ensure very good adhesion of the thick copper layer, film conditioning was essential when copper strike was used. The same trend was observed when anodising was carried out in oxalic-sulphuric acid electrolyte. Since the object was to get a thick insulating barrier between the copper deposit and aluminium, 25p thick anodised oxide film was used. Hence the effect of film thickness from this bath on coverage and adhesion was not studied. However, we could not achieve insulation between copper layer and aluminium even with this thick anodic film as tested by electrical conductivity measurement between the top copper layer and aluminium metal. Actually. zero resistance was shown indicating probably the penetration of the bottom barrier layer by the electrodeposits. The oxalic acid-sulphuric acid anodised film could ensure an adherent electroplate with acid copper or Watt's nickel as the initial layer when a cyanide film conditioning treatment is given. DISCUSSIONS Studies on the coverage of an initial electrodeposit on the sulphuric acid (concentration of H2SQ 1516) anodised and phosphoric acid anodised films conclude [b ]  that the thickness of the oxide film formed in phosphoric acid does not affect the degree of coverage by copper deposit and that the high degree of coverage in these cases is indicative of high density of nucleation sites on the oxide film for subsequent plating. It is also presumed that 'the initial deposition starts at the peak around the pit groups on the anodic film'. This formation of nucleation sites is related to the retention factor of the impurity constitutents like iron present in the alloy and a chemical or electrochemical conditioning treatment of the phosphoric acid anodised film has been developed [6] to ensure excellent adhesion of the deposit on 1100 alloys. It has been shown 171 that the adhesion increases with increasing voltage, and with complete filling of the pores in the anodic films. thereby concluding that there is no penetration of the barrier layer by the plated metal. There is a critical pore diameter below which unacceptable adhesion results and that the strength of the oxide film is a major factor in electrodeposit adhesion [El. From the S. E. hi. photopaphs of the anodic films on aluminium it is assumed 19) that the probable pore diameter should be around 1030 f i n  250 g / l  phosphoric acid. However, the covemge on high purity aluminium was poor whereas it was complete on 7029 T6 alloy (both anodised at 20°C, 80 V in 250 g /  l H2P04, 20 coulombs. cm-'). The pore diameter on 7029 Ti may be larger due to chemical and electrochemical dissolution of the lalloy constituents during anodising 191. But it is not clear how the pore diameter of anodic film is influenced by the impurity contents. It has been established 191 experimentally that no only the film strength but also an 'open structure' of the film fonned are necessary criteria for improving the adhesion. Bullet~n ol Eleclrochem~stry 2 (3) May -June, 1986 274 Kanakaraj et el- Plating on anodized aluminium The following condition8 am auggerted to get limiting film thickness (IIT) [9] : Phosphoric acid 250 gpl, temperature 20-2S°C, cumntdensity 1-2 ~.dm-', voltage 60- 80 volts and time 30 minutea for 7 0 2 m  anodisiagfor a longer time for high purity alloys and a lower temperature with higher voltages for highly alloyed metal. I t  h~ alro been established thatihe pore filling by the electrodepait i s  essential for good adhesion. Addition of oxalic acid to a sulphuric acid anodising electrolyte modifiea the anodic film with a lamer pore diameter and increast in hardness and strend. The cyanide c&dkoning treatment or the fluoride conditioning treatment probably attach the surface oxide film to produce an open structure'at its limiting film thicheas which w u  reached in 40 minutes anodising time. The complete filling of the pores i s  ensured M i s  evident from zero reabtance between copper layer and aluminium substrate. Thus the two easential criteria for good adhesion namely, an optimum strength/structure combination with an 'open structure' produced by surface conditioning treatments are achieved for the oxalic-sulphuric acid electrolyte runs parallel to the behaviour of phosphoric acid-sulphuric acid in this regard except that the latter does not require film canditioning b t m e n t  to ensure electroplate adheaion probably because of the pore diheter and structure i s  well around the optimum even without film conditioning. In the present work i t  appear9 that the metal deposit has penetrated the barrier layer (zero resistance between copper and aluminium) whereas the earlier workem 17, 91 have concluded that there i s  no penetration of the barrier layer by the electrodeposit. I t  i s  to be confirmed whether the film modification treatment electrolytes have also attacked the barrier byer at the bottom of the pores while opening up the surface pores. Except for this difference, the present work makes i t  possible to plate vev adherent deposits on anodised 1100 aluminium alloys. CONCLUSIONS A non-phosphoric anodising electrolyte namely sulphuric acid-oxalic acid electrolyte produces an anodic film receptive to adhermt electroplates when the film i s  suitably modified. Copper can be deposited adherently on this modified film from an acid copper sulphate bath. Nickel can be deposid with very good adhesion from 4 low pH Watt's bath on this film with or without film conditioning treatment. The behaviour of the oxalic acid-modified sulphuric acid bath i s  similar to 8ulphuric acid modified phosphoric k i d  bath in  ppducing an optimum strength/structure combination at LF.T. The work supports the mechanism of adheaion of electroplates to anodised aluminium surface due to complete filling of the pores of'an open structured anodic film, but the metal deposits have penetrated the barrier alro to establish a direct metallic bond-with substrate aluminium. Although the main object of having an insulating layer between the copper deposit and aluininium substrate i s  not achieved, yet a method for plating adherent electrodeposits on thick anodised film has been found. REFERENCES 1. S Wemick and R Pinner, The sufact f r t a h n l  andfinuhing of aluminium aria ifs alloys, R Draper Ltd., Vol. 2 (1972) Ch. 13 2. Aluminium Co. of America, Eltmophting of aluminium and its  alloy^ (1 946) 3. R V Vandenberg, Trans Inst of Mt&l Finishing, 45, (1967) 167 4. WJ Trwers, Trans Anur Eltcfrochem Soc, 75 ( 1939) 201 5. WJ Travers, Aw A m  Eltclrophh~'  Soc, 29 (1942) 110 6. A Akiyama, K Yasuhara, K Tanaka andT Soji, Plating +Sur, Fin, 58 (6) (1971) 594 7. DS Laahmore, Mt&l Finuhing (USA) 78 (4) (1980) 21 9. DJ Schardein, aid, 71 (2) (1984) 64 10. S Wemick and R Pimer, Ref. 1. Ch. 16. p 962 FORMULATION OF SELF-REGULATIN~ HIGH SPEED (SRHS) CHROME SALT Process developed at : Central Electrochemid lereakh Institute, Karaikudi - 623 006, Tamil Nadu, India Chromium plating is being carried out in several major electroplating institutions in the country and abroad to meet the requirements such as corrosion resistance, abrasion resistance, lowcoefficient of friction and non-wettability tocertain sticky materials. The industrial applications are wide and they include automobiles, chemical industries, defence, electrical, food, paper, plastic, printing. railways, rubber, tools etc. A majority of the industries are already employing chromium plating. either decorative or hard chromium, to meet their demand. The conventional chromic acid-sulphuric acid type baths are being replaced by new salts and a switch over to the use of modem chrome baths is taking place due to the following advantages: (a) A higher production rate is possible (by about 30%). (b) The deposits are smoother, brighter and harder, (c) The accessories like jigs are less frequently required, (d) The need for control of the concentration of critical constituents like sulphate is avoided. The modern chrome salts are however, not available in the country and are being imported. Process developed With the view to help towards import substitution, a catalyst-additive mixture has been developed. This when added along with chromic acid gives the modem SRHS chrome bath, which has been observed to have a performance similiar to the bath made out of imported salt for both decorative and hard chrome platers. Consumer acceptability trials on a 900-litre bath have been carried out at Diesel Locomotive Works, Varanasi. The bath prepared as per this know-how has a performance similiar to the bath prepared out of imported salts. The essential items of plant and machinery are ball mill with procelain jars, balls for the jars, trays, air oven and a balance. All are indigenously available. J Bulletin of Eleclrochemistry 2 (3) May -June, 1986 

Plating on anodised aluminium

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