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4 Metal Roofs, Siding, and Flashing
confer longitudinal stiffness. These roofs are suitable for buildings with
design lives of the order of 20 years, such as supermarkets, light industrial
premises etc. The basic surface protection, galvanizing for steel and application of conversion, and baked paint coatings on aluminum, is applied to
the sheet by the metal manufacturer when flat and it must withstand the
subsequent deformation in profiling.
The galvanized steel sheet is typically coated with 275 g m–3 of electrodeposited zinc and then further coated with a 200 µm thick film of polyvinyl
difluoride on the outside and a 25 µm thick film of lacquer on the inside.
The alternative material, aluminum alloy sheet is produced from a
strain-hardening alloy, such as AA 3004 in medium hard temper. The alloy
selected must be free from copper to avoid exfoliation corrosion. It is protected by a chromate or chromate-phosphate conversion coating as
described in Section 184.108.40.206 and supplemented by a baked paint coating.
Similar material is used for siding, i.e., cladding on the exterior of low-rise
domestic property. If the cut ends are left untreated, as they often may be,
corrosion working in from the ends gradually undermines the protective
coatings and they peel back progressively.
Fully Supported Roofs and Flashings
Pure lead and copper sheet are traditional roofing materials used for
buildings with a long life. The sheet is not rigid enough for unsupported
spans and is supported on timber or other suitable substrate. A related use
of supported lead sheet is for flashings to seal valleys in pitched tiled roofs
and for joints between roofs and chimneys or vents; it is well suited to this
function because it is soft and easily shaped to conform with awkward
Lead roofs exposed to the outside atmosphere develop films composed
of lead carbonate, PbCO 3, and lead sulfate, PbSO 4, that are insoluble and
electrically insulating so that protection can be established even in atmospheres polluted with sulfurous gases. In contrast, the film formed on the
underside from condensing water vapor is predominantly the unprotective oxide, PbO, so that most failures of lead roofs are from the inside.
Because lead is so soft, it can also suffer erosion corrosion from constant
flow or dripping of water laden with grit. Copper roofs, similarly exposed,
exhibit the familiar green patina of basic copper carbonate and sulfate,
CuCO 3 · Cu(OH)2, CuSO 4 · Cu(OH)2, that is both protective and aesthetically pleasing.
The lives of all roofs that depend on the establishment of a natural protective coating on originally bare metals are determined inter alia by the
initial and early conditions of exposure. Aggressive species such as chloride ions contaminating the carbonate, sulfate or oxide layers during their
evolution reduce their protective powers.
13.5 Plumbing and Central Heating Installations
Supply waters vary considerably, depending on sources, contact with substrates, biological activity and artificial treatment. They may be hard or
soft as described in Section 2.2.9 with pH values usually in the range 6 to
8; they contain various concentrations of dissolved oxygen and carbon
dioxide and other soluble species. The corrosion resistance of metals used
in plumbing and central heating systems depends critically on all of these
aspects of composition and different metals are selected to suit different
Galvanized steel, copper and austenitic stainless steels are all used for
pipes. The choice between them is based mainly on experience of what
works and what does not in particular localities.
Zinc coatings are unreliable in soft acidic waters and galvanized steel is
best suited to hard waters that it resists well due to precipitation of a tenacious calcareous scale supplementing the natural passivity of zinc; more
failures of galvanized steel pipe in hard waters are due to furring, i.e.,
reduction of internal diameter by accumulated scale, than by corrosion. In
cold water, zinc sacrificially protects steel exposed at gaps but this does not
apply to hot water because there is a polarity reversal at 70°C and at higher
temperatures, the zinc coating can stimulate attack on exposed steel.
Copper is a current standard material for tube used in plumbing and central heating circuits, usually with 1 mm wall thickness. It is tolerant of
most water supplies but there are certain recognized causes of corrosion
failure, type 1 pitting, type 2 pitting and dissolution in certain waters that
can slowly dissolve copper.
Type 1 pitting occurs in cold water and is associated with a very thin carbon film on the inside wall formed due to lack of care in manufacturing the
tube, as described in Section 220.127.116.11. The film acts as a cathodic collector
stimulating the dissolution of copper exposed at gaps. The effect is well
known and responsibility for it lies squarely with the manufacturer, who
accepts liability, typically by guaranteeing the product for 25 years.
Type 2 pitting occurs in hot water and is associated with particular locations, where the water contains traces of manganese. A deposit of manganese dioxide accumulates during several years, forming a cathodic surface
that stimulates corrosion of copper exposed at gaps.
Soft acidic waters with low oxygen contents can dissolve copper,
i.e., they are cuprosolvent. If the effect is small the copper is not impaired
but any base metals over which the water subsequently flows can suffer
indirectly stimulated galvanic attack by the mechanism described in Section 18.104.22.168. This can cause failure of downstream galvanized steel in the
system and of aluminum cooking utensils that are filled from it. This is a
good example of where care must be taken not only in laying out a system
so that water does not flow from more noble to less noble metals but also
in advising clients who use it.
Austenitic Stainless Steels
Where waters are so cuprosolvent that they can damage copper pipes, austenitic stainless steel pipes are used instead. The cost differential is not prohibitive but it is more difficult to make joints in stainless steel.
Many older installations used galvanized steel for both cold and hot water
tanks. Causes of premature failure of cold water tanks could often be
attributed to differential aeration, either at the water line or at the sites of
debris that had fallen in. It is now usual to install reinforced plastic tanks.
Cylinders formed from copper sheet are now standard for hot water tanks.
They are of course compatible with the copper tubing used in modern
One of the advantages of copper tubes is the ease with which joints can be
made, either by fittings containing rings of solder or by compression fittings. Soldering is the most reliable and least expensive method and is preferred where the heat does no damage; current trends are towards leadfree solders. The copper must be fluxed with a material that enables the
solder to wet the metal. There are various fluxes but since their function is
to dissolve the copper oxide that covers and protects the metal, they must
be rinsed away; corrosion can sometimes be observed in the track of flux
that trickled from a joint in a vertical pipe.
A water circuit in a central heating system is almost inevitably a mixed
metal system because of differences in the functions of the components
and the most economic means of manufacturing them. The boiler in which
water is heated, usually by gas or oil flames, is an iron casting; radiators
are constructed by welding pressed steel panels that are painted on the
outside but are uncoated inside; brass castings serve for pump and valve
bodies; the circuit is connected by copper tubing for ease of installation.
The mixed metal system survives because the circuit is closed. Oxygen
in the charge of water is depleted by initial corrosion but is not then
replenished. If the water is hard, a thin calcareous scale also affords protection. The system can usually run uninhibited but if necessary inhibitors
can be added to the water. Since the system has more than one metal, a
mixture of inhibitors is required such as sodium nitrite and mercaptobenzothiazole to protect the iron and copper, respectively.
Most failures occur through inadvertent and probably unsuspected
aeration during service due to poor maintenance. The most common fault
is an improperly balanced circulating pump that continuously expels
some of the water through the overflow; another fault is neglect in sealing
slight leaks that drain the water charge. Either of these faults opens the
closed system to a constant supply of fresh aerated water to replenish that
which is lost.
13.6 Corrosion of Metals in Timber
Building entails extensive use of metals in contact with and in close proximity to woods. Woods can promote corrosion in two different ways:
1. Providing an aggressive environment for metals in contact with
it, especially fasteners e.g., nails, screws, and brackets.
2. Emitting corrosive vapors.
Woods are botanical materials that vary in properties both between and to
a lesser extent within species. One of their chief characteristics is the ability
to absorb and desorb water with corresponding dimensional changes.
They are neutral or acidic media with pH values generally in the range
3.5 to 7.0. Among other solutes they can contain acetic, formic and oxalic
acids and carbon dioxide solutions derived from bacterial transformation
of starch and sugars. Although woods vary in chemical characteristics
even within the same species, there is a recognized hierarchy in their ability to promote corrosion. Generally, harder woods are more acidic and
more corrosive than softer woods; some qualitative examples are given in
Table 13.1. Electrochemical processes causing corrosion of contacting metals proceed in the aqueous phase in the wood and the more water that is
present the more damage ensues. Woods are at their most corrosive when
they are damp, when they are new and when the atmosphere is humid. It
is advisable to maintain the moisture content of timber below that in
equilibrium with 60 to 70% relative humidity. New oak and sweet chestnut are among the more aggressive woods and ramin, walnut, and African
mahogany are among the least. Iron, steel, lead, cadmium, and zinc are the
most susceptible metals and stainless steels, copper and its alloys, aluminum and its alloys and tin are less vulnerable.
Qualitative Comparison of Environments in Some
Treatments given to woods in contact with metals can exacerbate their
aggressive nature. Some preservatives with which they are impregnated
to protect against biological attack are water-borne and increase the electrolytic conductivity. Alternative formulations based on oxides or organic
solvents are less harmful. Fire retardant preparations based on halogens
used to impregnate wood can also be aggressive to metal fixings.
When steel nails, screws, or bolts, corrode in wood, there are two concurrent damaging processes that weaken the fixture. Not only does steel
lose cross-section but the voluminous corrosion products, iron hydroxides, and iron salts, disrupt and soften the wood, an effect sometimes
called nail sickness. For this reason, unprotected steel should not be in contact with wood exposed outside. Nails used to secure battens and clay tiles
to wooden roof trusses should at least be galvanized but it is better to use
stainless steel or brass.
Corrosion by Vapors from Wood
Some woods emit acidic vapors that can corrode metals in their vicinity.
There are several situations in building where problems can be anticipated
and appropriate precautions taken. Red cedar is a popular material for use
as shingles, i.e., wooden tiles, on roofs or walls, but its emissions are particularly aggressive to metals in the immediate vicinity. New oak is an
attractive wood for interior fittings such as panelling, shelving and
window surrounds but its vapors can damage associated metal fittings
and the metal parts of adjacent equipment and furnishings.
13.7 Application of Stainless Steels
in Leisure Pool Buildings
Stainless steels are applied extensively in swimming pool buildings, both
for structural members and for accessories like balustrades and ladders.
The austenitic stainless steels, AISI 304 and AISI 316 have a good service
record in traditional unheated swimming pools providing facilities for
exercise and sport. Public swimming pools are now evolving into more
comprehensive leisure centers based around the water. More people use
them and spend longer times in the water imposing the following changes
in the environment that have increased its hostility towards materials of
1. The water is heated to temperatures in the range 26 to 30°C.
2. The water is turbulent in features such as water slides and
3. Higher concentrations of chlorine-based disinfectants are used.
The first two factors stimulate evaporation and hence condensation on
cooler surfaces, particularly when the pool is closed.
Greater use of disinfectants increases the aggression of condensates by
reaction with organic species in body fluids discharged into the water.
Chlorine and some materials containing chlorine interact with urea and
other substances to produce chlorinated nitrogenous substances of the
generic type, chloramines, based on the simplest member chloramine,
NH2Cl; in more complex chloramines the hydrogen atoms are replaced by
organic radicles containing carbon and hydrogen atoms. They are formed
by overall reactions represented tentatively by:
CO · (NH 2) 2 (urea) + 2Cl 2 + H 2O = 2NH 2Cl (chloramine) + CO 2 + 2HCl
The chemistry of these interactions is complicated and the nature of the
particular products formed is sensitive to the pH of the water. Chloramines are very volatile and unstable; their presence is manifest by a pungent
odor characteristic of swimming pools. Two aspects of the problems they
cause, safety-critical damage and area degradation of the building have
stimulated reassessment of the selection and use of the steels.
Safety-Critical Damage by Stress-Corrosion Cracking
A particular concern is stress-corrosion cracking. Attention was drawn to
the problem as recently as 1985, by the collapse of a suspended concrete
ceiling in Switzerland through failure of the stainless steel supporting
structure. The volatile chloramines can carry chlorine species to condensates in parts of the building remote from the pool, where they decompose into more stable species that can be concentrated by repeated
2NH 2Cl + H 2O = 3HCl + HClO + N 2
Typical structures at risk are roof supports, wire suspensions and bolt
heads. The stress may be applied by external loads or imparted internally
by fabrication or pulling up and tightening bolts. The danger is the insidious progress of incubation preceding crack initiation.
Area damage is due to depassivation of the steel by the chloride condensate. On open panels, the effect is unsightly rust staining from dissolved
iron. Undetected pitting on hidden surfaces can develop into perforation
of sheet in ventilation ducts and other services. Corrosion is confined
mainly to areas where evaporation can concentrate condensates or fine
spray; metal that is fully immersed or frequently washed is less vulnerable.
As with other structures, corrosion control begins with good geometric
design to eliminate not only traps for liquid water but also traps for condensate remote from the pool with special attention to load-bearing structures and devices. Where possible and appropriate, the materials should
be stress-relieved after shaping.
Steels can be selected to suit different situations. The less expensive austenitic steels, AISI 304 and AISI 316 still have a useful role in non-critical
applications in direct contact with the pool. More specialized steels are
needed for critical structures and some other areas sensitive to condensation. Steels with higher molybdenum contents are less vulnerable to stresscorrosion cracking. These include AISI 317, an austenitic steel with 3 to 4%
molybdenum, and duplex steels with 3% molybdenum, listed in Table 8.3.
Duplex steels have an advantage in the more resistant ferrite they contain
but AISI 317 may prove to have the best pitting resistance.
Condition monitoring of the structure is now strongly recommended,
especially for buildings that were erected before the full extent of the problems were fully appreciated. The first concern is safety and although
stress-corrosion cracking cannot be anticipated during its incubation
period, the onset of cracking can be detected before it becomes catastrophic, provided that inspection is targeted, detailed and at short intervals. Other damage can be reduced by inspection for condensation on
open and hidden surfaces and cleaning them regularly to remove aggressive substances.
Glaser, F. P. (ed.), The Chemistry and Chemistry Related Properties of Cement, British
Ceramic Society, London, 1984.
Portland Cement Paste and Concrete, Macmillan, London, 1979.
Page, C. L., Treadaway, K. W. J. and Barnforth, P. B. (eds.), Corrosion of Reinforcement
in Concrete, Elsevier Applied Science, London, 1996.
Berke, N. S., Chaker, V. and Whiting, D. (eds.), Corrosion of Steel in Concrete, ASTM,
Philadelphia, PA, 1990.
Wernick, S., Pinner, R. and Sheasby, P. G., The Surface Treatment of Aluminum and
its Alloys, ASM International, Metals Park, OH, 1990.
Standards for Anodized Architectural Aluminum, Aluminum Associaton,
Washington, D.C., 1978
Short, E. P. and Bryant, A. J., A review of some defects appearing on anodized
aluminum, Trans. Inst. Metals Finish., 53, 169, 1975.
Emley, E. F., Continuous casting of aluminum, International Met. Reviews,
21, 75, 1976.
Franks, F., Water, The Royal Society for Chemistry, London, 1984.
Butler, J. N., Carbon Dioxide Equilibria and Their Applications, Addison-Wesley,
Reading, MA, 1982.
Oldfield, J. W. and Todd, B., Room temperature stress corrosion cracking of stainless steels in indoor swimming pool atmospheres, Br. Corros. J., 26, 173, 1991.