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?
Engineer finds corrosion in collapsed bridge at North Carolina speedway (2000)
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 + H Oxidation reaction flow of e- 2+ Zn Zn + H in the metal Acid + H Zinc - + solution 2e H + H + H H (gas) 2 + H reduction reaction • Other reduction reactions in solutions with dissolved oxygen: -- acidic solution -- neutral or basic solution Corrosion of zinc in an acid solution • Two reactions are necessary: -- oxidation reaction: -- reduction reaction:
-- Electrodeposition - - e e - + - 2e H ne + H n+ M Platinum ions metal, M 25°C n+ + n+ + 1M M sol’n 1M H sol’n 1M M sol’n 1M H sol’n -- Metal is the cathode (+) (relative to Pt) (relative to Pt) Standard Electrode Potential Standard Hydrogen Electrode • Two outcomes: -- Corrosion - - e e H2(gas) - - ne 2e n+ M + H Platinum metal, M ions + H 25°C -- Metal is the anode (-)
• Metal with smaller Vcorrodes. • EMF series o V o metal metal metal 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 • Ex: Cd-Ni cellV < V Cd corrodes o o - + Ni Cd more cathodic o DV = 0.153V Cd Ni 25°C 1.0 M 1.0 M more anodic 2 + 2+ Cd solution Ni solution Standard EMF Series
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.
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 cathodic (inert) more anodic (active) Galvanic Series • Ranking the reactivity of metals/alloys in seawater
Solution Concentration and Temperature • Ex: Cd-Ni cell with non-standard solutions - + n = #e- per unit oxid/red reaction (= 2 here) Cd Ni T F = Faraday's constant = 96,500 C/mol. X M Y M 2 + 2+ Cd solution Ni solution • Ex: Cd-Ni cell with standard 1 M solutions - + 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).
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:
c16f08 • 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).
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
Useful Parameters • The potential, polarization resistanceand 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.
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.
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