Corrosion Protection
Basic corrosion theory and
protection methods
Author: Dr. Thomas J. Langill
© 2006 American Galvanizers Association
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 (n)
The chemical or electrochemical reaction
between a material and its environments that
produces a deterioration of the material and its
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).
Bimetallic Couple
Conventional
Current
Electrons Electrons
External
Circuit
Return Current
+ Path
-
Cathode Anode
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 in
electrical potential sets up a corrosion cell or a bimetallic couple
Corrosion Costs
Direct Costs Indirect Costs
NACE, CC Catastrophe
Technologies, & Public safety, property
FHWA jointly damage, environmental
produced a report in contamination
2001 detailing the Natural Resources
costs of corrosion Waste production,
$276 billion USD increased energy
annually consumption
3.1% of US GDP Public Outcry
(1998) Traffic, inconvenience
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 the
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
Cathodic Protection
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 expectancy.
Cathodic Protection
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 to the
structure to be protected
Example: Docked Naval Ships
Cathodic Protection
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
Galvanic Zinc Applications
Zinc-rich Paints
Zinc Metallizing
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 Degreasing/Caustic cleaning
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 Zinc bath removal
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.
Metallurgical Bond
Edge Protection
Same thickness at corner
Micrograph of galvanized edge
Influencers of Coating Development
Steel Surface Conditions
Steel Chemistry
Silicon
Phosphorous
The Sandelin Curve
Coating Appearance
Newly Newly
Galvanized Galvanized
No Spangle Dull Coating
Newly Newly
Galvanized Installed
Highly Spangle Shiny & Dull
Coating
The Zinc Patina
Forms as zinc reacts with the
environment
Consists of zinc oxide, zinc hydroxide,
and zinc carbonate
Protects the galvanized coating by
providing an additional layer of corrosion
resistance
Passivation Cycle
Time
0 – 48 hrs. 1
48 hrs. – 6 mo. 2
6 mo. – 2 yrs. 3
Environmental Performance
Atmospheric
Liquid (Chemicals, Fresh H2O, Salt H2O)
Soil
High Temperature
Low Temperature
Concrete
Atmospheric: Service Life of HDG
Liquid: Effect of pH on HDG steel
Performance in Soil
> 200 different soil types
Complex corrosion kinetics in soil
Variables include:
Porosity
Resistivity
Organic material
Moisture content
pH
Temperature
Performance in Various Temps
High Temperature
< 392 F (200 C)
Low Temperature
> -75 F (-60 C)
Concrete: Rebar Corrosion
Staining Cracking
Spalling Complete Failure
Concrete: Galvanized Rebar
Unprotected Rebar
Galvanized Rebar
Zinc is Natural
Air
Soil
Water
Features of HDG Coatings
Zinc-iron intermetallic layers
Harder than the substrate steel
Zinc patina
Barrier protection
Cathodic protection
Metallurgical bond to the substrate steel
Paintable
Edge and corner protection
Zinc is a natural and healthy metal
Benefits of HDG Coatings
Maintenance-free for 50 – 100 years in
most atmospheric environments
Long term performance in soils, water,
and chemical environments
No touch-up required
High & Low temperature performance
Application independent of weather
100% recyclable
Dry Bridge Road Bridge
Date Galvanized
1999
Sector
Bridge &
Highway
Environment
Rural
Location
Alexander, NY
Harrisburg Airport Transportation Facility
Date Galvanized
2004
Sector
Building &
Architecture
Environment
Urban
Location
Harrisburg, PA
AES-PR Total Energy Power Plant
Date Galvanized
2002
Sector
Electrical, Utility &
Communication
Environment
Industrial
Location
San Juan, Puerto Rico
Leprino Foods
Date Galvanized
2002
Sector
Food & Agriculture
Environment
Rural
Location
Waverly , NY
Aspinwall Water Treatment Plant
Date Galvanized
2001
Sector
Water & Marine
Environment
Industrial
Location
Pittsburgh, PA
