III Oil and Petrochemical Industries
The use of aluminium bronzes in
the oil and petrochemical industries is largely restricted to
pumps, heat exchangers and valves in cooling water systems, to
which the comments under Section 4(i) and Section 4(ii) generally
apply.
Aluminium bronze (CA102 or UNS
60800) tubes are sometimes used with tubeplates of CA105 or CA106
in heat exchangers especially when these are operating under
relatively high pressure and it is desired to weld the tubes to
the tubeplates. For most purposes, however, heat exchangers are
tubed with aluminium brass or, for particularly severe service
conditions, with titanium and the tubeplates are of naval brass
or aluminium bronze CA105 or CA106.
For coolers dealing with high
pressure product streams, tube-and-shell condensers are used with
sea water on the shell side and the product passing through the
tubes. The shells and baffles are fabricated from CA106
("Alloy D") plate 12 to 15 mm thick, using a duplex (9
to 10% Al) alloy as the weld filler but with a capping run of
CA106. The tubeplates are also of CA106; with tubes of 70/30
copper-nickel or titanium.
Aluminium bronze bolts are usually
employed on submersible cooling water pumps. Monel is more
commonly used for bolting flanges on large diameter undersea oil
or gas pipes but there is no reason why aluminium bronzes should
not be used for this purpose. Since the pipes themselves are of
steel and are cathodically protected, an alternative procedure
which is often used is to fit steel bolts and flanges with a
"bolt protector" filled with grease fitted round the
bolt shanks, and to rely upon the cathodic protection to take
care of the exposed ends.
Sea water piping ranging from
approximately 50 to 350 mm diameter is used on offshore oil
platforms to convey cooling water and water for injection back
into the well. The first generation of North Sea platforrns used
cement-lined steel for these but 90/10 copper-nickel is now
frequently employed since it gives better corrosion resistance
and a considerable reduction in weight. Welded aluminium bronze
tubes would give at least equally good corrosion resistance and
greater weight savings since the strength/weight ratio of
aluminium bronzes is higher than that of 90/10 copper-nickel.
One important application of
aluminium bronze in the oil industry is its use for fans in the
inert gas protection systems on board oil tankers.
These fans are used to maintain
the flow and pressure of the inert gas blanket over the oil cargo
so as to obviate the danger of explosion or fire. The inert gas
used is produced from the exhaust gas from the main engines,
auxiliary engines or sometimes from a special generator, by
"scrubbing" with sea water. The fan operating
conditions can be very corrosive and far from easy to predict,
involving salt-laden water vapour, sulphurous gases and traces of
carbon. Several materials have been used to construct the fans,
many of which are large and run at high speeds. Only titanium and
aluminium bronze have been found to give reliable service and, of
these, aluminium bronze is far less expensive. The smaller fans
may use cast impellers but the larger ones are fabricated by
welding.
iv Specific Corrosive Chemical
Environments
The general policy in the chemical
manufacturing industries is to construct equipment of mild steel
wherever possible and to use stainless steel for those parts
where the corrosion resistance of mild steel is inadequate.
Copper-base alloys tend to be used mainly in cooling water pumps
and valves. Aluminium bronzes are, however, also used for small
items in a very wide variety of chemical environments where high
resistance to corrosion and erosion are required.
Acidic
Environments
Aluminium bronzes are often used
in processes employing sulphuric acid. Their corrosion resistance
depends upon the composition and metallurgical structure of the
alloy as well as upon temperature, concentration and degree of
aeration or presence of oxidising agents in the acid. Single
phase aluminium bronzes generally show the highest resistance to
sulphuric acid but alpha-beta alloys are satisfactory provided
that they are free from gamma 2 phase. Nickel aluminium bronzes
are also satisfactory provided that they are free from retained
beta but, unless the higher strength of these alloys is required,
a single phase alloy is generally preferred. The effects of
temperature and of concentration are not simple and can only be
determined experimentally. Figure 3 shows the relationship
between temperature, sulphuric acid concentration and corrosion
rate for nickel aluminium bronze in the presence of oxygen.
Aeration increases the rate of corrosion of aluminium bronzes in
sulphuric acid and greater acceleration is caused by the presence
of oxidising agents such as ferric salts, dichromates or nitric
acid.
The corrosion rate of aluminium
bronzes in hydrochloric acid is very much higher than in
sulphuric acid and aluminium bronzes are not usually suitable for
handling hydrochloric acid solutions (See Table 8). One exception
to this general rule is the use of cast AB2 aluminium bronze as
pickling hooks for descaling coils of steel wire rod in warm,
moderately strong hydrochloric acid. This, however, depends upon
the sacrificial cathodic protection which is afforded to the
aluminium bronze by the steel.
Table 8 gives corrosion rates for
Cu 9%Al in hydrochloric acid. Note that the presence of oxidising
agents increases the attack.
Table 9 (also from Rabald) gives
corrosion rates in pure 20% and 60% phosphoric acid in long term
tests.
Table 8 Corrosion Rates for Cu. 9%Al in
Hydrochloric Acid
Table 8 is taken from E. Rabald,
Corrosion Guide, Second Edition, Elsevier Publishing Company,
Amsterdam-London-New York, 1965. Rabald comments "The
greater attack at 100°C and 15% HCI is caused by the higher air
content" and notes that the solubility of oxygen at 100°C
is higher in 15% HCl than in 30% HCI.
Table 9 Corrosion Rates for Cu 10%AI in
Phosphoric Acid mm/year
The good resistance of aluminium
bronzes to corrosion by hydrofluoric acid is exploited in the
"frosting" of glass bulbs for electric lamps. Nickel
aluminium bronze is used for the nozzles which spray acid into
the bulbs and the trays which collect the acid from the process.
Aluminium bronze can be used with
other non-oxidizing acids - including most organic acids - and
high concentrations of phosphoric and acetic acid have been
handled industrially at elevated temperatures. Nitric acid
solutions are strongly oxidising and produce rates of attack on
aluminium bronze which preclude their use in that environment.
Figure 3. Corrosion rate of nickel
aluminium bronze, CA 104, in sulphuric acid in the presence of
oxygen.
Corrosion rates for aluminium
bronzes in non-oxidizing conditions depend upon acid
concentration and temperature. Figure 3 gives data for nickel
aluminium bronze in sulphuric acid. This is from information
provided by Langley Alloys Limited and was previously published
in "The Aluminium Bronzes. Properties and Production
Processes", Copper Development Association. London, 1938.
Alkaline
Environments
Strongly alkaline environments
such as caustic soda or caustic potash solutions remove the
protective alumina film from aluminium bronzes and result in
steady and moderately high dissolution rates. Less strongly
alkaline solutions and moist alkaline chemicals such as sodium or
potassium carbonates can be handled satisfactorily. In common
with all copper alloys aluminium bronze suffers general
corrosion, and shows suceptibility to stress corrosion cracking,
in environments containing ammonia.
Table 10 Uses of Aluminium Bronzes in
Corrosive Chemical Environments
Table 10 presents information
taken from E. Rabald, Corrosion Guide, Second Edition, Elsevier
Publishing Company, Amsterdam-London-New York, 1968. It lists
industrial uses of aluminium bronzes in contact with corrosive
chemicals where corrosion rates of < 2.4 g/m2 per
day (0.008 mm per year) were recorded. The list by no means
covers all industrial uses of aluminium bronzes in chemical
industry and excludes cases where aluminium bronzes have been
used successfully but with corrosion rates somewhat higher than
those specified above. The information provided by Rabald does
not always include details of the particular aluminium bronzes
concerned. Where this is the case, a dash has been inserted under
the heading "Alloy" in Table 10.
v Resistance to Atmospheric Corrosion
and Oxidation
Aluminium bronzes preserve their
original golden colour for long periods under normal atmospheric
exposure conditions which would produce green patination on
copper, brass and most other copper alloys. Little quantitative
information is available on the long-term corrosion resistance of
aluminium bronzes in the atmosphere but a 7% aluminium, 2%
silicon aluminium bronze, similar to UNS 63900, was included in
ASTM twenty-year atmospheric exposure tests commenced in 1957.
The final results have yet to be published, but after seven
years, this alloy showed average corrosion rates lower than those
of any other copper alloys tested.
The high resistance of aluminium
bronzes to atmospheric corrosion combined with their ability to
carry heavy loads at low rubbing speeds without undue wear or
distortion makes them particularly suitable for bearing bushes in
aircraft frames. Nickel aluminium bronze, CA104, is widely
employed for this purpose.
Sulphur dioxide pollution is an
important cause of corrosion of most metals under atmospheric
exposure conditions and, as noted in Section 4(iv) the resistance
of aluminium bronzes to sulphuric acid is high. It is not,
therefore, surprising that aluminium bronze is used for access
fittings for high chimneys. Another very important property of
aluminium bronzes for this purpose is their low susceptibility to
stress corrosion cracking. The high-temperature oxidation
resistance of aluminium bronze results from the formation of a
thin protective aluminium oxide film and increases with the
aluminium content of the alloy. Figure 4 shows that the
improvement in oxidation resistance over copper is most marked as
the aluminium content is increased to about 6% - further increase
of aluminium content above that level producing relatively little
further improvement.
Figure 4. The influence of aluminium
content on the oxidation of copper-aluminium alloys at 650°C.
vi Aluminium Bronzes in Building
The combination of high strength
with good general corrosion resistance and low susceptibility to
stress corrosion cracking presented by aluminium bronzes make
them a preferred material for load bearing masonry fixings. For
fixings made from wrought rod or sections the best alloy is
CA104; the corresponding cast alloy AB2 is generally used for
cast fixings and sockets though AB1 is sometimes employed.
The appearance and
tarnish resistance of aluminium bronzes make them suitable for
decorative architectural features but their relatively high price
compared with brasses and high tensile brasses limits their use
for this purpose. A specialized application for aluminium bronze
in building is as reinforcement bars or clamps used in the repair
of old stonework. Restorations of stonework carried out in the
nineteenth century were commonly made using cast or wrought iron
for reinforcement and subsequent rusting of the reinforcement has
resulted in severe further damage to the stonework. Aluminium
bronze is the most suitable of the copper alloys for this
purpose, not only because of its high resistance to general
corrosion and to stress corrosion cracking but because its high
strength enables bars and clamps of relatively small section to
be used, with correspondingly less damage to the old stonework
during installation of the reinforcement.
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