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.
5.1 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
5.2 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.
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.
7. 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.
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 @ 6 V.
If you only had 12V available, the same current requirement would result in 22.5 W 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 2 V, but the controller would run a lot cooler at six volts.
7.2 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.
7.3 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.
7.4 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).
7.5 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:
- The chassis cover must be well ventilated/perforated;
- 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;
- Use short lengths of 2OGA wire where indicated on the schematic and solid soldered joints;
- 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);
- The .001 de-coupling disc ceramic capacitors are best soldered directly across each transistor base and emitter leads after installing the transistor; and,
- 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.
7.6 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.
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.