Corrosion - Lecture 4 - Material Engineering, Lecture notes of Material Engineering

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Open Circuit Potentials

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.

Polarization Resistance, Rp

Potentiodynamic Curves

Tafel Extrapolation

Proposed Relationship between Corrosion Rate and Remaining Service Life icorr (A/cm 2 ) 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

 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 O 2 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.

Corrosion Prevention

steel

zinc zinc

Zn2+

2 e -^2 e -

e.g., zinc-coated nail

Galvanized Steel Metal (examples: Al, stainless steel) Metal oxide

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)

• 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

Summary

"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 Fe 2 O 3. And where the ocean meets the shore, You'll find there's Fe 3 O 4. '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.