COPPER AND COPPER ALLOYS are widely used in many environments and
applications because of their excellent corrosion resistance,
which is coupled with combinations of other desirable properties,
such as superior electrical and thermal conductivity, ease of fabricating
and joining, wide range of attainable mechanical properties,
and resistance to biofouling...
COPPER AND COPPER ALLOYS are widely used in many environments and
applications because of their excellent corrosion resistance, which is
coupled with combinations of other desirable properties, such as superior
electrical and thermal conductivity, ease of fabricating and joining,
wide range of attainable mechanical properties, and resistance to biofouling.
Copper corrodes at negligible rates in unpolluted air, water, and deaerated
nonoxidizing acids. Copper alloy artifacts have been found in nearly
pristine condition after having been buried in the earth for thousands
of years, and copper roofing in rural atmospheres has been found to
corrode at rates of less than 0.4 mm in 200 years.
Copper alloys resist many saline solutions, alkaline solutions, and
organic chemicals. However, copper is susceptible to more rapid attack
in oxidizing acids, oxidizing heavy-metal salts, sulfur, ammonia (NH3),
and some sulfur and
Copper and copper alloys provide superior service in many of the applications
included in the following general classifications:
- Applications requiring resistance to atmospheric exposure, such as
roofing and other architectural uses, hardware, building fronts, grille
work, hand rails, lock bodies, doorknobs, and kick plates
- Freshwater supply lines and plumbing fittings, for which superior
resistance to corrosion by various types of waters and soils is important
- Marine applications - most often freshwater and seawater supply lines,
heat exchangers, condensers, shafting, valve stems, and marine
hardware - in which resistance to seawater, hydrated salt deposits,
and biofouling from marine organisms is important
- Heat exchangers and condensers in marine service, steam power plants,
and chemical process applications, as well as liquid-to-gas or gas-to-gas
heat exchangers in which either process stream may contain a corrosive
- Industrial and chemical plant process equipment involving exposure
to a wide variety of organic and inorganic chemicals
- Electrical wiring, hardware, and connectors; printed circuit boards;
and electronic applications that require demanding combinations of
electrical, thermal, and mechanical properties, such as semiconductor
packages, lead frames, and connectors
Effects of alloy compositions on corrosion
Coppers and high-copper alloys (C 10100 - C 19600; C 80100 - C 82800) have
similar corrosion resistance.
They have excellent resistance to seawater
corrosion and biofouling, but are susceptible to erosion-corrosion at high
water velocities. The high-copper alloys are primarily used in applications
that require enhanced mechanical performance, often at slightly elevated
temperature, with good thermal or electrical conductivity. Processing for
increased strength in the high-copper alloys generally improves their
resistance to erosion-corrosion.
Brasses (C 20500 - C 28580) are basically copper-zinc alloys and
are the most widely used group of copper alloys. The resistance of
brasses to corrosion by aqueous solutions does not change markedly as
long as the zinc content does not exceed about 15%. Above 15% Zn,
dezincification may occur.
Susceptibility to stress-corrosion cracking (SCC) is significantly affected
by zinc content; alloys that contain more zinc are more susceptible.
Resistance increases substantially as zinc content decreases from 15% to 0%.
Stress-corrosion cracking is practically unknown in commercial copper.
Elements such as lead, tellurium, beryllium, chromium, phosphorus, and
manganese have little or no effect on the corrosion resistance of coppers
and binary copper-zinc alloys. These elements are added to enhance such
mechanical properties as machinability, strength, and hardness.
Tin Brasses (C 40400 - C 49800; C 90200 - C 94500). Tin additions significantly
increase the corrosion resistance of some brasses, especially resistance
Cast brasses for marine applications are also modified by the addition of
tin, lead, and, sometimes, nickel. This group of alloys is known by
various names, including composition bronze, ounce metal, and valve metal.
Aluminum Brasses (C66400-C69900). An important constituent of the corrosion
film on a brass that contains few percents of aluminum in addition to
copper and zinc is aluminum oxide (A1203), which markedly increases
resistance to impingement attack in turbulent high-velocity saline water.
Phosphor Bronzes (C 50100 - C 52400). Addition of tin and phosphorus to
copper produces good resistance to flowing seawater and to most
nonoxidizing acids except hydrochloric (HCl). Alloys containing 8 to 10%
Sn have high resistance to impingement attack. Phosphor bronzes are much
less susceptible to SCC than brasses and are similar to copper in
resistance to sulfur attack. Tin bronzes-alloys of copper and tin-tend
to be used primarily in the cast form, in which they are modified by
further alloy additions of lead, zinc, and nickel.
Copper Nickels (C 70000 - C 79900; C 96200 - C 96800). Alloy C71500
(Cu-30Ni) has the best general resistance to aqueous corrosion of all
the commercially important copper alloys, but C70600 (Cu-3ONi) is often
selected because it offers good resistance at lower cost. Both of these
alloys, although well suited to applications in the chemical industry,
have been most extensively used for condenser tubes and heat-exchanger
tubes in recirculating steam systems. They are superior to coppers and
to other copper alloys in resisting acid solutions and are highly
resistant to SCC and impingement corrosion.
Nickel Silvers (C 73200 - C 79900; C 97300 - C 97800). The two most common
nickel silvers are C75200 (65Cu-18Ni-17Zn) and C77000 (55Cu-18Ni-27Zn).
They have good resistance to corrosion in both fresh and salt waters.
Primarily because their relatively high nickel contents inhibit dezincification,
C75200 and C77000 are usually much more resistant to corrosion in saline
solutions than brasses of similar copper content.
Copper-silicon alloys (C 64700 - C66100; C 87300 - C 87900) generally have
the same corrosion resistance as copper, but they have higher mechanical
properties and superior weldability. These alloys appear to be much more
resistant to SCC than the common brasses. Silicon bronzes are susceptible
to embrittlement by high-pressure steam and should be tested for
suitability in the service environment before being specified for
components to be used at elevated temperature.
Aluminum bronzes (C 60600 - C 64400; C 95200 - C 95810) containing 5 to 12%
Al have excellent resistance to impingement corrosion and high-temperature
oxidation. Aluminum bronzes are used for beater bars and for blades in
wood pulp machines because of their ability to withstand mechanical
abrasion and chemical attack by sulfite solutions.
In the most of practical commercial applications, the corrosion
characteristics of aluminum bronzes are primarily related to aluminum
content. Alloys with up to 8% Al normally have completely face-centered
cubic structures and a good resistance to corrosion attack. As aluminum
con tent increases above 8%, a-b duplex
Depending on specific environmental conditions, b phase or eutectoid
structure in aluminum bronze can be selectively attacked by a mechanism
similar to the dezincification of brasses. Proper quench-and-temper
treatment of duplex alloys, such as C62400 and C95400, produces a
tempered (b structure with reprecipitated acicular a crystals, a
combination that is often superior in corrosion resistance to the normal
Nickel-aluminum bronzes are more complex in structure with the introduction
of the K phase. Nickel appears to alter the corrosion characteristics of
the b phase to provide greater resistance to dealloying and
cavitation-erosion in most liquids.
Aluminum bronzes are generally suitable for service in nonoxidizing
mineral acids, such as phosphoric (H3PO4),
sulfuric (H2SO4), and HCl; organic acids,
such as lactic, acetic (CF3COOH), or oxalic;
neutral saline solutions, such as sodium chloride (NaCI)
or potassium chloride (KCl); alkalies, such as sodium hydroxide (NaOH),
potassium hydroxide (KOH), and anhydrous ammonium hydroxide (NH4OH);
and various natural waters including sea, brackish, and potable waters.
Environments to be avoided include nitric acid (HNO3); some
metallic salts, such as ferric chloride (FeCl3) and chromic
acid (H2CrO4); moist chlorinated hydrocarbons;
and moist HN3. Aeration can result in accelerated corrosion
in many media that appear to be compatible.