What Is Electroplating or Electro-Deposition?
Electroplating or electro-deposition is a process whereby atoms or molecules (the smallest indivisible portions of a substance having the properties of the substance) of metal or some other material are precipitated from a liquid solution, known as the electrolyte or plating bath, onto the surface of some other piece of material such as a metal part through the action of electricity externally applied to the system.
This process is carried out with attendant difficulties peculiar to each metal or material deposited, and chromium is probably the most difficult metal to successfully deposit.
How Is Hard Chromium Plating Performed?
In order to more fully understand the basic principles underlying the hard chromium plating process, let us examine Fig. , which is a schematic diagram of the plating tank arrangement.
The equipment required is
- a plating tank containing the plating bath or electrolyte;
- a source of direct current electricity such as a battery, motor-generator set or rectifier depending upon the power requirements and plant facilities;
- a double pole-double throw switch for reversing current;
- a rheostat for controlling the current going through the circuit;
- a conforming anode to distribute the current field uniformly over the surface of the part to be plated;
- an ammeter and voltmeter for visual observation of the behavior of the circuit and an operator highly skilled and experienced in the “art” of hard chromium plating.
The part to be plated is connected in one leg of the circuit, and the conforming anode is connected into the other leg as shown with the switch open and no current flowing in the circuit.
The entire surface to be plated must, of course, be completely submerged in the plating bath since the bath or electrolyte completes the circuit when the switch is closed.
(In actual practice, the anode is mounted very close to the surface of the part to be plated, as will be discussed later, but for purposes of clarity in drawing Fig., the relative location of the anode and part is out of proportion).
It is usual practice in most instances to reverse-current etch the surface to be plated immediately prior to plating.
This is accomplished by closing the switch so that the part to be plated is electrically positive for about 3 to 5 seconds; then the switch is thrown over to the plating position in which the part to be plated is electrically negative.
The purpose of the reverse-current etch is to clean the surface immediately prior to deposition, and in so doing to microscopically etch the surface, thus providing a stronger bond between the chromium deposit and the surface of the basis metal.
The switch is then left closed in the plating position for a sufficient length of time to produce the desired thickness of chromium deposit.
Mathematically the current flowing in the electrolytic circuit is given by the well known Ohm’s Law, namely
- where I=current flowing in amperes,
- E=applied direct current voltage in volts,
- R0=rheostat resistance in ohms and,
- Rs —resistance offered by plating bath or solution in ohms.
The current (I) and voltage (E) are visually indicated by the meters.