The Principle Mechanisms of Corrosion

William HawkesGraduate Diploma: Spring Term: 2013-2014.

Conservation Of MetalworkWord Count: 2912

Describe the principle mechanisms of the oxidization andcorrosion of ferrous and copper alloy objects, and explain how

storage and display environments affect this. Identify anddiscuss two case studies demonstrating the good and badeffects of long-term storage and /or display of ferrous or

copper alloy objects.

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&docid=TxSVaaRi1dCqzM&tbnid=lzORZwKXxFzBnM:&ved=0CAUQjRw&url=http://newsolio.com/bronze-metal-working-how-is-a-bronze-sculpture-made,17&ei=xz00U4HCEua50QXDpICYAQ&bvm=bv.63808443,d.ZGU&psig=AFQjCNEaXd-rKIKijo2r4Pl_EAe3zCFvbw&ust=1396018977473890

William HawkesGraduate Diploma Spring Term 2013-2014.Conservation Of MetalworkWord Count:

Corrosion is the deterioration of any metal as a

result of chemical reactions between it, and the

surrounding environment. This subject cannot

possibly covered in any detail due to the

restrains of this paper, but this is a brief

overview of the principles involved

This paper will discuss what corrosion is, how it

occurs and its varying types. Finally this paper

will look at the ways in which the environment

can affect metals and how we can control

corrosion.

Corrosion and the environment:

Some metals occur naturally in the metallic state

and are stable under normal conditions (room

temperature and in fresh air), while others will

rapidly tarnish and corrode on exposure to air.

The ease, with which metals will react and form

compounds, by giving up some of their electrons,

is often expressed by their electrode potential

(E°). Metals with a high positive value are

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stable and it may even be difficult to form

compounds with these metals, while those with a

high negative value will react spontaneously

(Dungworth, 2012).

Table 3: Electrode Potentials of Common Metals:Name (symbol) (E° (v))

Gold (Au) +1.50

Mercury (Hg) +0.85

Silver (Ag) +0.80

Copper (Cu) +0.34

Antimony (Sb) +0.10

Lead (Pb) - 0.13

Tin (Sn) - 0.14

Iron (Fe) -0.44

Arsenic (As) - 0.68

Zinc (Zn) - 0.76

As can be seen in the fabrication of iron

objects, the flash corrosion of the surface is a

result of the iron having a relatively high

negative electrode potential. In an indoors

environment, this oxide layer on the surface of

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the metal is what stops further corrosion. This

layer is usually stable at room temperature and

at a Relative Humidity (RH) of 65% or less.

However when the RH goes above the 65% mark, a

layer of active corrosion, which is bright

orange, friable and considered active, quickly

forms. This corrosion product is usually

lepdiocrosite (Selwyn, 2004).

Galvanic Cell Reactions:

When we consider the corrosion of copper or iron,

the reactions we see the results of, in the form

of corrosion products, are galvanic cell or

electrochemical reaction. As in all chemical

reactions, corrosion reactions occur through an

exchange of electrons.

In electrochemical reactions, the electrons are

produced by a chemical reaction in one area, the

anode area. The electrons travel through a

metallic path, usually the parent metal, and are

consumed through a different chemical reaction in

another area, the cathodic area.

4


Anodic Reactions.

The generic chemical reaction for this metal loss

at anodic sites is:

M- M+ + e-

Where M is an uncharged metal atom at the metal

surface, M+ is a positively charged metal ion in

the electrolyte and e- is an electron that

remains in the metal.

This type of chemical reaction is called metal

oxidation, even though it does not directly

involve oxygen, but only results in an increase

in positive charge on the atom undergoing

oxidation or loss of electrons. This resultant

ion is dissolved into the water.

More than one electron can be lost in the

reaction, as in the case of iron where the most

common anodic reaction is:

Fe- → Fe2+ + 2e-

Where Fe is metallic iron. Fe2+ is a ferrous ion

that carries a double positive charge.

5


Cathodic Reactions.

The electrons that are produced at anodic sites

are consumed at cathodic sites. The type of

chemical reactions that consume electrons are

called reduction:

One of the most common cathodic reactions is the

reduction of oxygen:

O2 + 4e- + 4H2O- → 4OH-

Further to this, the four OH- ions are free in

the water to combine with the Fe2+ ions to form

Fe(OH)2 following the equation:

Fe2+ + OH- →Fe(OH)

This whole process can be seen in the following

diagram (diagram 1) where the end product is iron

hydroxide, or what we commonly call “rust”.

6


Diagram 1: Galvanic reaction with water as the

solvent. (1)

Types of corrosion:Corrosion comes in many different forms and can

be classified by the cause of the chemical

deterioration of a metal:

7


1. General Attack Corrosion

“General attack” corrosion on a capstan. (1)

This is the most common type of corrosion and

is caused by a chemical or electrochemical

reaction that results in the deterioration of

the entire exposed surface of a metal.

Ultimately, the metal deteriorates to failure.

General attack corrosion accounts for the

greatest amount of metal destruction by

corrosion, but is considered a “safe” form of

corrosion, due to the fact that it is

predictable, manageable and often preventable.

8


2. Localised Corrosion:

Crevice corrosion in steel (2)

Localised corrosion specifically targets one

area of the metal structure. Localised

corrosion is classified as three types:

Pitting: Pitting results when a small cavity,

forms in the metal, usually as a result of

“de-passivation” of a small area. This area

becomes anodic, while part of the remaining

metal becomes cathodic, producing a localised

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galvanic reaction. The deterioration of this

small area penetrates the metal leading to

failure. This form of corrosion is often

difficult to detect due to the fact that it is

usually relatively small and may be covered,

hidden by corrosion products

Crevice corrosion: Crevice corrosion also

occurs at a specific location. This type of

corrosion is often associated with a stagnant

microenvironment, like those found in the low

points of sculptures. Acidic conditions, or a

depletion of oxygen in a crevice can lead to

crevice corrosion.

Repousse low points can form a microenvironment in which

crevice corrosion occurs (3).

10


Filiform corrosion: Occurring under painted

or plated surfaces when water breaches the

coating, filiform corrosion begins at small

defects in the coating and spreads to cause

structural weakness.

3. Galvanic Corrosion:

Galvanic corrosion occurs when two different

metals are located together in a corrosive

electrolyte. A “galvanic couple” forms between

the two metals, where one metal becomes the

anode and the other the cathode. The anode, or

sacrificial metal, corrodes and deteriorates

faster than it would alone. The cathode

deteriorates more slowly than it would

otherwise.

Three conditions must exist for galvanic

corrosion to occur:

Electrochemically dissimilar metals must be

present such as copper together with iron

The metals must be in electrical contact, and

The metals must be exposed to an electrolyte

like water.

11


Galvanic corrosion is discussed under “Galvanic

cell reactions” see above.

4. Environmental Cracking:

Environmental cracking in steel sheet (4)

Environmental cracking is a process that

results from environmental conditions

affecting the metal. Chemical, temperature and

stress-related conditions can result in the

following types of environmental corrosion:

Stress Corrosion Cracking

Corrosion fatigue

Hydrogen-induced cracking

12


Liquid metal embrittlement

13


5. Flow-Assisted Corrosion:

Flow assisted corrosion in a textured sheet of steel (5)

Flow-assisted corrosion, or flow-accelerated

corrosion, results when a protective layer of

oxide on a metal surface is dissolved or

removed by wind or water, allowing the metal to

further corrode and deteriorate. Also known as:

Erosion-assisted corrosion

Impingement

Cavitation

14


6.Inter-granular corrosion:

A microscopic image of inter-granular corrosion (6)

Inter-granular corrosion is an electrochemical

attack on the grain boundaries in metal. This

often occurs due to impurities in the metal,

which tend to be present more often near grain

boundaries. These boundaries are more

vulnerable to corrosion than the actual crystal

of the metal.

9.De-Alloying:

15


De-alloying, or selective leaching, is the

selective corrosion of a specific element in an

alloy. The most common de-alloying is “de-

zincing” of brass. The result of this is a

deteriorated and porous copper.

16


9. Fretting corrosion:

Fretting corrosion in metal links (7)

Fretting corrosion occurs as a result of

repeated wearing, weight and/or vibration on an

uneven, rough surface. Corrosion, resulting in

pits and grooves, occurs on the surface.

Fretting corrosion is often found on surfaces

exposed to vibration during transportation.

17


Iron Oxides, Chlorides, Hydroxides, Sulphides and

Sulphates:

Iron forms a large variety of compounds as

corrosion products, and exactly which compounds

are formed are dependent on which elements are

present at the time. The following table (Table

1) shows the most commonly encountered iron

corrosion products and their chemical

composition.

Table 1: Corrosion Products of Iron

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(Selwyn, 2004)

Iron (II)

oxide:

Wustite FeO Grey

blackIron (II,

III) oxide

Magnetite (Fe3O4). Black

Iron (II,

III) oxide

(Fe4O5)

Iron (III)

oxide

Haematite α(Fe2O3) Red,

BlackIron (III)

oxide

Maghemite γ(Fe2O3) Red-Brown

Iron (II)

Chloride

FeCl2 White

Iron (III)

Chloride

FeCl3 Green

Iron (III)

Hydroxide.

Goethite α-Fe(OH) Yellow-

BrownIron (III)

Hydroxide.

Akaganeite β-FeO(OH) Yellow-

BrownIron

(III) ,Hydr

oxide.

Lepidocroc

ite

γ-FeO(OH) Orange

Iron (II)

Sulphide

Pyrrhotite

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Copper Oxides:

There are two stable copper oxides, copper (I)

oxide (Cu2O) and copper (II) oxide (CuO). These

are stable due to the electron configuration

shown by each of the molecules. Copper by itself

can lose either one or two electrons making it

either a Cu1+ ion or a Cu2+ ion.

With Copper (i) oxide, the oxygen, which has two

spare electrons in its outer valence or electron

shell, and is therefore O2-, is balanced

electrically by the two copper ions each of which

is Cu1+:

2Cu+ + 02- = Cu2O

(Both sides of the equation balance the numbers

of ions and charges). This reaction is called the

oxidation of copper.

20


Copper (I) oxide, also known as cuprous oxide or

cuprite, is insoluble in water and organic

solvents.

When exposed to oxygen, copper naturally oxidises

to copper (I) oxide, but this takes considerable

time. Artificial formation is usually

accomplished at high temperature or high oxygen

pressure. With further heating, copper (I) oxide

will form copper (II) oxide:

2Cu2O +O2 → 4CuO

Copper (ii) oxide, a black solid, melts above

1200 °C with some loss of oxygen. Heating copper

in air can form copper (ii) oxide:

2Cu + O2 → 2CuO

Copper (i) oxide forms on silver-plated copper

items exposed to moisture when the silver layer

is porous or damaged; this kind of corrosion is

known as “red plague”. This is seen on silver

plated objects and electroplated nickel-silver

21


pieces, which are of a copper-based alloy

construction

Copper (II) oxide, or cupric oxide (CuO), is the

higher oxide of copper, known as Tenorite.

Table 2: Oxides of Copper.Copper (I) Oxide: (Cuprous Oxide).

Also known as “Cuprite”

Dark red to orangein colour.

Copper (II) Oxide: (Cupric Oxide). Also known as “Tenorite”.

Dull-black colour.

(Scott, 1997).

Copper Chlorides, Hydroxides, Sulphides and

Sulphates:

Again, copper forms a large number of corrosion

products, each dependant on the elements present

at the time of the reaction.

Typically, copper (ii) oxide dissolves in mineral

acids such as hydrochloric acid, sulphuric acid

or nitric acid to give the corresponding copper

(II) corrosion products or salts, represented in

the following equations:

22


CuO + 2HNO3 → Cu(NO3)2 + H2O

(Copper nitrate and water)

CuO + 2HCl → CuCl2 + H2O

(Copper chloride and water)

CuO + H2SO4 → CuSO4 + H2O

(Copper sulphate and water)

Passivation layers and relative humidity (RH):

Corrosion relies on the metal being exposed to

the various ions of corrosion, often in gaseous

form, and in an electrolyte. As a result, if a

barrier forms which is impermeable to moisture,

the electrolyte is unable to make contact with

the metal surface. The same can be said for the

ions, if the gaseous ions cannot make contact

with the metal then they cannot react with it.

Therefore the passivation layer is an oxide layer

that can greatly reduce, if not prevent, the

contact of corrosive ions with the underlying

metal’s surface. Freshly exposed metallic

23


surfaces will adsorb and react with oxygen

present in the atmosphere almost instantaneously.

E.G:

4Fe + 3O2 → 2Fe2O3.

This reaction occurs spontaneously in air at room

temperature, however, as there is no abundance of

moisture or water to form an electrolyte the

corrosion stops and is deemed passive.

24


Case Studies:

The Statue of Liberty, on Bedloe’s Island, New York (8).

1. The statue of Liberty: in the most hostile

environment imaginable for metals.

Standing tall in New York harbour since 1886, the

Statue of Liberty is the tallest statue ever made

entirely of metal. Its arms are 42 feet long and

its torch is 21 feet in height. Its index fingers

are eight feet long and nose is 4-foot 6-inches

long.

25


The Statue of Liberty is made entirely of copper

and iron and as such, especially given the fact

that it stands beside a marine environment

comprising salt water on one side and one of the

busiest cities in the world on the other, is

destined to corrode, most prolifically through

“galvanic cell corrosion”.

Lady Liberty is comprised of a copper shroud

consisting of plates of copper, which makes up

the detail of the design. These copper plates are

supported internally by an armature structure of

iron. The copper plates are attached to the

armature by copper straps riveted in place.

26


Plans of the iron skeleton of armatures created by Gustave Eiffel in

1885 (9).

In 1984 the process of restoration and

conservation started with a thorough examination

of the structure and all of its components. It

was found that the copper plates, and the straps

that held them to the iron, were relatively

intact and that the patina on the surface of the

copper had acted as a passivation layer and

protected the statues detail to a very high

27


degree. However, where the passivation layer had

been breached, the copper was shown to be pitted

and porous as a result of reaction with the

atmosphere. Due to a galvanic reaction between

the copper and the iron, the iron had corroded

preferentially to the copper and formed a

sacrificial anode. Because of this reaction the

iron structure, in many places, had lost more

than half of its thickness. As a measure to

isolate the two metals from each other a foam

strapping was used. This, in theory, was to

prevent the preferential corrosion from

occurring. In practice, however, the foam under

the strapping became waterlogged and created an

electrochemical cell where the water in the

sponge was the electrolyte. (Bellante, 1987).

28


Diagram 2: Iron armatures holding the copper shroud together with

copper rivets and saddle straps.

Due to the nature of the work to be carried out

the conservation team needed to remove most of

the iron armatures and replace them, while others

could be pacified through cleaning and chemical

treatments. During this process, it was found,

through analysis, that the initial expectation of

29


the corrosion products being chlorides, due to

the marine environment in which the statue

stands, was actually wrong. The greatest

quantities of corrosion products were sulphides,

derived from the pollution of New York. Thus the

green colour that we know and relate to the

statue is copper sulphates and sulphides, which

have formed on the surface of the copper shroud.

The fact of the statue being outdoors will always

mean that the moisture in the air will be

variable, however the statue being so close to a

body of water means that it will be biased on the

high side. In order to prevent iron corrosion

entirely the RH should never exceed 11%, a

condition that is entirely impossible in these

circumstances (Rimmer, 2013). The marine

environment also causes the water in the

atmosphere to be salty and therefore an even

better electrolyte in which the corrosion

reactions occur. Because of this, the interior of

the statue was coated in a bituminous layer to

try to exclude the humid atmosphere from

30


contacting the metal. This had previously been

done in 1911 but with the use of coal tar instead

of bitumen.

The nature of the statue means that it can never

be protected from the elements and effects of the

environment, however the patina on the copper,

and the bituminous layer inside the statue

provide some protection by reducing the chances

of a galvanic cell being created. The statue is

now under a regimen of monitoring and this is

critical to the early detection of problems and

the ability to be able to prevent more

interventive processes being necessary.2. A bronze statue in a museum environment:

Any bronze in a museum environment must be kept

at a maximum RH of 50%, and that is only where

the object is of mixed media. In fact the RH

should be consistently around the region of 42-

46% for the benefit of the bronze.

As with any museum object there is a need to

inspect the bronze on a regular basis to identify

any corrosion early. The type of corrosion most

31


often seen in the museum environment is “general

attack corrosion”, and this is what we are aiming

to prevent more often than not indoors.

The most common corrosion on bronzes displayed

indoors is copper chloride. The chlorine present

in the atmosphere is in the form of chloride ions

(Cl-) that can easily be dissolved into an

electrolyte.

Another, far more controllable, source of

chloride ions is through contact from hands.

Human hands are virtually never free from oils

and perspiration, which is acidic in nature and

deposits on the surface of the metal, and can

remain mobile across its surface for up to two

weeks (Edgecumbe1 - pers comm.)

The further reaction between water, copper

chloride and sulphur dioxide results in the

formation of copper chloride, hydrochloric acid

and sulphuric acid.

2 CuCl2 + SO2 + 2 H2O → 2 CuCl + 2 HCl + H2SO4

1Richard Edgecumbe: Victoria and Albert Museum 2014.

32


As a result the surface of the bronze is attacked

and etched by both of the acids and the end

result is “bronze disease”. In this situation it

is necessary to place the objects in to a dry

environment with the RH as low as possible but

certainly below 40%, in order to stop the

activity of the cupric chloride.

(Williamstown, 2009)

So what do we do?Given the number of corrosion products that can, and do

occur on both iron and copper, given that the air we

breathe is comprised of the very reactants that are

responsible for these corrosion products, and finally,

given that the earth is so laden with water, we as

conservators must accept that our endeavor is not to stop

objects from corroding, that they will eventually corrode

away is inevitable, but what we do is manage that

degradation and corrosion so that we can have the maximum

benefit and enjoyment form the object possible.

The principle of this management is to control the

elements responsible for the corrosion. The ways in which

33


this can be achieved are to remove reactants, to remove

moisture and to be vigilant in our regimen of

observation.

Passivation layers in the forms of oxidation layers,

patinas and other layers, which we deliberately impart

upon the surfaces of the objects, form an impermeable

layer that prevents the reactant ions form forming and

electrical connection with the metal. Ions, such as

nitrates, sulphates and sulphides, chlorides, hydroxides

and carbonates, all need an electrolyte to form the

electrical connection with the metal. As such we can

eliminate the electrolyte as another way to prevent

corrosion. This, in practice, means the prevention of

moisture getting in contact with the metal.

34


Bibliography:

BELLANTE, E. L. A. C., E.B. 1987. Restoring the Statue of Liberty: Construction or Conservation?, Washington, Internation Council on Monuments and Sites.

DUNGWORTH, D. 2012. Metals and Metalworking: a research framework for archaeometallurgy. Historical Metallurgy Society. November ed.: Historical Metallurgy Society.

RIMMER, M., THICKETT, D., WATKINSON, D. AND GANIARIS, H. 2013. Guidelines for the Storage and Display of Archaeological Metalwork, Swindon, English Heritage.

SCOTT, D. A. 1997. Copper Compounds in Metals and Colorants: Oxides and Hydroxides. IIC. Studies in Conservation, 42, 93-100.

SELWYN, L. 2004. Metals and Corrosion, A Handbook for the Conservation professional, Ottawa,, Canadian Conservation Institute.

WILLIAMSTOWN, A. C. C. 2009. Care and handling of Bronze Objects,WACC.

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Diagrammatic and Picture Credits:

Diagram 1: Galvanic corrosion diagram:

http://www.corrosionist.com/Iron_Corrosion.htm

(Accessed 27/3/2014)

Diagram 2: Armature structure and attachments:

http://corrosion-doctors.org/Landmarks/statue-

saddle.htm

(Accessed on 27/3/2014

Title page. A bronze sculpture of a Greek God:

http://newsolio.com/bronze-metal-working-how-is-a-

bronze-sculpture-made,17


1. General attach corrosion on a capstan:

www.corrosionanalysisnetwork.org


2. Crevice corrosion:

davidnfrench.com

(Accessed 30/3/2014

3. Repousse bronze box:

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http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=16mbdSCgHVJwLM&tbnid=QswoNhYgGnZnrM:&ved=0CAQQjB0&url=http%3A%2F%2Fdavidnfrench.com%2Fpitting-corrosion.html&ei=1O03U8GLIeaV7Abi34HwBA&bvm=bv.63808443,d.ZGU&psig=AFQjCNHqv7rd7fMlwtE1opsrOjPAIgJF2A&ust=1396258935372610

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=BXFs2UNzAOwrNM&tbnid=nHhSeiEibxCBMM:&ved=0CAQQjB0&url=http%3A%2F%2Fwww.corrosionanalysisnetwork.org%2Fweb%2Fcan%2Fimage-gallery%2F-%2Fjournal_content%2F56%2F10183%2FUNIFORMCORROSION-3%2FIMAGE-TEMPLATE&ei=K-03U7TQMs6O7Qaa9IA4&bvm=bv.63808443,d.ZGU&psig=AFQjCNF_fDmanUaEgQEj7Z1a5K99weDhug&ust=1396258880641469

http://newsolio.com/bronze-metal-working-how-is-a-bronze-sculpture-made,17

http://newsolio.com/bronze-metal-working-how-is-a-bronze-sculpture-made,17

http://corrosion-doctors.org/Landmarks/statue-saddle.htm

http://corrosion-doctors.org/Landmarks/statue-saddle.htm

http://www.corrosionist.com/Iron_Corrosion.htm


http://www.rubylane.com/item/419645-2009-1020A/

Large-Art-Nouveau-French-Bronze.


4. Environmental Cracking:

www.integratedglobal.com


5. Flow assisted corrosion:

www.cospp.com


6.Inter-granular corrosion:

www.cospp.com


7.Fretting Corrosion:

corrosion.ksc.nasa.gov


8. The statue of Liberty on Bedloe’s Island:

http://en.wikipedia.org/wiki/

File:0328Jersey_City_Statue_of_Liberty.JPG

(Accessed on 27/3/2014)

9. The armature structure design of the statue of

liberty:

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http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=q7Bjozm4njtukM&tbnid=VqPQa5EM33nJvM:&ved=0CAQQjB0&url=http%3A%2F%2Fcorrosion.ksc.nasa.gov%2Ffretcor.htm&ei=UO83U42lCq7N7AaTk4GQAw&psig=AFQjCNGljOBpa7BCAE7pTKyMqZKTwRrzRA&ust=1396259039209696

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=K-J8DZ-8VC2dfM&tbnid=AHctM_2NV-GGKM:&ved=0CAQQjB0&url=http%3A%2F%2Fwww.cospp.com%2Farticles%2Fprint%2Fvolume-14%2Fissue-4%2Ffeatures%2Fbe-alert-to-flow-accelerated-corrosion-in-your-hsrg.html&ei=_uU3U5G4JuvQ7AbB6ID4DQ&bvm=bv.63808443,d.ZGU&psig=AFQjCNFWFuejTKt6VQIq3j-su3AS4QLSxA&ust=1396258683855934

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=K-J8DZ-8VC2dfM&tbnid=AHctM_2NV-GGKM:&ved=0CAQQjB0&url=http%3A%2F%2Fwww.cospp.com%2Farticles%2Fprint%2Fvolume-14%2Fissue-4%2Ffeatures%2Fbe-alert-to-flow-accelerated-corrosion-in-your-hsrg.html&ei=_uU3U5G4JuvQ7AbB6ID4DQ&bvm=bv.63808443,d.ZGU&psig=AFQjCNFWFuejTKt6VQIq3j-su3AS4QLSxA&ust=1396258683855934

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=DYRLCir-woev9M&tbnid=__jMHgxsWk5kxM:&ved=0CAQQjB0&url=http%3A%2F%2Fwww.integratedglobal.com%2Fstress-corrosion-cracking-scc.html&ei=de43U_bAIrTT7AacvYDYBQ&bvm=bv.63808443,d.ZGU&psig=AFQjCNE6p1g_IF6OKSWP0El4M7vo7sGVQA&ust=1396259740826339

http://en.wikipedia.org/wiki/File:0328Jersey_City_Statue_of_Liberty.JPG

http://en.wikipedia.org/wiki/File:0328Jersey_City_Statue_of_Liberty.JPG

http://www.rubylane.com/item/419645-2009-1020A/Large-Art-Nouveau-French-Bronze

http://www.rubylane.com/item/419645-2009-1020A/Large-Art-Nouveau-French-Bronze


http://www.cr.nps.gov/history/online_books/hh/11/

hh11e.htm


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http://www.cr.nps.gov/history/online_books/hh/11/hh11e.htm

http://www.cr.nps.gov/history/online_books/hh/11/hh11e.htm

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The Principle Mechanisms of Corrosion