peter southwell-keely - unsw chemistry · 20 9.6 shipwrecks and salvage 3.2 distinguish between and...

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9.6 Shipwrecks and Salvage

Peter Southwell-Keely

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Two types of Chemical Reaction

• Oxidation-Reduction (Redox)

• The others

3

Shipwrecks and Salvage

is

Redox Chemistry

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Identification and Production of Materials (core)

Shipwrecks andSalvage (option)

Industrial Chemistry(option) major

overlaps

AcidicEnvironment (core)

ChemicalMonitoring and Management (core)

Biochemistry of Movement (option)

ForensicChemistry (option)

Chemistry of Art (option)

minoroverlaps

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Identification and Prod'n of Materials (org and inorg)

Shipwrecks andSalvage (inorg)

Industrial Chemistry(inorg)

majoroverlaps

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Shipwrecks andSalvage (inorg)

AcidicEnvironment (inorg)

ChemicalMonitoring and Management(org and inorg)

Biochemistry of Movement (org)

ForensicChemistry (org)

Chemistry of Art (inorg)

minoroverlaps

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Oxidation Reduction

• Loss of electrons (inorg)

• Gain of oxygen (org)

• Loss of hydrogen (org)

• Increase in oxidationnumber (org and inorg)

• Gain of electrons (inorg)

• Loss of oxygen (org)

• Gain of hydrogen (org)

• Decrease in oxidationnumber (org and inorg)


1.1 Identify the origins of minerals in oceans..

Conservation of Marine Archaeological Objects, C. Pearson.

Analyses of the principal types of pelagic sediments

(weight % oxides)Red clay Calcareous

oozeSiliceousooze

SiO2 53.93 24.23 67.36TiO2 0.96 0.25 0.59Al2O3 17.46 6.60 11.33Fe2O3 8.55 2.43 3.40FeO 0.45 0.64 1.42MnO 0.78 0.31 0.19CaCO3 0.39 56.73 1.52


1.2 Outline the role of electron transfer in oxidationreduction reactions cf 8.3.2.s4; 9.2.4.1; 9.2.4.3; 9.2.4.4;9.2.4.5

Formation of sodium chloride

Oxidn: 2Na ---->2Na+ + 2e

Redn: Cl2 + 2e ---> 2Cl-

Overall 2Na + Cl2 ---> 2NaCl


1.2 (cont’d)

Oxidation of methane

Oxidn: CH4 + 2H2O --->CO2 + 8H+ + 8e

Redn: 2(O2 + 4H+ + 4e ---> 2H2O)

Overall: CH4 + 2O2 ---> CO2 + 2H2O


1.2 (cont’d)

Fermentation of glucose (CH2O)6

Oxidn: (CH2O)2 + 2H2O ---> 2CO2 + 8H+ + 8e

Redn: (CH2O)4 + 8H+ + 8e ---> 2C2H5OH + 2H2O

Overall: (CH2O)6 ---> 2C2H5OH + 2CO2


1.3 ...redox reactions can occur when ions are free to movein solid and liquid electrolytes cf 9.2.4.4; 9.2.4.5;9.2.4.6; 9.2.4.7; 9.2.4.8; 9.2.4.s1; 9.2.4.s2; 9.2.4.s3;9.2.4.s4; 9.5.3.6; 9.5.3.s2; 9.5.4.1; 9.5.4.2; 9.5.4.3;9.5.4.s1; 9.5.4.s3


1.4 Describe the work of Galvani, Davy and Faraday(NEW!) (www.ask.com---> Luigi Galvani etc)

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Shipwrecks and Salvage

Luigi Galvani Humphry Davy Michael Faraday


2.1 Account for differences in corrosion of active andpassivating metals

• On the basis of their reduction potentials, most metals except goldshould oxidise in air.

• However, most metals form a thin oxide coating, which protectsagainst further oxidation. This is passivation.

• Aluminium shows this best. Its reduction potential is -1.7V.However, Al coated with Al2(OH)6, “passive” Al, has a reductionpotential of -0.6V.


2.2 Identify iron and steel as the main metals used in ships

2.3 Identify the composition of steel and explain how thepercentage composition of steel can determine itsproperties (see p. 1012 Zumdahl 5th edit, 2000)


2.4 Describe the conditions under which rusting occurs andexplain the process of rusting

Fe dissolvesforming a pit anodic area

cathodic area

water droplet

rustO2

anode reaction: Fe(s) Fe2+ + 2e

cathode reaction: O2 + 2H2O + 4e 4OH-

2Fe(s) + O2 + 2H2O 2Fe2+ + 4OH-

Fe2+

e

4Fe2+(aq) + O2(g) + (4 + 2n) H2O(l) 2Fe2O3.nH2O(s) + 8H+(aq)Rust


3.1 Define the terms galvanic cell and electrolytic cell cf9.2.4.3; 9.2.4.4; 9.2.4.5; 9.2.4.6; 9.2.4.7

• A galvanic cell uses a spontaneous redox reaction to produce acurrent that can be used to do work.

• An electrolytic cell uses electrical energy to produce a chemicalchange that would otherwise not occur naturally.


3.2 Distinguish between and give examples of galvanic andelectrolytic cells cf 9.2.4.4; 9.2.4.5; 9.2.4.6; 9.2.4.7

Zn2+

Zn2+

Zn2+

Zn2+

Zn electrodeanode

Cu electrodecathode

conducting wire

KCl salt bridge

Zn2+

M ZnSO4

electronsleaving cell

e-

Cl- K+

e-electronsentering cell

e-

e-

e-

e-

e-

Cu2+

1M CuSO4

SO42-

Oxidation Zn(s) Zn2+ + 2e Reduction Cu2+ + 2e Cu(s)

Galvanic Cell


3.2 Distinguish between and give examples of galvanic andelectrolytic cells cf 9.2.4.4; 9.2.4.5; 9.2.4.6; 9.2.4.7

directcurrentsource

Na+

Na+

Na+

Na+Na+

e-

e-

e-e-

e-

e-

Cl-Cl-

ooooo

oooo

oooo

Cl2Cl2

Na+

Oxidation2Cl- Cl2(g) + 2e

ReductionNa+ + e Na

Diaphragm allows ions to flowbut separates Cl2 and Na

inertgraphite electrode

inertgraphite electrode

liquid Na metal

Molten NaCl(+CaCL2)

Electrolysis of molten sodium chloride

Addition of CaCl2 to NaCl lowers the melting point enough to make the cell workable.

Cell reaction 2NaCl(l) 2Na(l) + Cl2(g)


3.3 Describe using half-equations, what happens at theanode and cathode during electrolysis of selected aqueoussolutions cf 9.2.4.6

Electrolysis of brine

possible anode reactions

2Cl- ---> Cl2 + 2e E0 = - 1.36V

2H2O ---> O2 + 4H+ + 4e E0 = - 1.23V

The most favourable reaction is the oxidation of water. In dilutesolution oxygen is formed but in concentrated solution chlorine isformed and is the basis of the chlor-alkali process.


3.3 Describe using half-equations, what happens at theanode and cathode during electrolysis of selected aqueoussolutions cf 9.2.4.6

Electrolysis of brine

possible cathode reactions

Na+ + e ---> Na E0 = - 2.71V

2H2O + 2e ---> H2 + 2OH- E0 = - 0.83V

The most favourable reaction is the reduction of water to hydrogenand that is what happens both in dilute and concentrated brine.However in the mercury cell process, sodium is reduced.


3.4 Describe factors that effect an electrolysis reaction...• magnitude of current

• time of current flow

• concentration of solutions

• overvoltage

• nature of electrode

• nature of electrolyte


3.5 Define Faraday’s first law of electrolysis (NEW!)

“The extent to which an electrochemical reaction occurs is in directproportion to the number of moles of electrons transferred.”

q = I x t

charge (coulombs) current (amps) time (sec)

1 mole of electrons = 96,500 coulombs (1 faraday)


4.1 Identify ways in which a metal hull may be protectedincluding corrosion resistant metals, development ofsurface alloys, new paints.

• Instead of forming a metal alloy such as stainless steel, which hasthe same composition throughout, a cheaper carbon steel is treatedby ion bombardment to produce a thin layer of stainless steel, orother desirable alloy on the surface.

In this process a “plasma” or “ion gas” of the alloying ions isformed at high temperatures and directed onto the surface of themetal.



• Rustmaster paint is a water-based polymer that prevents corrosionin two ways:

• Firstly, the polymer layer forms a barrier impenetrable to bothoxygen and water vapour.

• Secondly, the chemicals in the coating react with the steel surface toform a complex mineral inter-layer called pyroaurite between themetal and the polymer coating.



• Pyroaurite has the form [M1-xZx(OH)2]x+ where M is a 2+ ion

(Mg2+, Fe2+, Zn2+, Co2+ or Ni2+), Z is a 3+ ion (Al3+, Fe3+, Mn3+,

Co3+, or Ni3+) and x is a number between 0 and 1. The anions inpyroaurite are typically CO3

2-, Cl- and/or SO42-.

The pyroaurite layer grows into the neighbouring polymer layerpreventing the movement of ions between the anodic and cathodicareas on the surface of the steel.


4.2 Predict the metal that corrodes when two metals forman electrochemical cell using a list of standard potentials(in old syllabus)


4.3 outline the process of cathodic protection, describingexamples of its use in both marine and wet terrestrialenvironments (in old syllabus)

• Cathodic protection is used to protect steel in buried fuel tanks,pipelines and ships.

• A metal such as magnesium is connected by a wire to the tank orpipeline creating an electrochemical cell in which the moist soil isthe electrolyte. As magnesium is the more active metal, it becomesthe anode and is preferentially oxidised, donating electrons to thesteel and maintaining it in the reduced state. Thus steel is thecathode in this system.


4.3 outline the process of cathodic protection, describingexamples of its use in both marine and wet terrestrialenvironments (in old syllabus)

• Thus in cathodic protection, the metal being protected is made theinert cathode of an electrochemical cell.

• The hulls of ships are protected in the same way by using bars oftitanium steel (as anodes) connected by wire to the hull.


4.4 Describe the process of cathodic protection in terms ofthe oxidation reduction chemistry involved (in oldsyllabus)

anode: Mg ----> Mg2+ + 2e

cathode: O2 + 2H2O + 4e ---> 4OH-

overall: 2Mg + O2 + 2H2O ---> 2Mg2+ + 4OH-


5.1 Outline the effect of temp. and pressure on the solubilityof gases and salts (NEW!)( gases/temp SI Data Book, 4thed., p.141; Conserv’n of Marine Archeological Objects,C. Pearson, p.5)

Oxygen solubility in sea water (ml/l)SalinityTemp

0 10 20 36

0 10.22 9.54 8.91 7.995 8.93 8.36 7.83 7.0410 7.89 7.41 6.95 6.2815 7.05 6.63 6.24 5.6520 6.35 5.99 5.64 5.1425 5.77 5.45 5.15 4.7030 5.28 4.99 4.73 4.3335 4.85 4.60 4.36 4.01


5.2 Identify that gases are normally dissolved in the oceansand compare their concentrations in the oceans to theirconcentrations in the atmosphere (NEW!).

% Concnin air (0°C)

% Concn inwater (0°C)

Oxygen 21 0.0015

Nitrogen 78 0.0023

Carbondioxide 0.04 0.014


5.3 Compare and explain the solubility of selected gases atincreasing depths in the oceans (NEW!).(c.f C. Pearson)

1000

2000

3000

0 1 2 3 4 6 875

Oxygen concentration (ml/l)

Dep

th (

m)

Pacific Ocean

Arctic(south ofGreenland)


5.4 Predict the effect of low temperatures at great depths onthe rate of corrosion of a metal (NEW!).


6.1 Explain that shipwrecks at great depths are not corrodedby electrochemical reactions but by anaerobic bacteria(NEW!).

At great depths oxygen depletion occurs

• due to pollution or high microbiological activity

• when the object is buried beneath the seabed and oxygen diffusionfrom the overlying water is restricted

• when the artefact becomes covered by marine growth


6.1 Explain that shipwrecks at great depths are not corrodedby electrochemical reactions but by anaerobic bacteria(NEW!).

I believe this statement is incorrect because it implies thatelectrochemical reactions do not occur.

What actually happens is that the cathode reaction changes from

O2 + 2H2O + 4e ---> 4OH-

to

2H+ + 2e ---> H2


6.1 (cont’d)The reaction 2H+ + 2e ---> H2

is usually slow on most metals in the seawater pH range.

Sufate-reducing bacteria speed up this reaction as they producesulfide ions which react with metal ions to form sulfides, such asFeS, and elemental sulfur on which the hydrogen evolution reactioncan occur more rapidly than on the metal surface.

The sulfate-reducing bacteria may also increase the corrosion rateby forming acids which lower the pH.


6.2 Describe the bacteria as sulfur-reducing species whosewastes produce acidic environments around deep wrecks(NEW!).

4H2 + SO42- + 2H+ ---> H2S + 4H2O


SO42- Adenylyl

Sulfate

HSO3-

cyt c3 (red)

cyt c3(oxid)

H2

2H+

Hydrogenase

H2SH2

2H+

Hydrogenase

cyt c3 (red)

cyt c3(oxid)

ATP PPi

Production of H2S by Sulfate- reducing bacteria

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Bacteria of the species Thiobacillus are able to oxidisesulfide, elemental sulfur, thiosulfate and sulfite to sulfate.These are the types of organisms which deplete the oxygenon the ocean floor and make it anaerobic.

S2- S0

S-SO32- SO3

2- SO42-

H2O 2H+sulfiteoxidase


6.3 Explain that acidic environments accelerate corrosionin non-passivating metals cf 8.3.2.1; 8.3.2.2; 8.3.2.3;8.3.2.4; 8.3.2.s3; 8.3.2.s4; 9.2.4.1.

• Formation of ferrous ions is promoted by an increase in the [H+](ielowering of pH). The presence of CO2 in rain water causes Fe torust much more rapidly than if it were absent.

• O2(g) + 4H+ + 4e ---> 2H2O(l) 1.23V

O2(g) + 2H2O(l) + 4e ---> 4OH- 0.41V


7.1 Explain that artefacts from long submerged wrecks willbe saturated with dissolved chlorides and sulfates

Ion Amount in water(g/kg)

Cl- 19.35

Na+ 10.76

SO42- 2.71

Mg2+ 1.29

Ca2+ 0.41

K+ 0.39

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9.6 Shipwrecks and Salvage7.1 (cont’d)

Composition of concretions (Batavia wreck)

Iron concretion (wt %)Inner red zone (4 cmbelow seaward surface)

Fe (total) 53.2

Fe3+ 47

Ca2+ 0.22

Mg2+ 0.07

Na+ 0.12

S (total) 2.2

P 0.18

Cl- 6.35


7.2 Describe the processes that occur when a saturatedsolution evaporates and relate this to the potential damageto drying artefacts


7.3 Identify the use of electrolysis as a means of removingsaltThe two most significant problems in marine metal conservationare:

1. Retention and stabilisation of corrosion products

2. Removal of chlorides

• After the removal of an iron object from water, the presence ofchlorides can bring about the same amount of corrosion in a fewdays as takes months or years under water.


7.3 (cont’d)• The corrosion product on partially corroded cast iron contains

FeO(OH), iron chlorides, SiO2, Fe3C and graphite.

• On drying in air the iron chlorides decompose to Fe2O3, FeCl3, HCland FeO(OH) generating ideal conditions for rapid corrosion.

• The principal aim of all marine iron treatment is to removechlorides from corrosion products which is achieved by

1. Washing

2. Heat treatment


7.3 (cont’d)Washing

Fe(OH)Cl(s) + X-(wash water) --> Fe(OH)X(s) + Cl-(wash water)

(where X = OH-, HCO3-, CO3

2-)

The most effective means of increasing Cl- diffusion within thecorrosion products is to increase their porosity by converting theiron compounds to a denser state. Reduction of iron compoundsincreases their density


7.3 (cont’d)Washing (cont’d)

Reduction of the iron compounds can be achieved by

1. Electrolysis

2. Alkaline sulfite

3. Hydrogen furnace

e.g 3FeO(OH) + H+ + e ---> Fe3O4 + 2H2O


7.3 (cont’d)Heat treatment

in air at 400°C

4FeOCl + O2 ---> 2Fe2O3 + 2Cl2

3FeOCl ---> Fe2O3 + FeCl3

4FeCl2 + 3O2 ---> 2Fe2O3 + 4Cl2


7.3 (cont’d)Heat treatment

in H2/N2 at 100 - 300°C

10FeOCl + 3H2 ---> 2Fe3O4 + 4FeCl2 + 2HCl + 2H2O

FeCl3.H2O ---> FeOCl + 2HCl

FeCl2 + H2 ---> Fe + 2HCl

Fe3O4 + 3H2 ---> 3Fe + 4H2O


7.4 Identify the use of electrolysis as a means of cleaningand stabilising strong metal artefacts (NEW!).

• Silver reacts very readily with very low concentrations of Hydrogensulfide to form the very insoluble silver sulfide

2Ag(s) + H2S(aq) ----> Ag2S(s) + H2(g)


7.4 (cont’d)

The reaction

2Ag(s) + H2S(aq) ----> Ag2S(s) + H2(g)

seems improbable until one examines the reduction potentials

O2(g) + 4H+ + 4e ---> 2H2O(l) 1.23V

Ag+ + e ---> Ag 0.80V

AgCl + e ---> Ag + Cl- 0.22V

2H+ + 2e ---> H2 0.00V

Ag2S + 2H+ 2e ---->2Ag + H2S -0.037V

Ag2S + 2e ---->2Ag + S2- -0.705V


• 7.4 Identify the use ofelectrolysis as a meansof cleaning andstabilising strongmetal artefacts(NEW!).

Powersource

Na+

OH-

H2OCoin coated with Ag2S

e-e-

Anode Cathode

Ag2S + 2e- 2Ag + S2-

2H2O + 2e- H2(g) + 2OH-


7.5 Discuss the range of chemical procedures which can beused to clean preserve and stabilise artefacts from wrecksand, where possible, provide an example of the use ofeach procedure (NEW!).(Chem. in Brit.1995, 5-7.

Nat. Maritime Museum---> PORT database)

• Some bronze objects (cannons) can be simply washed with cleanwater to remove corrosive salts from sea water.

• Smaller artifacts of bronze, copper brass, lead, pewter and gold arepreserved either by electrolysis or hydrogen reduction as describedbelow for iron.


7.5 (cont’d)• The patina on bronzes consists of a range of compounds of which

malachite CuCO3.Cu(OH)2 and azurite Cu(OH)2.2CuCO3 arecommon constituents.

• “Bronze disease” occurs when deep seated CuCl reacts withmoisture to form e.g. Cu2(OH)3Cl which the breaks through andblisters the patina and causes gradual disintegration of the bronze.


7.5 (cont’d)• Usual treatment of “bronze disease” involves

1. Removal of any surface concretions (carbonates). This is oftendone with citric acid.

2. Hydrolysis of CuCl and Cu2(OH)3Cl with a solution of sodiumcarbonate/sodium bicarbonate to remove the chloride.

3. Impregnation with benzotriazole.


7.5 (cont’d)• As an alternative to the alkaline wash, electrolysis can be used to

liberate the chlorides.

CuCl + e ---> Cu + Cl-

Cu(OH)3Cl + 4e ---> Cu + 3OH- + Cl-

• A recent treatment for bronze disease is to dust the surface withzinc which stabilises the site and prevents further decay. (Znreduces Cu2+ back to metal (?))


7.5 (cont’d)• For iron objects, as much oxidation as possible is removed by

heating the object in a reducing atmosphere of hydrogen.

The hydrogen is derived by cracking liquid ammonia.

2NH3(l) ----> N2(g) + 3H2(g) (Mary Rose)

Fe2O3 + 3H2 ---> 2Fe + 3H2O (800°C)

FeCl2 + H2 ---> Fe + 2HCl (800°C)

Fe2(SO4)3 + 3H2 ---> 2Fe + 3H2SO4 (800°C)


7.5 (cont’d)• Wooden objects, such as the hull of the Mary Rose, are stabilised

by spraying with succesive lots of polyethylene glycol (PEG) ofgradually increasing molecular weight.

peter southwell-keely - unsw chemistry · 20 9.6 shipwrecks and salvage 3.2 distinguish between and...

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