Titanium

HISTORY

The element Titanium was discovered as a component of beach sands by a William Gregor in 1790. It was named Titanium after Titan, a giant in Greek mythology, by M.H. Klaproth in 1795. Since then, it has been widely studied, and the basic research for the development of the titanium industry have been accomplished with the magnesium reduction process of titanium tetrachloride invented by W.J. Kroll in 1938. In 1947 mass production of titanium metal was started by the US Bureau of mines.

INTRODUCTION

Titanium is lightweight, the specific gravity is 4.5, which is approximately half and 60% those of nickel or copper and steel, respectively. Titanium melts at 3,045*F. Titanium can be TIG and MIG welded in inert gas. It is ductile only when it is free of Oxygen. The specific strength (tensile strength per specific gravity) of titanium is 3 times that of aluminum and even higher than that of stainless steel. This superior property is maintained virtually at the same level even at a temperature of 500*C. Titanium is equivalent to platinum in resistance to corrosion by seawater and is mostly superior to stainless steel in corrosion resistance.

Titanium is present in meteorites and in the sun. Titanium oxide bands are prominent in the spectra of M-type stars. Titanium is the fourth most abundant structural metal and ninth most common element in the earth's crust. It is refined from rutile (TiO2) ores mined in the U.S., Australia, Africa, China, and Russia. Rutile is most commonly recognized as the gold fibrous crystals in rutilated quarts. Titanium is almost always present in igneous rocks and in the sediments derived from them. It occurs in the minerals rutile, ilmenite, and sphene, and is present in titanates and in many iron ores. Titanium is present in the ash of coal, in plants, and in the human body.

Titanium, when pure, is a lustrous, white metal. Titanium metal is considered to be physiologically inert. When pure, titanium dioxide is relatively clear and has an extremely high index of refraction with an optical dispersion higher than diamond. It is produced artificially for use as a gemstone, but it is relatively soft. Star sapphires and rubies exhibit their asterism as a result of TiO2. Titanium dioxide is extensively used for both house paint and artist's paint, as it is permanent and has good covering power. Titanium paint is an excellent reflector of infrared. Titanium oxide pigment accounts for the largest use of the element.

Due to its high strength (especially in the alloys) and light weight, Titanium has many applications in the aerospace industry. Excellent corrosion resistance makes this metal highly desirable for chemical and food processing, also bone replacement and other body implants. This hypo-allergenic metal is safe for sensitive wearers.

Titanium is available in some 20 standard grades and alloys. Grade #1, commercially pure (CP) titanium is the best suited to jewelry applications. It is ductile and surprisingly slow to work harden. Although the other alloys and less pure grades will color, they are too hard for jewelry work. Titanium needs to be freshly etched to produce its most vivid colors.

Titanium may be either hot or cold forged. At approximately 1640*F titanium goes through a structural phase change and becomes very ductile. As it drops below this temperature it will suddenly harden. The main disadvantage to hot work is the build up of very tough surface oxides. These are dark and may extend deep into the metal surface. They must be ground off and the surface finished before coloring.

COLOR

Reactive is a descriptive term used in connection with a family of metals that react to their environment. Titanium is one of the most common of these metals. They react by oxidizing, especially when excited with heat or electricity in an electrode. The colors produced by these metals are known as interference colors. There are no pigments or dies involved. They are generated by a transparent oxide film grown on the metal surface, which results in the appearance of color. The metal does not actually change color, rather the thin, transparent oxide generates interference colors. The colors develop when part of the light striking the surface reflects and part pass through the film to reflect off the metal below. When the delayed light reappears and combines with the surface light waves they may either reinforce or cancel. This generates a specific color. The thickness of the oxide film dictates the color. In nature these colors can be found in the eddies of an oily wet street and in the iridescent colors of some insects. The colors are controlled by the voltage being applied through a bath, a brush or sponge.

All forming and surface finishing must be done before coloring. Most surface finishing techniques can be adapted to these metals. Finishing will be more time consuming than with traditional metals. Consideration should be given to sanded and textured surfaces. Light gathering scratches and marks will add flash and variety to the work. Textured surfaces add some protection against abrasion. Although harder than the parent metal, the extreme thinness of the oxide dictates that it is not a strong wearing surface. Items that normally receive heavy abrasion should not be considered unless the metals are protected by other design elements. Titanium requires a chemical etch to prepare the surface for high voltage anodizing.

Coloring can be achieved in two ways; thermal oxidation and electrolytic excitation, the metals react with oxygen to form a thin transparent film. Thermal oxidation (heat coloring) is simple, but difficult to control. Anodizing is infinitely more predictable and is the only effective way to control color. The colors produced appear in up to five repeating orders. Most of the current jewelry is produced within the first two orders. All the colors of the light spectrum are not produced. True red and forest green are not generated. When the oxide is of a thickness to generate interference colors, its depth is measured in angstroms (A=1/100,000,000 centimeter.) This layer can vary from 500 to 1,000A+ depending on the color. It is not the oxide itself that is perceived by the viewer, but its effect on light.

Anodizing most closely resembles standard electroplating. When a reactive metal is suspended in an electrolytic bath as an anode (+) and current is passed through the bath, oxygen is produced at the anode surface. This oxygen reacts with the metal to form a thin oxide film that generates colors. The transparent oxide increases in thickness in relation to the amount of voltage applied. At any given voltage the oxide will grow to a specific thickness (i.e. color) and stop, having reached a stage where current will no longer pass. This phenomenon of voltage controlled growth means that the color is also voltage controlled. An area of oxide produced with a high voltage will not pass current from a lower voltage. In other words an area anodized at 60 volts will not need masking when an adjacent area is anodized to 40 volts. It follows that multiple anodizing processes should proceed in decreasing voltages. Working in descending order will save masking and generate fewer errors. While oxygen is generated at the anode (+), hydrogen is formed at the cathode (-). Titanium and stainless steel make most convenient cathodes. This process does not have much throwing power and it is necessary to have a cathode equal to or larger than the anode. The electrolytic solution can be almost any liquid capable of carrying current. Such diverse solutions as cola, Sparex, sulfuric acid, ammonium sulfate (fertilizer), magnesium sulfate (Epsom salts), trisodium phosphate, dish detergents and even wine will work. Recommended is a solution of 3 to 10% by weight trisodium phosphate (T.S.P.) in solution with distilled water. The percentage of chemicals in the solution will determine to some extent the length of time for the desired reaction to be completed. Slowing the reaction can be achieved by lowering the concentration of chemical in solution. The power supply required for anodizing has a much greater range of voltage control and lower range of current capabilities than plating rectifiers. The requirements are 0-150 volts DC variable in one volt increments and from 2-5 amps. Larger capacity power supplies may be necessary for work larger than jewelry and in high volume production.

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