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ASAC REFRIGERATION DIVISION
COPPER TUBE CORROSION INFORMATION
Colin Hewetson 10/12/02
Reproduced in part from Process Cooling & Equipment 2002
We are aware of increasing tube failures and tube corrosion on various sites in Saudi Arabia and on different
manufacturers chillers. A sample of failed tubes was sent by us to an independent laboratory for analysis,
which indicated such failures appeared to be caused by under deposit scale corrosion, which is indicative of
poor water treatment, tube inspections and tube cleaning. The common factors in all cases was that most of
the affected tubes were internally rifled or enhanced. The enhancement is designed to increase heat transfer
wetted surface area and reduce Kw/TR by creating turbulence, which increases the water to refrigerant heat
transfer efficiency. Under deposit corrosion can occur in both Cu and Cu/Ni tubes and particularly so with
internally rifled tubes and particularly if tubes are not annealed and incorrect or inadequate water treatment is
undertaken by the owner.
Internal helical ridges of an enhanced condenser tube act to induce turbulent water flow and increase surface
area, both of which improve heat transfer. Enhanced copper condenser tubes are designed to improve the
condenser water to refrigerant heat transfer efficiency. Condenser tubes enhanced with refrigerant side fins on
the outside work well to improve chiller efficiency and raise few corrosion concerns. However, many chiller
manufacturers are increasingly designing and producing chiller heat exchangers using internally enhanced as
well as on the outside. Machine rifling or spiraling of the internal tube surfaces can contribute to early
corrosion because contaminants, suspended and dissolved solids are often found in open cooling water
systems.
Premature corrosion or failure of internally enhanced tubes is related to their design and fabrication.
Generally, internally enhanced tubes are susceptible to deposition, under-deposit corrosion and
microbiologically influenced corrosion because of their increased surface area and small helical grooves.
High-efficiency condenser tubes demand special care. To help protect and maximize the service life of these
capital investments, it is important to be aware of the potential hazards. The cost of chemical treatment,
filtration and manual cleaning is insignificant when compared to the capital cost of replacing tubes or entire
chillers.
Even though the internally enhanced tubes are designed to create turbulent water flow and increase efficiency,
the increase in localized water turbulence can also affect the protective, microscopic film barrier on the
helical ridges. Copper corrosion inhibitors (Azoles such as TTA and BZT) protect tubes by forming a very
thin protective film on the metal surface. If this film is disrupted by turbulent water flow and not rapidly
replaced by free Azole inhibitor in the water, corrosion will ensue.
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Tube fabrication methods also must be considered when diagnosing the premature failure of internal machine
rifled tubes. First, the cold working (such as rifling) of any metal component will distort and fragment its
grain structure and produce significant residual stresses. Second, the rifling process also can lead to many
scratches and fissures on the tube surfaces, especially after tube cleaning. Both of these factors result in "builtin" corrosion sites. Such scratching and fissures have been evidenced on tubes in all chillers we have been
requested to repair
A close-up view of an enhanced tube shows localized corrosion cells that are covered with greenish/blue
copper corrosion products. High efficiency chiller tubes have a greater tendency for corrosion (pitting) and
deposition. Again such corrosion cells have been evident in tubes removed from all the sites where we have
been requested to repair the chillers.
Specifications. The high residual stresses of the cold-worked, machine-rifled tubes can be relieved by
annealing (heat treating) the tubes. Annealing will improve the corrosion resistance and general toughness of
the metal after cold working. When replacing or installing a new condenser bundle, specifications should
require the supply of proper annealed enhanced tubes. As energy becomes a more important area of
concentration, chiller manufacturers have opted to use more energy efficient heat exchanger tubes. These
tubes alter the flow of water as it moves through the heat exchanger bundle, have increased surface area, and
appear to be thinner at the weakest point on the tube.
The Water Treatment Industry has failed to be proactive in its response to the new enhanced tubes and the
manufacturers also have been slow to respond to the potential risks. Enhanced tubes, under certain conditions,
have a much higher potential for deposition than smooth bore tubes. If deposited, the enhanced tubes will
have dramatic useful life reductions. We now have a number of cases where enhanced tubes have pitted inside
the first one to two years of operation, been subject to excessive metal loss or have prematurely failed.
PROBLEM AREAS:
Flow seems to be a major contributor to the deposition in Enhanced tubes. Many customers periodically layup chillers during winter time and many plants have 100% standby chillers which are only rotated once a
week or month to even up the operating hours. In Saudi Arabia cooling towers are likely to induce wind borne
dust, sand and salts leading to high dissolved and suspended solids content in the recirculating tower water.
Additionally from analysis of many sites the make up water can be aggressive and itself has high TDS and
suspended solids. The problem of increased corrosion within one year of operation may well be due to no
water flow through the heat exchanger for extended periods which increases deposition potential. The problem
is more likely to develop in condenser tubes but undoubtedly evaporator tubes are also subject to the same
problem as the raw water used to fill the chilled water circuit is the same as the condenser cooling water
circuit. Too, many customers assume wrongly that the chilled water circuit does not require water analysis or
corrosion and biocide additives dosed on a regular annual basis.
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Once deposited, internally rifled enhanced tubes may be subjected to Biologically Induced Under-deposit
corrosion conditions. Some customers inspect and clean the evaporator and condenser tubes annually or bi
annually and many do not open the heat exchangers until a problem arises. Heat exchangers require
cleaning at least once per year or more often depending on the design fouling factors used and scale
potential of the circulating water. Rifled tubes require special brushes to be effective in cleaning the
tubes and the manufacturer should provide guidance on the cleaning regime and brush requirements.
Neither the tube manufacturer nor the water treatment Industry has, in general, made any allowances for the
deposition potential of Internally rifled tubes.The Industry uses precipitation, in various forms, as a corrosion
inhibitor for mild steel. One of the more common treatments involves Ortho Phosphate, or products, which
revert to Ortho Phosphate, as the mild steel corrosion inhibitor. With enhanced tubes, this may be the formula
for disaster.
No allowances are being made to insure a good corrosion barrier exists on the enhanced tubes. The only
product used to protect Copper in a system is the Azole usually in the form of Tolytriazole. The water
treatment suppliers are depending on stoichiometric balancing of the treatment program where the mild steel
corrosion inhibitor, or tracer, is balanced against the Tolytriazole component of the Chemical Drum.
Invariably, this does not work as the Tolytriazole is decomposed or consumed at higher rates, leaving the
system unprotected from copper corrosion. As a check, the Treatment Supplier may utilize Copper Corrosion
Coupons but these are "after the fact" and do not account for the heat flux of the tube. Chemical reactions
generally occur faster as heat is added and the tube may be as much as 50 Degrees F higher temperature than
the Corrosion Coupon.
CORRECTIVE ACTION:
1. The customer should utilize independent laboratory analysis of the Treatment Program to investigate
potential problem areas, which may damage equipment. Remember the water treatment company are hardly
likely to highlight any deficiency arising out of their treatment programme and unless the customer requests
them or an independent laboratory to undertake regular weekly water sample analysis covering complete
Anion & Cation, TDS + suspended solids with a certified report, he generally will not have a clue as to what
is happening chemically in his cooling system until a leak arises. High ppm of Cu & Fe are indicative of
corrosion taking place in the cooling system.
2. Enhanced tube heat exchangers must be cleaned properly using the correct brushes at regular intervals and
at a minimum once/year. They should be subjected to Boroscopic examination prior to and after each cleaning
to insure removal of all deposits. At least once every 2 years tubes should be Eddy current tested on a random
basis to pick up advance warning of corrosion metal loss or pinholing. After brushing, immediately repassivate the tubes with 20+ ppm Azole(TTA or BZT). Remember, the rifled grooves and any surface
scratches are magnets for silt and deposit accumulations..
3. Enhanced tubes must have sufficient flow at all times, to reduce deposition potential. Standby plant heat
exchangers should be drained where it is not possible to circulate water regularly through the heat exchanger.
Water stagnation or low flow conditions are particularly troublesome for these tubes because these
conditions allow suspended solids to drop out of solution and deposit on the tube surfaces. Once
deposits have formed, a mechanism for chemical-concentration-cell corrosion or differential-aerationcell corrosion immediately is established. Deposits also facilitate biological growth and subsequent
microbiologically influenced corrosion. Where chillers are planned to be shut down or layed up for
extended periods of time or act as standby without regular rotation with stagnant tower water in the
condenser tubes, the potential for corrosion and deposition is at its highest. Treated water must be
circulated through the unit each day to help maintain passivation of the tube metal and prevent
settlement of suspended solids.
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4. Enhanced tubes must have sufficient Copper Corrosion Inhibitor azole (Tolytriazole) present at all times
to reduce the chances of pitting.
5. Enhanced tubes must use water treatment technology, which reduces the chances of deposition based on
Raw water make up characteristics and system conditions and COC.
6. Enhanced Tube Condensers must have a very good alternating biocide program designed for both bulk
water sterilization and penetration without increasing copper corrosion potential.
7. System Cleanliness is essential. Inspection and cleaning at manufacturers recommended intervals or as site
examination dictates must be followed to ensure durability of enhanced chiller tubes. Where high suspended
solid counts arise particularly in Saudi Arabia, sidestream solids filters should be installed to maintain a low
concentration of suspended solids. For example after the Aramco failures we convinced the customer to
install side-stream filters for the new chiller installation and these are removing the majority of suspended
solids.
A close-up view of the internal ridges of an enhanced condenser shows deposits and debris that will lead to
localized corrosion (pitting).
8. Biological Control. An alternating biocide program must be implemented to control biofilm formation and
general micro-organism growth. Biocides should be dosed near their maximum legal levels to ensure
biological control. In particular, the stressed areas on the enhanced tube surface (from cold working) are very
susceptible to oxidizing biocides. A free chlorine residual that exceeds 0.3 ppm during idle operation
(stagnation) can be particularly damaging to the passive film created by the Azoles.
9) Chemical Inhibitors. The Azole copper corrosion inhibitor levels should be tested independently of the
normal cooling water sample analysis or control test (i.e., ortho-PO4, molybdate, phosphonate). Detectable
azole concentration levels such as BZT or TTA always must be present. As a general rule of thumb, the azole
concentration in the recirculating water should be at least 2.0 ppm, or (Total Copper residual x 2) + 2 ppm
whichever is higher. If the cooling water formulation does not contribute the necessary levels, additional
Azole should be fed into the system. Large facilities need to operate as efficiently as possible, so if enhanced
tubes are used, they must be kept very clean at all times.
10)Recommendations
For all water cooled applications in Saudi Arabia, 90/10 or 70/30Cu/Ni with Cu/Ni cladding on the tube sheets
is recommended. Tubes should where possible be plain bore rather than internally rifled. Where internally
rifled tubes are proposed to be used to reduce Kw/TR this can be achieved by selecting units with a larger
number of straight bore tubes to provide a greater surface area. The customer should be advised of the tube
cleaning requirements, absolute need for chemical corrosion inhibitor for steel and Copper plus regular
weekly water sample analysis. The thickest wall tubes should be used e.g. 0.035" to provide the maximum
lifecycle and reserve against corrosion metal loss. It is the customers responsibility to ensure that proper
water treatment, independent water analysis, tube inspections and cleaning regimes are implemented,
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failing which any tube failure is unlikely to be accepted under the warranty terms should a failure arise
during the warranty period.
In order to determine if there is enough copper corrosion inhibitor in a system, you must first run a copper
test. You multiply the copper residual by two to find out the demand of the water for copper corrosion
inhibitor. You then run a Copper Corrosion Inhibitor test and subtract the demand. If the result is positive, you
have protection.