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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.