2 Eleetroplating and metal finishing The processes and solutions described in this section are intended to give a general guide to surface finishing procedures. To operate these systems on an industrial scale would normally require recourse to one of the Chemical Supply Houses which retail properietary solutions. This particularly applies to electroplating baths containing brighteners. 32.8 Polishing compositions The following abrasive powders are used for polishing metal. ALOXITE Aluminium oxide made by fusing bauxite. Used for cutting down in the same way as emery. ALUMINA Certain grades of alumina are used for polishing stainless steel and chromium. The material is generally used in the form of a composition in which the powder is mixed with stearines or other fats. EMERY POWDER Used principally in cutting down and for preliminary operations. It is applied to the mop by means of an adhesive, usually glue. Emery powder is an impure aluminium oxide containing about 50-60% A1,0,, 3040% magnetite and small amounts of ferric oxide, silica, chromium, etc. Emery powder should never be used on magnesium or aluminium components because af the adverse effect on corrosion resistance. TRIPOLI A calcined diatomaceous earth used for polishing brass, steel and aluminium. It is used generally in tbe intermediate stages, and is usually compounded with stearines and paraffin wax to make a polishing composition which can be used directly on a mop. CROCUS POWDER A polishing composition consisting essentially of ferric oxide, of coarser grade than rouge, used for polishing iron and steel. and also, tin. Usually compounded with stearine and used with a mop or fibre brush. 32-1
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Electroplating and Metal Finishing From Smithlles Metals Reference Book-2
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2 Eleetroplating and metal finishing
The processes and solutions described in this section are intended to give a general guide to surface finishing procedures. To operate these systems on an industrial scale would normally require recourse to one of the Chemical Supply Houses which retail properietary solutions. This particularly applies to electroplating baths containing brighteners.
32.8 Polishing compositions
The following abrasive powders are used for polishing metal.
ALOXITE
Aluminium oxide made by fusing bauxite. Used for cutting down in the same way as emery.
ALUMINA
Certain grades of alumina are used for polishing stainless steel and chromium. The material is generally used in the form of a composition in which the powder is mixed with stearines or other fats.
EMERY POWDER
Used principally in cutting down and for preliminary operations. It is applied to the mop by means of an adhesive, usually glue. Emery powder is an impure aluminium oxide containing about 50-60% A1,0,, 3040% magnetite and small amounts of ferric oxide, silica, chromium, etc. Emery powder should never be used on magnesium or aluminium components because af the adverse effect on corrosion resistance.
TRIPOLI
A calcined diatomaceous earth used for polishing brass, steel and aluminium. It is used generally in tbe intermediate stages, and is usually compounded with stearines and paraffin wax to make a polishing composition which can be used directly on a mop.
CROCUS POWDER
A polishing composition consisting essentially of ferric oxide, of coarser grade than rouge, used for polishing iron and steel. and also, tin. Usually compounded with stearine and used with a mop or fibre brush.
32-1
32-2 Electroplating and metal finishing
ROUGE
A high-grade ferric oxide supplied in various degrees of fineness. It can be used in the form of a paste directly on to a soft mop or can be made into a composition with stearine. It is used essentially for finishing to obtain a very high polish on gold, silver, brass, aluminium, etc.
BLACK ROUGE
This consists of black oxide of iron and is sometimes used for finishing operations,
GREEN ROUGE
Chromic oxide used for polishing chromium and stainless steel and can be used either in the form of a composition mixed with stearine or as a paste applied directly to the mop.
VIENNA LIME
Used for making the white finish for polishing nickel, etc. It consists of a calcined dolomite and contains about 60% calcium oxide and 40% magnesia.
CARBORUNDUM
Silicon carbide used for low tensile strength materials, e.g. brass, copper, aluminium, etc. and also brittle metals, such as hard alloys and cast irons.
32.2 Cleaning and pickling processes VAPOUR DEGREASING
Used to remove excess oil and grease. Components are suspended in a solvent vapour, such as tri- or tetrachloroethylene. Note: Both vapours are toxic and care should be taken to ensure efficient condensation or extraction of vapours.
EMULSION CLEANING
An emulsion cleaner suitable for most metals can be prepared by diluting the mixture given below with a mixture of equal parts of white spirit and solvent naphtha.
Pine oil 62 g
Triethanolamine 1.2 g Ethylene glycol-monobutyl ether 20 g
Oleic acid 10.8 g
This is used at room temperature and should be followed by thorough swilling.
Table 32.1 ALKALINE CLEANING SOLUTIONS
Composition of solution Temperature Metal to be cleaned ozgal-' g l - ' "F "C Remarks
All common Sodium hydroxide metals (NaOH) 6 37.5 180-200 80-90 For heavy duty other than Sodium carbonate aluminium and (Na,CO,) 4 25.0 zinc, but Tribasic sodium including phosphate magnesium (Na,PO, . 12H,O) 6.2
1.5 Wetting agent
CIeuning and pickling processes 32-3 Table 32.1 ALKALINE CLEANING SOLUTIONS-continued
Composition of solution Temperature Metal to be cleaned ozgal-I gl- ' "F "C Remarks
Electrolytic cleaner, 6 V Current density l00/A ft-' (1O/A drn-') Article to be cleaned may be made cathode or anode or both alternately
May be used electrolytically
Table 32.2 PICKLING SOLUTIONS
Composition of solution Temperature Metal to be pickled ozgal-I g l - ' "F "C Remarks
Aluminium For etching (wrought) Sodium hydroxide
(NaOH) 8 56 104-176 40-80
Aluminium Nitric acid, s.g. 1.42 1 gal 11 Room Room (cast and Hydrofluoric acid 1 gal 1 1
Water 8gal 81 wrought) (52%)
Articles dipped until they gas freely, then swilled, and dipped in nitric acid 1 part by vol. to I of water (room temperature)
Articles first cleaned in solvent degreaser. Use polytheile or PVC tanks
Note-: It is almost universal practi- to use an inhibitor in the pickling bath. This ensures dissolution of the scale with practically no attack on the metal. inhibitors are usually of the long chain amine type and often proprietary materiais. Examples are Galvene and Stannine made by ICI.
32-4 Electroplating and metal ,finishing
Table 32.2 PICKLING SOLUTIONS-continued
Composition of solution Temperature Metal to be pickled ozgal-' g l - ' "F "C Remarks
Aluminium and other non- ferrous metals
Copper and copper alloys
Iron and steel
Bright dip Chromic acid Ammonium bifluoride Cane syrup Copper nitrate Nitric acid (s.g. 1.4) Water (distilled) to
Semi-matt dip Sodium dichromate Sulphuric acid* Water
Slow pickle to loosen heavy scale Sulphuric acid' Glue
To remove scale Sulphuric acid*
0.84 oz 0.72 oz 0.68 oz 0.04 oz 4.8 oz 1 gal
8.4 gal 0.6 gal
1 gal 4 gal
1 gal 12 02
4 gal
2 gal 1 gal 1 gal 0.5 oz
1 gal 1 gal 2 lb
3 02 18 oz 1 gal
2 Yo 0.25%
10%
5.2 g
4.2 g 0.25 g 30 ml 1 1
4.5 g
9.41 0.61
11 41
11 75 g
41
21 11 11 25 ml
11 11 2 m g
19 g 114 g 11
- -
-
195 90
195 90
150-170 65-75
70-175 20-75
Room Room
160-180 70-80
Room Room
Room Room
120-180 50-80
Immerse for 15 min. Solution has limited life. AR chemicals and deionized or distilled water should be used
Immerse for several min. Agitate work and solution. Good ventilation necessary. Addition of acetic acid useful with some alloys
After pickling articles can he dipped in sodium cyanide: 40zgal- ' (25gl-') to remove tarnish
This solution leaves a slight passive film which helps to prevent tarnish
If any scale first dip in spent bright dip. Remove stains by dipping in sodium cyanide 4 oz gal- (25 g 1-I)
If the finish is too fine add nitric acid. If too coarse add sulphuric acid
Leave for several hours or overnight
Or hydrochloric acid 10-20%
* Sulphuric acid, pure cornel. grade, s.g. 1.84. Note: It is almost universal practice to use an inhibitor in the pickling bath. This ensures dissolution of the scale with practically no attack on the metal. Inhibitors are usually of the long chain amine type and often proprietary materials. Examples are Galvene and Stannine made by ICI.
Cleaning and pickling processes 32-5 TabUe 32.2 PICKLING SOLUTIONS--continued
Composition of solution Temperature Metal $0 be pickled ozgal-' g l - ' "F "C Remarks
Iron and steel Bright dip conrinued Oxalic acid crystals 4 oz
Hydrogen peroxide 2 oz
Sulphuric acid (10%) 0.02 oz Water to 1 gal
Anode etching Sulphuric acid* 1 gal Water 2 gal
(100 vol.)
25g Room 13 g
0.1 g 11
Room This solution has so far only been used on an experimental basis
11 Not 21 above
75
Not Current density: above 2 0 0 A K 2 25 (20Adm-')
For polished work Sulphuric acid* - Not
above 75
- Not Density must not fall above below 1.61 g 01 25 work will he etched
Magnesium General cleaner and magnesium Chromic acid alloys
16-32 100- Up to 200 b.p.
Up to b.p. films, corrosion
For removal of oxide
products, etc. Should not be used on oily or painted material
Sulphuric acid pickle Sulphuric acid* 3 Yo - Room Room Should be used on
rough castings or heavy sheet only. Removes approx. 0.002 in. in 20-30 s
* Sulphuric acid, pure comcl. grade, s.g. 1.84. Note: It is almost universal practice to use an inhibitor in the pickling bath. This ensures dissolution of the scale with practically no attack on the metal. Inhibitors are usually of the long chain amine type and often proprietary materials. Examples are Galvene and Stannine made by ICI.
32-6 Electroplating and metal jnishing
Table 32.2 PICKLING SOLUTIONScontinued
Composition of solution Temperature Metal to be pickled ozgal-' gl- ' "F "C Remarks
Stainless steel To loosen scale Sulphuric acid* 13-30 80-180 130-160 60-70 Use prior to scale Hydrochloric acid 6-20 40-120 removal treatment, for (s.g. 1.16) heavy scales
White matt finish Ferric sulphate 13 80 160-180 70-80 5-15 min [Fex(so4),1 Hydrofluoric acid 6 40 (52% HF)
Zinc and zinc Bright dip alloys Chromic acid (CrO,) 40 250 Room Room 5-30 s. If yellow film
Sodium sulphate 3 19 persists after rinsing ( N a W 4 ) dip in sulphuric acid:
1 floz per gal (6 ml1-I) and rinse again
* Sulphuric acid, pure comcl. grade, s.g. 1.84. Nore: It is almost universal practice to use an inhibitor in the pickling bath. This ensures dissolution of the scale with practically no attack on the metal. Inhibitors are usually of the long chain amine type and often proprietary materials. Examples are Galvene and Stannine made by ICI.
Anodizing and plating processes 32-7
32.3 Anodizing and plating processes Table 32.3 ANODIZING PROCESSES FOR ALUMINIUM Good ventilation above the bath and agitation of the bath is advisable in all cases.
Temp- Current COmpOSiliOfl of SOhtiOn rrature density Time
02 ampft-2 and gal-'g I - ' "F - "C (A dm-2) voltage Cathodes Vat Hangers Remarks
Chromic acid 5-16 30-100 103- 38- Current tl-10 min Tank or Steel Pure Slight agitation is (CrO,), chloride content must not exceed 0.2 g I-', sulphate less than 0.5 g 1-' (After Bengough- Stuart)
Sulphuric acid (sg. 32 200 1.84)
Hard anodizing Hardas process Sul?huric acid
Elosui G X process Oxalic acid (COOH),.2H20
Eloxal WX process Oxalic acid
108 42 con- 0 4 0 V stain- (ex- a h - required trolled increased less hausted) minium by in steps steel voltage. of 5 V Average 5-35 min 3-4 Maintain (0.34.4) at 40 V d.c. 3-5 min
Increase gradually to 50 V 4-5 min Maintain at 50 V
60- 15- 10-20 12-18V Alum- Lead 75 24 (1-2) d.c. 20-40 inium or lined
min lead pla- steel tes (tank if lead lined)
32 200 23- -5- 25-400 40- 41 + 5 (2.540) 120V
d.c.
12.8 80 70 20 10-20 50 V (1- 2) d.c. 30-60
min
12.8 80 75- 25- 20-30 20-60 95 35 (2-3) a s . V 40-
60 min
Lead Lead lined steel
Vat Lead lining lined
steel
Vat Lead lining lined
steel
or This process titanium cannot be used
with alloys containing more than 5% copper
Pure The current alumin- must not ium or exceed 0.2 titanium A l - ' of
electrolyte
Alumi- Agitation nium required. or Gives coating titanium 1-3 thou.
thick
Alum- Oxalic acid pro- inium cesses are more ex- or pensive than sul- titanium phuric acid anodiz-
ing; but coatings are thicker and are coloured.
Alum- inium or titanium
Integral colour Anodizing Kalcoior process
Sulphosalicylic acid 16 100 (3) d.c. 2 M 5 Lead Lead Alum- Aluminium level in Sulphuric acid 0.8 5 72 22 30 25-60 V
min lined inium solution must be steel or maintained between
titanium 1.5 and 3 gl-'
t Period according to degree of protection. Complete cycle normally 40 min
32-8 Electroplating and metal ,finishing
Table 32.4 ANODIZING PROCESSES FOR MAGNESIUM ALLOYS
Tempera- Composition of solution ture Current
density Time 0 2 A ft-a and gal-' g I- ' "F "C (A dm-') voltage Cathodes Vat Hangers Remarks
HAE process Potassium hydroxide Aluminium Potassium fluoride Trisodium phosphate Potassium manganite
Cr 22 process Chromic acid Hydrofluoric acid (G%, H m 4 (85%) Phosphoric acid
Ammonia solution 25- 160- 30 180
19.2 120
1.7 10.4 5.5 34 5.5 34
3.2 20
39 232
16 100
14 88
4 25 4 25
13.5 84
MEL process Fluoride anodize Ammonium 16 100 bifluoride
<95 t 3 5 12-15 90 min Mg Mild Mg Matt hard, brittle, (1.2-1.5) at 85 V alloy steel or alloy corrosion resistant.
approx. for a.c. rubber dark brown 25- ax. Mg or lined 50 pm thick, pre- steel if abrasion resistant ferred d.c. used
160- 70- 5-50 180 85 (0.5-5)
165- 75- 15 205 95 (1.5)
<86 <30 5-100 (0.5-10)
10-100 Mg Mild Mg Matt dark green, min up alloy steel or alloy corrosion to for a.c. rubber resistant, 25 pm 110 V Mg or lined as. steel abrasion or d.c. for d.c. resistant
thick approx.,
12 min - Mild Mg Matt dark green, 380 V steel alloy corrosion as. resistant, 25 pm
thick approx.
30 min Mg Rubber Mg Principally a 120 V alloy lined alloy cleaning process a.c. for ax. to improve pre- Mg or corrosive ferred steel resistance by
for d.c. dissolving or ejecting cathodic particles from the surface
32.4 Electroplating processes
Table 32.5 PLATING PROCESSES ~ ~~
Qperating conditions
Temperature Current density Current efficiency
Metal Type and composition ozgal-' g l - ' "F "C Aft-, Adm-2 % Voltage pH Anodes Vat Remarks
The treatment time is 3-5min at a temperature of 175-185°F (80-85T) with mild agitation. The pH of the bath should be 10-10.4.
The steps of the complete process are:
1. Solvent or vapour degreasing. 2. Hot caustic soda clean or cathodic cleaning in alkaline cleaner. 3. Pickle $-1imin in 1% hydrochloric acid and rinse. 4. Zinc immersion bath as above without drying off from the rinse. 5. Cold rinse and immediately apply copper strike as under.
3040 A ft-' ( 3 4 A dm-') for 9-1 min, reducing to 15-20 A ft-2 (1.5-2 A dm-') for 5 min or longer.
If required, the copper thickness from the above strike can be built up in the usual alkaline or proprietary bright plating baths. Following the above steps, further plating may be carried out in conventional electroplating baths.
ELECTROLESS PLATING ON MAGNESIUM
Deposits of a compound of nickel and phosphorus can be obtained on magnesium alloy components by direct immersion in baths of suitable compositions. Details of the process may be obtained from the inventors, The Dow Chemical Co. Inc., Midland, Michigan, USA.
'GAS PLATING' OF MAGNESIUM (VAPOUR PLATING)
Deposits of various metals on magnesium components (as on other metals) can be produced by heating the article in an atmosphere of a carbonyl or hydride of the metal in question.
32-18 Electroplating and metal .finishing
32.6 Electroplating process parameters
Table 32.6 AVERAGE CURRENT EFFICIENCIES OF PLATING SOLUTIONS The figures given below are ap- proximate
Cadmium (oxide) Chromium Copper (acid) Copper (cyanide) Copper (Rochelle) Gold Indium (cyanide) Indium (sulphate) Iron Lead Nickel Silver Tin (acid) Tin (stannate) Rhodium Zinc (acid) Zinc (cyanide)
Copper (cuprous) Copper (cupric) Gold (auric) Indium Iron (ferrous)
Lead Nickel Palladium Platinum Rhodium
Silver Tin (stannous) Tin (stannic) Zinc
0.335 1.515 2.096 0.323 5 1.099
2.372 1.186 2.452 1.428 1.042
3.866 1.095 1.990 3.642 1.920
4.025 2.215 1.108 1.220
0.011 8 0.053 4 0.073 9 0.0114 0.038 8
0.083 7 0.041 8 0.086 5 0.050 3 0.036 8
0.136 3 0.038 6 0.070 2 0.1284 0.067 7
0.1294 (Troy) 0.078 1 0.039 1 0.043 0
1356 2 989 299 659 216 476 701 1545 413 911
191 42 1 383 844 185 408 318 701 435 959
117 258 414 913 228 503 125 276 236 520
113 (Avoir.) 350 205 451 409 902 372 820
Miscellaneous coating processes 32-19
32.7 Miscellaneous coating processes
(1) AUTOCATALYTIC PLATING
Autocatalytic plating is a form of electroless plating in which metal is deposited via a chemical reduction process (as opposed to immersion plating in which thin coatings are formed by electrochemical displacement of the coating metal).
Processes exist for the autocatalytic deposition of a large number of metals, particularly nickel, gold, silver and copper. Basically, the solutions contain a salt of the metal to be deposited and a suitable reducing agent (most commonly hypophosphite, but also hydrazine and boranes etc.). When a metal substrate, which is catalytic to the solution, is introduced into the bath, it becomes covered with a layer of the coating metal which is itself catalytic and thus the process can continue. This mechanism results in an extremely even distribution of deposit on the substrate, i.e. these solutions have a high 'throwing power'.
The most widely used autocatalytic process is nickel-phosphorus; a typical acid bath is as follows:
nickel chloride 4.8 oz gal-' (30 g I - l )
sodium glycollate 8.0 oz gal - ' (50 g 1-') sodium hypophosphite 1.6 oz gal-' (10 g 1-1)
The solution is operated at temperatures between 75 and 100"C, at pH 4-6, giving deposition rates up to 0.6 thou per hour (15 iun h-l). The deposit is an alloy of nickel and phosphorus, containing about 7-10% phosphorus. A useful property of this material is that it can be hardened (typically by heat treating for 1 hour at 400°C) so as to increase the as-deposited hardness from 400 HV to almost 1000 HV. Thus, autocatalytic nickel coatings find engineering applications, often as a replacement for hard chromium electrodeposits.
(2) COMPOSITE COATINGS
Composite coatings can be electroplated or electroless plated deposits into which a uniform distribution of a second phase material is dispersed. The incorporated material can be hard, ceramic particles for increased wear resistance, as in the Tribomet process carried out by BAJ Ltd. Alternatively, PTFE particles can be used to increase surface lubricity as in the Niflor process from Norman Hay Engineered Surfaces Ltd.
(3) ELECTROSTATIC AND ELECTROPHORETIC METHODS OF PAINT APPLICATION
These are methods which have been developed for the economic application of paint to articles of complicated shapes (and often skeleton structure) in large numbers.
When an article like a metal chair is sprayed by a conventional spray gun procedure, much of the spray overshoots the surfaces and is wasted. In the electrostatic method a very high electrical potential is developed between the gun and the article being painted. The droplets of paint assume a charge of opposite sign to the workpiece and are attracted to it. This ensures more uniform coverage, less overspray, and thus greater economy in operation.
Electrophoresis has been utilized in a somewhat similar way. When a direct current is applied to an aqueous emulsion, large dispersed molecules and even oily particles are caused to move towards one d the electrodes. In the electrophoretic process a paint is provided as an aqueous emulsion and current is applied in such a manner that the globules of paint move towards and attach themselves to the object to be painted. The process can be made automatic and continuous and results in very uniform build-up even on points and sharp edges. It is only suitable for large- scale operations but is very economical in paint.
(4) COATING WITH CERAMIC MATERIAL§
Just as metals can be sprayed, certain refractory oxides and silicates and the like can be applied by flame gun. Coating thickness and uniformity can be controlled by suitable means and hard, dense coatings can be built up. The coatings resemble biscuit-ware rather than a vitreous glaze, that is, they are absorbent: their chief use is to provide abrasion resistance and to delay heat transfer.
32-20 Electroplating and metal jnishing
(5 ) MECHANICAL PLATING
Mechanical plating is a method of plating Which utilizes mechanical energy to deposit metal coatings on to metal parts. Parts, glass beads, water, chemicals and metal powder are tumbled together in a barrel at around room temperature to obtain the desired coating.
The process is used primarily to provide ferrous-based parts with sacrificial coatings of zinc, cadmium, and co-deposits with tin. Parts treated by this method are most often fasteners, springs, clips and sintered iron components which are typically handled in bulk.
Parts which have been degreased, descaled, and copper-flashed are tumbled in rubber-lined barrels with water, glass bead impact media, promoter chemicals, and a finely divided powder of the metal to be plated. The promoter chemical serves to clean the metal powder and controls the size of the metal powder agglomerates that are formed. The mechanical energy generated from the barrel's rotation is transmitted through the glass impact media and causes the clean metal powder to be cold welded to the clean metal parts, thereby providing an adherent, metallic coating.
Due to the absence of an impressed current during coating, the process does not produce hydrogen diffusion into the steel substrate. Thus, a post-electroplating bake, in order to preclude hydrogen embrittlement of high strength steel components, is not required for mechanically plated deposits.
32.8 Plating formulae for non-conducting surfaces
(1) METAL POWDERS
The article to be treated may be coated with a metal powder. The best powder for this purpose is a finely ground copper, which is generally sold under the commercial title of bronze powder. This may be applied by mixing it with a cellulose lacquer to which has been added five parts by volume of thinner and spraying it on the object concerned. Alternatively, the object may first be sprayed with lacquer and before it is quite dry may be brushed over with the bronze powder using a soft camel-hair brush. After this treatment the article can be struck over in an acid copper solution.
Waxes (for gramophone records, etc.) may be coated directly by brushing with bronze powder and a soft brush. After brushing with bronze powder they may be treated in the following manner to improve the conductivity and reduce the time of covering in the plating bath
1. Brush with a soft brush and a 50% mixture of methylated spirit and distilled water. 2. Immerse in a solution containing 30 g I- ' of sodium cyanide and 6 g 1-' of silver nitrate. 3. Make up two solutions as follows: (a) Pyrogallic acid 7 g 1-', citric acid 4 g 1-' and (b) silver
nitrate, 40 g I - ' . Take four parts of solution (a) and one part of solution (b) and mix together and immerse article in this solution for about 10 min.
4. Swill and place in plating bath.
(2) SILVER REDUCTION
A number of articles can be treated by directly reducing silver on the surface. This process is particularly applicable to plastics and glass. Any process which will form a good silver mirror may be used, but the following will be found satisfactory for most purposes.
1. The surface of the article must be very thoroughly cleaned and completely free from grease. Glass and porcelain may be cleaned by using concentrated acid and alkali alternately. Plastics may be treated by brushing or barrelling with a mixture of Vienna lime and pumice, by treating with a suitable solvent or by immersing in a solution of chromic acid.
2. Priming. After cleaning the article is immersed in a 10% stannous chloride solution. Alternately, the solution may be swabbed on to the surface with cotton wool. The article is then thoroughly swilled.
3. The article is then silvered by immersing in a silvering solution, the formula for which is given below. The silvering operation generally takes about 20 min and the temperature must be carefully controlled during this period; usually about 21°C will he found the most satisfactory. The solution should be slightly agitated during the process.
4. The articles are thoroughly swilled and struck over in a copper tartrate bath. After being coated over with copper any desired plating can be made upon it.
Stripping electroplate coatings 32-21
Tte silvering solution is prepared from the following:
Solution (a) 100 g I - ' Rochelle salt. Solution (b) 10 g 1-' silver nitrate. Solution (c] 200 cm3 per litre ammonia (sp. gr. 0.880).
Take 100 cm3 of solution (b) rand add solution (c) carefully a little at a time until the precipitate which first forms just redissolves. If too much is added and the solution becomes quite clear. add a few drops of solution (b) until a very faint turbidity is produced. Then add 20 cm3 of solution (a), thoroughly mix and use immediately.
[N.B. The brown precipitate formed by adding ammonia to silver nitrate is explosive if allowed to dry. Care should be taken therefore to see that this does not happen.]
(3) 'VACUUM METALLIZING'
This process is carried out at less than 0.0007 mmHg pressure and can only be applied to objects which are stable under these conditions. The metal to be deposited is heated until it evaporates and there are several ways in which this is achieved. The vapour recondenses on the first cool surface it encounters, and can be made to form a thick dense coating. The 'throwing power' is very poor since the evaporated metal travels in straight lines and steps must be taken to rotate or xanipulate objects exposed to it in order to achieve uniform coating. By controlling the temperature of the work piece the crystal structure of the deposit can be varied.
32.9 Methods of stripping electroplated coatings
CADMIUM OR TIN FROM STEEL
Coatings may be stripped from steel by immersing the article in a solution containing 1 gal (4.5 litres) of hydrochloric acid, 2 oz (57 g) of antimony trioxide and f pint (280 ml) of water. After stripping and rinsing the article will probably require wiping to remove smuts.
CHROMIUM
Chromium may be stripped from non-ferrous metals by dissolving it in dilute hydrochloric acid. From steel it is best stripped by making it the anode in a solution of sodium hydroxide. If the base metal is zinc or zinc base diecastings, it is best to strip the chromium by making it the anode in sodium carbonate solution as this will not attack the exposed zinc. The conditions of operation are not critical.
COPPER FROM STEEL
Copper can be stripped from steel by immersing in a solution containing 5 lb gal-' (500 g I-') of chromic acid and 8 oz gal-' (50 g I - ' ) of sulphuric acid. This solution will work at room temperature but strips the copper very quickly if heated.
Alternatively, the article can be made anodic (2-6 V) in a solution containing 14 oz gal-' (90 g 1-') sodium cyanide and 2.4 oz gal-' (15 g 1-I) sodium hydroxide.
COPPER FROM ZINC AND ZINC BASE DIECASTINGS
Copper may be stripped from these materials by immersing in a solution prepared by dissolving 18 g of sulphur and 250 g of sodium sulphide (Na2S.9H20) in a litre of solution. The solution works very rapidly if warmed. The sludge formed on the surface of the qbject will require removing from time to time by brushing or immersion in a 120 g I - ' sodium cyanide solution.
32-22 Electroplating and metal jinishing
COPPER, NICKEL, ETC., FROM MAGNESIUM
Most metal deposits can be removed from plated magnesium by submitting the component to the fluoride anodizing process. Alternating current is used in a bath of 10% ammonium bifluoride, in which the plating gradually dissolves without affecting the magnesium. About P 1 0 V are used until most of the plating has disappeared. Finally, the voltage is raised to 120 V to complete the process and cleanse the magnesium from remaining traces of foreign metal. Magnesium hangers must be used with firm connections. The bath should be operated cold. Direct current may be used if the work piece is made the anode using mild steel or magnesium cathodes.
LEAD FROM STEEL, COPPER AND BRASS
Immersion of article in a solution of 95 vol. % glacial acetic acid, 5 vol. % hydrogen peroxide (30 wt %). Dilute solutions may be used although this can lead to pitting of steel.
NICKEL FROM COPPER AND BRASS
Nickel may be stripped from copper by an anodic treatment in either 60 vol. % sulphuric acid or 15 g 1-' hydrochloric acid. Care must be taken with the concentration of the acids as this can affect the pitting of the substrates.
NICKEL FROM STEEL
Nickel can be anodically stripped in sulphuric acid, as for nickel from copper. Copper sulphate (30g 1-') or glycerine (3Ogl-') can be added to reduce pitting of the steel. An immersion process involves the use of fuming nitric acid (85-95% HNO,, sp. gr. 1.50) from which water is excluded.
SILVER FROM BRASS
Silver may be stripped from brass by immersing the object in a mixture of 1 vol. conc. nitric acid and 19 vols. conc. sulphuric acid heated to 175 "F (80 "C). The silver is dissolved in a few minutes. The articles should be immediately removed and swilled.
ZINC FROM STEEL
The reagent used for stripping cadmium may be used. Alternatively zinc may be stripped from steel in either warm dilute hydrochloric or sulphuric acid, or l0-15% ammonium nitrate solution, or hot sodium hydroxide solution.
32.10 Conversion coating processes
(1) PHOSPHATING
Phosphating solutions are used to produce corrosion-resistant coatings on ferrous metals and also zinc, cadmium and aluminium. Probably the most important application for these coatings is to act as bases for subsequent painting operations.
Basically, phosphate solutions comprise metal phosphates dissolved in carefully balanced solutions of phosphoric acid. When a clean metal surface is dipped into the solution, the free acid present reacts with the metal, liberating hydrogen and causing the pH of the solution, adjacent to the metal, to rise. This unbalances the solution, resulting in the precipitation of metal phosphates which form a film, chemically bonded to the substrate.
Due to the complexity of modern phosphate solutions, these processes are normally proprietary, examples of which are listed in section 32.11.
There are four main types of phosphate solution: iron, zinc, heavy zinc and manganese, and these produce increasing weights of coating from 30-90mgft-' for iron phosphate solutions to 1000-4000 mgft - * for manganese phosphate solutions.
Conversion coating processes 32-23
(2) CHROMATING
Chromating solutions contain hexavalent chromium ions and a mineral acid and are used to increase the corrosion resistance of metals, in particular zinc, cadmium, aluminium and magnesium, by forming a surface layer containing chromium compounds.
The process is usually performed by immersion, although spraying or brushing processes are also used. A wide variety of proprietary solutions are available which produce coatings of different thicknesses. These coatings are often distinguishable by their colour which can vary from clear, to blue, to iridescent yellow and finally to black.
(3) COLOURING OF METALS
The following solutions and operating conditions will produce coloured conversion coatings, as detailed:
(i) Copper and Brass Black
Copper carbonate Ammonia 2pt (950 ml) Water 5pt (2.4 1)
1 lb (454 g)
The copper carbonate and ammonia are mixed before adding the water. The solution is operated at 175'F (80°C).
The blue black colour may be fixed by dipping in 2$% sodium hydroxide solution.
Green Water Sodium thiosulphate Nickel ammonium sulphate
or Ferric nitrate Temperature
Potassium chlorate Nickel sulphate Copper sulphate Water Temperature
Brown
(ii) Iron and steel Black
Sodium hydroxide Sodium nitrate Sodium dichromate Water Temperature
Ferric chloride Mercuric nitrate Hydrochloric acid Alcohol Water
Blue
1 gal (USA) 8 oz 8 oz 1 oz 160°F
53 oz 2: 02 24 oz 1 gal (USA) 195-212°F
8 lb 14 oz 14 02. 1 gal. 295°F
2 oz 2 oz 2 oz 8 0 2 8 oz
Room temperature. Parts are immersed for 20 minutes, removed and allowed to stand in air for 12 hours. Repeat and then boil in water for 1 hour. Dry, scratch-brush and oil.
Brown Copper sulphate Mercuric chloride Ferric chloride Nitric acid Alcohol
3 oz gal (20 g 1 - 1 )
0.8 oz gal-' (5gl-I) 5 oz gal-' 25 oz gal-' 93 fl oz gal-'
(30 g 1- ( 150 g 1- ) (700 ml 1 - I )
32-24 Electroplating and metal jinishing
Dip in solution, place in hot box at 175°F (80°C) for 30 minutes, stand in steam box at 150°F (65°C) until coated in red rust, immerse in boiling water to form black oxide, dry and scratch brush. Repeat this operation three times before oiling with linseed oil.
(iii) Stainless steel Black
Sulphuric acid Water Potassium dichromate Temperature
(iv) Zinc Black
Ammonium molybdate Ammonia
180 parts 200 parts 50 parts 210°F (99°C)
4 02 gal-' (24gl-') 6 fl 02 gal-' (45 mll- ' )
Heat solution to obtain a deep black. Rinse in cold and then hot water; allow to dry and harden.
(v) Aluminium Black
Potassium permanganate Nitric acid Copper nitrate
1.6 oz gal-' ( logl-1) 0.5 fl oz gal-' 4 oz gal-'
(4 mll- ' ) (25 g 1-')
Operate at 70°F (24°C) for 10 minutes. Blue
Ferric chloride 60 oz gal-' (360 g 1-') Potassium ferricyanide 60 oz gal-' (360 g 1-') Temperature 150°F (66°C)
32.11 Glossary of trade names for coating processes 32.11.1 Wet processes (1) PHOSPHATE PROCESSES
Processes by which a coating of phosphate is produced on the surface of steel or zinc base alloys by treatment in or with a solution of acid phosphates. For rustproofing, the metal must receive a finishing treatment with paint, varnish, lacquer or oil; examples of typical finishing treatments are given under Parkerizing but it should be understood that firms using or marketing other proprietary phosphate processes may apply different designations or use different media for the necessary finishing treatment.
Bonderizing A proprietary phosphate process applied to steel and zinc, marketed by Ardrox Pyrene Ltd. (similar to Parkerizing for steel but produces a thinner and less protective coating; synonymous with Parkerizing for zinc alloys).
Coslettizing The original phosphate process for steel, introduced in 1903. Electro-granodizing A proprietary phosphate process applied to steel, marketed by IC1 Ltd.
(Paints Division). The chemical action of the solution is assisted by electrolysis. Granodizing Proprietary phosphate processes applied to steel and zinc marketed by IC1 Ltd.
(Paints Division). Lithoform A proprietary phosphate solution applied to zinc, marketed by IC1 Ltd. (Paints
Division). Parkerizing A proprietary phosphate process applied to steel and zinc, marketed by Ardrox
Pyrene Ltd. Examples of subsequent finishing treatments are:
P20 Dewatering black finish P41 Black shellac finish P75 Oiled finish P96 Oiled finish SP55 Mineral oil finish
Walterizing A proprietary phosphate process applied to steel and zinc base alloys, marketed by the Walterisation Co., Ltd.
Glossary of trade names for coating processes 32-25
(2) ALKALINE OXIDATION PROCESSES
Processes by which a black oxide film is formed on steel by treatment in a strongly alkaline solution containing an oxidizing agent. For rustproofing, the metal must receive a finishing treatment which is usually carried out with oil.
Black Magic Blackening processes marketed by M&T Chemicals Ltd. Blakodizing Ebonol
Jerul An American process marketed by Technic Inc.
Black chemical finishes on steel, marketed by Tool Treatments Ltd. Range of conversion coating processes for both ferrous and non-ferrous metals, marketed
by OMI-Imasa (UK) Ltd.
(3) CHROMATE PROCESSES
Alocrom Enthos Chromating solutions marketed by OMI-Imasa (UK) Ltd. Kenoerr Chromating solutions marketed by the 3M Company Ltd.
Chromating solutions marketed by IC1 Paints Ltd.
(4) ANODIC OXIDATION OF ALUMINIUM AND ITS ALLOYS (ANODIZING)
For protection against corrosion and wear, for decoration, for aiding heat emission and for miscellaneous uses based on the absorptive properties of the oxide film when freshly made; used in conjunction with electrolytic ‘polishing’ for producing reflectors. Processes involving electrolytic treatment in solution, generally of chromic, sulphuric or oxalic acid with the production of a relatively thick film of oxide.
Anobrite Bright anodizing process operated by Anobrite Ltd. Bengough-Stuart process (chromic acid) The first anodizing process patented in 1923. Brytul process A process of producing reflectors introduced by British Alcan Aluminium. Eloxal A generic term used in Germany for anodic oxidation. Phosbrite process Chemical polish for bright anodizing marketed by Albright & Wilson Ltd.
(5) IMMERSION PROCESSES FOR THE TREATMENT OF ALUMINIUM ALLOYS
Alocrorn process A process of priming a thin greenish yellow film in a cold acid solution containing chromates. Marketed by IC1 Ltd. (Paints Division).
Alumon Zincating process marketed by Enthone/Imasa Ltd. Bondul Zincating process marketed by W. Canning Ltd. Decorul Oxidizing process giving electrically conducting coatings capable of being coloured by
A4BVprocess (Modified Bauer Vogel) A process of forming a thin oxide film by immersion in
P.vlumin process A similar process marketed by Ardrox Pyrene Ltd.
dyeing. Marketed by Lea Manufacturing Co. Ltd.
an alkaline solution containing chromates.
(6) NON-ELECTROLYTIC PROCESSES
Enplate Electroless nickel plating solutions, marketed by Enthone/Imasa Ltd. lliklud Electroless nickel plating solutions, marketed by Lea Manufacturing Co. Ltd. Sylek Electroless nickel-boron plating solutions, marketed by Imasa Ltd. Transiflo Mechanical plating process, marketed by the 3M Company Ltd.
(7 ) ELECTROPLATING AND ELECTRODEPOSITION PROCESSES
Achrolyte Tin-cobalt alloy plating process marketed by Udylite/Oxy Metals Ltd. Alecra 3000 Brylunizing
Chromonyx
Fescdizing
Trivalent chromium plating process marketed by Albright and Wilson Ltd. Zinc coating of wire by the ‘Bethanizing’ electroplating process of the Bethlehem
Steel Co. Black chromium plating process, often used for coating solar panels. Marketed by
Harshaw Chemicals Ltd. A term applied by FescoI Ltd., to any electrodeposition process carried out by them
(incomplete without mention of the metal referred to, e.g. Fescolizing in chromium, etc.). Now carried out by British Metal Treatments Ltd.
32-26 Electroplating and metal finishing
Listard process-Van der Horst process Patented processes of hard chromium plating for protecting the cylinders of internal combustion and other engines from wear. These processes give an oil-retaining surface to the chromium. The processes are operated by British Metal Treatments Ltd.
Niron Bright nickel-iron plating process, marketed by OMI-Imasa (UK) Ltd. Rovalizing A term applied by International Corrodeless Ltd., to any protective coating process
used or marketed by them. Incomplete without description of the process referred to (e.g. Roval cadmium, etc., see also phosphate processes).
Tryposit A name used by Thomas Try Ltd., to indicate their special process of electrodepositing heavy nickel or hard chromium for engineering purposes.
Zartan Alloy deposit, used as an alternative to decorative chromium. Marketed by M&T Chemicals Ltd.
32.11.2 Dry processes
(1) THERMAL PROCESSES
The processes described below involve heating of the object to be coated in contact with the coating metal (in the form of powder or as a coating to secure inter-penetration) or with a compound of the coating element. Used for coating steel except where otherwise stated.
Aluminizing A process involving spraying with aluminium and heating to cause alloying. Bower Barff A process of coating steel with a black oxide by heating in contact with steam. Calorizing A process of coating with aluminium, similar to sherardizing (see below) but using
Chromizing A process of coating with chromium involving heating to a high temperature in
Galvanizing A process of coating with zinc by dipping pickled steel into molten zinc
Zhrigizing A process of coating with silicon by heating to a high temperature in contact with
Nitriding A process of forming a hard layer on steel involving heating in a suitable atmosphere
Sherardizing A process of coating with zinc involving heating in contact with zinc powder in
aluminium in place of zinc.
contact either with a vapour of chromous chloride or with metallic chromium.
(electrogalvanizing is sometimes used to mean electroplating with zinc).
the vapour of silicon tetrachloride.
(usually ammonia vapour) to form a surface layer rich in nitride.
revolving drums (introduced by Sherard Cowper Coles).
(2) METAL SPRAYING PROCESSES
A method of coating consisting of projecting a stream of molten metallic particles at high velocity against the surface to be coated. Mainly for protection against corrosion but also used for restoring the dimensions of undersized parts.
Wire or Schoop process A process marketed by Metallization Ltd., and Metallizing Equipment Co. Ltd., in which the coating metal in the form of wire is melted and atomized.
Schori or powder process A process marketed by Schori Metallizing Process Ltd., in which a stream of the powdered coating metal is fed into a flame and blown on to the surface to be coated.
Mellosing or molten metal process A process in which a molten metal is fed into a jet of heated compressed air which serves to atomize it and to project it against the surface to be coated.
Arc spray process A process in which the metal feed material is melted by an electric arc and then atomized by a stream of compressed air.
Plasma sprayprocess A process in which a very high temperature plasma is produced by blowing gas through an electric arc. Metal wire or powder is melted by passage through the plasma and is projected by the gas on to the surface to be coated.
Detonation or D-gun process Oxygen, acetylene and the material to be plated, are introduced into a detonation chamber where a spark ignites the mixture. A detonation wave travelling at supersonic speed, forces the powdered material heated to 1 3 5OO0C, on to the substrate. A special building is required for sound insulation. Very high density coatings of refractory materials like tungsten carbide, can be plated by the process.