Corrosion Protection
Steel
is an abundant, efficient building material that provides specifiers design
freedom. However, for projects exposed to the atmosphere and other harsh
environments, it is critical to coat the steel for corrosion protection. Often
large construction projects target a 50-100 year design life, highlighting the
need fordurable, long lasting corrosion protection. There are countless
examples demonstrating the proven protection of hot-dip galvanizing in some of
the harshest environments.
The reason for the extensive use of hot-dip galvanizing (HDG) is the three-fold protective nature of the coating. As a
barrier coating, it provides a tough, metallurgically-bonded zinc coating that completely covers the steel surface and seals the
steel from the corrosive action of the environment. Additionally, zinc’s
sacrificial behavior protects the steel, even where damage or a minor
discontinuity in the coating occurs. Finally, the natural weathering of the
coating results in the development of an additional layer of protection on the
surface.
Barrier
Protection
Barrier protection is perhaps the
oldest and most widely used method of corrosion protection. It acts by
isolating the base metal from the environment. Like paints, the hot-dip
galvanized coating provides barrier protection to steel. As long as the
barrier is intact, the steel is protected and corrosion will not occur.
However, if the barrier is breached, corrosion will begin.
Because a barrier must remain intact
to provide corrosion resistance, two important properties of barrier protection
are adhesion to the base metal and abrasion resistance. The tightly-bonded,
impervious nature of zinc makes galvanizing a very good barrier
coating.Furthermore, zinc corrodes approximately 1/10 to 1/40 the rate of steel
depending on the environment, making the corrosion rate of a thin zinc coating
equivalent to a much thicker steel piece. Coatings such as paint that have pin
holes are susceptible to penetration by elements causing underfilm corrosion to spread rapidly.
Cathodic
Protection
The Galvanic Series of Metals (right) lists metals and
alloys in decreasing order of electrical activity
Cathodic protection is a more
effective method of resisting corrosion. It requires changing an element
of the corrosion circuit by introducing a new corrosion element, thus ensuring
the base metal becomes the cathodic element of the circuit.
Hot-dip galvanizing protects steel
cathodically similarly to the sacrificial anode method. Basically, a
metal (zinc) anodic to the base metal (steel) is placed in the circuit to
corrode in place of the base metal. The Galvanic Series of Metals is a
list of metals arranged in order of electrochemical activity in seawater (the
electrolyte). This arrangement of metals determines what metal will be the
anode and cathode when the two are put in an electrolytic cell. Metals higher
on the list are anodic to the metals below meaning they provide cathodic or
sacrificial protection when the two are connected.
From the list, it is apparent in the
case of hot-dip galvanizing, zinc will protect preferentially corrode to
protect the underlying base steel. In fact, thecathodic protection of HDG
ensures even if the coating is damaged to the point bare steel is exposed (up
to ¼ inch in diameter), no corrosion will begin until all the surrounding zinc
is consumed.
The
zinc of the hot-dip galvanized coating will sacrifice itself to protect the underlying
base steel until all the surrounding zinc is consumed.
Zinc
Patina
Different than barrier and cathodic
protection which resist corrosion of the steel itself, the zinc patina protects
the zinc coating. Zinc, like all metals, begins to corrode when exposed
to the atmosphere. Thus, freshly galvanized steel progresses through a
natural weathering process when exposed to wet and dry cycles in the
environment.
As galvanized coatings are exposed
to free flowing air, the formation of the zinc patina begins with a thin layer
of zinc oxides. Then as the piece is exposed to moisture – rainfall, dew,
humidity – the zinc oxide particles react with the water to form a porous,
gelatinous zinc hydroxide. Then during dry cycles, carbon dioxide reacts
with the zinc hydroxide and converts to a thin, compact, and tightly adherent
layer of zinc carbonate. The rate of the patina formation varies
according to the environmental conditions, but it typically takes approximately
6-12 months to fully develop.
The fully developed patina is a
passive, stable film that adheres to the zinc surface and is not water soluble
so it does not wash off in the rain or snow. Because of this, the zinc
patina corrodes very slowly and protects the galvanized coating underneath
retarding the corrosion rate to about 1/30th the rate of steel in the same
environment. As the zinc patina develops, the galvanized coating will
turn a matte gray color.
The zinc patina formation is
critical in the long lasting corrosion resistance of hot-dip
galvanizing. Because the patina development
relies on natural wet and dry cycles found in the environment, the results of
salt spray tests, which rely on constant wet exposure, are not accurate in
predicting the life of galvanized coatings in the real world.
http://www.galvanizeit.org/hot-dip-galvanizing/why-specify-galvanizing/corrosion-protection
Leopad Group a leading provider of corrosion
protection services ranges from the scope of blasting and painting, insulation,
thermal spray application, passive fire protection, refractory and other
services such as scaffolding, cable tray systems and cathode protection.
We are a
Malaysian company with close to 3000 staff and over 10 offices and fabrication
yards throughout the country. Leopad Group is dedicated to being the market
leader for corrosion protection and provide the highest standards in the
industry with the convenience of providing multi-disciplinary services through
a single point of contact.
For further
enquiries on our services, please contact our Business Development Department
at +603-22600200 , website www.leopad.com or email at hq@leopad.com
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