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Corrosion, Coatings and Hydropower
Steve Reiber, Ph.D.
sreiber@HDRINC.COM
The Pat Tillman Bridge: Under Construction 2009
Completed 2011
Acknowledgements
Materials kindly supplied by:
Society for Protective Coatings (SSPC) NACE International Wasser High Tech Coatings Sherwin Williams
• Corrosion Control Services – Bolt On Engineering Services
– Add Value to New Designs and Extends Life of Existing Structures
• Condition Assessment Services – Similar to Master Planning
• Laboratory Services and Forensic Analysis – Answers nagging questions and
operational adjustments
The Business of Corrosion (a.k.a. Rust)
• Broad based applicability to engineering systems – Corrosion is ultimately the life limiting mechanism for engineered
structures/systems.
• Multidisciplinary by nature. – Chemistry/Electrochemistry
– Physical Material Science
– Stress/Mechanics
• Limited availability of pre-qualified personnel – On the Job Training Key to Success
– Civil, Chemical, Mechanical, Electrical and Other
Corrosion Engineering is…
Drivers for corrosion and condition assessment: the laws of nature.
Second Law of Thermodynamics: Increasing entropy or tendency towards chaos.
DSuniverse ≥ 0
Corrosion Engineering saves money and preserves assets
ROI between $5 - $50 to $1
U.S. Cost of Corrosion is about 3.1% of GDP (50% to 70% Related to Civil Engineering)
2010 US GDP = $14.72 Trillion
U.S. Cost of Corrosion = $460 Billion
Five Parts of a Corrosion Cell
Common Corrosion Morphologies
Not-so-common: Microbial Influenced Corrosion (MIC)
• Material Selection/Design Details
–Choose materials compatible with environment.
–Do not create corrosion cell through
design/construction details.
• Corrosion Inhibitors
–Alter the environment adjacent to metal to
passivate and protect metal.
–Concrete or mortar on steel are inhibitors
Four Basic Methods Corrosion Control
• Cathodic Protection
–Electrochemically alter the surface condition
of the metal to move the anodic reactions
elsewhere.
• Coatings (exterior) and Linings (interior)
–Provide a barrier to the electrolyte and
protect the metal. Usually dielectric material
that prevents electron and ionic current flow.
Four Basic Methods Corrosion Control (continued)
Effect of Cathodic Protection Current
ANODE
-0.65 volt
CATHODE
-0.50 volt
Icorr= 1 mA ANODE
-0.65 volt
CATHODE
-0.60 volt
Icorr= .3 mA
Before Cathodic Protection After Cathodic Protection Reduction
in
corrosion
current
C.P. CURRENT
Cathodic Protection: Making Anodes into Cathodes
Cathodic Protection is Limited by the “Throw” of the Current (especially in fresh water)
Not always useful on complex structures
What is Microbially-
Influenced Corrosion (MIC)?
MIC is a form of
localized corrosion,
controlled in part
by the growth of
biofilms (slime
layers).
Biofilms and MIC are inseparable!
SRB activity on stainless steel
All steel surfaces support
biofilm growth!
Biofilm growth on a supposedly
sterile surface
• Colonization of steel
surfaces is inevitable
in natural water
systems.
• Where there is a
carbon source, there
is a biofilm.
• Disinfectants
suppress microbial
growth – they do not
sterilize.
Biofilm Development
(4 steps)
Corrosion scale formation Early colonization
A corrosion scale is usually necessary for the
organisms to anchor themselves.
Biofilm Development
Multiple species proliferate Formation of a cohesive film
Exopolymers are the glue and the building blocks of
the biofilm.
How do biofilms promote
corrosion
• Create a reducing environment
• Generate sulfides
• Release volatile acids
• Depress the pH
• Metal complexation by exopolymers
• Create discontinuities on the pipe surface
By creating and holding a corrosive micro-
environment in contact with the metal surface.
MIC Damage on Steel
Characteristic circular pit
MIC Damage on Steel
Cross-section of pit
Weld seam biofilm
Rivet Head Corrosion – Splash Zone
A Short History of Paint
5000 B.C. Egypt First synthetic pigments.
1500 B.C. Egypt First solvent-based lacquer.
1100 A.D. China Oil-based varnish.
1700 A.D. America Colonists make paint using eggs, skimmed milk
and earth pigments.
1867 A.D. America Sherwin and Williams market first prepared paint.
1900 A.D. America Red-lead/graphite/linseed oil protective coating
system developed.
1960 A.D. America Epoxy systems developed.
1980 A.D. America Urethane systems developed.
2002 A.D. California Bans VOC Solvents -- decides to return to the
days of eggs and earth pigments.
Brunnel’s Firth of Forth Bridge
Red Lead: A Century of Service
SURFACE PREPARATION Cleaning and Profiling
Why is it so important?
Poor surface preparation is the most frequent cause of premature coatings failure.
Surface preparation is the most expensive operation of a painting project (> 60%).
HAND TOOLS (Stainless Steel or Aluminum) POWER TOOLS VACUUM POWER TOOLS ABRASIVE BLASTING
SAND RECYCLABLE STEEL SHOT AND GRIT SLAG (Black Beauty) SPONGE
WATER BLASTING OR JETTING CHEMICAL STRIPPING
DEGREE OF CLEANILINESS SSPC SURFACE PREPARATION STANDARDS
SP1 Solvent Cleaning SP2 Hand Tool Cleaning SP3 Power Tool Cleaning SP5 “White Metal” Blast (NACE 1) SP6 “Commercial” Blast (NACE 3) SP7 “Brush-Off” Blast (NACE 4) SP10 “Near White” Blast (NACE 2) SP12 High and Ultrahigh (NACE 5) Pressure Water Jetting
PROFILE (ANCHOR PATTERN)
CORRECT TOO DEEP TOO ROUND
2-3 mil depth
Still the best …..
• Zinc containing primers are
still the best foundation for
a long lasting advanced
paint system
– Cathodic protection
– Self repairing
– Reducing risk of
undercutting corrosion
A Primer on Primers
Zinc Primers
Spray Coatings are Nearly the Equivalent of Hot-Dip Galvanizing
Galvanized Coating
Spray Coating
The ability of providing cathodic protection
Zinc primer
Topcoat
Superior adhesion prevents coating undercut
Moisture allows Zinc to ionize and
cathodically protecting the steel
Damage in coating to steel surface
The zinc primer reacts to protect the steel substrate when the topcoat is damaged
Steel Zn
2+
Mid coat
Chemistry - zinc primers
Inorganic : Network of zinc, oxygen and silicon atoms chemically bonded to each other and to the steel substrate
Organic : Metallic zinc particles "floating" in a cured epoxy binder network. The high content of zinc ensures metallic contact between the zinc and the steel substrate, and between the individual
zinc particles.
• In general the inorganic binders have better electrical conductivity than the organic binders and will give a metallic conductivity all through the coating
• Industry guidelines for minimum zinc requirements is given by organisations and specifications (80 – 90%)
Properties - zinc epoxy
• Chemically curing
• Good corrosion protection
• Good adhesion
• Pre-treatment: UHPWJ to WJ2
or Sa 2½
• Good mechanical strength
• May be recoated with all types of
paint, except alkyd
• Dry heat resistant up to 120ºC
• Short over-coating time
• Temperature dependent
• 2-component (water borne 3-comp)
• Film thickness: 25 - 90 m
• Not acid- and alkaline resistant
(Resistant between pH 6-10)
Advantages Considerations
Properties - zinc silicate primers
• Very good corrosion protection
• Very good solvent resistance
• Very high heath resistance
(max 400oC)
• Very high mechanical strength
• Very good adhesion to blast
cleaned steel
• Relatively good recoatability
• May be recoated with all types of
paint, except Alkyd
• Requires humidity for curing
• 2-pack
• Max. DFT: 100 µm – (alkalisilicate 200 µm )
At higher DFT tendency of mud-cracking or checking
• Curing to be checked before over-coating (ASTM 4752)
• Use a tie coat or mist-coat/full-coat technique for the first subsequent coat
• Shelf life : 6 months at 23ºC (alkali silicate 18 – 24 months)
Advantages Considerations
Moisture-cured
urethane is
rapidly
becoming the
coating system
of choice in the
water industry.
Topcoats: Urethane vs. Epoxy
Advantages
• single component
• applied in humidities to 99%
• cures in freezing temperatures
• excellent adhesion, toughness,
corrosion resistance
Disadvantages
•cost
•cures very rapidly
Urethanes Offer a Broad Application Range
On-shore (fresh water) Typical Coating Systems (gold std.)
Two Coating System (above the splash-zone)
• Epoxy Zinc-Rich Primer (3 to 4 mils) • Polyuretahe Topcoat (4 to 5 mils)
Three Coating System (immersion)
• Epoxy Zinc-Rich Primer (2 to 4 mils) • Epoxy Intermediate Coat (4 to 6 mils) • Polyuretahe Topcoat (2 to 4 mils)
Typical Marine (off-shore) above the Splashzone Coating Systems
Three-Four Coating System (gold std.)
• Zinc-Rich Primer ( 2 to 4 mils) • Epoxy Intermediate Coating (two
costs at 4-6 mils) • Polyurethane Top Coat (2-4 mils)
Cathodic Protection Required for Immersion Coatings
Take Away Points
• Only use corrosion protection on those assets you want to keep – corrosion engineering doesn’t cost, it pays.
• MIC is more common than generally recognized, and can be serious if not properly managed.
• Surface preparation is always key to an effective recoat project.
• A thirty year recoat system is only possible with a zinc-rich primer.
• Moisture–cured urethanes offer both durability and application advantages.
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