macroscopic.

Visible at magnifications to 25x.

 

macrostructure.

The structure of metals as revealed by macroscopic examination of the etched surface of a polished specimen.

 

magnetite.

Naturally occurring magnetic oxide of iron (Fe₃O₄).

 

martensite.

A generic term for microstructures formed by diffusionless phase transformation in which the parent and product phases have a specific crystallographic relationship. Martensite is characterized by an acicular pattern in the microstructure in both ferrous and nonferrous alloys. In alloys where the solute atoms occupy interstitial positions in the martensitic lattice (such as carbon in iron), the structure is hard and highly strained; but where the solute atoms occupy substitutional positions (such as nickel in iron), the martensite is soft and ductile. The amount of high-temperature phase that transforms to martensite on cooling depends to a large extent on the lowest temperature attained, there being a rather distinct beginning temperature (Ms) and a temperature at which the transformation is essentially complete (Mf).

 

mechanical plating.

Plating wherein fine metal powders are peened onto the work by tumbling or other means.

 

metal dusting.

Accelerated deterioration of metals in carbonaceous gases at elevated temperatures to form a dustlike corrosion product; a unique form of high temperature corrosion which forms a dust-like corrosion product and sometimes develops hemispherical pits on a susceptible metal surface; simultaneous carburization is generally observed.

 

metal ion concentration cell

A galvanic cell caused by a difference in metal ion concentration at two locations on the same metal surface.

 

metallic glass.

An alloy having an amorphous or glassy structure. See also amorphous solid.

 

metallizing.

(1) The application of an electrically conductive metallic layer to the surface of nonconductors. (2) The application of metallic coatings by nonelectrolytic procedures such us spraying of molten metal and deposition from the vapor phase.

 

meteor perforation

Perforation of material in outer space resulting from meteor strikes.

 

microbial corrosion.

See biological corrosion.

 

microscopic.

Visible at magnifications above 25x.

 

microstructure.

The structure of a prepared surface of a metal as revealed by a microscope at a magnification exceeding 25x.

 

mill scale.

The heavy oxide layer formed during hot fabrication or heat treatment of metals.

 

mixed potential.

The potential of a specimen (or specimens in a galvanic couple) when two or more electrochemical reactions are occurring. Also called galvanic couple potential.

 

molal solution.

Concentration of a solution expressed in moles of solute divided by 1000 g of solvent.

 

molar solution.

Aqueous solution that contains 1 mole (gram-molecular weight) of solute in 1 L of the solution.

 

mole.

One mole is the mass numerically equal (in grams) to the relative molecular mass of a substance. It is the amount of substance of a system that contains as many elementary units (6.023 exp23) as there are atoms of carbon in 0.012 kg of the pure nuclide C12; the elemenary unit must be specified and may be an atom, molecule, ion, electron, photon, or even a specified group of such units.

 

moneypenny-strauss test.

Corrosion testing in a copper-sulfate solution containing sulfuric acid. Used to detect the susceptibility of stainless steel to intergranular corrosion.

 

monomer.

A molecule usually an organic compound, having the ability to join with a number of identical molecules to form a polymer.

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natural aging.

Spontaneous aging of a supersaturated solid solution at room temperature. See also aging. Compare with artificial aging.

 

Nernst equation.

An equation that expresses the exact electromotive force of a cell in terms of the activities of products and reactants of the cell.

 

Nernst layer, Nernst thickness.

The diffusion layer or the hypothetical thickness of this layer as given by the theory of Nernst, is defined by:

i_d = n F D (C^o - C)/d

where, i_d = the diffusion limited current density, D = the diffusion coefficient, C^o = the concentration at the electrode surface, and d = the Nernst thickness. It is a hypothetical thickness which has been found to be 0.05 cm in many cases of unstirred aqueous electrolytes.

 

neutron embrittlement.

Embrittlement resulting from bombardment with neutrons, usually encountered in metals that have been exposed to a neutron flux in the core of a reactor. In steels, neutron embrittlement is evidenced by a rise in the ductile-to-brittle transition temperature.

 

nitriding.

Introducing nitrogen into the surface layer of a solid ferrous alloy by holding at a suitable temperature (below Ac1 for ferritic steels) in contact with a nitrogenous material, usually ammonia or molten cyanide of appropriate composition. Quenching is not required to produce a hard case.

 

nitrocarburizing.

Any of several processes in which both nitrogen and carbon are absorbed into the surface layers of a ferrous material at temperatures below the lower critical temperature and, by diffusion, create a concenteration gradient. Nitrocarburizing is performed primarily to provide an antiscuffling surface layer and to improve fatigue resistance. Comparc with carbonitriding.

 

noble.

The positive direction of electrode potential, thus resembling noble metals such as gold and platinum.

 

noble metal.

(1) A metal whose potential is highly positive relative to the hydrogen electrode. (2) A metal with marked resistance to chemical reaction, particularly to oxidation and to evolution by inorganic acids. The term as often used is synonymous with precious metal.

 

noble potential.

A potential more cathodic (positive) than the standard hydrogen potential.

 

normalizing.

Heating a ferrous alloy to a suitable temperature above the transformation range and then cooling in air to a temperature substantially below the transformation range.

 

normal solution.

An aqueous solution containing one gram equivalent of the active reagent in 1L of the solution.

 

normal stress.

The stress component perpendicular to a plane on which forces act. Normal stress may be either tensile or compresssive.

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