Page 184 - Materials Chemistry, Second Edition

171
3.1. Mining and Processing of Metals

The industry standard for galvanized coatings is a minimum thickness of 70 mm,
2
or 505 g of Zn/m . A zinc coating may be applied using either electrogalvanization
or a hot-dip galvanization process. Whereas the former applies a thin layer of
metallic zinc, hot-dipping deposits a thicker coating that is more desirable for the
undercarriage of automobiles or building nails, for instance. Thermal diffusion
galvanizing is a new process that applies Zn powder to the desired part within a
slowly rotating sealed drum, heated to temperatures of ca. 600–850 C. [4] The Zn/Fe

alloying takes place at a lower temperature relative to hot-dipping, resulting in a
more uniform and wear-resistant coating. This process also eliminates the need for
caustic, acidic, and flux baths required to prepare parts for hot-dipping. A coating of
Zn may also be deposited by mechanical galvanization, in which zinc powder is
pressed onto the surface of steel via the interaction of sand or glass beads within a
rotating drum at elevated temperatures (ca. 300–350 C). It should be noted that no

galvanization process is sufficient to protect the steel in highly corrosive environ-
ments (e.g., seawater). For applications within this media, stainless steel is preferred
wherein the chemical composition of the steel is appropriately doped with Cr to
attain corrosion resistance (see Section 3.2).

3.1.1. Powder Metallurgy
Although the origin of fabricating metallic materials through flame sintering dates
back to ca. 3,000 B.C., this method was not widely applied until the late eighteenth
century. The earliest foundations of metallurgy focused on doping and strengthening
bulk metallic materials; however, powders are now frequently used as precursors for
metallic materials. For instance, tantalum powder is used in the fabrication of
capacitors for electronics and telecommunications, including cellular phones and
computer chips. Iron powder is used as a carrier for toner in electrostatic copying
machines; also, over 2 million pounds of iron powder is incorporated each year in
iron-enriched cereals! Copper powder is used in antifouling paints for boat hulls and
in metallic pigmented inks for printing and packaging. Indeed, the list of applica-
tions for metal powders goes on and on, and must constantly be updated as new
applications arise.
Modern powder metallurgy consists of placing a metal powder(s) into a closed
metal cavity, or die, compacting under high pressure (typically 200–300+ MPa),
and sintering in a furnace to yield a metal with the desired porosity and hardness.
The sintering process effectively results in the welding together of powder particles
to form a mechanically strong finished material.
Metal and alloy powders may be produced through the following routes, with the
last three accounting for the most common methods currently employed:
1. Grinding and pulverization of a metallic solid or oxide-based ore
2. Reductive precipitation from a salt solution
3. Thermal decomposition of a chemical compound, or precursor
4. Electrodeposition
5. Atomization of molten metal