set2 chemistry of corrosion

7/31/2019 Set2 Chemistry of Corrosion

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mistry of corrosion

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Chemistry of Corrosion

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osion chemistry

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Corrosion ChemistryModule Two of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

Lesson Objectives

Explain the instability of metals

Discuss the factors that can trigger corrosion

Explain the chemistry of corrosion

Review the definition of acidity

Compare corrosion reactions of some common metals

Required Reading

his Module consists of three Web pages of required reading. The pagination is visible at the bottom of each page

irect links to adjacent pages.

Additional information can be found in sections 2.1, 2.2, 2.3, and 2.4 of the reference textbook (Corrosion

ngineering: Principles and Practice).

ntroduction

he driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic foro reach this metallic state from their occurrence in nature in the form of various chemical compounds (ores), it is

ecessary for them to absorb and store up for later return by corrosion, the energy required to release the metals froheir original compounds.

ee Why Metals Corrode?

Exam ple p rob lem 2 .1

Compare the energy required to produce one metric ton of magnesium from its oxide to the energy requiro convert enough copper oxide to produce one ton of metallic copper.
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Exam ple prob lem 2.2

Discuss the energy values presented in the Table shown on the page describing why metals corrode inelation to the order in which metals and associated alloys appeared in the h is tory o f mank ind .

When discussing the ionic content of an aqueous medium, the question often arises as to how acid (or alkaline) is tolution. Quite simply, this refers to whether there is an excess of H+ (hydrogen) or OH- (hydroxyl) ions present. T

H+ ion is acid while the hydroxyl ion is alkaline or basic. The other ionic portion of an acid or alkali added to wate

an increases its conductivity or change other properties of the liquid, but does not increase or decrease its acidity. nstance, whether a given amount of H+ ion is produced in water by introducing hydrochloric (HCl), sulfuric (H 2SO

r any other acid is immaterial. The pH of the solution will be the same for the same number of dissolved hydrogentoms. (reference)

he pH may be measured with a meter or calculated if certain parameters are established. Water itself dissociates t

mall extent to produce equal quantities of H+ and OH- ions displayed in the following equilibrium:

H , originally defined by Danish biochemist Sren Peter Lauritz Srensen in 1909, is a measure of the

oncentration of hydrogen ions. The term pH was derived from the manner in which the hydrogen ion concentratioalculated, it is the negative logarithm of the hydrogen ion (H+) concentration:

where log is a base-10 logarithm and aH+ is the activity (related to concentration) of hydrogen ions. The "p" in

quation stands for the German word for "power", potenz, so pH is an abbreviation for "power of hydrogen".


A solution is made up to contain 0.01 M HCl. What is its pH?


A solution is made up to contain 0.01 M NaOH. What is its pH?

A higher pH means there are fewer free hydrogen ions, and that a change of one pH unit reflects a tenfold change i

he concentrations of the hydrogen ion. For example, there are 10 times as many hydrogen ions available at pH 7 tht pH 8. The pH scale commonly quoted ranges from 0 to 14 with a pH of 7 considered to be neutral.

ubstances with a pH less that 7 are considered to be acidic and substances with pH equal to or greater than 7 to beasic or alkaline. Thus,a pH of 2 is very acidic and a pH of 12 very alkaline. However, it is technically possible toave very acidic solutions with a pH lower than zero and concentrated caustic solutions with a pH greater than 14.uch solutions are in fact typical of many ore extracting processes that require the digestive power of caustics and

cids.

ow pH acid waters accelerate corrosion by supplying hydrogen ions to the corrosion process. Although even

bsolutely pure water contains some free hydrogen ions, dissolved carbon dioxide (CO 2) in the water can increase t
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ydrogen ion concentration. Dissolved CO2 may react with water to form carbonic acid (H2CO3) as shown in equat

where Keq is the reaction equilibrium expressed as a ratio.

Carbonic acid subsequently dissociates in bicarbonate and carbonate ions as expressed respectively in the following

quations:

Exam ple p rob lem 2 .5

A solution contains a mixture of sodium bicarbonate (0.05 M) and sodium carbonate (0.2 M).What is its p

Care must be taken when quoting and using the dissociation constant in equation. This equilibrium value is correct he H2CO3 molecule, and shows that it is a stronger acid than acetic acid or formic acid as might be expected from

nfluence of the electronegative oxygen substituent. However, carbonic acid only exists in solution in equilibrium warbon dioxide, and so the concentration of H2CO3 is much lower than the concentration of CO 2, reducing the

measured acidity. The equation may be rewritten as follows:

ven more acidity is sometimes encountered in mine waters and in water contaminated by industrial wastes. Manyalts added to an aqueous system also have a direct effect on the pH of that mixture through the following process ydrolysis shown here for the addition of ferric ions to water:

n this particular example the equilibrium is established between ferric ions, water, ferric hydroxide or Fe(OH)3 and

he acidity of the water. This particular example is quite useful to explain the severity of a situation that can develoonfined areas such as corrosion pitting and crevices.

previous) Page 1 of 3 (n

nformation Module

Introduction

Corrosion in acids

Corrosion in neutral or alkaline environments

ee also CCE 513: Corrosion Engineering
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osion in acids

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Module Two of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

Corrosion in Acids

One of the common ways of generating hydrogen in a laboratory is to place zinc into a dilute acid, such asydrochloric or sulfuric. When this is done, there is a rapid reaction in which the zinc is attacked or dissolved anydrogen is evolved as a gas.

Rapid evolution of hydrogen bubbles during the corrosion of a zinc strip in a 1 M HCl acid solution

hese reactions are described in the following equations to:
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hese equations are the chemical shorthand for the statement: One zinc atom + two hydrochloric acid moleculesissociated as ions H+ and Cl- and becomes one molecule of zinc chloride in the first equation and written as a solualt in the form of Zn2+ and Cl- ions in the second equation + one molecule of hydrogen gas which is given off asndicated by the vertical arrow. It should be noted that the chloride ions do not participate directly in this reaction,lthough they could play an important role in real corrosion situations.

imilarly, zinc combines with sulfuric acid to form zinc sulfate (a salt) and hydrogen gas as shown in the followingquations:

Note that each atom of a substance that appears on the left-hand side of these equations must also appear on the rigand side. There are also some rules that denote in what proportion different atoms combine with each other. As inreceding reaction, the sulfate ions that are an integral part of sulfuric acid do not participate directly to the corrosiottack and therefore one could write these equations in a simpler form:

Many other metals are also corroded by acids often yielding soluble salts and hydrogen gas as shown in Equations or respectively iron and aluminum:

Note that zinc and iron react with two H+ ions, whereas aluminum reacts with three. This is due to the fact that bothinc and iron, when corroding, each lose two electrons and display two positive charges in their ionic form. They aaid to have a valence of +2 or II, whereas aluminum loses three electrons when leaving an anodic surface and henisplays three positive charges and is said to have a valence of +3 or III. Some metals have several common valencthers only one. The following Figure shows Some of the oxidation states found in compounds of the transition-mlements.


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Oxidation states found in compounds of the metalic elements. A solid circle represents a common oxidatio

state, and a ring represents a less common (less energetically favorable) oxidation state

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osion in Neutral or Alkaline Environments

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Module Two of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

Corrosion in Neutral or Alkaline Environments

he corrosion of metals can also occur in fresh water, seawater, salt solutions, and alkaline or basic media. In almo

ll of these environments, corrosion occurs importantly only if dissolved oxygen is also present. Water solutionsapidly dissolve oxygen from the air, and this is the source of the oxygen required in the corrosion process. The mo

amiliar corrosion of this type is the rusting of iron when exposed to a moist atmosphere. (reference)

n this equation, iron combines with water and oxygen to produce an insoluble reddish-brown corrosion product th

alls out of the solution, as shown by the downward pointing arrow.

During rusting in the atmosphere, there is an opportunity for drying, and this ferric hydroxide dehydrates and formamiliar red-brown ferric oxide (rust) or Fe2O3, as shown below:

imilar reactions occur when zinc is exposed to water or moist air followed by natural drying.

he resulting zinc oxide is the whitish deposit seen on galvanized pails, rain gutters, and imperfectly chrome-plated

athroom faucets. It also familiarly called 'white rust' a non-protective and even destructive form of corrosion thatttacks incompletely passivated galvanized steel material or galvanized components subjected to marine atmospher
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White rust on seaside road railing

As discussed previously, the iron that took part in the reaction with hydrochloric acid in had a valence of 2, wherea

he iron that takes part in the reaction shown in the previous equation has a valence of 3. The clue to this lies in thexamination of the equation for the corrosion product Fe(OH)3. Note that water ionized into H

+ and OH-. It is furth

nown that hydrogen ion has a valence of 1 (it has only one electron to lose). It would require three hydrogen ions

with the corresponding three positive charges to combine with the three OH - ions held by the iron. It can thus beoncluded that the iron ion must have been Fe3+ or a ferric ion.

Also note that there is no oxidation or reduction (electron transfer) during either reaction. In both cases the valences

he elements on the left of each reaction remain what it is on the right. The valences of iron, zinc, hydrogen, andxygen elements remain unchanged throughout the course of these reactions, and it is consequently not possible to

ivide these reactions into individual oxidation and reduction reactions.

Answers to example problems

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set2 chemistry of corrosion

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