Corrosion and Degradation of Materials Chapter 16

CORROSION AND DEGRADATION OF MATERIALS Cost of Corrosion Fundamentals of Corrosion Electrochemical reactions EMF and Galvanic Series Concentration and Temperature (Nernst) Corrosion rate Corrosion prediction (likelihood) Polarization Protection Methods

What is the…. Cost of Corrosion? 3

The Cost of Corrosion

Significance of Corrosion on Infrastructure

Engineer finds corrosion in collapsed bridge at North Carolina speedway (2000)

Corrosion & Catastrophic Failure.

A Concrete bridge failure

Fundamental Components Corrosion can be defined as the deterioration of material by reaction to its environment. Corrosion occurs because of the natural tendency for most metals to return to their natural state; e.g., iron in the presence of moist air will revert to its natural state, iron oxide. 4 required components in an electrochemical corrosion cell: 1) An anode; 2) A cathode; 3) A conducting environment for ionic movement (electrolyte); 4) An electrical connection between the anode and cathode for the flow of electron current. If any of the above components is missing or disabled, the electrochemical corrosion process will be stopped.

Electrochemical Corrosion Corrosion of zinc in an acid solution • Two reactions are necessary: -- oxidation reaction: -- reduction reaction: Zinc Oxidation reaction Zn 2+ 2e - Acid solution reduction reaction H + 2 (gas) flow of e- in the metal • Other reduction reactions in solutions with dissolved oxygen: -- acidic solution -- neutral or basic solution

Standard Hydrogen Electrode • Two outcomes: -- Corrosion -- Electrodeposition -- Metal is the cathode (+) M n+ ions ne - e 25°C 1M M sol’n 1M H + Platinum metal, M H 2e (relative to Pt) - - e e H2(gas) - ne - 2e n+ M + H metal, M ions Platinum + H 25°C 1M M n+ sol’n 1M H + -- Metal is the anode (-) (relative to Pt) Standard Electrode Potential

Standard EMF Series - • EMF series • Metal with smaller Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K +1.420 V +0.340 - 0.126 - 0.136 - 0.250 - 0.277 - 0.403 - 0.440 - 0.744 - 0.763 - 1.662 - 2.363 - 2.714 - 2.924 metal V o more anodic more cathodic metal o • Metal with smaller V corrodes. Cd o Ni • Ex: Cd-Ni cell V < V  Cd corrodes - 1.0 M Ni 2+ solution Cd 2 + 25°C DV = 0.153V o

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Driving force A driving force is necessary for electrons to flow between the anodes and the cathodes. The driving force is the difference in potential between the anodic and cathodic sites. This difference exists because each oxidation or reduction reaction has associated with it a potential determined by the tendency for the reaction to take place spontaneously. The potential is a measure of this tendency.

Galvanic Series • Ranking the reactivity of metals/alloys in seawater Platinum Gold Graphite Titanium Silver 316 Stainless Steel (passive) Nickel (passive) Copper Nickel (active) Tin Lead 316 Stainless Steel (active) Iron/Steel Aluminum Alloys Cadmium Zinc Magnesium more anodic (active) more cathodic (inert)

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Solution Concentration and Temperature • Ex: Cd-Ni cell with standard 1 M solutions • Ex: Cd-Ni cell with non-standard solutions n = #e- per unit oxid/red reaction (= 2 here) F = Faraday's constant = 96,500 C/mol. - + Ni Y M 2+ solution X Cd 2 T - + Cd 25°C Ni 1.0 M 1.0 M 2 + 2+ Cd solution Ni solution

Kinetics, Polarization, Corrosion Rates While it is necessary to determine corrosion tendencies by measuring potentials, it will not be sufficient to determine whether a given metal or alloy will suffer corrosion under a given set of environmental conditions. Even though the tendency for corrosion may be high, the rate of corrosion may be very low, so corrosion may not be a problem. Corrosion rates are determined by applying a current to produce a polarization curve (the degree of potential change as a function of the amount of current applied) for the metal surface whose corrosion rate is being determined. The variation of potential as a function of current (a polarization curve) enables the study of concentration and activation processes on the rate at which anodic or cathodic reactions can transfer electrons. Polarization measurements can thereby determine the rate of the reactions that are involved in the corrosion process (the corrosion rate).

Anodic Polarization Curve -1 c16f12 Anodic Polarization Curve -1 This curve is usually scanned from 20 mV below the Eoc (open circuit potential) upward. The curve can be used to identify the following corrosion regions:

The degree of polarization is a measure of how the rates for anodic and cathodic reactions are slowed by various environmental factors (concentration of metal ions, dissolved oxygen in solution, diffusion limitations; referred to as concentration polarization) and/or surface process (activation polarization). All electrochemical reactions consist of a sequence of steps that occur in series at the interface between the metal electrode and the solution. Activation polarization is where the reaction is limited (controlled) by the slowest rate reaction of the steps (adsorption H+, film formation, ease of release of electrons, called the activation polarization). c16f08

Types of Corrosion Uniform Attack – General Corrosion Galvanic Corrosion Crevice Corrosion Pitting Intergranular Corrosion Selective Leaching Erosion Corrosion Stress Corrosion

Uniform Corrosion Formerly a ship

Galvanic c16f14 Dissimilar metals are physically joined in the presence of an electrolyte. The more anodic metal corrodes. Bilge pump - Magnesium shell cast around a steel core.

Aluminum Alloys Traditionally, structural aluminum alloys in aircraft have been 2024-T3 in damage critical areas and 7075-T6 in strength critical areas. As aircraft structures became more complex, skin materials became an integral part of the structure and SCC became more prevalent. The high performance aircraft designed since 1945 have made extensive use of skin structures machined from thick plates and extrusions. The residual stresses induced by heat treatment in conjunction with those from machining made these materials sensitive to SCC.

Stress Corrosion Cracking, SCC A structure that has SCC sensitivity, if subjected to stresses and then exposed to a corrosive environment, may initiate cracks and crack growth well below the yield strength of the metal. Consequently, no corrosion products are visible, making it difficult to detect or prevent; fine cracks can penetrate deeply into the part.

Crevice Corrosion c16f15 Narrow and confined spaces.

Pitting c16f17 Pitting is a localized form of corrosive attack.  Pitting corrosion is typified by the formation of holes or pits on the metal surface.  Pitting can cause failure, yet the total corrosion, as measured by weight loss, may be minimal. 304 stainless steel / acid chloride solution 5th Century sword Boiler tube

Intergranular c16f18 Corrosion along grain boundaries, often where precipitate particles form.

c16f20 Erosion-corrosion Combined chemical attack and mechanical wear (e.g., pipe elbows). Brass water pump

c16f21 Selective Leaching Preferred corrosion of one element/constituent [e.g., Zn from brass (Cu-Zn)]. Dezincification.

Energy Technology Developments Using Gamry Electrochemical Instrumentation Electrochemical cells used in energy technology include: Batteries Fuel Cells Supercapacitors Solar Cells Batteries are the ultimate electrochemical device, so typically, battery scientists understand and use electrochemistry as a routine tool to develop and improve their products.  The challenge for these engineers is to higher energy densities at lower prices.  A battery is a very active electrochemical device, so safety is an important issue. 

Corrosion Test Methods 1: The measurement of the open circuit potential is very easy and inexpensive, but is not considered to be very reliable, since the potential tells nothing about the kinetics of the process. 2: Linear polarization measurements are encumbered by “IR” effects from the concrete; there is so much potential drop in the concrete, that an accurate determination of the potential of the rebar surface is very difficult. 3: Electrochemical impedance spectroscopy (EIS) can overcome the difficulties of the concrete resistance. 35

Electrochemical Basics Corrosion is an electrochemical phenomena The simultaneous combination of electrical & chemical processes Techniques involve either or both of: Measuring voltage difference (thermodynamic) Measuring current flow (kinetic) Working electrode Equipment material Reference electrode Maintains constant potential Even at large currents Counter (Secondary) electrode Allows infinite current

Test Samples

EG&G Instruments: Potentiostat/Galvanostat Model 273A

Concrete Exterior & Interior

Concrete Interior (untreated)

Reinforced Concrete

Useful Parameters The potential, polarization resistance and current density data can provide useful information about: Corrosion state of the metal (active or passive). Estimates of the Tafel constants for input into LPR analysis, corrosion rate measurement or cathodic protection criteria.

Open Circuit Potentials 46

Polarization Resistance, Rp This electrochemical technique enables the measurement of the instantaneous corrosion rate. It quantifies the amount of metal per unit of area being corroding in a particular instant. The method is based on the observation of the linearity of the polarization curves near the potential (Ecorr). The slope expresses the value of the polarization resistance (Rp) if the increment is close to zero. This Rp value is related to the corrosion current (Icorr) by means of the expression: Where A is the area of metal surface evenly polarized and B is a constant that may vary from 13 to 52 mV. For steel embedded in concrete, the best fit with parallel gravimetric losses results in B= 26 mV for actively corroding steel , and a value of B= 52 mV, when the steel is passivated.

Galvanic Corrosion Tests 48

Potentiodynamic Curves 49

Tafel Extrapolation 50

Electrochemical Impedance Spectroscopy (EIS) EIS has been successfully applied to the study of corrosion systems and been proven to be a powerful and accurate method for measuring corrosion rates. To access the charge transfer resistance or polarization resistance  that is proportional to the corrosion rate at the monitored interface, EIS results have to be interpreted with the help of a model (see simple circuit model) of the interface. An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. To make an EIS measurement, a small amplitude signal, usually a voltage between 5 to 50 mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000 Hz. The EIS instrument records the real (resistance) and imaginary (capacitance) components of the impedance response of the system. 51

Proposed Relationship between Corrosion Rate and Remaining Service Life icorr (mA/cm2) Severity of Damage <0.5 no corrosion damage expected 0.5-2.7 corrosion damage possible in 10 to 15 years 2.7-27 corrosion damage expected in 2 to 10 years >27 corrosion damage expected in 2 years or less

Corrosion Prevention Metal (examples: Al, stainless steel) Metal oxide Use metals that passivate, form a thin, adhering oxide layer that slows corrosion. Use metals that are relatively unreactive in the corrosion environment. Use inhibitors (substances added to solution that decrease reactivity); slow oxidation/reduction reactions by removing reactants like O2 gas by reacting it w/an inhibitor). Slow oxidation reaction by attaching species to the surface. Apply physical barriers: films and coatings, paint Reduce T (slows kinetics of oxidation and reduction) Cathodic (or sacrificial) protection; attach a more anodic material to the one to be protected. steel zinc Zn 2+ 2e - e.g., zinc-coated nail Galvanized Steel

Passivation Process Stainless steel was “discovered” around 1900–1915. A result of multiple scientific efforts in England, France and Germany on alloys with compositions that would later be known as the 410, 420, 430, 442, 446 and 440C grades. Stainless steels must have a very low level of carbon; difficult to obtain (low carbon) for many years, which explains the late arrival of good ferritic grades in the 1980s. Chromium (Cr) is by far the most important alloying element in the production of stainless steel. It forms the “passive” surface film (chromium oxide) that makes stainless steel corrosion resistant and increases scaling resistance, wear resistance and tensile strength. A minimum of 10.5% chromium content (by weight) is required for the protective, self-repairing surface layer of chromium oxide to form reliably. The higher the chromium content, the stronger the passive layer. If the stainless steel surface is machined or accidentally damaged, the passive layer quickly re-forms, in the presence of air or water.

Sacrificial Anodes This field is located in Viosca Knoll, block 786, southeast of New Orleans.  It lies in water depths of approximately 1754 feet (535 meters).  Petronius is the largest free-standing structure in the world.  Texaco's choice was Galvotec-CW-III Aluminum Sacrificial Anodes for their Petronius cathodic protection system.  http://www.galvotec.com/img/texaco.jpg

"Salt water isn't good for anything." A man blamed a low-flying pelican and a dropped cell phone for veering his million-dollar (French-built Bugatti Veyron) sports car off a road and into a salt marsh near Galveston. The car was half-submerged in the brine about 20 feet from the road when police arrived (Nov 11, 2009). WORLD'S FASTEST: Bugatti Veyron Busts Out With 1,000-hp and $1.3 Million Price Tag  The Veyron's 16-cylinder engine develops a shade over 1,000 horsepower, giving it a 0-60 time of fewer than 3 seconds and a 252-mph top speed. Those staggering stats make the Veyron the world's fastest production car. It's also the most expensive (2005 stats). $1.95 Million (2009)

Summary -- materials selection -- reducing the temperature • Metallic corrosion involves electrochemical reactions -- electrons are given up by metals in an oxidation reaction -- these electrons are consumed in a reduction reaction • Metals and alloys are ranked according to their corrosiveness in standard emf and galvanic series. • Temperature and solution composition affect corrosion rates. Increasing T, speeds up oxidation/reduction reactions. • Forms of corrosion are classified according to mechanism • Corrosion may be prevented or controlled by: -- materials selection -- reducing the temperature -- applying physical barriers -- adding inhibitors -- cathodic protection using metals that form a protective oxide layer Painting/coating

"Rust's A Must" Mighty ships upon the ocean Suffer from severe corrosion, Even those that stay at dockside Are rapidly becoming oxide. Alas, that piling in the sea Is mostly Fe2O3. And where the ocean meets the shore, You'll find there's Fe3O4. 'Cause when the wind is salt and gusty, Things are getting awful rusty. We can measure, we can test it, We can halt it or arrest it. We can gather it and weigh it, We can coat it, we can spray it. We examine and dissect it, We cathodically protect it We can pick it up and drop it. But heaven knows we'll never stop it! So here's to rust, no doubt about it, Most of us would starve without it. The origin of this epic poem is a bit fuzzy.  We have seen a reference to the late Mr. T. R. B Watson of Corrosion Services Co., Ltd. in Toronto and we believe that he is the author. 

More Information http://www.electrochem.org/ The Journal of The Electrochemical Society (JES) is the leader in the field of solid-state and electrochemical science and technology. This peer-reviewed journal publishes an average of 450 pages of 70 articles each month. Articles are posted online, with a monthly paper edition following electronic publication in the following areas: Batteries and Energy Storage Fuel Cells and Energy Conversion Corrosion, Passivation, and Anodic Films Electrochemical/Chemical Deposition and Etching Electrochemical Synthesis and Engineering Physical and Analytical Electrochemistry Dielectric Science and Materials Semiconductor Devices, Materials, and Processing Sensors and Displays: Principles, Materials, and Processing Nanostructured Materials, Carbon Nanotubes, and Fullerenes Interdisciplinary Topics