Civil Engineering Reference
In-Depth Information
For zinc coating, metal is dipped into molten zinc at a temperature of at least
450°C. Zinc and iron bind with each other giving a solution which forms a hard
alloy layer. Galvanizing is an electrolytic process. The metal to be coated acts as
a cathode, and the material which coats the metal acts as an anode. A thin metal
layer is formed on all the free surfaces without any chemical reaction.
These two processes, zinc coating and galvanizing, are considered serious
environmental polluters. In both cases there is an emission of organic solvents,
cyanides, chrome, phosphates, fluorides etc., mainly in the rinse water. These
pollutants could be precipitated in a sludge by relatively simple means, which
requires treatment as a special waste. Most of the galvanizing industries do not
take advantage of these possibilities. Processes do exist that do not produce
waste water, or have a completely closed system.
One method for relatively pollution-free galvanizing is a process making use
of the natural occurrence of magnesium and calcium in sea-water. The technique
was patented in 1936 and quite simply involves dropping the negatively charged
iron into the sea-water and switching on the electricity. The method has proved
effective for underwater sea structures. It is, however, not known whether this
technique gives lasting protection from rust for metal components that are later
exposed to conditions on land.
Treating surfaces of steel and metals with a ceramic coating would also give a
better result environmentally. These methods are currently only used on materi-
als in specialized structures.
Steel reinforcement is not galvanized. Concrete provides adequate protection
against corrosion. But even concrete disintegrates in time and the reinforcement
is then exposed. Correct casting of concrete should give a functional life span of
at least 50 years. The most corrosive environment for galvanized iron and rein-
forced concrete structures is sea air and the air surrounding industrial plants and
car traffic.
Aluminium
Aluminium is one of the newcomers amongst metals, and was produced for the
first time in 1850. It is used in light construction and as roof and wall cladding.
The use of aluminium in the building industry is increasing rapidly.
Aluminium is usually extracted from the ore bauxite. The Norwegian compa-
nies Elkem and Hydro import their bauxite from Brazil, Surinam and Venezuela,
which are important rainforest areas. Extraction occurs mainly in opencast quar-
ries after clearing the vegetation, which causes a great deal of damage to the local
ecosystems. Production of aluminium entails a highly technological process of
which electrolysis is an integral part. Building efficient production plants
requires high capital investment, and countries with low and medium industri-
alization with large reserves of bauxite have mostly been forced to export the ore
