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
The Business of Corrosion (a.k.a. Rust) 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
Corrosion Engineering is 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
Drivers for corrosion and condition assessment: the laws of nature. Second Law of Thermodynamics: Increasing entropy or tendency towards chaos. DS universe 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)
Four Basic Methods Corrosion Control 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 (continued) 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.
Cathodic Protection: Making Anodes into Cathodes Effect of Cathodic Protection Current C.P. CURRENT ANODE -0.65 volt Icorr= 1 ma CATHODE -0.50 volt ANODE -0.65 volt Icorr=.3 ma CATHODE -0.60 volt Before Cathodic Protection Reduction in corrosion current After Cathodic Protection
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). SRB activity on stainless steel Biofilms and MIC are inseparable!
All steel surfaces support biofilm growth! Colonization of steel surfaces is inevitable in natural water systems. Where there is a carbon source, there is a biofilm. Biofilm growth on a supposedly sterile surface 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 By creating and holding a corrosive microenvironment in contact with the metal surface. Create a reducing environment Generate sulfides Release volatile acids Depress the ph Metal complexation by exopolymers Create discontinuities on the pipe surface
MIC Damage on Steel Characteristic circular pit
MIC Damage on Steel Weld seam biofilm Cross-section of pit
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.
Red Lead: A Century of Service Brunnel s Firth of Forth Bridge
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 2-3 mil depth TOO DEEP TOO ROUND
A Primer on Primers Zinc Primers 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
Spray Coatings are Nearly the Equivalent of Hot-Dip Galvanizing Galvanized Coating Spray Coating
The ability of providing cathodic protection Damage in coating to steel surface Moisture allows Zinc to ionize and cathodically protecting the steel Superior adhesion prevents coating undercut 2+ Zn Steel Topcoat Mid coat Zinc primer The zinc primer reacts to protect the steel substrate when the topcoat is damaged
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 Advantages 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 Considerations Temperature dependent 2-component (water borne 3-comp) Film thickness: 25-90 m Not acid- and alkaline resistant (Resistant between ph 6-10)
Properties - zinc silicate primers Advantages Very good corrosion protection Very good solvent resistance Very high heath resistance (max 400 o C) Very high mechanical strength Very good adhesion to blast cleaned steel Relatively good recoatability May be recoated with all types of paint, except Alkyd Considerations 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)
Topcoats: Urethane vs. Epoxy Moisture-cured urethane is rapidly becoming the coating system of choice in the water industry.
Urethanes Offer a Broad Application Range Advantages single component applied in humidities to 99% cures in freezing temperatures excellent adhesion, toughness, corrosion resistance Disadvantages cost cures very rapidly
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.