Chrome Plating

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INTRODUCTION TO THE CHROME PLATING PROCESS

Chrome plating is not difficult providing the part has been properly cleansed and the following requirements met:

Preparation of the chromic acid (CrO3) solution..(Do not acquire the hydrogenated [ H2CrO4 ] chromic acid crystals)
Temperature control of the bath (plating solution)
Preparation of lead anodes (peroxide)
Agitation method of the bath (bubbles)
Plating current density control and duration (controller)
Ventilation (for safety)

All that remains is the requirement of time - so don't let the apparent complexity of the task discourages you because the results are very worthwhile, indeed.

I have studied the industrial processes involved, reduced them to pint-size applications for model engineering, and experimented enough to be able to tell you what works. We have a lot to learn and the process has been laid out for you in ten easy steps. So, here we go!

1. DECORATIVE CHROME, OR HARD CHROME?

All chromium is about the same hardness; 800 to 1000 VHN - very hard! The main difference lies in the thickness of the deposit.

For decorative purposes, chrome sits best on nickel which itself adheres very well to copper - this combination also offers the best corrosion protection resistance. Decorative chrome thickness will vary from a few hundredths of a mil to 1 mil. The mirror finish will only be as good as the finish you put on the surface before you put on the chrome.

For functional purposes, to take advantage of the extremely low chrome coefficient of friction, or for wear build-up (bearing surfaces or pistons, as examples), hard chrome is plated in thickness as required from 1 to 50 mills.

When used as a bearing surface. Chrome must be micro-finished (more on this later) and will then provide a coefficient of friction lower than any other metal when used against steel, iron, brass, bronze, babbitt, or aluminium alloys. Do not use chrome against chrome. Because chrome is also much harder than casehardened steel, we then have a perfect set-up for longwearing working surfaces. Chrome will resist mostly all organic and in organic compounds and acids, except hydrochloric acid (muriatic).

2. CONTROL OF THICKNESS OF DEPOSIT

Given fixed parameters for temperature, plating solutions, anodes, set-up, and current density, thickness is a function of time. Expect around .75 to 1.2 mil per hour of plating time.

I have plated up to 20 mills successfully at home - admittedly this was by accident because I was aiming for 3 mills deposit to refinish a piston! It had previously taken six hours using a particular chromic acid solution to deposit 3 mills of excellent chrome. I thought to shorten plating time I would increase the current density

from 600 mA to 800 mA and the temperature of the solution was tweaked from 450 C. to 500 C. (1 13oF to 122o F). I then plated, with agitation, for five hours and wound up with an hour-glass shaped piston, due to a 13 mill chrome deposit measured at mid-skirt level and 21 mils on the edges (formed by the bottom of the skirt and the piston crown).

Let that be a lesson to all of us: Never change more than one parameter at a time.

Subsequently, grinding of the same piston was successfully carried out; which attests to the excellent adhesion of the chrome to the base metal (steel) as prepared earlier (see "Cleaning the work").

Of course, the piston was then lapped to a perfect fit in the re-lapped bore (no rings involved in that .020 cu.in. engine). We'll come to the grinding and lapping notes later. Chrome will lap to a superb finish, to a degree of precision obtainable by no other method and limited only by the machinist's patience and skills.

3. PREPARATION OF THE CHROMIC ACID BATH

NOTE: The chemical formulations given in this article are in avoirdupois ounces per gallon of solution (avoir. oz./gal). To convert these to metric measure, simply multiply the oz/gal number by the conversion factor of 7.5 to obtain grams per litre.

3.1 BRIGHT CHROME

I use the basic formulation of 100:1 chromic acid/sulphuric acid proportions:

Chromic acid crystals = 33 oz. (936 grams)

Sulphuric acid fluid = .33 oz. (9.36 millilitre)

Distilled (or demineralized) water to make 1 gallon (3.79 litre).

Of course, you can vary these proportions in accordance with the quantity you wish to make up. So, to make up one pint for small work, simply divide everything by eight The dilution ratio of the sulphuric acid as purchased has to be taken into account and the amount used in the bath must be one of pure H2SO4tO 100 Cr03.

Be very accurate in this process; and:

ALWAYS ADD ACID SLOWLY TO WATER -- NEVER ADD WATER TO ACID --

If you have access to demineralized water from your home dehumidifier (of course, clean filter if required). This is a good substitute for the recommended distilled water.

Also, I recommend the use of surgical rubber gloves when handling any of the chemicals called up in this article. Pharmacies (Chemists) carry them and they are much easier to replace than the skin of your hands.

The chromic acid crystals yield about 52% pure chrome metal. For reasons, which must remain unexplained at this stage, a freshly mixed solution will only deposit passably good chromium. The same solution, like a good wine, improves with age... So use it for experimentation when first mixed, before you undertake any serious plating - I keep mine in a sealed glass container and it is good for years. Filter as required between uses - plating current will be around 0.75 A/sq.in. For bright chrome and up to 1.4 A/sqin. for dull 'hard chrome'.

3.2 BLACK CHROME

Black chrome can also be plated in the same way and still have similar characteristics to the bright chrome. For aesthetic or anti-reflective applications, it may be preferable in some cases. I have not yet used it, but the formula is as follows:

Chromic acid 33 oz (936 g)

Acetic acid = 28.2 oz (800 g)

Barium acetate =1.0 oz (28 g)

Distilled (or demineralized) water to make 1 gal. (3.79 litre).

Operation of this bath will be at 90 to 115 F (32.2 C. to 46.1 C.) and at a current density of 0.25 to 0.63 AIsq.in. (More on how to set this up later).

3.3 TEMPERATURE CONTROL

Temperature is critical for good (or any) results. This is best maintained automatically by using a thermostatically controlled electric heater right in the bath. A simple and cheap expedient for this requirement is to use a tropical fish-tank heater available at any pet store. And, while you're there, pick up a fish tank air pump, plastic piping to suit, and one air valve control, too.

The 115 V heater comes in a quartz tube with a temperature control knob on top. This acts on a bi-metal strip contact tension and can easily be cranked up to maintain the required 45 C. to 50 C. (1130F. to 122 F). A thermometer covering this range is also required.

It is important this temperature range be maintained throughout plating times.

4. ANODES

These are made of lead strips. Various sources can be found, but the material I use is about 1/8" (3.175 mm) thick cut or formed into the required shapes from a 4" (101.6 mm) waste pipe section available from plumbing/hardware stores. Joiner sections can be bought about 8" (203.2 mm) long.

Lead is attacked by chromic acid and an insulating layer of lead chromate (yellow in colour) will be formed if the anodes are not first treated. This lead chromate interferes with the plating current flow, but is easily avoided by transforming the working surface of our anodes into lead peroxide. The oxide prevents the formation of lead chromate, whilst allowing the passage of our plating current.

4.1 PEROXIDING THE ANODES

 

NOTE: For current density required for this treatment, the current is 5A/sq ft. (0. 035A/sq. in., or 0.55A/DM2) of total wetted anode areas.
Make up a 5% solution of sulphuric acid in water
Clean by scraping the lead anodes
Connect two of these lead anodes to the DC power supply, one positive, one negative
Adjust the current density to 5 A and maintain this for 15 minutes
Reverse the polarity of the DC supply and maintain 5 A for 15 minutes
Finally, reverse the polarity of the DC supply one more time and maintain 5 A for another 20 minutes

 

A dark brown coating on the anodes will indicate the presence of our protective layer of lead peroxide. Dry the prepared anodes and put them aside, ready for use. This process will be repeated on any anodes which tend to develop yellow spots during use (because of the lack of the lead peroxide coating).

4.2 SHAPE AND SIZE OF THE ANODES

A good rule to follow is to allow for at least 50% more area of anodes in the bath than the surface area being chromed This will avoid problems due to the hexavalent balance of the solution from developing, and we'll not go into that here. Just use generously sized anodes.

The anodes must also be spaced equidistantly from the cathode (the part) being plated and following the shape of the part. They may be interconnected in the bath by soldered lead strips of narrow section (also for lead-outs) or above the solution level by soldered copper flex Due to the importance of an even deposit on pistons, or cylinders, for these I use a cylindrical-shaped anode. Spacing of cathode to anode is not critical, as long as it is even and wide enough to allow free flow of the solution by the action of agitation.

5. AGITATION

This is necessary in order to continually refresh the chrome solution plating the cathode-connected workpiece. Shaking the jar around will only succeed in disrupting contacts to electrodes; upsetting anode spacing -or worse still -spilling the chromic acid.

Agitation can be accomplished by mechanical means such as a propeller in the bath or a motorised crank made to move the cathode up and clown at about one motion per second, or slower. I prefer air agitation because it is so simple:

An inexpensive fish tank air pump connected to 1/4" (6.35 mm) plastic tubing, terminated in the bottom of the plating solution container and the end blocked but a series of pin holes placed in the tube at the bottom. These holes emit a stream of air bubbles under the cathode workpiece and serve to constantly pump fresh solution from the bottom of the tank upwards. This pumping motion also maintains an even temperature in the bath by helping circulate the plating solution around the tank heater business end.

A simple control valve in series with the air supply pipe is adjusted to limit the stream of bubbles to below the splash level on the surface of the bath. This method is just great in our small tanks.

6. DC CURRENT SUPPLY AND CONTROL

For chrome plating direct current is required, strictly. The reason is that chromic acid has a strong etching action on most metals when no current is flowing and chrome is deposited on the work-piece only when current is flowing. We cannot use a battery charger; or unfiltered power supplies. Full wave rectification of AC remains at 60 cycles/sec. resulting in a pulsing DC current, which reduces to zero volts, or no current flow. 120 times a second. The chrome would be dissolved away as quickly as it was being plated if this type current were to be used.

Smooth DC is required, such as that obtained from batteries or well filtered AC power supplies - and it must be controlled at the high-current levels required for chrome plating. I happen to have, and therefore use, a filtered AC current regulated bench power supply capable of delivering five amps DC at various voltages. The readers who have such a supply will find it ideal for chroming small parts up to 6.5 sq. in. For larger parts, and for readers who do not possess such a power supply, we'll examine a practical alternative at the 10-amp level.

6.1 BATTERIES

Because of the current requirements, about the only choice left here is the common lead/acid car type battery - the problem is that we do not always need 12 volts, but rather need to be able to select the voltage required to drive the desired current through the resistance of the plating bath. Of course, if the reader has access to individual six-volt batteries, which were common at one time, these can be used hooked up in series and a simple tap taken oft between the two. In order to use a regular 12-volt car battery we can tap into the lead-interconnecting strip between cells with a brass screw. Battery terminal grease applied at the threads will help to prevent sulphate build-up problems.

You can, of course, use a 12-volt supply with an adjustable series controller, but because you most often will not require more than six volts to obtain the required current density, it is not sensible to be dissipating the other six volts as heat in the controller. A simple switch will add the other six-volt cells only when required.

A variable resistance (rheostat) in series with the battery and an ammeter to tell you what is flowing is all that is required. The problem is that a rheostat of the power dissipation required would be a very large heat-dissipating component.

Let us assume that you have a work piece to chrome plate and you have calculated the surface area to be 2.5 sq. in.

The current density you need is going to be 0.75 x 2.5= 1.875A, or ll.25W @6V.

If you only had 12V available, the same current requirement would result in 22.5W and half of this would be dissipated as heat in the rheostat. Of course, if, as is usually the case, you only need a lower voltage to drive the plating current through the low resistance of the bath, the difference would be dissipated as additional heat in the rheostat at six or 1 2V, but the controller would run a lot cooler at six volts.

6.2 CONTROL OF THE PLATING CURRENT

An electronic controller makes a lot more sense than the unobtainable power rheostat. I have made up one and tested it for this article and propose the simple schematic here. It requires the minimum of electronic knowledge and can be made up essentially of Radio Shack components

Peak current is limited to a maximum of ten amps, which is sufficient for chroming up to 13 sq. in. The ammeter can be any zero to one amp DC ammeter - with internal shunt and of the common moving coil type. Moving iron ammeters (el cheapos) are too inaccurate for our use.

We are going to make it read 0-10 amp by using an external shunt made up of resistance wire.

The shunt resistance in ohms = meter resistance in ohms over (multiplication factor - 1).

So, if we take the suggested 0-1 amp meter, which has a resistance of .05 ohm, the additional shunt required for the 10 amp range is: 0.05 / 9 = .0055 ohm

This is a very small resistance which can conveniently be made of common 14GA (0.064") iron wire and safely carry our ten amp max current.

From the wire tables, bare iron (baling) wire measures 0.015 ohms/ft. If it is galvanised, rub this down to bare iron or the resistance will be different.

We will need exactly: (12 / 0.015) x .0055 = 4.4 inches

Due to unseen variations, the exact and final shunt length should be finely adjusted and solidly soldered to binding posts, close to the ammeter and range switch. A Multimeter with a suitable range (10 or 20 amps) will serve for this calibration. Every component, including this shunt and the meter, is hooked up using the recommended 12 GA copper wire. You can, of course, apply this shunt calculation to the ammeter you may happen to have access to, as long as the full-scale deflection current and the internal meter resistance are known parameters.

6.3 PANEL POLARITY REVERSING SWITCH

I have found it handy to provide a polarity reversing switch in the DC controller by now you know that to deposit metal on the workpiece, it must always be connected to the negative (cathode) terminal. The polarity of this set-up is only reversed and the workpiece made positive (anode) for treating steel prior to plating. This polarity-reversing switch is more convenient than changing leads around but must be correctly marked and not left in the wrong position. I recommend using even the cheapest portable voltmeter to make sure of this polarity and avoid frustrations.

6.4 PANEL VOLTMETER

A panel-mounted voltmeter is optional. It is not needed for the plating process because we only need to know about and control the current. A voltmeter will indicate what voltage (pressure) is required to drive the desired current (flow) through the plating solution, and that the battery charge is not dropping off. The reason I use one is that it also indicates the plating solution normal resistance, in series with all our hook-up wires to the cathode and anodes: If the voltage is high and we can't drive enough current through the bath, it indicates bad contacts or something wrong with the solution or anodes (high resistance).

6.5 DC CONTROLLER CONSTRUCTION NOTES

First, you couldn't get all the parts you needed at Radio Shack. They used to carry good-size utility chassis and business-like heat sinks, but no more; and they don't have a 1-amp meter, which we do need. We'll also need a 2.5 K 114 in. shaft potentiometer and a common garden PNP driver transistor such as a 2N1 131 or 2N2905. These, for mysterious reasons, are not stocked by Radio Shack either. So, get them from your electronics’ components stockist.

You can bend up your own chassis or buy the one suggested.

The layout is not critical, but here are a few pointers to bear in mind:

1. The chassis cover must be well ventilated/perforated;

2. Do not skimp on the size of the heat sink for the power transistors or they will overheat and burn out. If you use the RS insulating mounting hardware, make sure the transistor case is insulated from the heat sink by doing a continuity check. If you use insulating Teflon mounts, the heat sinks must be insulated from the chassis. Ensure this by doing a continuity check before wiring. The transistor cases are the collectors;

3. Use short lengths of 2OGA wire where indicated on the schematic and solid soldered joints;

4. The RS#274-661 5-lug tie down strips are used to mount resistors/capacitors as required. The centre lug is bolted to chassis ground, so don't use it. One of these insulated tie-down strips will also support the 10A shunt and another the 2N5783 driver transistor (or 2N1 131 or 2N2905);

5. The .001 de-coupling disc ceramic capacitors are best soldered directly across each transistor base and emitter leads after installing the transistor; and,

6. The current control potentiometer from Electro Sonic (2.5K ohm) is a sealed quality component and linear. For good control reliability, do not substitute this for an open frame 'cheapo' from the junk box, unless you happen to have a similar good quality one. The 2.5K value is worked into the bias requirements for the driver transistor; so don't substitute the value either, unless you want to re-bias the transistor differently.

6.6 LEAD-ACID BATTERY CARE

A few recommendations may be appropriate here because we don't want the battery going flat during the plating operation.

Neglected lead-acid batteries develop a layer of insulating lead sulphate on the battery's plates. This is only removed by deep charging (time) and monitored by the use of a battery hydrometer. As the offending lead sulphate is transformed back into sulphuric acid by the charging process, the hydrometer reading will increase to the fully charged reading of

SG 1275/1280 slowly.

A low hydrometer reading will indicate a weak cell.
A long, slow charge is much better for continued battery life than a fast charge - and uses a hydrometer.
Battery voltage is a good indication but does not tell you much about remaining capacity; the hydrometer reading does.
A fully charged cell will measure a nominal 2.2V. We have six in series for a 12V battery
A fully discharged cell will drop to 1.8V and if left there will build up sulphate.

 

Following the above instructions and charging correctly will give us 13.2V for a fully charged battery (any over-voltage present immediately after a charge will soon level off) and at 10.8V the battery is fully discharged.

CAUTION: Remember the hydrogen and oxygen gasses produced by these batteries IS VERY EXPLOSIVE! (Ed note: This is not the time to say to yourself 'Ah, it won't happen to me. You could have your battery explode in your face. I don't believe I need tell you the consequences to life and limb in such an event).

Protect yourself and others by simply making sure the charger Is OFF before connecting /discharging clip. At the battery terminals - then switch ON and adjust the charge. The same simple rule applies to the feeder lines from the battery: Make sure the controller switch S2 is OFF before connecting/disconnecting at the battery.

DO NOT DO ANYTHING THAT COULD CAUSE A SPARK AT THE BATTERY TERMINALS!

Observation of these simple requirements should lead to a happy relationship between you, your chrome plating current requirements, and your lead-acid battery cells.

7 CLEANING THE WORK

Step 1: The purpose of this vital step is to remove surface contaminants of various types which would otherwise prevent proper adhesion of the metal plating to the surface and/or to cause the finish obtained by electroplating to lift oft completely.

Following your required mechanical surface preparation, such as machining, grinding, sanding, and buffing (remember, you polish the chrome before you put it on), the metal may look perfectly clean - but it isn't - for electroplating, we require bare metal - CHEMICALLY AND MICROSCOPICALLY Clean.

I usually follow up any heavy smut/oil/grease/dirt removal from used parts with a hot detergent wash and rinse under running water. The detergent treatment is also required on newly machined parts due to cutting oil residues and fingerprints.

Solvent degreasers such as isopropyl alcohol and trichloroethylene can also be used at this stage to assist in the removal of heavy oily or waxy deposit -trichloroethylene leaves no residue.

At this point is where you reach for your favourite pair of stainless steel tweezers to handle the parts because any fingering of the part will mean having to return it to the start of STEP 1. Failure to comply with this recommendation will resuIt in part of the plating unceremoniously peeling off in the none-too-distant future.

7.1 ELECTRO-CLEANING THE WORK

Step 2: This is where we make an apparently clean part even cleaner. Without this step, plating problems such as bare spots, poor or no adhesion, or even no deposit may develop.

The process is simple enough:

In a one-gallon container, mix 2 to 10 ounces of caustic soda or sodium hydroxide with one gallon of warm water. If you're operating from a pint-sized stainless steel container, that’s OK because the solution grows dirty in use and can be replenished from the handy pre-mixed plastic one-gallon container of fresh solution.

7.2 CATHODIC CLEANSING OF TIN, BRASS, ALUMINUM, MAGNESIUM, NICKEL, & CHROME.

In this process, the part will be suspended in the above-mentioned alkaline solution at room temperature, connected to the negative DC supply terminal. The positive DC terminal will be connected to the stainless steel container.

Increase the DC current through the bath until the part is gassing freely (generating hydrogen). Maintain this for one to two minutes; this bubble scrubbing action will also assist in the cleaning process.

The DC current will be in the range of .1A to 1 Amp/sq.in. For an applied voltage of 3 to 1 2V DC -not critical, just enough current to induce free gassing at the cathode connected workpiece. Excessive current will cause localised surface burning, so a little restraint is called for when cranking up the current to obtain the 'free gassing.'

NOTE: Ventilating the produced hydrogen is MANDATORY because the stuff has a well-earned reputation of going off with a bang (exploding) if given the chance to accumulate anywhere.

The following safety recommendation is valid for all-metal plating, anodising, or Electro-cleaning activities:

VENTILATION OF THE HAZARDOUS OR EXPLOSIVE GASSES PRODUCED BY THESE PROCESSES IS A MANDATORY STEP IN THE PROCEDURES TO BE FOLLOWED IN ANY KIND OF PLATING.

One further note on safety concerns the poisonous/corrosive nature of many of the nasty chemicals used: Note the CAUTIONS printed on the labels of chemical containers when purchased - and do as told there!

ALSO, disconnect the DC supply at the power controller before making or breaking electrical contacts at the working end. This will avoid sparking around the bath and the possible accidental ignition of the gasses generated.

This valuable disconnecting advice is also applicable to the charging of common lead-acid batteries.

7.3 ANODIC CLEANSING OF FERROUS PARTS BEFORE PLATING

This is the preferred cleansing process since metal is caused to migrate off the workpiece - oxygen will bubble up at the anode (positive DC supply terminal now connected to the workpiece) and the negative DC supply terminal connected to the stainless sheet container. Proceed as indicated above for cathodic cleaning of other metals, but with the polarities reversed; i.e.. The workpiece is now connected to the anode (positive lead).

The parts are then dipped in a commercial grade of hydrochloric or sulphuric acid, for a few seconds -don't forget the clean, cold-water rinse before going into and when coming out of the acids

An effective cleanliness test at this stage consists of simply observing the surface of the part as you hold it with your stainless steel tweezers. As it drains (for about 30 seconds), the surface must remain wetted all over without any tendency to break free anywhere on the part.

Ferrous parts thus treated cannot be left to lie around at all because they will quickly develop a layer of red oxide. Proceed with the plating while the part is still wet.

NOTE: The anodic treatment of metals requiring a chromic acid etch may be done in a separate tank or in the plating tank, as desired.

7.4 RECOMMENDATION FOR THE PRE-TREATMENT OF VARIOUS COMMON METALS

Stainless steel - 400 series

Step 1 - Alkaline Anodic

Step 2- Etch dip

Chromic acid - 3 minutes

Stainless steel - 300 series

Cathodic

Sulphuric acid - 2 minutes

Nickel alloys

Cathodic

Sulphuric acid - 2 minutes

Carbon & Nitrided steels
Low carbon
High carbon
Nitrided

Anodic

Chromic acid
Chromic acid -3 mm.
Chromic acid - 1 mm.
Chromic acid - 15 sec.

Copper, brass. bronze

Cathodic

Hydrochloric acid - 15 sec.

Cast iron Special process for good adhesion, irrespective of the material quality as follows:
1) Mechanically finish and cleanse the surface as required.
2 Light alkaline clean.
3) Rinse.
4) Cathodic etch in sulphuric acid.
5) Water rinse

6) Hi-flash in chromic acid bath with a preconnected cathode (50% higher current for 4 to 5 seconds)

7) Plate at 2A to 3Asq.in. to build up required chrome thickness

 

7.5 ALUMINUM PLATING PREPARATION

This presents special problems for plating due to the instantaneous formation of a surface oxide film in the presence of oxygen in the air. A Zincate-dip method is recommended but the degree of success varies with different alloys; experimentation may well be required, along with a degree of patience:

1). Clean in alkaline solution (see Electro-cleaning the work Section 7.1)

2). Cold water rinses.

3). Nitric acid dip (50% by volume) - room temperature.

4). Cold water rinses.

5). Hydrofluoric acid dip (5% by volume) - room temperature.

6). Water rinse.

7). Zincate dips (see formula at upper right).

8). Nitric acid dips (repeat of Step 3 above)

9). Zincate dip to build up final zinc coat.

0). Water rinse and plate immediately.

7.6. ZINCATE DIP FOR ALUMINUM ALLOYS:

ZINCATE DIP FOR ALUMINUM ALLOYS:

Sodium Hydroxide 6.7 oz / gaI

Zinc oxide 0.67oz / gal

Rochelle salt 6.7oz / gal

Ferric chloride crystals 0.27oz / gal

Sodium nitrate 0.13oz / gal

Demineralized water to make 1 gallon

Immersion time of the work is to be no more than 30 seconds at room temperature (70~75o F.). I am not aware of possible short cuts in this treatment - as outlined, it works!

7.7 MAGNESIUM PLATING PREPARATION:

These metal presents other degrees of complexity in processes beyond those experienced with aluminium alloys. Preparation in phosphoric and ammonium acids is required before the zincate dip. Furthermore, preliminary copper and nickel plating before the chrome plating must then follow this. So, magnesium plating will not be covered in this article. I may cover copper and nickel plating later, if SIC readers are interested.

7.8 PLATING CHROME OVER CHROME

This process is particularly useful when the chroming operation has been interrupted to measure the thickness deposited and the workpiece must be returned to the plating bath for the continued build-up of a thicker plate. Returning the work to the bath without preparation will result in a separation of the second deposited thickness from the initial build-up:

1). Anodic cleansing per previous procedure (see Section 7.3)

2). Cold water rinses.

3). Anodic treatment in the chromic acid plating bath until uniform gassing is obtained over the part.

4). Let stand in the chromic acid plating bath with no current flow for ten minutes.

5). Cathodic connections to the DC, gradually increasing the current to the required plating current density.

Then proceed as outlined for chrome plating.

8. BLOCKING OFF / MASKING AND LAYING UP THE WORK IN THE TANK:

This is simply the process of covering over those areas of the workpiece, which we do not need or want to plate with chrome. This may include cathode lead-out supports, usually made of copper or brass. These uncovered areas would otherwise also receive deposited chrome and upset the calculation of .75A per sq. in. current density at the workpiece; i.e., the area over which chrome is required. The leadout supports/contacts may not represent a significant consideration with respect to the area of the work-piece but they sometimes 00.1 like to slip a suitable diameter of heat-shrink plastic tubing over the cathode leadout and thereby eliminate the problem.

If the workpiece, say a rocker cover box, is to be chromed on the outside surface only, the inside can be varnished/enamelled or simply covered over with adhesive plastic tape. I don't recommend waxes (which are used in industry), because I haven't found one that doesn't tend to melt and contaminate the plating solution at 500C.

In other words. we apply a removable electrically insulating layer of material, which is able to support the operating temperature. This layer will allow the selective plating of chrome over the required areas only.

Undesirable masking of the surface being plated may take place due to the presence of the cathode supports to the work-piece. Say, we want to chrome the rim of a brass flywheel, the centre areas will have been blocked off with shellac or enamel and allowed to dry. This part would conveniently be hung in the plating tank (see Fig.'s 1 & 2) by using a solid copper wire sling with a single coil around each end of a temporary shaft laid horizontally through the flywheel. The copper sling, our cathode leadout, shadows half the diameter of the flywheel on each side and if allowed to remain in this position for the duration of the plating period, would result in a thinner deposit of chrome on the 'shaded' area .To prevent this, simply rotate the flywheel on the shaft (use a probe) several times during the process. For this example, flat lead anodes would have been used each side of the flywheel. Chrome deposit would be thickest on the cheeks and thin nest on the outer rim, due to the position of the anodes relative to the cathode workpiece.

Alternatively, (Fig. 2) the flywheel could be supported by one central cathode shaft protruding from the solution. The flywheel would then be horizontal and Centred up inside a lead anode strip formed to a larger diameter, with two or three anode lead-outs protruding from the top of the plating solution and tank. With this arrangement no rotation is necessary. The chrome would leave a thicker deposit on the rim and a thinner one on the cheeks of the flywheel. Let's take a look at these two alternatives in the plating tank:

While Fig. 2 shows plating a flywheel, we can see it would also be the ideal set-up for plating a piston skirt, with an equidistantly spaced anode all around.

To keep the work piece centred, I have also used insulated solid copper wire bent around the cathode rod and reaching out to the anode. Two of these at 900 from each other works fine.

8.1 THE PLATING TANK:

Corrosion attacks the handy alligator clips used for anode/cathode connections at the tank, and it is good to keep a pack of new ones at hand. Plastic clothes pegs (pins) are very handy to set up and stabilise the anode/cathode supports around the rim of the glass Tank.

The tank can be just about any suitable size glass container. I used wide-mouth Mason jars. Stove enamelware and the thicker gauge plastic container also make suitable chrome plating tanks.

In summary, we have to use our imagination and improvise the mounting requirement for differently shaped parts as the cases present themselves; remembering to satisfy the basic requirements in any set-up:

1. Maintain electrical insulation between CATHODE & ANODES.

2. Leave sufficient space for the heater and agitator.

3. Ensure solid electrical contacts to the workpiece and all ANODE & CATHODE connections.

4. ANODE strips can be bent above or below the workpiece to distribute the required plate, as long as they are all interconnected electrically and their surface area exceeds that of the cathode by about 50%.

5. It helps to set up in a dry tank first, then fill the tank with the plating solution (not forgetting to leave space for liquid displacement), bring up to temperature, and adjust the thermostat setting to keep it there (free usage of thermometer), then drop in the previous set-up and start plating.

6. Connect the DC controller leads to CATHODE & ANODE contacts. With the CONTROL knob turned down, Turn the power switch on, and adjust the CONTROL to the calculated plating current indicated on the Ammeter.

10. CHROME FINISHING OPERATIONS

10.1 Chrome Plating Problems

It is difficult to predict everything that may go wrong in any given set-up, but here are a few pointers gleaned from technical references, which were of help to me:

Problem:

1. No Chrome Deposited.

The chromic acid grade purchased is impure or a gross mistake was made with the sulphuric acid proportion used. It must be exactly 100:1(1 is the acid!). Be careful of the dilution ratio of the acid purchased and work this figure into your mix calculations carefully.

2. Milky Looking Deposit:

The plating current density used was too low. You probably forgot to work in the exposed areas of the cathode supports into your calculation - increase the plating current.

3. Grey Matt Deposit:

The base metal was not polished; i.e., it too had a rough finish. The plating current could be too high. Check your cathode area calculations - there may be too much acid in the solution. If this is the case, you can salvage your solution by making up another without sulphuric acid, and keep adding this to the original solution until satisfactory results are obtained. The remaining acid4ree solution can then be measured and the correct 100:1 solution made up with sulphuric acid. The two solutions are then mixed together for corrected and increased total volume. If you have a Baume hydrometer the correct reading for a 33oz/gal mix is 21.5 deg. Be at room temperature.

10.2 GRINDING THE DEPOSITED CHROME

To relieve the hydrogen embrittlement characteristic of hard chrome over high carbon steels, bake the workpiece in an oven at 300 to 500' F for one to five hours, dependant on the thickness of the plate. Grinding is carried out using fine grades of 'B' vitrified bond silicon carbide or alumina wheels at around 5000 sfpm wet or dry.

Very small feeds are indicated - don't use anything else: From .0001" to .0002" will avoid overheating the chrome.

Dry grinding chrome is OK but higher traverse speeds and Very light cuts must be taken to avoid overheating/ cracking problems. Wet grinding is preferred and produces much better results.

The following grinding wheel specifications are recommended for good commercial finishes:

Cylindrical grinding: 3860-L5BE
Surface grinding: 3860-I8BE
Internal grinding: 3860-K8BE

 

The 60 number in the above specifications is the grain size. Smaller grain sizes may be used if higher finishes are required. The numbers go all the way to 500.

10.3 LAPPING THE DEPOSITED CHROME

Fine grades of Alundum work well, with a kerosene or WD-40 lubricant. Six hundred grit breaks down against the chrome and leaves a fine finish for those special parts such as slowly bringing a lapped piston to size. I finish off with a touch of jewellers diamond dust on the lap and a lot of lubricant. Very high finishes are obtained.

10.4 BUFFING DECORATIVE CHROME:

The finest finish obtainable must be imparted to the workpiece before chroming.

Satin finish: On a loose buff, use lubricated silica greaseless compound at around 400 sfpm. For finishing, use chromes green oxide or unfused alumina on a loose buff at around 700 sfpm.

High finish: On sewed canvas or felt buff, use aluminium oxide or silicon carbide greaseless compound at around 6000 sfpm.

CONCLUSION:

Having the chrome plating facility right there when he needs it is very valuable to the home machinist. Of course there is a learning curve and some basic investment in dollars but this investment is mostly non-recurring and will give you the benefit of this process from now on.

Coupled with other surface finishes such as anodising aluminium (already covered in SIC) and the plating of other prime metals such as copper, nickel, silver, or gold which we may yet cover, we can really dress up and protect our creations!

The equipment described here will cover the requirements for plating other metals (chrome was the most demanding) - all we will have to change will be our tank chemicals, anodes, and process knowledge

 

 

 

QTY

RADIOSHACK PART NUMBER

DESCRIPTION

3

271-128

0.1 ohm 5W wirewound resistor

2

272-126

.001 50V disc ceramic capacitor (2 per package)

1

272-1051

.01 250V metal film capacitor

2

275-8058A

15A dpdt centre off switch for S1 & 52

1

2761372

Silicone(heatsink)

3

2761371

TO-3 power transistor mounting hardware kit

3

2762041

2N3055 NPN power transistor TO-3)

1

271 319

470 ohm resistor (2 per package)

1

271-030

4.7K ohm resistor (2 per package)

1

274-402

1.5" instrumentation knob with 1/4' shaft (2 per package)

1

270-384

11/4 x 1/4 panel fuse holder

1

274-457

3 terminal safety barrier strip

I

270-344

2 1/4' battery clips (2 per package)

1

270-347

2 1/4' alligator clips (10 per package)

1

27~0-1754

23/4'. ISV voltmeter (optional)

1

274-730

Banana plugs (soldered) (2 per package)

1

274-451

Banana jacks (soldered) (2 per package)

I

274-688

5-lug tie down strip (4 per package)

1

270-8013

SPST toggle switch (53)

1

271-1325

2.2K ohm resistor (5 per package)

1

276-080

LED panel holder

I

276-041

T 1 3/4 LED (pilot) (3 per package)

1

276-1289

12 gauge heavy-duty hook-up wire (20 ft. per package)

I

276-1627

Heatshrink tubing from 1/16' to 1/2' diameter

1

276-1302

18 gauge solid copper wire (60 ft. per package)

1

276-1303

18 gauge stranded copper wire (45 ft. per package
Model U225 UM2 - 1ADC 2 11/16' panel meter
IA DC Ammeter (.05 ohm) 2 112' panel meter
Moulded composition 2.5K ohm potentiometer
TO-5 / T0-39 heat sink
Ventilated Hammond Instrument cabinet 10.25" L x 6.1" D x6" H rubber feet