Hard Chrome Plating Training Course

Table of Contents

Appendix 11. Glossary

Alternating current. Alternating current (AC) refers to the form in which electricity is delivered to businesses and residences. In alternating current the movement (or flow) of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again.

Ampacity. The amperage capacity of a metal cable or bar used for fixtures and racks. Before designing a rack or fixture for a particular part, the necessary amperage
is calculated. Then, the size of the bars and cables is determined by knowing the ampacity of the available materials. For example, alloy 110 copper, a commonly used fixture metal, has an ampacity of 1,000 amps/in².

Catalyst. A chemical substance that either increases or decreases the rate of a chemical reaction. Unlike other chemicals that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. The catalyst used in the standard hard chrome bath is sulfate, which is added to the bath as sulfuric acid. Fluoride is a catalyst used along with sulfate in mixed catalyst hard chrome baths.

Cathode efficiency. The cathode efficiency of a plating bath is the ratio of the weight of metal actually deposited to the weight which ought theoretically to have been deposited considering only the electricity consumed. It is expressed as a percentage. The cathode efficiency of most non-chromium plating baths (e.g., nickel and copper) is above 70%. The efficiency of a standard hard chrome bath is usually around 13%. It is affected by current density, temperature, chromic acid and sulfate concentrations, and by other variables.

Chrome. A slang word for chromium often used in the electroplating sector.

Chromium trioxide. A dark red/orange brown solid chemical compound with the formula CrO³. It is usually purchased in flake form. When mixed with water the resultant solution is referred to as chromic acid. Chromium trioxide is mainly used for chromium electroplating.

Conforming anodes. Lead alloy that is shaped in the same manner as the part being plated and used in the hard chrome plating process. Most often, the lead is cast in a gridmat mold that gives it a waffle shape. It is then cut and formed to the desired shape and the seams are welded together. Conforming anodes proved significant benefits over common stick anodes, including faster plating rates and more even deposits.

Corrosion. The disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. Most commonly this means a loss of electrons of metals reacting with water and oxygen. Corrosion of iron due to oxidation of the iron atoms is a well-known example of electrochemical corrosion. This is commonly known as rusting.

Covering power. The ability of a chrome bath to initiate metal deposition at low current density areas.

Current density. The current density (amperage of the electroplating current divided by the surface area of the part) in hard chrome plating strongly influences the deposition rate, plating adherence, and plating quality. This density can vary over the surface of a part, as outside surfaces will tend to have a higher current density than inside surfaces (e.g., holes, bores, etc.). The higher the current density, the faster the deposition rate will be, although there is a practical limit enforced by poor adhesion and plating quality when the deposition rate is too high.

Decorative chrome. Electroplated chromium coating applied for decorative purposes. Deposit thicknesses are usually much less than 0.05 mils. Both hexavalent and trivalent chromium baths are in common use. Decorative chrome is typically applied over undercoats of nickel and or copper.

Direct current. The unidirectional flow of electric charge. Direct current (DC) is produced by such sources as batteries or electroplating rectifiers. Direct current may flow in a conductor such as a copper bars or wires, but can also be through conductive liquids (electrolytes). With DC the electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was Galvanic current.

Electrical resistance. The electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. Excessive electrical resistance in busing can be caused by dirty connections. Heat will build up in the area of the resistance.

Electrochemical. A chemical reaction driven by an external applied voltage from a source such as a battery or rectifier.

Electrons. Subatomic particles that carries a negative electric charge. An electron has a mass that is approximately 1,836 times less than that of the proton. A metal consists of a lattice of atoms, each with a shell of electrons. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field.

Freeboard. The space between the liquid surface of a bath and the upper lip of the tank. Freeboard space is needed with hard chrome tanks to allow for displacement of solution when large parts are placed into the tank. Having sufficient freeboard also improves the efficiency of air exhaust systems.

Fume suppressant. A chemical that is added to hard chrome plating baths to lower the surface tension of the solution and/or create
a foam blanket with the objective of reducing chromic acid mist generation.

Galling. A form of surface damage arising between sliding solids, distinguished by microscopic, usually localized, roughening and creation of protrusions (i.e., lumps) above the original surface.

Hard chrome. Electroplated chromium coating applied for functional purposes such as wear resistance and corrosion protection. Deposit thicknesses range for 0.1 mils to 50 mils or more (note that local standards may restrict thickness on fatigue critical parts, such as aircraft engine components). Hard chrome is typically plated directly onto the base metal, although a nickel undercoat is sometimes used. Presently it is applied from a hexavalent chromium bath, although trivalent bath research is underway.

Heat exchanger. A device built for efficient heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact. One common example of a heat exchanger is the radiator in a car, in which the heat source, being a hot engine-cooling fluid, water, transfers heat to air flowing through the radiator.

Hertz. The hertz (symbol: Hz) is a unit of frequency. It is defined as the number of complete cycles per second. When used in conjunction with electricity it refers to the alternating positive and negative cycle of alternating current. With U.S. generated electricity, one cycle takes 1/60th of a second, which is equivalent to 60 cycles per second (written 60 Hz). European countries generate electricity with a frequency of 50 Hz. In computing, most central processing units (CPU) are labeled in terms of their clock speed expressed in megahertz or gigahertz (109 hertz).

Hexavalent chromium. Chromium atoms with six missing electrons. Usually written Cr⁺⁶ or Cr VI.

High current density region. Areas of parts being plated that receive more than the average current density. These are points or edges of the part. As a result, the chromium deposition rate will be more than average for these areas. High current density areas are subject to potential burning and the formation of trees.

Hydrogen embrittlement. Hydrogen gas is adsorbed into the base metal during hard chrome plating. When this occurs, the fatigue strength of the steel is reduced so much that the part may fracture in service, if the hydrogen is not removed ("relieved") by baking after plating. This phenomenon is referred to as hydrogen embrittlement. Hardened and high-strength steels are particularly susceptible to hydrogen embrittlement.

Ion exchange. A technology used for purification of liquids, such as hard chrome baths. This is achieved by passing a liquid containing the ions to be removed (for example tramp metals) through a bed of ion exchange resin. In this case, the tramp metal ions, which have a positive charge are attracted to a resin with a negative charge (cation resin) and removed from the solution. Ion exchange resins are reusable once they have been regenerated with an appropriate acid or base and rinsed with water.

Ions. Each atom has a specific number of electrons, protons and neutrons. But no matter how many particles an atom has, the number of electrons usually needs to be the same as the number of protons. If the numbers are the same, the atom is called balanced, and it is very stable. Some kinds of atoms have loosely attached electrons. An atom that loses electrons has more protons than electrons and is positively charged. An atom that gains electrons has more negative particles and is negatively charge. A "charged" atom is called an "ion."

Low current density region. Areas of parts being plated that receive less than the average current density. These are generally concave or recessed areas. As a result, the chromium deposition rate will be less than average
for these areas and in extreme cases, no deposition will occur.

Macrocracks. Generally speaking, a crack structure which is comprised of a high population density of narrow, shallow cracks (microcracks) is desirable for hard chrome deposits, because the deposit tends to have a lower stress, higher lubricity, good wearability and better corrosion resistance. If the conditions during plating cause the cracks to be coarse in nature, they are often referred to as macro-cracks, which may be visible to the naked eye. Usually, chromium with this type of microstructure exhibits less desirable properties in service. It should be noted that macro-cracking can occur in chromium deposited over any type of substrate, not just those that are already stressed in tensile.

Masking. The process of or the material used for covering areas of parts where chromium plating is not desired. Common materials used for this purpose include wax, metal tapes and foils, plastic tapes, plastic plugs, and lacquer.

Microcracks. Almost all electroplated hard chromium deposits are cracked. Cracking occurs during the plating cycle when internal stress exceeds the tensile strength of the chromium, which is hard and brittle. The width, depth and population density of these microcracks varies widely and is influenced by the following: the type of plating chemistry used (single-catalyst, mixed catalyst, proprietary), chromic acid concentration, type and concentration of catalyst, chromium-to-catalyst ratio, plating current-density, bath temperature, concentration of bath impurities (iron, copper, zinc, nickel, trivalent chromium, etc.) chromium deposit thickness surface condition of substrate.

Neutrons. Subatomic particles with no net electric charge and a mass slightly larger than that of a proton.

No-mask anodes. A custom hard chrome plating fixture that eliminates the need to mask a part with wax, tapes or foils. They consist of a PVC housing that holds a conforming anode and attaches to the part being plated. They provide the same benefits as conforming anodes, plus they greatly reduce labor and improve the deposition at the ends of the part (e.g., no trees, better run-outs)

Plating fixtures. A loose term that refers to any mechanical device used to hold a part and/or anode during plating and/or attach them to the tank bus bars. Fixtures are often "make-shift" for plating an occasional odd part or custom designed for higher production parts. They are fabricated from various types of metal, but mainly steel, copper or aluminum and must be sized properly to carry the plating amperage. Depending on the type of metal used and the design, certain fixtures require masking before plating to prevent electrical communication with the part or anode.

Plating to size. A term used to describe plating a part to the desired final dimension, thereby eliminating the need for machining. This practice is commonly performed for new parts that require less than 3 mils thick of chromium. Parts being plated for refurbishment are usually not plated to size because they typically require thick deposits (greater than 3 mils) that create an opportunity for some deposit roughness or uneven deposition, which subsequently demands grinding.

Porosity. A measure of the void spaces in a material. It is commonly measured as a fraction, between 0–1, or as a percentage between 0–100%.

Porous pot. A technology commonly used for hard chrome bath maintenance. It is most effective for converting trivalent chromium to hexavalent chromium. It also removes tramp metals, but with limited effectiveness.

Protons. Subatomic particles with an electric charge of +1 charge. It is found in the nucleus of each atom.

Pumice. A rock produced by erupting volcanoes. Finely ground pumice is used to clean the surface of parts before hard chrome plating. It is also added to some heavy-duty hand cleaners (such as Lava soap) as a mild abrasive.

Ramp up. A plating procedure that is sometimes used at the start of a plating cycle, where the amperage or voltage setting is initially set below normal and subsequently raised in "steps" until the final setting is achieved. Ramping up usually takes place over several minutes time. It is most often used when plating new chromium over an existing chromium deposit. The value of ramp up in other situations is suspect.

Rectifier. An electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers are commonly referred to as power supplies within the electroplating industry.

Smut. A black granular substance consisting wholly or principally of carbon. Smut may be created during the reverse etch process, where the carbon in the steel is brought to the surface. The presence of excessive carbon smut may impact the quality of the chromium deposit.

Solenoid valve. An electromechanical valve for use with liquid or gas controlled by running or stopping an electric current through a solenoid (a coil of wire), thus opening or closing the valve. The operation of a solenoid valve is similar to that of a light switch, but typically controls the flow of air or water, whereas a light switch typically controls the flow of electricity.

Strike. Starting the chrome plating process with a higher than normal amperage setting. A strike is often used when low current density areas are present. In this case, a strike lasting several minutes may help to throw (see throwing power) the chromium into those regions. Following the strike, the rectifier is set to normal operation. This type of strike should not be confused with a special plating deposit called a "strike" or "flash". These are used in nickel, silver and many other electroplating processes to form a very thin (typically less than 0.1 micrometer thick) plating with high quality and good adherence to the substrate. This serves as a foundation for subsequent plating processes.

Throwing power. The ability of an electroplating solution to deposit metal uniformly on an irregularly shaped part, including into recesses. Concave areas of parts are referred to as low current density regions.

Tramp metals. Metals such as iron, copper and aluminum are considered tramp metals in hard chrome baths. These are unwelcome impurities that will reduce the quality of plating if they reach sufficiently high concentrations.

Trees. Metallic chromium outgrowths from chromium deposits that resemble the branches of a tree. Trees usually occur at the ends of parts during hard chrome plating, which are high current density regions.

Trivalent chromium. Chromium atoms with three missing electrons. Usually written Cr⁺³ or Cr III.

Wild chrome. Inappropriate masking materials and/or poor masking techniques can lead to the lifting or loss of maskants during plating. This will cause unintended deposition of chromium (i.e., wild chrome).

Zincate. A process commonly used to prepare aluminum parts for hard chrome plating, whereby a thin sacrificial coating of zinc is applied to an aluminum part to prevent aluminum oxide formation.