[Illustration: Fig. 7.—Relations of Copper and Arsenic.]
_Arsenic in Copper._—When arsenic and copper are melted together
chemical combination occurs, and a series of arsenides is produced;
the system, which has been investigated by Friedrich (from whose work
the following diagram has been constructed), Hiorns, Bengough & Hill,
and others, being one of considerable complexity. With proportions of
arsenic such as are usually present in commercial coppers, the compound
produced is probably Cu_{3}As (28·3 per cent. of arsenic), which
passes into solution in the excess of metal, and on solidification the
copper retains this arsenide in solid solution. As in the case of all
such solid solutions, the solidification takes place over a range of
temperature represented between the liquidus and solidus curves; the
purer metal crystallising out first, followed gradually by crystals
of copper which become progressively richer and richer in arsenic
(still in solid solution). In the case in question, diffusion of the
arsenic throughout the crystalline mass proceeds but slowly, and as
a result, the metal, as usually obtained in the cast state, shows
fringes of such arsenic-rich copper. By annealing, diffusion is greatly
assisted, and the material gradually becomes homogeneous, as is seen
on microscopic examination. There appears further to be some decrease
of this solubility with fall of temperature when the arsenic is high,
leading sometimes to a separation of the arsenide itself at the crystal
boundaries.
_Antimony_ appears to form an analogous compound, Cu_{3}Sb, also
capable of passing into solid solution in the copper, but to a
rather smaller extent than the corresponding arsenide. The fringes
are therefore more pronounced, and the decrease of the solubility on
further cooling is also more marked.
_Bismuth._—The influence of even minute quantities of bismuth on
copper is notorious. Bismuth appears to be soluble in liquid copper,
but not in the solid metal. In consequence, when copper containing
bismuth solidifies, the copper crystals separate first, whilst the
liquid bismuth still remains between them, until the metal reaches a
temperature of about 268° C.—the melting point of bismuth—when it too
solidifies _in situ_. The presence of such envelopes of very brittle,
fusible, and limpid bismuth material explains much of the harmful
effect of this impurity. These envelopes are found to consist almost
entirely of practically pure bismuth. Oxygen converts the bismuth into
a more compactly crystalline oxide, much less fusible and harmful.
Arsenical copper tends to the scattering of the bismuth globules
among the fringes which are formed during the gradual process of
solidification over the range of temperature already indicated, and
thus renders this impurity to some extent less dangerous.
_Lead_ behaves in apparently much the same way as bismuth, and the
effects produced upon it by the presence of oxygen and arsenic are
probably similar.
_Selenium and Tellurium_ probably exist in the form of selenides
and tellurides, which are characterised by marked brittleness and
fusibility.
=Mechanical Properties of Copper.=—The mechanical properties of
commercial copper are influenced to a vital degree by the conditions
associated with working practice, such as composition, previous
mechanical and thermal treatment, temperature of working, etc. As has
been already indicated, it is the possession by the copper of certain
mechanical qualifications which leads to its employment by engineers,
and it is, therefore, necessary to consider the influence of the above
conditions, when reviewing the mechanical properties of the metal.
Much of the copper employed for general engineering work (apart from
electrical and alloying purposes) is of the quality designated as
“tough-pitch” copper. This tough copper generally contains certain
impurities which render the metal exceedingly useful for mechanical
service, and their presence is, indeed, almost essential in copper
intended for such purposes. At the same time, such elements would
render it absolutely unfit for the other uses just specified, where
purity is practically the first necessity.
The standard works and the papers indicated in the appended list of
references should be consulted for details concerning the effect of
each circumstance on the several mechanical properties; certain general
considerations must, however, be noted here.
Not only should the composition of the metal be carefully considered,
but attention must be directed to the actual condition and distribution
of each constituent. Owing largely to the difficulties of determining
the oxygen contents in copper, and to a want of definite knowledge as
to the condition, amount, and effects of the dissolved gases in the
metal, the information at present available is not sufficiently concise
to allow of a systematised statement being made as to the direct
influence of the constituents on the mechanical properties. This is
more especially the case since the other attendant circumstances of
working practice may react through these to a considerable extent.
Many of the more general results have, however, long been known to
engineers from practical working, and these have been placed on record
from time to time.
The _malleability_ and _ductility_ of copper are considerable. Cold
rolling and hammering causes a reduction in this respect, and the
metal is hardened, but the properties are restored by annealing.
The annealing effect commences at about 300° C., but proceeds
more effectively at higher temperatures, the factors of annealing
temperature and duration necessary for annealing being inversely
connected. The impurities which influence these properties most
adversely are bismuth and tellurium. The effect of other constituents,
oxygen _per se_, sulphur, and iron, in the quantities usually present
in commercial copper, is very small. Arsenic and antimony up to 0·4
or 0·5 per cent. have no deleterious effect on the malleability and
ductility of copper _of the correct pitch_, and may even improve the
metal when tested in the cold; the hot malleability is, however,
somewhat decreased.
The presence of impurities raises the temperature required to
bring about the full effects of annealing after the metal has been
hardened by mechanical work. This action is probably explained by
the interference of the impurities upon the molecular freedom of the
metal, which controls the mechanism of annealing. The conditions,
whether reducing or oxidising, during annealing, may exert an important
influence on the results.
_Hardness._—Pure copper is a comparatively soft metal. It is hardened
by mechanical work—the hardness of rolled copper, determined by the
Brinell Test, being 74 compared with mild steel as 100—and by the
presence of even small quantities of impurities, tin possessing a
particularly marked effect in this connection. The worked metal is
softened on annealing.
_Tensile Strength and Elongation._—The strength of copper, being a
property of such practical importance, has been the subject of much
extended investigation. The work has, however, been conducted under
such a great variety of conditions, many of which have been left
unrecorded, that co-ordination of the results is barely possible,
and does not allow of establishing on a definite basis the effect of
different influences on this property of the metal. Later work, some
already published, some still in progress, should eventually allow
of more general standardisation than is at present possible. The
tensile strength of pure cast copper is 8 to 9 tons per square inch.
Mechanical work causes an increase in the value up to 14, or even 16
tons, cold work exerting a still more marked influence; whilst 33 tons
and more per square inch has been recorded with cold-drawn fine wire.
The elongation varies according to the mechanical work which the metal
has undergone; the amount ranges from 35 to 40 per cent. and upwards,
measured on a 3-inch length.
Tensile strength is reduced on annealing, but never to so low a
degree as that of the cast material, the usual figure being 12 to 14
tons per square inch. The effect of temperature in reducing tensile
strength, especially when impurities are present, is important from the
industrial point of view. The reduction in strength caused by annealing
appears to be considerably smaller in the presence of arsenic and
antimony.
Arsenic increases the tensile strength of copper when the metal
is of the correct pitch, generally to well over 15 or 16 tons,
in the presence of the proportions usually found. Antimony has a
similar effect. Some workers state that, within certain limits, the
strengthening effect of this element is even more pronounced. Excess
of antimony exerts, however, a much more adverse influence than does
excess of arsenic. The elongation is increased by the presence of
moderate quantities of arsenic.
Oxygen _per se_, when present in moderate quantity in copper, has but
little effect on the tenacity. Bismuth, tellurium, sulphur, and lead
are the impurities which lower the strength, even when present in
minute quantities, and especially on heating. Bismuth in the proportion
of 0·005 per cent. lowers the malleability and ductility considerably,
and recent reports state that 0·02 per cent. bismuth renders copper
cold short, that 0·05 per cent. makes it red short, and that 0·005
per cent. is the limit for electrolytic copper which is to be rolled.
The deleterious effects of bismuth are, as already explained, to some
extent masked by the presence of arsenic and by oxygen.
The strength is increased by the presence of nickel, tin, and zinc in
the proportions usually present in the commercial metal; these are,
however, generally small.
From the foregoing review, indications will be afforded of the reasons
for the choice by engineers of “tough-pitch” copper for much of their
work, and the explanation for the 0·3 to 0·5 per cent. arsenic often
particularly specified for. The frequent use of arsenical coppers for
such purposes as fire-box plates will also be understood, since the
arsenic not only improves the mechanical properties of the metal, but
ensures the retention of rigidity and strength at the high working
temperatures required, to a greater degree than would have been the
case had pure copper been employed.
The effect of the above factors on the elastic limit of copper, is
also very marked and of much importance, the influence being closely
analogous to that produced on the other mechanical properties.
=Chemical Properties.=—The atomic weight of copper is 63·57. The metal
is unchanged in dry air at ordinary temperatures; in the presence of
moisture and of carbon dioxide a green coating of basic carbonate is
produced. When heated in air, a black scale, consisting of cuprous
oxide, Cu_{2}O, is obtained, which is readily detached by quenching
and hammering. Water at ordinary temperatures is without effect upon
copper; concentrated sulphuric and nitric acids have little action
upon it in the cold, but attack it on heating. The best solvent for
the metal is dilute nitric acid, which dissolves it very readily.
Copper is liable to corrosion when subjected, whilst hot, to the
action of chlorine or hydrochloric acid gas; this action has provided
an explanation of the corrosion of copper boiler tubes where the coal
employed had been exposed to sea water.
Copper is deposited from solution as a dull red, spongy mass, by iron,
zinc, or aluminium, but it is more electro-positive than gold or
silver, and readily precipitates these metals from solutions of their
salts, these effects being extensively made use of in practice. The
metal possesses a powerful affinity for sulphur, and this property has
very important applications in the smelting processes.
Copper readily alloys with gold, silver, tin, zinc, and nickel, but not
with lead or iron.
References.
Composition and Properties of Metal for Railway and Locomotive Work
(p. 20).
_Proc. Inst. Mech. Eng._, 1893; Dean, p. 139; Blount, p. 164;
Watson, p. 168; Gowland, p. 176; Aspinall, p. 193;
Tomlinson, p. 182.
Webb, F. W., “Locomotive Fire-box Stays.” _Proc. Inst. C.E._, 1902.
Milton, J. T., “The Treatment of Copper for Steam Pipes.”
_Inst. Marine Eng._, 1908–9.
Hughes, G., “Non-ferrous Metals in Railway Work.”
_J. Inst. Metals_, Sept. 1911.
Law, E. F, “Alloys.”
Influence of Impurities on the Electrical Conductivity of Copper
(p. 24).
Lawrence Addicks,
_Trans. Amer. Inst. Elect. Eng._, 1903, vol. xxii., pp. 695–702;
_Electro-Chemical Industry_, 1902–3, pp. 580–583;
_Trans. Amer. Inst. Min. Eng._, 1906, vol. xxxvi., p. 18.
Walker, A. L., _Mineral Industry_, 1898, vol. vii., p. 248.
T. Johnson, “Some Features in the Metallurgy of Copper.”
_Proc. B’ham. Met. Soc._, 1906.
Hiorns and Lamb, “Influence of Arsenic and Antimony on Copper.”
_Journ. Soc. Chem. Ind._, May, 1909.
Condition and Influence of Impurities on the Mechanical Properties
of Copper (p. 32).
┌─
│ _Zeitschrift für Berg. Hutten and Sal. Wesen_,
Hampe ─┤ 1873, xxi., 218; 1876, xxiv., 26
│ _Chemiker Zeitung_, 1892, No. 42, p. 16.
└─
┌─
Heyn, E., │ _Reports, Royal Tech. Testing Institute._
“Copper and Oxygen.” ─┤ Charlottenburg, 1900, p. 315.
│ _Metallographist_, vol. vi., 1902, p. 48.
└─
Arnold, _Engineering_, vol. lxi., p. 176. Feb. 7, 1896.
Roberts-Austen, Second Report, Alloys, Research Committee.
_Proc. Inst. Mech. Eng._, April, 1893, p. 114.
Rudeloff, _Mittheil. König. Tech. Versuchs. Anstalt._, 1894, ii.
(_b_), pp. 292–330; 1898. 16_a_, pp. 171–219.
Lawrie, _Bull. Amer. Inst. Min. Eng._, 1909, pp. 857–66.
“Bismuth in Wire Bar Copper.”
Johnson, F., “Impurities in Tough Pitch Copper containing Arsenic.”
_Proc. Inst. of Metals_, 1910, vol. iv.; No. 2, p. 163, _et seq._
Johnson, F., “The Influence of Impurities on the Properties of
Copper.” _Metallurgical and Chemical Engineering_, Oct. 1910,
p. 570.
“Annealing of Copper and Diseases of Copper.” _Ibid._,
Feb. 1911, p. 87.
“Notes on the Metallurgy of Wrought Copper.” _Ibid._,
August, 1911, p. 396.
See also Standard Specifications for Copper Wire-Bars (recommendations
by the Committee of the American Society for Testing Materials). _Eng.
and Min. Journ._, Jan. 20, 1912, p. 181.