Surface textures and process characteristics of the electrolytic photoetching of annealed AISI 304 stainless steel in hydrochloric acid

Surface textures and process characteristics of the electrolytic photoetching of annealed AlSl 304 stainless steel in hydrochloric acid D.M. Allen*, T.N. Talib* and D.F. Hornet Electrolytic photoetching of A ISl 304 stainless steel surfaces In 10% (w/w) hydrochloric acid has been studied. A surface texture value of 1.35 pm (R a) has been measured for a commercially acceptable rate of etch (10 #m/min). Comparisons of this etching system with FeCl 3-HCI-H2 0 spray systems used in conventional photochemical machining have a/so been made

Keywords: electrolytic etching, photoresist process, surface texture Results forming part of a general investigation .nto quantltative aspects of photoetchlng are presented here This manufacturing process, also known as photochemical machining (pcm) is a multi-stage process I employing photographic and chemical etching techniques (Fig 1) for the productlon of components and devices in a wide range of metals and alloys. Stainless steels are of particular importance in production eng,neermg and may be spray etched through photoresist stencds with modihed aqueous ferric chloride solutions. The resultant surface fimsh 2 and edged profiles have now been studied m detail 3'4 Usually, mt is desirable that when recesses are etched, smooth umform finishes are obtained especially where product aesthetics are of prime ~mportance, such as m decoratlve applications. Although photoetched parts are burr-free, care must be taken to ensure that profiles of etched edges are approximately straight (Fig 2). Deviations from a true straight edge are controlled to within 20% of the metal thickness by the pcm industry. Ferric chloride solutions are relatively inexpensive, but unfortunately disposal is a problem. To increase the working lifetime of the etchant, chemical regeneration with chlorine gas is now being practised in the largest commercial photoetchlng companiess . In the past, a suggested alternative method to spray etching was electrolytic etching. In particular, hydrochloric acid solutions have been recommended for the rapid electrolytic etching of stainless steels6 although no quantltatlve investigations appear to have been made of this system. In conventional pcm it is important to be able to control the rate and depth of etch so that specific component dimensions and satisfactory edge profiles can be obtained 7 . As a general rule, dimensional tolerances in production runs can be kept to -+10% of the metal thickness down to -+0.025 mm. As centre-to-centre dimensions are independent of etching time, the associated tolerances reflect the accuracy with which the orlg.nal enlarged artwork Is drawn. Typically, the tolerance =s -+0.05% of the centre-to-centre dimensions of the part, eg 50 -+ 0.025 mm. The objective of this paper is therefore to present quantitative data on the *School of Production Studies, Cranheld Institute of Technology. Cranheld, Bedford, UK. MK43 OAL tPresent address: 6 Green End Road, Boxmoor, Hernel Hernpstead. Hefts, UK. HPI 1QW

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electrolytic photoetching of AISI 304 sta,nless steel in a hydrochloric acid electrolyte so that comparisons can be made with conventional pcm using sprayed ferric chloride as an etchant.

Passivatingsystems Passwlty is a state where a metal or alloy in a certain environment shows a slow corrosion rate when, thermoSubstrate selection I

Enlarged artwork Microphotography ' step and repeat' pnntlng (and r~glstrat~on for doublesided etching )

Chemical cleaning

Surface preparation Photoreslst coating Sensitization of the substrata

Working 'tool' photographic image

I

I I Sensitized substrote exposed through 'tool' and development Durable stencil(s)

~ Etching Reslst-covered

component

I Resist stripping Finished component

Fig I pcm process stages

/

Photoresist =

(

Undercut

Fig 2 Development of etched edge profiles (a) break through point (b) biconvex (c) straight (d) biconcave

0141-6359/83/020051 -06 $ 0 3 . 0 0 © 1983 Butterworth & Co (Publishers) Ltd

51

Allen, Talib and Home - electrolytic photoetchmg of stainless steel dynamically, ~t would be expected to corrode more rap=dly In general, it =s associated w~th ox=dizing med=a and the formation of thin protectwe films on the surface by the contmuous reaction between the metal or alloy and ~ts environment Therefore changes in the env=ronment (eg temperature, concentration) may cause =t to corrode more rapidly. Examples of metals that exhib=t pass=vity are ~ron, chromium, mckel, t~tamum and alloys containing these metals A polanzat=on d=agram of a typical pass=vatmg system (F~g 3) dlustrates that under galvanostatic etching and at current densities m the proximity of Ja, b,c, any of three etching mechanisms may take place whose anode potentials are Ea, Eb, and E c. At Ea, lying in the active zone, etching would be uniform whereas at Ec, pitting and mtergranular attack may occur. E b hes in the trans=t~on zone between the actwe and the passive zones, where the anode potential =s unhkely to remain at E b due to =ts instability.

Procedures and experimental details Potent=ostatic and galvanostat=c modes of electrolytic etching may be employed. In potentiostat=c etching, a reference electrode measures the potent=al at the anode surface and maintains ~t at a constant level by use of a potent=ostat. Such equipment is difficult to set up and hence ~s unsu=table for industrial production work. Alternatively, w~th galvanostat~c etching, control of the depth and rate of etch is readdy achievable as the mass of metal removed ~s proportional to the product of current and time (Faraday's First Law) Galvanostat=c etching of metals that exhibit no passw~ty is relatweiy s=mple, yet w~th pass=vating materials such as stainless steels difficulties may occur at current densities m the region of Ja,b,c (F~g 3) causing Inconsistencies m I L I I

results Modfflcatlonstotheenvlronment by altermgelec trolyte temperatures or concentrations may extend the current deqsJtles achievable in the actwe zone and/or ehmmate passivity Polarization dDagramsfor AISI 304 m 10% w/w HCI under potentJodynamJc conditions were obtained to explain the reactions on the anode surface. Regions of passwlty were ~dentlhed and the effects of changes m temperature upon itdetermlned Higher electrolyte concentratlons were felt to be unsuitable for industrial appllcatrons for both practical and 'health and safety' reasons Experimental

details

Details of the equ=pment and materials used throughout the experimental work are listed =n the Append=x. Square p=eces of flat stainless steel 51 mm square x 1 63 mm thrck were carefully degreased w=th acetone and then immersed m a CD/70 solut=on for 10 minutes. These were then washed and dried for 20 minutes at 90°C and =mmedlately laminated on both sides w~th dry film photoresist The coated samples were then exposed through a double-sided mask and developed to produce nine 10 mm square apertures on both faces of the steel Theres~st was developed for 60 seconds at 35~C in spray developing equipment The res=st stencd pattern allows the same sample to be electroetched 18 times under different etching conditions thus eliminating the possibility of sample van at=ons PVCelectncal msulatmg tape was found to be the most suitable maskant for protecting apertures not under test during etching The working electrode (anode) and the secondary electrode (cathode) were held verbcally in the electrolyte m a glass cell. Th~s =n turn was ~mmersed partially =n a water bath for temperature control. A paddle st=rrer was used to gently agitate the electrolyte to ensure constant temperature Figure4 dlustrates the cell The anode potentJal was swept w~th the aid of a linear sweep generator driving a potent=ostat Measurement of the anode potential was with a saturated calomel electrode (sce) and both current density and anode potential were plotted s=multaneously on an X-Y recorder glwng the potentiodynamic polarization d=agrams of the materials usedS

¢J o

I

<

I

l Working electrode

Ec

Potentlostat

Ep

I I

Effect of

ON

I

~ ternperoture increase

->

Eb

l Reference J electr°de E [

]Linear sweep generator

IYx

j

'X- Y' recorder

~

Sotd KCl Salt b r i d g e - -

Ea ~

o

I

<

Stirrer

Cathode -

~

r i

Ja b c

Current dens~t

FiE 3 Schematic polarization diagram of a pa~sivating system

52

Fig 4 Schematic layout of the cell and equipment used for potentiodynamic polarization measurements

I

APR 1983 V Q L 5 NO 2

Allen, Talib and Home - electrolytic photoetching of stainless steel

2o~c 25°C

i•i

1000

800

30°C

35

~E 3c <

60C

E U

~D

2C

8

lC

40C

20C 0

<

P

-400

5

t)

-20C I 0

I

I I 50 100 Current density, A /dm 2

I

o - 300-200

150

Fig 5 Anodic polarization diagrams for A l S l 304 stainless steel in 10% (w/w) HCl

o

Galvanostatic etching Galvanostatic etching was carried out utilising the same type of workpiece and masking techniques used in the potentiostattc work. Similar equipment was used except for the use of a constant current source and the elimination of the reference electrode Etching was carried out at an electrolyte temperature of 25°C and each aperture was etched indwidually The two parameters that varied were the current density (in steps of 10 A/din 2 in the range 1 0 - 9 0 A/dm 2 ) and the etch duration (in steps of 5 minutes m the range of 5 - 2 0 rain). The mass of metal removed was measured by weighing the workpiece before and after etch-

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ENGINEERING

200 400 600 Anode potenttal, mV

800

lOOO

Fig 6 Current density standard deviation versus anode potential

Results A series of potentlodynamlc anodtc polarization measurements was repeated eight ttmes for electrolyte temperatures i o of 15, 20, 25 and 30 C. The means and standard deviations in current density of the measurements are shown in Ftgs 5 and 6 respectively. The anode potentials were swept at a rate of 150 m V / m i n from - 0 . 3 V ~ +1.0 V (versus sce) as this range was predicted to include the most suitable current densities for electrolytic photoetching. Passivatton of AISI 304 stainless steel in 10% HCI can be seen to occur at a temperature of 15°C at an anode potential between - 5 0 and --100 mV (versus sce) (Fig 5). Increasing the electrolyte temperature reduces the tendency of the material to passivate. It also has the effect of pushing the anodic polarization curve to the right (Fig 3) and this increases the current densities achievable in the active region where uniform etching is expected. Although passlvatton of the anode surface at 20°C ts apparently ehminated (Fig 5), nevertheless, due to the inherently high values of standard deviation only the probability of passtvation is reduced with temperature increase. Therefore, high etching temperatures are desirable, but for practtcal etching, temperatures in excess of 25°C are not recommended, due to the rapid evolution of HCI gas from the electrolyte. In Fig 6 it can be seen that the standard deviation in current density increases rapidly with anode potenttal in the area corresponding to the acttve zone.

o

32 o 20 rams z~ 15 rains • 10 mfns • 5 rains

8 E

016

o

E

E

o (

I

I

I

!

I

I

50 Current denstty, A/din 2

I

100

Fig 7 Mass of metal removed versus current density for various etching times mg each aperture. The results (Fig 7) illustrate that the mass of metal removed increases linearly wtth current density. The geometric profiles of the etched apertures were plotted using a Talysurf 3 suitably modified wtth a st;/lus extension arm to reduce magnification. The sweep was carried out horizontally (normal to the etching position) and at the centre of the aperture. The profiles plotted in Ftg 8 are in positions corresponding to their actual posttions in the 3 x 3 array of apertures on the workpiece. A Talysurf 4 was used to measure the surface texture of the etched apertures, the R a (cut off length 0.8 mm) and peak-to-valley measurements are superimposed on the relevant geometrical profile plots of Fig 8. The effect of current density upon the surface texture produced after

53

Allen, Talib and Home - electrolytzc photoetching of stainless steel 15 m m etching is tllustrated m Figs 9 and 10. This clearly shows t h a t surface textures deteriorate raptdly at current d e n s i t ~ e s a b o v e 6 0 A / d m 2 This ls due t o mtergranular attack coupled w i t h vertical striattons produced f r o m the by-products of the process streaming d o w n the face of the aperture Blacking of the surface occurred stmultaneously w i t h etching at c u r r e n t densities up to 20 A / d m 2 This black layer was easily removed by rubbing w h i l s t sttll wet, but on d r y i n g the f i l m adhered strongly to the u n d e r l y i n g steel No analysis was carried o u t upon the c o m p o s i t t o n of t h B layer but tt ts t h o u g h t to be an iron h y d r o x i d e i n i t i a l l y which then oxtd~zes m atr to f o r m an adherent layer The degree of blacking ts d e p e n d e n t upon the current dens=ty and the d u r a t i o n of etch. I t ~ s n o t a passive layer, as~t can be seen f r o m the geometric p r o f i l e of Fig 8 t h a t at such c u r r e n t densities metal removal stdl takes place

Current distribution Due to the anode area being relatively small compared to that of the cathode, a non-umform current distributton on the anode surface ts produced. Th=s results m greater amounts of metal being removed from the edges of the anode aperture gtvmg a characteristic 'W' shaped profile (Fig 8). The pos=tton of the anode aperture relative to the

t

PV

ill

0

5

100

9

Ra

12

40 80

cathode also has an effect on the profile produced ShJftbng the aperture f r o m the centre of the cathode causes a f u r t h e r d i s t o r t i o n to the 'W' shape as even greater metal removal ts o b t a m e d at the edge closest to the larger cathode area (Figs 1 1 a a n d 11b)

Conclusions U n t f o r m dissolution of AISI 304 stainless steel can be achieved if the e l e c t r o l y t e and ~ts t e m p e r a t u r e are selected to prevent passtvatlon of the steel. Galvanostattc etching m 10% HCl at 25°C was carried out atJ<60A/dm 2,thellmittoumformetchlng Etchmg and surface blackening was noted a t J < 20 A / d m 2 w i t h break-up of the black surface layer commencing at stencil edges and a t J ~> 20 A / d m 2 The R a was shown to increase w~th current denstty m the range 20 < J < 60 A / d m ; w t t h typ=cal values of R a being 0.4 t o 2 0 / l m To achieve an a c c e p t a b l e R a a n d a practical metal removal rate a current denstty of 50 A / d m 2 is recommended, where an etch rate of 1 0 # m / m i n is obtained together w t t h an R a of 1 35 # m . This etch rate is a p p r o x i m a t e l y half t h a t recorded for spray etching w i t h aqueous ferric chloride solutions and the surface t e x t u r e produced ts much rougher than that obtained by spray etching ( m i n t m u m R a = 0 . 3 5 # m ) 2 F r o m these resultstt

c~ t

PV

Ra

120 200 300 400

'J=7~

5

19 32

40

15

4 8

02 041 5

60

160

~

0 20

1o6

200

d = 10 A/drn 2

j = 40A/dm 2

0

0

100

40

5

68

11

80

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12B

"~20

15

8

134

160

o

6

20 40

10

200

2O0 300

60 80 100 120

240

J = 20Atdm 2 J = 50 A/din 2

400 * Ra > lOl~rn (off scale) 0

5

100 200

5

28

0 20

10 5

45

40 60

100 1o

70

80 100

200

120

300 300

400 J=gOA/dm 2 Re > 10 lirn (off scale)

28

J=6OA/dm 2

140 160

L_--10

f

9 J=

58

o "1

lu

1 o

B5

1,4

30A/din

2

Fig 8 Profiles produced by electrolytic photoetching at various current densities together with time o f etch ( t, ram), peak-tovalley measurement ( P V , / ~ m ) and surface texture ( Ra, #m). Y-axis is depth o f etch (pro). The matrix corresponds to the positions o f the apertures in the resist stencil on the stainless steel anode

54

APR 1983 VOL 5 NO 2

Allen, Talib and Home - electrolytic photoetching o f stainless steel 10 100 -

9

9O

8

80

7

70-

6

E 6O

Off scale

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5

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5O o > 0

4O I

3C

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lo

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o

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20

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30 40 50 60 Current densrty, A / d i n

I

70

I

80

5" 6'0 7'0 8'0 9o

Current density, A/dm 2

I

90

2

Fig 10 Relationship between

PV

and current density for

an etch time o f 15 min

Fig 9 Relationship between Ra and current density for an etch time of 15 min Cathode appears that electrolytic photoetchmg has no advantages over conventional pcm for surface etchmg AISI 304 stainless steel. However, electrolytic photoetchmg may be advantageous if (1) high etch factors and good edge profiles are obtamed when etching completely through metalhc substrates, (2) complex alloys and materials resist ferric chloride etchants but are susceptible to electrolytic attack. These two aspects are being mvest~gated currently together w=th electrolytm metal removal processes which produce polished fmzshes9 .

res,,t layer

~

Anode---~

,

,

\1111 ~ I I ,Ill v

~

-~

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/

Acknowledgements The authors w=sh to acknowledge the help given by Dr J.E. Strutt, Dr. M.J. Robinson and Mr. W.H. Turner of the School of Industrial Science, Cranheld Institute of Technology, who kindly lent various items of equipment ll'l

I" I'i

I

A ooe - - ' F b

Appendix Materials Stainless steel:

annealed AISI 304 (C, 0.06%; Si, 0.6%; Mn, 0.8%; Cr, 17.5-19%; Ni, 9-11%) supplied by Knight Strip Metals, Potters Bar, Herts, UK

Cathode:

Lead

Electrolyte

1 volume of GPR Conc. HCI (36% w/w, sg 1.18) diluted with 3 volumes of distilled water.

Photoreslst

25 #m thick dry film Laminar AM supplied by Thiokol Chemicals Ltd, Warrmgton, Cheshire, UK

Cleaner and deoxidizer:

1 volume of CD/70 diluted with 5 volumes of tap water. CD/70 is a product of Robertsons Chemicals, Diss, Norfolk, UK

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Fig 11 Stylised current density distributions obtained in electrolytic photoetching when resist apertures are: (a) symmetric with respect to cathode area (b) asymmetric with respect to cathode area

Equipment The potentlostat (Type 703A) and linear sweep generator (Type L S U l ) were supplied by Chemical Electronics (Birtley) Ltd. Talysurf 3 and Talysurf 4 are surface texture mstruments manufactured by Taylor Hobson, Rank Prems~on Industries, Leicester, UK

References 1

Allan D.M. Design and production of small hole proftles in thin materials. Chartered Mechanical Engineer, (March 1981), 3 7 - 4 0

55

Allen, Talib and Home - electrolytic photoetching o f stainless steel 2

Allen D.M., Hegarty A.J. and Horne D.F. Surface textures of annealed AISI 304 stamless steel etched by aqueous ferric chloride-hydrochloric acid solutions, Trans. Inst. Metal Finishing, 59 (1981), 2 5 - 2 9

3. Allen D.M., Horne D.F. and Stevens G.W.W.Q.uantttatlve

Machining Institute Journal, No 6 (1981) 7 - I / 6

Apphcattons Data for Kodak Photosensttwe Resists (Kodak Pubhcation No. P-91, Eastman Kodak Co., Rochester, NY, 14G50, USA. December 1956)

7

Allen D.M , Home D.F , Lee H G. and Stevens G W.W. Production of sprtng steel camera shutter blades by photoetching. Prectslon Engmeermg 1, No. 1 (1979) 2 5 - 2 8

exammat=on of photofabr=cated profiles. Part 2 Photoetched profiles In stainless steel J. Photogr. Sct. 26, No. I, (1981),

72-- 75 4. Allen D.M., Home D.F., Masum S. and Stevens G,W.W. Ouant=tat=ve examinat=on of photofabricated profiles Part 5 Effect of stencd integrity on etch factor and the deep etch=ng of stmnless steel J. Photogr. Sc/. 28, No. 3 f1980) 140--144 5

Wible P.M, Regenerat=on of etchants The Photo Chemical

56

8. Standard reference method for making potent~ostat~c and potentlodynam~c anodlc polarization measurements

ANSl/ASTM G5-78 (1978) 9

Allen D.M. and Tahb T.N. The manufacture of stainless steel edge fdters -- a possible apphcatlon of electrolytic photopohshmg Precision Engmeermg 5, No. 2, (1983) 57--59

APR 1983 VOL 5 NO 2