Kuppan T. Heat Exchanger Design Handbook
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774
Chapter
I.?
1.
Forging and extrusions should be machined to their final dimensions prior to heat treat-
ment.
2.
Stressing of extrusions in the short transverse direction of extrusion should be avoided.
3.
Cathodic protection in the form of Alclad layers on the surface of high-strength sheet
material may be used to prevent general corrosion and
SCC.
4.
For most commercial alloys, stress-relieved tempers have been developed that provide
a
high degree of immunity to SCC.
Standard Test Method for Determining Susceptibility to SCC of High-Strength Aluminum
Alloys Products: ASTM
G47.
This is the standard practice for determining the susceptibility
to SCC of high-strength 2xxx (with 1.8-7.0% Cu) and 7xxx(with 0.4-2.8% Cu) aluminum
alloy products, particularly when stressed in the short transverse direction relative to the grain
structure.
Exfoliation Corrosion.
Exfoliation is subsurface corrosion that begins on a clean surface but
spreads below it. It is well known in aluminum alloys and to a certain extent
to
copper-nickels.
The attack differs from pitting
in
that the attack has a laminated appearance. It proceeds gener-
ally along the grain boundaries, laterally from the sites of initiation along planes parallel to
the
surface. The corrosion attack causes a leafing or delamination and sometimes a blistered sur-
face. Due to exfoliation, whole layers of material are eaten away. The end of a sample looks
like a pack of cards in which several cards have wasted away.
Susceptible Alloys.
In certain tempers, wrought products of some aluminum alloys are
subject to exfoliation corrosion, which causes a leafing or delamination of the products. This
type of corrosion develops in products that have a pronounced directional structure,
in
which
the grains are elongated highly and thin relative to their length and width. Alloys of the
~XXX,
the 5xxx series with higher magnesium, and 7xxx series are most prone to exfoliation corro-
sion. It is rare in wrought alloys
of
the non-heat-treatable type (lxxx, 3xxx) and heat-treatable
type 6xxx. Exfoliation can be overcome by heat treatment and alloying [173,178].
Standard Test Method for Determining Exfoliation Corrosion Susceptibility.
ASTM
G34
is the immersion test method for determining exfoliation corrosion
in
2xxx and 7xxx series
aluminum alloys (EXCO test). This test method is applicable to wrought products such
as
plates, sheets, extrusions, and forgings. ASTM G66 is an immersion test
of
5xxx series alumi-
num alloys for exfoliation corrosion, known as the ASSET test.
Intergranular Corrosion.
Aluminum alloys that do not form second-phase microconstituents
at grain boundaries, or those in which the constituents have corrosion potentials similar to the
matrix, are not susceptible to intergranular corrosion. Aluminum alloys such
as
1100,
3003,
and 3004, aluminum-magnesium alloys (5xxx) containing less than
3%
magnesium, are not
affected by intergranular corrosion. The 6xxx series alloys generally resist this type of corro-
sion [175]. Thermal treatment in
~XXX,
Sxxx with more than 3% Mg, and 7xxx series
alloys
that cause grain boundary precipitation is susceptible to intergranular corrosion. The level of
susceptibility to corrosion increases with (1) magnesium content, (2) the amount of cold work,
and (3) exposure time at temperature
[
1731. Resistance to intergranular corrosion is achieved
by use
of
heat treatments that cause precipitation to be more uniform throughout the grain
structure or by restricting the alloy element that causes the intergranular corrosion.
Creiice
Corrosion.
Crevice corrosion of aluminum is negligible in fresh water. In seawater,
it takes the form of pitting and the corrosion rate is low.
Erosion-Corrosion, Cavitation, and Impingement Attack.
Aluminum and its alloys are sus-
ceptible to cavitation, impingement, and erosion-corrosion.
Corrosion Fatigue.
Aluminum alloys, like many steels, have relatively low corrosion fatigue
strength, about half the fatigue strength in air and about 25%
of
the original ultimate strength
775
Material Selection and Fabrication
of material. Corrosion fatigue failures of aluminum alloys are characteristically transgranular
and differ from
SCC
failures, which are usually intergranular. Localized corrosion of the alumi-
num surface provides stress rises and greatly lowers fatigue strength and hence fatigue life
[173].
Corrosion
of
Aluminum in Diesel Engine Cooling Water System. Uniform corrosion may
take place in poorly inhibited or uninhibited glycoVwater mixtures. Localized corrosion is
initiated with the breakdown of the passive film by chloride ion. Pitting and crevice corrosion
may be prevented by the use of inhibitors. Anions commonly used to reduce the effect of
corrosion may be classified into two classes: (1) oxidizing, those that form a passive film on
the metal surface; and (2) nonoxidizing, those that act as blocking agents by forming insoluble
precipitates with aluminum ions
[
1791. Benzoate, phosphate, and silicate are examples of non-
oxidizing inhibitors. Oxidizing inhibitors include nitrates and chromates.
Corrosion Prevention and Control Measures
A number of corrosion preventive measures have already been described. Important methods
of preventing corrosion of aluminum include [173,175]: (1) alloy and temper selection,
(2)
design considerations of equipment, (3) inhibitors,
(4)
cathodic protection,
(5)
use of Alclad
products, (6) modification of environment, and (7) thickened surface oxide film and organic
coating. Some of these methods are discussed next.
Alloy and
Temper
Selection. In general, aluminum-magnesium alloys Sxxx series have the
best corrosion resistance, followed by commercial-purity alloys series
1
xxx and
~XXX,
and
6xxx in that order. Alloys 2xxx and 7xxx are usually given a protective coating such as clad-
ding. Temper selection gives better resistance
to
intergranular corrosion and exfoliation corro-
sion for alloys Sxxx, whereas in alloys
~XXX,
improved resistance to
SCC
is being achieved
[
1731.
Design Aspects. The design aspects that influence corrosion behavior include improper choice
of alloy or temper, galvanic couple, failure to provide sealants in the crevices to avoid crevice
corrosion, and selection of the joining method and filler metals.
Inhibitors. Phosphates, silicates, nitrates, fluorides, benzoates, etc. have been recommended
for use with aluminum in some services. If copper is present
in
a closed system, use sodium
mercaptobenzothiazole to prevent copper corrosion and subsequent depostion corrosion of the
aluminum.
Cathodic Protection. In some applications, aluminum alloy parts are protected by cathodic
protection, either by sacrificial anode or impressed current method. Since cathodic reaction
produces hydroxyl ions, the current on these alloys should not be high enough to make the
solution sufficiently alkaline to cause significant corrosion
[
1801.
Alclad Alloys. The use of Alclad products to resist corrosion has been well established. Al-
clad alloys is a composite product in which a thin surface layer of one aluminum alloy (anodic),
usually
5
to 10% of the total thickness, is metallurgically bonded to the main core alloy (ca-
thodic), which was selected to provide the desired strength and the cladding to provide the
maximum resistance, mostly to perforation by pitting. The difference in the potential between
the core alloy and the cladding alloy provides cathodic protection to the core. The cladding
generally used is alloy 7072, which contains 1% zinc and has a solution potential of -0.96V,
which is at least 100 mV more anodic than typical core alloys such as
3003
and 6951. Since
the cladding are anodic to the core, preferential corrosion of the cladding takes place until the
core and cladding interface. After reaching the interface it spreads laterally, avoiding localized
corrosion. Alclad products are primarily available in the form of sheets and tubes, coated on
one or both sides. For example, Alclad tubes are used in brazed aluminum radiators to avoid
776
Chapter
I3
pitting corrosion of tubes on the water side. Typical combinations of Alclad products generally
used in heat exchanger applications are given in Table
35.
Modification
of
the Environment.
Such modifications are reducing the corrosivity of an envi-
ronment, adjustment of pH of a solution, deareation of water to minimize oxygen content and
hence to reduce the tendency to pit aluminum, etc.
Thickened Su&ce Oxide Film and Organic Coating.
Coatings by diffusion cladding or ther-
mal spray will act in the same way as the cladding layer and corrode sacrificially to protect
the core alloy
[
1751.
Aluminum Difused Steels in Petroleum Refinery Heat Exchangers.
Aluminum vapor diffu-
sion into the surface of iron-base or nickel-base alloys offers one method of protecting steel
tubes (the process is known as alonizing) and other components in heat exchangers from the
harmful effects of high-temperature oxidation, sulfidation, and carburization
[
18
I].
As a result,
this process finds broad application for heat exchangers used in chemical plants and petroleum
refineries.
20.3
Fabrication
Aluminum alloys can be joined by most fusion and solid-state welding process as well as by
brazing and soldering. Fusion welding is commonly done by GMAW, GTAW, FCAW,
SMAW, resistance spot, and seam welding. Plasma and electron beam welding are used for
special applications. Oxyfuel gas welding may be used for applications where high strength
and quality are not essential for the intended service. Flux shielding is unsuitable for aluminum
because flux tends to corrode the metal and produce spatter. Fusion welding of aluminum is
discussed next. Brazing and soldering are covered in the chapter on heat-exchanger fabrication.
Parameters Affecting Aluminum Welding
The parameters that influence welding aluminum and its alloys include surface oxide film,
thermal conductivity, thermal expansion, reflectivity, lack of color change during melting,
nonmagnetic, use of backing bars, surface preparation and cleanliness, gas shielding, welding
heat on base metal properties, and hot cracking. These parameters are discussed in detail by
Lancaster
[176]
and Young
[182].
Salient features of these points are discussed here.
Su$ace oxide film.
The surface oxide film on aluminum and its alloys
is
insoluble in the weld
metal and inhibits wetting by molten filler metals. Additionally, the dielectric nature of
the oxide film may prevent a stable starting of the welding arc. Therefore, to permit good
wetting and fusion, the surface oxide must be removed from its surface before welding,
brazing, and soldering.
Thermal conductivity.
The higher thermal conductivity of aluminum (approximately three times
that of steel) requires higher heat inputs. Thick sections may require preheating to reduce
the heat input.
Table
35
Alclad
Alloys
Core
alloy
Cladding
alloy
3003 7072
3004,
6061
7072
or
7013
695
1 7072
7075
7072, 7008,
or
701
1
7178 7072
777
Material Selection and Fabrication
Thermal expansion.
Higher thermal expansion diminishes the root gap in a butt joint; shrinkage
during solidification (about
6%)
narrows the root gap, and stresses try to crack the weld
metal and warp the weldment. Measures to prevent cracking and distortion include clamp-
ing, fixturing, use of chill bars, and changing welding techniques.
Reflectivity.
Aluminum sheets and plates reflect about
80%
of light and heat rays. When em-
ploying arc welding, therefore, the welders must
be
protected against burns from reflected
as well as direct rays. The intensity of bums is maximum when welding curved surfaces,
such as the inside of a cylinder.
Lack
of
color change during melting.
Aluminum does not assume a dark red color at
1200°F
(649°F) just prior to melting, as does steel, but it appears to collapse suddenly at the
melting point. The lack of color change makes it difficult to judge when the metal is
approaching the molten state. Hence, practice in welding is required to learn to control the
rate of heat input.
Nonmagentic.
Aluminum is nonmagnetic, and therefore, arc blow is not encountered when arc
welding with direct current.
Backing bars.
While welding, some form of backing for the molten pool is necessary. Remov-
able backing bars of steel or cast iron are satisfactory, provided that the welding groove
is of adequate depth
[
1761. Permanent backing bars are rarely used because they introduce
undesirable notch effect and welds are subjected to defects that originate at the root of the
joint.
Root gaps.
Large root gaps cannot be bridged.
Cleanliness.
When fabricating aluminum, maintain a high standard of cleanliness. The method
of edge prepartion and cleaning determines weld quality.
Gas shielding.
In a gas-shielded process, the maintenance of a satisfactory nonturbulent argon
cover in the vicinity of arc and weld pool is essential [182]. Failure to observe this leads to
weld defects like oxide inclusions, lack of fusion, and porosity. While employing GMAW,
puckering and tunneling occur in welding at high currents due to turbulent conditions in
the molten pool and entrainment of air in the gas shield. This is overcome by using an
auxiliary gas shield or by increasing the primary gas coverage to prevent air being sucked
in with argon.
EfSect
of
welding heat on base metal.
Welding heat weakens the base metal next to the weld;
the weak zone is about an inch either side of the center
of
the butt or fillet welds. Heat-
treatable alloys can be stengthened by heat treatment, but only cold working strengthens
non-heat-treatable alloys [7].
Hot Cracking.
Aluminum welds are susceptible to hot cracking. Sensitivity to hot cracking
is influenced by composition of the filler metal, the degree of dilution by the base metal, and
restraint on the weldment. The following meaures help to overcome hot cracking:
1.
Avoid weld metal compositions known to be sensitive to cracking. Combining magnesium
and copper in an aluminum weld metal
is
not desirable.
2.
Use welding procedures that minimize restraint
of
the alloy being welded.
3.
Select a filler metal of a compostion whose solidus temperature is close to that of the base
metal
[48].
4.
Use a filler metal of higher alloy content
than
the base metal.
Surface Preparation and Sueace Cleanliness.
Most porosity in aluminum welds is due to
surface contaminants like shop dirt, cutting fluids, oils, grease, and marking crayons. Therefore,
to obtain weld metal of desired quality, aluminum surfaces are to be clean. Surface cleaning
may be done by these methods [7]:
778
Chapter
13
Solvent degreasing.
Wiping, spraying or dipping, and vapor degreasing remove oil, grease,
dirt, and loose particles.
Mechanical cleaning.
This is oxide removal by stainless steel wire brushing, filing, milling,
rubbing with steel wool, sanding, rotary planing.
Chemical cleaning.
Use immersion
in
5%
sodium hydroxide (caustic soda) solution at 160°F
(70°C)
for 10-60
s,
followed by rinse in commercial desmuting solution at room tempera-
ture for
10-30
s,
followed by a hot-water rinse and air-dry.
Plate Cutting and Forming.
Cut aluminum to size and shape and prepare its edges with most
metal-working tools and methods. However, oxygas cutting won’t work with aluminum. Care
has to be taken while bending aluminum and its alloys with regard to scoring and mechanical
damage because of their softness. It is essential to avoid iron and other foreign metallic embed-
ments or inclusions into aluminum surfaces. To ensure this, cover the plates with plastic
or
sheets of paper and pasting paper over the rolls or by inserting a sheet of hardboard between
the plate and the rolls. Lift large sheets
or
plates with vacuum lifters.
Joint Design.
Joint design is important in selecting the joining method. If butt joints are
required, the choice is limited preferably to an inert gas
or
vacuum-shielded fusion welding
process or a solid-state welding process.
Joint Geometry.
In general, the recommended joint geometries for arc welding aluminum
are
similar to those for steel. However, smaller root openings and larger groove angles are gener-
ally used because aluminum is more fluid and welding gun nozzles are larger. Joint geometry
for aluminum welding is shown
in
Fig.
25.
Preheating.
Preheating is usually avoided, because the properties and metallurgy of alumi-
num almost always suffer from high heat. If preheating is unavoidable, heat aluminum
for
a
short while without exceeding 300°F (149°C).
Corrosion Resistance: Welded, Brazed, and Soldered Joints
Most aluminum base metal and filler metal combinations are satisfactory for service under
normal conditions. For best performance in specific corrosive environments, the filler metal
composition should nearly match the composition of the base metal. Some environments
or
chemicals may require the use of high-purity filler metals or tight limits on composition. For
example, for some saltwater applications, weld 1100 plate with
1100
filler weld.
Welding Filler Metals
Aluminum welding rods and bare electrodes are generally used with the gas tungsten arc, gas
metal arc, and oxyfuel gas welding processes. While selecting aluminum filler metal, consider
these factors:
(1)
ease of welding,
(2)
corrosion resistance,
(3)
resistance to high temperatures,
(4)
response to anodic treatments,
(5)
enhanced strength or ductility, and
(6)
minimized chances
for hot cracking. Choose a filler with higher alloy content than the base metal.
To
obtain the
desired weld metal properties, the filler metals should be free from gas and nonmetallic inclu-
sions and surface contamination. Keep the welding wire covered and store it
in
a dry area at
even temperatures.
Do
not open a new box of wire until ready to use it. Filler metals for the
most widely specified weldable grade alloys
5052,
5083,
5454,
and 6061 are given
in
Table
36.
Welding Methods
Gas-Shielded
Arc
Welding.
Gas metal arc welding (GMAW) and gas tungsten arc welding
(GTAW) have almost entirely replaced other arc welding processes for aluminum alloys. The
workhorse welding process is GTAW, argon shielded for root passes, followed by GMAW fill
Material Selection and Fabrication
779
1
11
1
II
'TEWORCIRY
BACK
IN6
\
TEMPORARY
BACKING-
SINGLE
-
v
-
WZOOVE
DOUBLL
-
J
-
mma
DOUBLE
U
-
GUWU€
SINGLE:
-
U
-
GROOM
Figure
25
Typical joint design for arc welding of aluminum.
Table
36
A
Guide to Aluminum Welding
Filler Metal Selection
[172]
Base
metal Filler metal
5052 5183, 5356, 5556
5083 5183, 5356, 5556
5454 5183, 5556
606 1 5183, 5556,4043
passes and in some cases a GTAW cover pass to dress the weld surface. Edge preparation is
typically a
45
to
73"
opening with a 1/16-in low.
Gas Tungsten Arc Welding Process.
The GTAW process solved the two problems associated
with aluminum, namely, high heat conductivity and aluminum oxide surface film, by concen-
trating large amounts of heat to remove the refractory oxide, then melting the base metal
rapidly. The GTAW process uses
a
consumable aluminum filler rod, and inert gas shielding
(usually argon, which protects the electrode, filler, and weld metal
from
oxidation).
An
alternat-
780
Chapter
I3
ing current (ac) is most often used, because it not only provides heat input to the base metal
but also removes the oxide surface film. Fluxes are not required. Welders usually restrict ac
operation to thickness less than 0.25
in
(6.35 mm), handling heavier gages by dcsp GTAW or
GMAW.
GTAW by Direct Current. GTAW
is performed with direct current straight polarity
(dcsp) or reverse polarity (dcrp). With straight polarity welding, current flows (electron) from
the electrode to the work, and the oxide film is not removed. To remove the oxide
film,
electrons must flow from the joint, and this is achieved with reverse polarity (dcrp).
Gas Metal
Arc
Welding.
GMAW is probably the most popular method for welding aluminum
in heavier gages other than GTAW. It is capable of much higher production rates than GTAW.
Higher current density, coupled with efficient heat transfer in the arc, give deep penetration,
high welding speeds, minimum distortion of base metal, and good mechanical strength. Weld
metal transfer modes include spray (the most desirable), globular, short-circuit, and pulsed
spray.
In
GMAW, direct current reverse polarity is used to establish arc between the consum-
able electrode and the work piece in a shield of inert gas (argon or helium).
Merits
of
Gas-Shielded
Arc
Welding Processes.
GMAW and GTAW processes result
in
opti-
mum weld quality and minimum distortion, and they require no flux. Since there is no flux is
used,
(
1)
difficult-to-reach places and completely inaccessible interiors are left without the danger
of corrosion from residual corrosive flux, (2) welding can be done in all positions, because there
is no slag to be removed out of the weld by gravity or by puddling, and
(3)
visibility is
god
because the gas envelope around the arc
is
transparent and the weld pool is clean.
21
COPPER
Metallurgy and Physical Properties.
Copper is a noble metal. It has excellent electrical and
thermal conductivity; it is malleable and machinable. Being a noble metal,
it
resists many
corrosive environments. Other useful properties include ductility, formability, easy joinability
by most conventional methods, nonmagnetic, availability in many products, proven service
performance, and moderate cost. However, it has low mechanical properties and must be cold
worked or alloyed to obtain strength. Typical alloying elements include zinc, aluminum, nickel,
silicon, tin, iron, and
so
on. There are hundreds of copper alloys. Copper combines with zinc,
tin, silicon, aluminum, and nickel to create the major copper alloy families. Copper is com-
pletely soluble in nickel and vice versa. Small additions of alloying elements ensure specific
purposes-for example, silver provides resistance to softening; beryllium, hardenability
;
lead,
machinability
;
phosphorus deoxidizes; phosphorus, antimony, arsenic, and tin give resistance
to dezincification; iron adds strength and resistance to erosion-corrosion; and nickel adds high
strength and resistance to various corrosion types
[
183,1841.
21
.I
Copper
Alloy
Designation
To bring order and to identify the many alloys, the Copper Development Association
[
1851,
together with the American Society of Testing and Materials and the Society of Automotive
Engineers, developed a five-digit system. This system is part of the Unified Numbering System
(UNS)
for metals and alloys. In this system, the numbers ClO0o0 through C79999 denote the
wrought alloys; cast copper and copper alloys are numbered from
C800o0
through C99999.
Wrought copper alloys are divided in the
UNS
system in the following groups.
Material Selection and Fabrication
781
Copper and high-copper alloys
c
1 xxxx
Zinc brasses (Cu-Zn) c2xxxx
Zinc-lead brasses c3xxxx
Zinc-tin brasses c4xxxx
Tin bronzes CSXXXX
Aluminum, manganese, silicon bronzes C6xxxx
Copper-nickel alloys c7xxxx
For selecting copper cast products, refer to Peters et al.
[
1861.
Wrought Alloys
In Table 37, the major alloy groupings used for pressure vessels and heat exchangers, the
UNS
number ranges, and the major alloying elements are given.
21.2 Heat-Exchanger Applications
Heat-exchanger materials require good thermal conductivity, strength, corrosion resistance,
ease of formability into various shapes like tubes, tubesheets, and fins by processes like draw-
ing, roll forming, bending, blanking, shearing, machining, etc., and joinability
.
Copper and
copper alloys offer good combinations of these properties. In addition, copper alloys have a
biostatic action, which gives tube surface antifouling properties. The three major areas of
applications of copper and copper alloys are in (1) steam condenser tubings, (2) automotive
heat exchangers such as charge air coolers, radiators, and oil coolers, and (3) condensers and
evaporators of refrigerators and air conditioners.
21.3 Copper in Steam Generation
Copper alloys such as admiralty brass, cupronickels, aluminum bronze, and the nickel-base
alloy Monel have found wide use in feedwater heaters. Their subsequent corrosion by the
feedwater, although minor, gives rise to the problem of carryover of copper into the steam
generator and possible blockage in turbine units. With the advent of the high-pressure supercrit-
ical units, the use of carbon steel tubes in feedwater heaters became common practice in order
to eliminate the main source of copper carryover into the steam generator [47,78].
21.4 Wrought Copper Alloys: Properties and Applications
In this section, wrought copper and copper alloys that are used for the construction of heat
exchangers and pressure vessels are discussed.
Coppers
High-purity coppers such as C10100, C10200, C10400, C10500, C10700, C10800, CllOOO,
Cl 1300, C11400, C11500, and C11600 are included in this category. They contain at minimum
Table
37
Alloy Groupings and UNS Number Ranges
Coppers
C
10
100-C
1
5760
>99% cu
High-copper alloys
C 16200-C 19600 >96%
Cu
Brasses
C205OO-C28580 Cu-Zn
Leaded brasses
C3 12WC38590
Cu-Zn-Pb
Tin brasses
C404oO-C49080 Cu-Zn-Sn-Pb
Aluminurn bronzes
C60600-C64400
Cu- Al-Ni-Fe-Si-Sn
Silicon bronzes
C647OGC66
100
Cu-Si-Sn
Copper-nickels
C70000-C79900
Cu-Ni-Fe
782 Chapter
13
99.3% copper. The elements like silver, arsenic, antimony, sulfur, phosphorus, lead, nickel,
cadmium, zirconium, magnesium, boron, and bismuth may be present singly or in combination.
These commercially available in various types (oxygen-bearing, oxygen-free, and deoxidized).
Due to their high thermal conductivity, these metals are entirely used for fins and tubes for
charge air cooler, intercooler, oil cooler, soldered radiator, and condenser and evaporator of
air conditioner, refrigerator, and heater coils. They have good resistance to atmospheric corro-
sion and galvanic corrosion.
0,xyen-Free Coppers.
The oxygen-free coppers have mechanical properties similar
to
those
of the oxygen-bearing coppers, but their microstructures are more uniform. They have excellent
ductility and resistance to fatigue, and can be joined readily by welding, brazing, and soldering.
Silver is sometimes added to oxygen-free copper to increase the strength at elevated tempera-
ture.
Phosphorus-Deoxidized Copper (C12000
to
CZ2300).
C12000 (DLP), C 12200 (DHP), and
C12300 (DHP) belong to this category. Due to good thermal conductivities, they are exten-
sively used as tubes in charge air coolers, condensers, and evaporators in sugar and fertilizer
industries, refrigerators, and air conditioners. Deoxidized copper is popular in the construction
of process equipment because it can be oxyacetylene welded, silver brazed, and soldered.
Deoxidized coppers suffer embrittlement when heated in a reducing atmosphere at 370°C or
above.
Fire-Refined Copper.
C12500, C12700, C12800, C12900, and C13000 are used for radiator
manufacture.
High-Copper Alloys
The wrought high-copper alloys C19200 and C19400 contain a minimum
of
96% copper.
C19400 is essentially, copper with the addition of about 2.4% iron. Iron addition enhances
strength and corrosion resistance. These alloys are known for good formability, excellent sol-
dering, brazing, and shielded arc welding, and good oxyacetylene welding. They are resistant
to SCC. The alloys are primarily used as seam-welded condenser tubing
in
desalting service.
Brasses (Copper-Zinc Alloys)
Brasses contain zinc as their principal alloying element. C23000, C24000, C26000, C26800,
C27000, and C28000 belong
to
this category. Other major alloying elements are lead, tin,
and aluminum. The addition of zinc to copper decreases the melting point, density, thermal
conductivity, and modulus of elasticity, among others. It increases the strength, hardness, duc-
tility, and coefficient
of
thermal expansion. These alloys are suspectable to dezincification and
SCC. Lead is added to improve machinability. The addition of tin, nominally about
I%,
in-
creases strength and resistance to dezincification. Aluminum is added to stabilize the protective
surface film.
Muntz
Metal
(60Cu-40Zn), C28000.
This has generally better resistance to sulfur-bearing
compounds than higher copper alloys. It shows poor cold-working but excellent hot-working
properties, and is the strongest of Cu-Zn alloys.
Leaded Brasses (Cu-Zn-Pb), C3 120O-C38590
Leaded Muntz metal or leaded brasses such as C36500, C36600, C36700, and C36800 belong
to this series, with good resistance to corrosion in both fresh and saltwater. C36500 is an
uninhibited alloy and susceptible to dezincification. Others, C36600, C36700, and C36800, are
inhibited alloys containing As, Sb, or
P
as an inhibitor element (0.02-0.1%), which imparts
high resistance to dezincification.
783
Material Selection and Fabrication
Tin Brasses (Cu-Zn-Sn-Pb), C40400-C49080
Admiralty brass, C44300, is extensively used in water-cooled condensers and coolers of petro-
leum refining and petrochemical operations. It is attacked by pitting, and ammonia SCC and
dezincification in environments containing high concentrations of ammonia and hydrogen sul-
fide. Cracking of admiralty brass tubes has been a recurring problem in a number of refinery
heat exchangers during shutdown when ammonia-containing deposits on the tube surfaces are
exposed to air. This problem can be overcome by spraying the tube bundle with a very dilute
solution of sulfuric acid, immediately after the tube bundle is withdrawn from the shell. Small
amounts (0.02-0.1 1
%)
of P, As, or Sb increase resistance to dezincification.
Inhibited Admiralty
Brass.
C44300, C44400, and C44500 exhibit resistance to dezincification
and good corrosion resistance in various service environments such as fresh water, seawater,
brackish water, steam and steam condensates, and sulfur compounds encountered
in
refinery
operation. However, they may fail in service due to high impingement velocity and ammonia
SCC.
Naval Brass-C46400, C46.500, C46600,
C46700.
Addition of 0.75%
tin
to Muntz metal
(60Cu-40Zn) results into naval brass. Naval brass exhibits good resistance to industrial, rural,
and marine atmosphere, petroleum products, alcohols, dry gases, and seawater. It has fairly
good resistance to weak bases, but generally poor resistance to solutions of cyanides and
ammonium compounds. It resists dezincification and offers good resistance to both fresh and
saltwater. Addition of inhibitor elements
P,
As,
or Sb in small amounts (0.02-0.1 1%) to
C46400 results in inhibited naval brasses--C46500, C46600, and C46700-which resist de-
zincification.
Aluminum Bronzes (Cu-Al-Ni-Fe-Si-Sn), C606oeC64400
Aluminum bronze contains typically
5-1
7%
aluminurn with or without iron, nickel, manga-
nese, and silicon. Typical wrought copper-aluminum alloys are C60800, C6
1000,
C6 1300,
C61400, C61500, C61800, C62300, C63000, and C63200. Alloys range from single-phase
composition containing up to about 7% aluminum to complex two-phase alloys containing up
to 11% aluminum with additions of iron, nickel, and manganese. They are available
in
cast
and wrought forms. They are generally suitable for service in alkalies, neutral salts, nonoxidiz-
ing acid salts, and many organic acids and compounds. They resist corrosion of water-pota-
ble, brackish, or seawater. Aluminum bronze and copper-nickel alloys represent the two most
important copper alloy groups for seawater applications. Salient characteristic features of alu-
minum bronzes are:
1. Aluminum bronzes resist oxidation and scaling at elevated temperatures due to the forma-
tion of aluminum oxide on the surface.
2.
Aluminum bronzes offer outstanding strength and resistance to erosion-corrosion.
3. They can be joined by welding and brazing.
4. Susceptible to SCC in moist ammonia and mercurous solution.
5.
Resist dealloying but to different degrees depending on alloy composition.
6. The corrosion characteristics
are
affected by the microstructure of the alloy.
A
small
amount of tin added to C61300 will inhibit intergranular stress corrosion.
Aluminum bronze tubesheets and wrought tubes are extensively used in condensers using
seawater. Tubesheets of these alloys are compatible with most condenser tube alloys like
C70600, C71500, C68700, and titanium. The cast from (C95400, C95500, C95800)
is
used as
heat exchanger tubesheets and water box.