CORROSION
PROTECTION
Basic corrosion theory and protection
methods
Corrosion & Corrosion Control
What is Corrosion
How/Why Does Corrosion Occur
Corrosion Costs
Forms of Corrosion
Corrosion Control Methods Hot-dip Galvanizing (HDG)
Process
Coating Characteristics
Performance in Corrosive Environments
Galvanized Steel in Action
What is Corrosion?
Corrosion
The chemical or electrochemical reaction between a
material and its environments that produces a
deterioration of the material and properties.
The Galvanic Series
ZINC Anode
STEEL Cathode
This arrangement of metals determines what metal will be
the anode and cathode when the two are put in a
electrolytic cell (arrangement dependent on salt water as
electrolyte).
Bare Steel Corrosion
Microscopic anodic and cathodic areas exist on a single
piece of steel.
As anodic areas corrode, new material of different
composition is exposed and thus has a different electrical
potential
Forms of Corrosion
General
Identified by uniform formation of corrosion products that
causes a even thinning of the substrate steel
Localized
Caused by difference in chemical or physical conditions
between adjoining sites
Bacterial
Caused by the formation of bacteria with an affinity for
metals on the surface of the steel
Galvanic/Dissimilar Metal
Caused when dissimilar metals come in contact, the
difference electrical potential sets up a corrosion cell or a
bimetallic couple
Corrosion Costs
Indirect Cost
Catastrophe
Public safety, property damage, environmental contamination
Natural Resource
Waste production increased energy consumption
Public Outcry
Traffic, inconvenience
Direct Costs
NACE, CC Technologies, & FHWA jointly produced a
report in 2001 detailing the costs of corrosion
$276 billion USD annually
3.1% of US GDP (1998)
Methods of Corrosion Control
Barrier Protection
Provided by a protective coating that acts as a barrier between
corrosive elements and the metal substrate
Cathodic Protection
Employs protecting one metal by connecting it to another metal that
is more anodic, according to galvanic series
Corrosion Resistant Materials
Materials inherently resistant to corrosion in certain environments
Barrier Protection
Paint
Powder Coatings
Galvanizing
Cathodic Protection
Impressed Current
Galvanic Sacrificial Anode
Galvanic Zinc Application
Zinc Metallizing
Zinc-rich Paints
Hot-dip Galvanizing
Impressed Current
External source of direct current power is connected (or
impressed) between the structure to be protected and the
ground bed (anode)
Ideal impressed current systems use ground bed material
that can discharge large amounts of current and yet still
have a long life expect
Galvanic Sacrificial Anode
Pieces of an active metal such as magnesium or zinc are
placed in contact with the corrosive environment and are
electrically connected structure to be protected
Galvanic Zinc Application
Zinc Metallizing (plating)
Feeding zinc into a heated gun, where it is melted and sprayed on a
structure or part using combustion gases and/or auxiliary compressed air
Zinc-rich Paints
Zinc-rich paints contain various amounts of metallic zinc dust and are
applied by brush or spray to properly prepared steel
Hot-dip Galvanizing
Complete immersion of steel into a kettle/vessel of molten zinc
Zinc Metalizing
Zinc Rich Paints
Hot Dip Galvanizing
Hot-dip Galvanizing Process
Surface Preparation
Galvanizing
Inspection
Surface Preparation
Zinc-iron metallurgical bond only occurs on clean steel
Degreasing
Removes dirt, oils, organic residue
Pickling
Removes mill scale and oxides
Fluxing
Mild cleaning, provides protective layer
Galvanizing
Steel articles are immersed in a bath of molten zinc (
830 F)
> 98% pure zinc, minor elements added for coating
properties (Al, Bi, Ni)
Zinc reacts with iron in the steel to form galvanized
coating.
Inspection
Steel articles are inspected after galvanizing to verify
conformance to appropriate specs.
Surface defects easily identified through visual inspection.
Coating thickness verified through magnetic thickness
gauge readings.
FIRE PROTECTION OF STEEL
STRUCTURES
There are four common methods of fire protecting
structural steelwork
Board based systems
Intumescent coatings
Sprayed fire protection systems
Concrete encasement or filling
Board based systems
Board based systems are used to form rectangular
encasements around steel members, such as internal
beams and columns. Paint or other finishes can be
applied directly to the boards.
The level of fire resistance achieved depends upon the
type and the thicknesses of the boards used and upon the
method of attachment.
There are broadly two families of board protection,
lightweight and heavyweight. Lightweight boards are
typically 150-250kg/m and are not usually suitable for
decorative finishes.
They are typically used where aesthetics are not
important and are cheaper than heavyweight equivalents.
Heavyweight boards are usually in the range 700950kg/m and will generally accept decorative finishes.
They are typically used where aesthetics are important.
Both types of board may be used in limited external
conditions but the advice of the manufacturer should be
sought. Detailed guidance on the installation of board
protection systems is available from the Association for
Specialist Fire Protection
Intumescent coatings
Intumescent coatings may be brushed or sprayed onto
steelwork rather like paint. The materials expand when
subjected to fire and form an insulating foam. Intumescent
coatings can achieve up to 120 minutes fire resistance,
and are used mostly on exposed steelwork.
Intumescent coatings are paint like materials which are
inert at low temperatures but which provide insulation as a
result of a complex chemical reaction at temperatures
typically of about 200-250C. At these temperatures the
properties of steel will not be affected. As a result of this
reaction they swell and provide an expanded layer of low
conductivity char.
Intumescent coatings can be divided into two broad
families: thin film and thick film. Thin film materials are
either solvent based or water based and are mainly used
for cellulosic fire conditions. Thick film intumescent
coatings were originally developed for the off-shore and
hydrocarbon industries but have been modified for use in
buildings.
Sprayed fire protection systems
Sprayed fire protection systems are usually based upon
cementitious materials and are applied directly onto the
surface of steelwork. They are generally low cost, but
cannot receive finishes owing to their coarse uneven
texture. Sprayed materials tend to be used where
steelwork is concealed or where appearance is
unimportant. Fire resistance is similar to that of board
based materials.
Spray protection is extensively used in the United States
but is less common in the United Kingdom. It has the
advantage that it can be used to cover complex shapes
and details and also that costs do not increase
significantly with increases in protection thickness.
Sprays are not suitable for aesthetic purposes. Also,
application is a wet trade and this may have impacts on
other site operations. Allowance may have to be made in
costing for the possible requirement for prevention of
overspray
Concrete encasement or filling
Structural Hollow Sections (SHS) can be fire protected by
filling with reinforced concrete. Concrete filled structural
hollow sections can achieve 120 minutes fire resistance.
Multi-storey frames requiring 30-60 minutes can have
40% of the floor beams unprotected by following the
recommendations of a special design guide.
Until the late 1970s, concrete was by far the most
common form of fire protection for structural steelwork.
However the introduction of lightweight, proprietary
systems such as boards, sprays and thin fill intumescent
coatings has seen a dramatic reduction in its use.
The principal advantage of concrete is durability. It tends
to be used where resistance to impact damage, abrasion
and weather exposure are important e.g. warehouses,
underground car parks and external structures. The
principal disadvantages are:
Cost - compared to lightweight systems;
Space utilisation (large protection thicknesses take up
valuable space around columns)
Weight.
Protection thicknesses
The section factor of a particular steel section is its
surface area per unit length divided by its volume per unit
length (A/V). This parameter defines how quickly a steel
section will heat up when subjected to fire. The section
factor for a member with box protection is lower than that
for a member with profile protection, and hence box
protected steelwork heats up more slowly and requires
less protection.
Typical spray or board thicknesses for a column in a multistorey building are as set out in the table below.
Typical spray or board thicknesses based on 254UC x 89
kg/m column in a multi-storey building.
Fire resistance(minutes)
Profile Protection(mm)
Box Protection (mm)
30
10
12
60
18
15
90
24
20
120
30
25
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MABUHAY!
