Corrosion Basics January 21st, 2009 Robert J. Sinko
Corrosion Experts! • “An expert is someone who carries a briefcase and comes from more than 50 miles away” (anonymous) • “An expert is a man who has made all the mistakes which can be made in a very narrow field” (Niels Bohr)
Outline of the Talk • A Little History • Corrosion Basics • Top Corrosion Mechanisms • Monitoring Corrosion and Obtaining Data • Materials Selection • Points to consider • Continuing Education • Questions
A Little History • Corrosion has been here since the copper age. • 18th century has Luigi Galvani and Allesandro Volta dabbling in electrochemistry. • A. Volta goes to make the 1st recorded battery or “voltaic pile”
A Little More History • Sir Humphry Davy in early 1800s passes current through substances to decompose them (electrolysis). • Concludes that the voltaic piles and electrolysis were same process. • Claims that current is generated only when the electrolyte and one of the metals was oxidized • Found that two metals are not required as reaction proceeds with zinc and carbon. • Initiated the use of zinc as a sacrificial anode for copper hulled British warships
A Little MORE History • Michael Faraday had been Sir Davy’s lab flunky but outgrew the role • in the 1830s he contributed by developing the theory of electrochemical action and coined the following words we use now. • Electrode • Electrolysis • Ion • Sir Davy tried to block Faraday’s election to the Royal Society • There are countless more that have moiled in the service
Corrosion Basics/History • Man sees ore in lowest energy state • Man transforms ore in useful object • Nature wants her dirt back • Nature transforms the object back to dirt. • Remember J.W. Gibbs (free energy diagram)? • He really worked with the 2nd Law of Thermo in the late 1800s. • (You can’t break even - entropy)
Smelted, heated and beated, forged, ground, drilled and machined Useful life! Left out in the rain Energy Rusted away Ore Time
Corrosion Basics • Corrosion is electrochemical • Anode (Oxidizing – losing electrons) Electrode • Cathode (Reducing – gaining electrons) Electrode • Need “Short circuit” for electrons between terminals • And need a medium for ion transport • Electricity and chemicals are main drivers • Influenced by other factors
Corrosion Basics • Usual Textbook Equations • Chemical: Zn + 2HCl = ZnCl2 + 2H • Electrical: Zn Zn+2 + 2e- (anode) • 2H+ + 2e- 2H (cathode) • Note: hydrogen is atomic, not diatomic • This can come back to get you • Generally, the electrical part is not shown • Oxygen reaction can be inserted as well
Note that atomic hydrogen forms on surface and becomes diatomic in solution. Hydrogen atoms can be absorbed into materials (Ti and carbon steel couple) Figure 2.6 – Electrochemical reactions occurring during the corrosion of zinc in air-free hydrochloric acid. Used by permission, NACE, Corrosion Basics An Introduction, NACE, 1984, pg 28.
Corrosion Basics- Other Factors • Corrosion rates are almost initially very high • Polarization – something to slow down reactions • Cathodic and anodic surface polarization • Film thickness of corrosion product • Rate of hydrogen or oxygen diffusion to and from surfaces • Rate of corrodant ion diffusion away • Areas of reaction (anode to cathode) • Oxygen Content (cathodic depolarizer) • Temperature – every 10°C = 2 x corrosion rate • Velocity effects – moving species to & fro
Corrosion Top Mechanisms • General • Pitting • Crevice • Underdeposit • Dealloying • Galvanic • Enviromental Cracking • Stress Corrosion • Hydrogen Embrittlement • Liquid metal embrittlement • Corrosion Fatigue • Cavitation • Erosion
Corrosion Top Mechanisms – General • The even removal of metal. • Allows great planning • Monitoring • Replacement and scheduling • Unfortunately, rare in the real world.
Corrosion Top Mechanisms - Pitting • Most common form of localized attack • Break down of protective scale • Localized attack in break • Pit sets up its own environment • Draws in chlorides and sulfates • Can form caps over pits • Low corrosion rates are deceitful
Corrosion Top Mechanisms - Crevice • Much like a large area pit. • Occurs in cracks or crevices • Think of flanged connections such as • Piping flanges • Column body flanges • Trays on tray rings • Car or truck doors • It will also set up its own environment
Crevice attack on titanium from fluorinated o-ring Severe crevice attack as well as general
Corrosion Top Mechanism - Underdeposit • Very similar to crevice corrosion but a larger • Usually an unplanned occurrence • Tools left on floor • River water silt buildup in bottoms • Sometimes called poultice corrosion • Sometimes called oxygen concentration cell
Corrosion Top Mechanism - Dealloying • Copper alloys • Brasses with >30% zinc (bath sink tap screws) • Copper nickel alloys (nickel removed) • Cast iron (graphitization) • Almost any alloy can have the problem • Two Theories • One element is “leached” from solution • Both elements corroded but more noble plates back.
Corrosion Mechanisms – Galvanic • Think dry cell battery • Carbon center cathode • Zinc jacket anode • MnOH (manganese hydroxide paste) • Switch short circuit provided by your flashlight • Galvanized water pipe to your house • Powerhouse soot blower of SS nozzle and steel pipe • Over the road trailers with Al sides and steel rivets • Your water heater with aluminum sacrificial anode
CSTL Pipe SS Nozzle Soot blower metallographic sample
Corrosion Top Mechanisms – Environmental Cracking • Stress Corrosion Cracking • Chlorides (aluminum, 300 series SS) • Caustic (cstl, 300 series SS, nickel alloys) • Ammonia (brass drain) • Hydrogen Embrittlement • Liquid Metal Embrittlement • Copper on stainless steel pipe • Zinc on stainless steel pipe
Weld metal Knife line attack Transgranular chloride SCC in 316 stainless steel
If your sink at home is a nice shiny chromium plated brass, do not pour your ammonia down this drain and let it sit overnight. It will stress corrosion crack! PVC/galvanized steel trap drains– go ahead – no problem with ammonia.
Corrosion Top Mechanism – Corrosion Fatigue • Starts with an alternating stress state • Protective oxide breaks open • Corrosive species attack and form products • Next cycle repeats: • crack growth • more corrosion product • accelerated fatigue failure • Seen in rotating shafts
Corrosion fatigue, cracks can be oriented the other direction depending on stress state of shaft.
Corrosion Top Mechanisms – Cavitation • Mostly found at • Pump impellor tips • Boat propellers • Constriction in fast fluids • Caused by formation of low pressure bubble • Bubble is a vacuum • Collapse of bubble slams the metal • Breaking protective oxide • Causing great mechanical damage
Piece of pump impellor with tip cavitation Valve trim diffuser with cavitation Centrifuge feed nozzle
Corrosion Top Mechanism - Erosion • Can be from • Gaseous vapor (steam cuts on flanges) • Liquid • Solids (Coal slurry) • Removes the protective oxide layer faster than it can heal
Look for “comet tails”! Water was flowing from right to left in copper water pipe.
Monitoring Corrosion • Visual examination for leaks • Lab testing • Field testing (Corrosion racks with coupons) • Corrosion probes • ER (electrical resistance) • LPR (linear polarization resistance) • New technology • Metals analysis in process fluids
Coupons • All kinds of materials and shapes • Metals • Plastics • Fiber reinforced plastics • Ceramics • Elastomers • Glass • Homemade or “store bought” coupons • Welds • Heat treatments
Corrosion Lab Testing • Coupons • Lab testing at many temps but low pressure • Heat flux testing to simulate exchangers • High pressure labs • Ingenious bench scale or pilot plant testing • Key question - What do you want to know?
Agitator blades as corrosion coupons Weld wires as coupons
Field Corrosion Tools • Corrosion racks • Electrical resistance probes • Linear polarization resistance probes • New technology • Using electrical noise • LPRs • ERs • Metals analysis in solutions
V = I*R Electrical resistance probe R = ρ*l/A
Linear Polarization Resistance Probe • Gives instantaneous corrosion rates • Only used in conductive solutions • Based on the current flow between two or more electrodes • Requires the surface to become passivated (or polarized) and current resistance is measured. • Sometimes probe has a reference electrode as well.
Honeywell’s SmartCET® uses a sensor for background electrochemical noise to detect pitting along with LPR probe and a sophisticated computer program. http://hpsweb.honeywell.com/NR/rdonlyres/8418C7B6-EBB9-4948-8441-C3803B06BA2E/44686/ChemEngJune07.pdf Newer Technology
Other Tools of the Trade • Electron Microscopy • Elemental analysis • Surface features • FTIR for identification • X-ray Diffraction • X-ray Fluorescence • Looking for clues by • Metals in fluids • Fluids in plastics • Corrosion products
Materials Selection • Basic Groups • Metals • Plastics • Ceramics • Elastomers • Coatings • Linings • Balance of “+”s and “-”s • Corrosion Resistance • Availability • Mechanical Properties • Cost • Code Compliance • Fabricability • Repair options
Materials Selection – Metals • Carbon steels • Aluminum (3000, 5000 and 6000 series) • Coppers, brasses, and bronzes • Stainless steels • Austenitic (200 and 300 series) • Martensitic (400 series) • Precipitation hardening (17-4 PH) • Duplex (2101, 2205, 255, and 2507) • Nickel Alloys (600 series, C, B, X, Inconel®, Hastelloy®) • Titanium alloys (common grades 2, 5, 7, and 11) • Zirconium
Materials Selection - Plastics • HDPE and Polypropylene (low end) • PVC and CPVC • Resins – epoxy, vinyl esters • Fluoropolymers • PTFE, PFA, FEP, ETFE, PVDF • PEEK (high end) • Can be used as monolithic or composite pieces in equipment
Material Selection - Coatings • Coatings (thin or thick films) • Many different technologies • Always have holidays • With or without reinforcement? • Linings (how to anchor) • What’s your permeation rate? • Differences in thermal expansion rates • How do you clean?