Anodizing
A process to produce an oxide film or coating on metals and alloys such as aluminium, niobium, tantalum, titanium, tungsten, zirconium etc by electrolysis is called anodizing. The metal to be treated is made the anode in an electrolytic cell and its surface is electrochemically oxidised. Anodizing can improve certain surface properties, such as corrosion resistance, abrasion resistance, hardness, appearance, etc. For each of these metals there are process conditions, which promote growth of a thin, dense, barrier oxide of uniform thickness. The thickness of this layer and its properties vary greatly depending on the metal, with only the aluminium and tantalum (and recently niobium) films being of substantial commercial and technological importance as capacitor dielectrics. Aluminium is unique among these metals in that, in addition to the thin barrier oxide, anodizing aluminium alloys in certain acidic electrolytes produces a thick oxide coating, containing a high density of microscopic pores. This coating has diverse and important applications including architectural finishes, prevention of corrosion of automobile and aerospace structures, and electrical insulation. In fact, it is this porous coating that is most often thought of as the product of anodizing.
Clear Anodize:
Clear anodize usually means sulphuric acid anodize followed water seal. This is the most widely used anodize coating. It is used on some aluminium alloys as the surface finish for automotive trim.
Immersing the anodize coating in a dye solution before sealing creates an attractive coloured surface for consumer products.
Hard Anodize:
Hard anodize is obtained by anodizing in sulphuric acid at low temperature. This produces a coating with large cells and small diameter pores. The coating is extremely hard and durable and is used for engineering applications such as bearing surfaces.
A thin phosphoric acid anodize coating is used as an adhesive bonding primer coat on aircraft and aerospace alloy sheets. This is an excellent surface for the epoxy adhesive and also improves corrosion resistance.
Chromic acid anodize is used for optimum corrosion resistance for severe applications, such as aerospace and military use.
Annealing
Annealing is a process in which glass, metals, and other materials are treated to render them less brittle and more workable. Annealing consists of heating the material and then cooling it very slowly and uniformly; the time and temperatures required in the process are set according to the properties desired. Annealing increases ductility and lessens the possibility of a failure by relieving internal strains.
Corrosion Protection
Corrosion is an electrochemical process and is usually defined as the detoriation of a material due to the transfer of electrons between a metal surface and an aqueous electrolyte solution. It results from the overwhelming tendency of metals to react electrochemically with oxygen, water, and other substances in the aqueous environment. The anodic and cathodic control measures are based on electrochemical driving force and are as follows:
Anodic (Corrosion) Protection: A process for corrosion protection of a metal or alloy achieved by impressing upon the metal an anodic current of sufficient magnitude to cause the formation of a passive film. Anodic protection is effective only for metals that are prone to passivate, such as stainless steel and titanium.
Cathodic (Corrosion) Protection: A process for corrosion protection of a metal or alloy achieved by impressing upon the metal a cathodic potential of sufficient magnitude to decrease the corrosion rate.
Anodic or cathodic protection is widely used for corrosion protection of steel structures such as ships, pipelines, tanks, and power plants equipment.
Chrome Plating
Chrome Plating are of two types, one is Hard Chrome Plating ( also known as engineering chrome plating)and the other Decorative chrome plating.
Hard Chrome Plating
Hard chromium plating is just chrome plating, but it is applied as a fairly heavy coating for wear resistance, lubricity, oil retention, and other 'wear' purposes. Some examples would be hydraulic cylinder rods, rollers, piston rings, mould surfaces, thread guides, etc. It is called hard chromium because it is thick enough that when a hardness measurement is performed the chrome hardness can actually be measured. It is almost always applied to items that are made of steel. It is not really shiny or decorative.
There are variations even within hard chrome plating, with some of the coatings optimized to be especially porous for oil retention, etc.
Many shops who do hard chromium plating do no other kind of plating at all, because their business is designed to serve only engineered, wear-type, needs.
Decorative Chrome Plating
Decorative chrome plating is sometimes called nickel-chrome plating because it always involves plating nickel before plating the chrome. The chrome plating in decorative chrome plating is exceptionally thin. It is still a very hard surface, but simple 'anvil' type hardness measurements don't detect the hardness because the anvil just punches through such a thin coating.
When you look at a decorative chromium plated surface, such as chrome plated wheel or truck bumper, most of what you are seeing is actually the nickel. The chrome adds a bluish cast (filtering the somewhat yellowish cast of the nickel), and it protects against tarnish, and minimizes scratching. But the point is, without the brilliant levelled nickel undercoating, you would not have a reflective, decorative surface.
Some metals, like zinc die-castings, cannot be directly nickel-plated but must be copper plated first. Other materials, like aluminium, cannot be copper plated until they have been zincated.
Chrome plating is hardly a matter of dipping an article into a tank, it is a long involved process that often starts with tedious polishing and buffing, then cleaning and acid dipping, zincating, and copper plating. This may be followed by buffing of the copper, cleaning and acid dipping again, and plating in two or three different types of nickel-plating solution, all before the chrome plating is done.
Electrolysis
A process that decomposes a chemical compound into its elements or produces a new compound by the action of an electrical current. The electrical current is passed through an electrolytic cell and oxidation/reduction reactions occur at the electrodes e.g., water can be decomposed into hydrogen and oxygen, or a metal can be electroplated by electrolysis.
Electroplating
Also called electro-deposition. It is the process that produces a thin, metallic coating on the surface of another metal (or any other conductor, e.g., graphite) by means of electric current. The metal substrate to be coated is made the cathode in an electrolytic cell where the cations of the electrolyte are the positive ions of the metal to be coated on the surface. When a current is applied, the electrode reaction occurring on the cathode is the reduction of the metal ions to metal. E.g., gold ions can be discharged form a gold solution to form a thin gold coating on a less expensive metal to produce "custom" jewellery. Similarly, chromium coating is often applied to steel surfaces to make them more "rust resistant." Electroplating is also used in the production of integrated circuits on computer chips and for other modern electronic instrumentation. The anode material can either be the metal to be deposited (in this case the electrode reaction is electro dissolution that continuously supplies the metal ions) or the anode can be of inert material and the anodic reaction is oxygen evolution (in this case the plating solution is eventually depleted of metal ions).
Electrolytic Pickling
Process for removal of oxide scales from metal surfaces in preparation for electroplating. The metal is made the cathode in an electrolytic cell containing strongly acidic (sometimes hot) solution that dissolves the oxide scales. The strong hydrogen evolution occurring on the cathode may reduce some of the oxides, and the strong bubble evolution removes the scales mechanically, while the agitation of the solution helps the chemical dissolution of the scales by the acid.
Electrophoretic Deposition (Painting)
Deposition of particles carried to a surface by electrophoresis. The loosely formed deposit layer typically needs compacting that can partially occur by electro osmotic removal of the liquid, or by other means. Electrode reactions occurring on the substrate surface can take part in "binding" the coating. Practical applications are surface coating and paint deposition (practiced on large scale in the automotive industry) and fabrication of ceramic products. Also called "electro coating."
Electrophoresis / Protein Separation
Also called Cataphoresis. It is the movement of small-suspended particles or very large molecules in a liquid driven by an electrical potential difference.
It is used to analyse and separate colloids (e.g.proteins) or to deposit coatings as on elements used in electron tubes. An electro analytical technique i.e. a separation method, for very large organic molecules (usually of biological origin) based on their different electrophoretic velocities through an "immobilized" liquid phase. The liquid can be immobilized by a variety of "supports", e.g.: paper, gelatinous material, capillary tubing.
Electro Refining
The electrochemical process that produces a purified metal from a less pure metal is called Electro-refining. The metal to be purified is made the anode in an electrolytic cell and it is dissolved by the application of a current into a usually acidic aqueous electrolyte or a molten salt. At the same time, the pure metal is deposited on the cathode. The process is carried out under conditions that most impurities will either precipitate as "sludge" or remain dissolved in the electrolyte. Copper is one metal that is often electro refined in aqueous solutions, and aluminium is electro refined using a molten salt electrolyte. Also called "electrolytic refining" and "metal refining."
Electro Winning
An electrochemical process that produces metals from their ores is called electro winning. Most metals occur in nature in oxidized form in their ores. While numerous ways exist to reduce the ores, for many metals electrochemical reduction is the most practical. The ore is dissolved (often following some chemical purification or pre-processing) in an acidic aqueous solution or in a molten salt and the resulting electrolyte solution is electrolyzed. The metal is deposited on the cathode (either in solid or in liquid form), while the anodic reaction is usually oxygen evolution. Copper and zinc are two metals that are often produced by aqueous electrolysis. Aluminium, magnesium, and sodium are some metals that can be produced by molten salt electrolysis. For aluminium, this is the only practically used production process. Also called "electroextraction."
Galvanic Cell
An electrochemical cell that converts chemical energy into electrical energy is called a galvanic cell. In the cell, chemical reactions occur spontaneously at the electrodes when they are connected through an external circuit, producing an electrical current. E.g., in a fuel cell hydrogen is oxidized at the anode by transferring electrons to the anode and the oxygen is reduced at the cathode by accepting electrons from the cathode. During this process the electrons are carried from the anode to the cathode through an outside electrical circuit where the electrical current can drive a motor, light a light bulb, etc. In contrast, in an electrolytic cell electrical power must be supplied to force the non-spontaneous reverse reaction, the electrolysis of water. Also called "voltaic" cell.
The free energy change of the overall cell reaction is negative.
Galvanizing
Galvanizing is a process for coating iron or steel with a thin layer of zinc for corrosion protection. It can be carried out electrochemically by electroplating (called "electro galvanizing") or by "hot-dip" galvanizing consisting of immersing the substrate into molten zinc.
Oxidation / Reduction
In a narrow sense, oxidation means the reaction of a substance with oxygen. Hydrogen can react with oxygen to be oxidized to water. Hydrocarbon fuels (gasoline, natural gas, etc) can react with oxygen to be oxidized to carbon dioxide and water. Iron can react with oxygen to be oxidized to "rust." During oxidation, the oxygen itself is being reduced. Oxidation and reduction always occur simultaneously.
During these reactions, electrons are transferred from the substance that is oxidized to the oxygen. In a wider sense, all electron-transfer reactions are considered oxidation/reduction. The substance gaining electrons ("oxidizing agent" or "oxidant") is oxidizing the substance that is losing electrons ("reducing agent" or "reductant"). In the process, the "oxidizing agent" is itself reduced by the "reducing agent." Consequently, the reduction process is sometimes called "electronation," and the oxidation process is called "de-electronation."