option shipwreck

8/12/2019 Option Shipwreck

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Shipwrecks, Corrosion and Conversation1. The chemcial composition of the ocean implies its potential roleas an electrolyte

Origins of minerals: Hydrothermal vents in mid-ocean ridges and leachingby rainwater from terrestial environments .

The impact of the work of Galvani, Volta, Davy and Faraday inunderstanding electron transfer reactions:

Galvani discovered that touching two different metals to the frog muscletissue could produce muscle contractions. He wrongly attributed thecontractions to "animal electric current and this became the basis for the

development of the battery in the name "Galvanic Cell" which electricity isproduced from two different electrodes.

Volta then disproved Galvanis idea by showing that the electric current wasactually produced by two different metals in most contact with each other,not the animal's muscle. This led to Voltas main contribution to science,where he was the first to generate electricity from a chemical reaction andestablished the connection between chemistry and electricity. He developeda primitive battery called a voltaic pile where society today would not haveall its portable electronic devices without the battery. Society still uses theterm voltage to honour Voltas work. His reactivity series was theprecursor of todays standard electrode potential table.

Davy used Voltas invention to perform chemical experiments andrecognized that a chemical reaction was the actual source of electric currentfrom the galvanic cell. He could decompose many compounds and isolatevarious elements and alkali metals such as sodium and potassium using animproved version of the galvanic cell. Thus he showed that there was adefinite connection between chemical bonds and electricity and referred to

his work as "electrochemistry".

Faraday continued Davys work by measuring the amount of electricityduring electrolysis and the amount of substances produced on the electrode.This led to his recognition that there was a quantitative relationship betweenthem, i.e. they were directly proportional. Many terms that we use todaywere invented by him, e.g. anode, cation and electrolysis. His work inelectrochemistry led others to develop a theory of ionic solutions, todetermine that electricity consists of individual charges called electrons, andthat oxidation and reduction were electron-transfer reactions.


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2. Ships have been made of metals or alloys of metals

Passivating metal : Aluminium, zinc, chromium and stainless steel. Theyare reactive metals that form an inactive, protective coating as a result of

reaction with substances such as water or oxygen.

Corrosion: is the degradation of a metal due to oxidation resulting in themetal becoming unsuitable for its intended purpose.

Aluminium is a passivating metal. When aluminium metal oxides in air, athin layer of aluminium oxide forms on the metal surface. This oxide bindsvery strongly to the metal and because this layer is impervious and nonporous to oxygen, further oxidation can not occur. It also prevents moreoxygen attacking the aluminium, so there is no further corrosion:

4Al (s) + 3O 2 (aq) 2Al2O3(s)

Iron is an active metal. It will readily oxidise with oxygen in the air as itmakes contact with the metal surface:4Fe (s) + 3O 2 (aq) 2Fe 2O3(s)

The oxides formed is porous so the oxygen can diffuse through it and keepoxidising the iron metal until most, if not all,of the iron is converted to oxide.

Iron or steel are used to construct ships.

The composition, properties and uses of a range of steels:Steels are alloys of iron and other elements. Steel makers must reduce theC content to 1.5% or less, so that steel is not brittle. This results in simplesteels which contain iron and carbon varying from 0.1-1.5%. As the carboncontent increases, the steel becomes harder, stronger and less ductile.

Pure iron is soft and malleable and corrodes very slowly therefore it is not

used commercially and widely

Mild steel (


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Tool steel (1.5% C) has more carbon content thus it is the strongest andharder than the mild steel. It has the ability to keep a sharpened edge andused for drill bits, high quality knives and tool blades.

Adding various proportions of other elements to iron also produces steelswith quite different properties.

Pig iron (4% C) and 1% Mn & Si in steel-making reduces the undesirableimpurities. However, it is brittle, subject to stress cracks and often fails.Thus it has limited use, such as cooking pots, Veradale rails and gates, etc.

15% Chromium and 10% nickel are added to make a stainless steel (75%Fe) which is hard, very resistant to corrosion, and takes a high polish. It isused in cutlery, medical instruments, sinks, etc.

The conditions under which rusting occurs:Rusting can result from iron being contact with both water and oxygen. Itoccured more quickly in the salt water rather than the tap water, and wasonly seen on the iron and not the steel. This involved oxidation of iron metalto produce Fe(II) ions or Fe(III) ions and the electrons released thenreduced the oxygen molecules:

It is a multi-step process where it involves:Oxidation of iron metal: Fe (s) Fe 2+ (aq) + 2e -

Reduction of dissolved oxygen: O2 (g) + 2H 2 O (l) + 4e - 4OH - (aq)Precipitation of Iron (II) hydroxide:Fe 2+ (aq) + 2OH - (aq) Fe (OH) 2 (s)Further oxidation of Iron (II) hydroxide:4Fe (OH) 2 (s) + O 2 (aq) 2Fe 2O3. H2O + 2H 2 O (l)The product (rust) is a porous, red-brown, flaky material.

2003 HSC

Fishermen face a problem of limiting corrosion of their steel fishhooks. A fisherman has the choice of stroring his steel fish hooks ina plastic, copper or aluminium container.Corrosion of the fish hooks requires oxygen and water. The rate of corrosionwill vary depending on what the container is made of:

Plastic- this is water resistant and will help to prevent corrosion provided onwater is present. The fish hooks however will have some moisture on them,and since there is oxygen available in the box, the oxygen will be reduced toform OH- and the iron will be oxidised to form Fe 2+ . These will form Fe (OH)2 and so some corrosion will occur.


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Copper- if two dissimialr metals are in contact, an electrochemical cell canoccur and corrosion of the more active metal occurs. Iron is more active thancopper, so iron becomes the anode and copper becomes the cathode. Thusoxidaton of iron will occur: Fe (s) Fe 2+ (aq) + 2e - and so corrosion of the fish

hooks will occur.

Aluminium - this is a more active metal than iron, so aluminium will act asthe anode and oxidise while the fish hooks acts as the cathode. This willprotect the fish hooks from corrosion. Therefore the aluminium box will bethe most effective in limiting corrosion.

Conditions Affecting Rusting:The presence of electrolytesIron rusts more in salty water than in fresh water. The presence of sodium

and chloride ions in the water film provides a conducting path between theanodic and cathodic sites on the surface of the iron.

PHAcidic environments promote rusting as hydrogen ions accept electronsreadily, forming hydrogen atoms.

TemperatureIncreasing temperature causes the rate of rusting to increase as collisionsbetween reactants become stronger and more frequent.

Regions of stress in the ironMechanical stresses such as bends or cracks weaken in the iron lattice, sooxidation occurs more easily at these weak points and rusting is accelerated.

Impurities in the ironThe presence of non-active metals impurities such as carbon and silicon canaccelerate rusting by causing internal stresses in the iron metallic lattice.This weakens the structure of the iron so that oxidation occurs readily. Since

steels are impure forms of iron, they tend to corrode more readily than pureiron.

Contact with metals less active than ironWhenever iron is joined to less active metals such as tin or copper, the rateof rusting of the iron is accelerated. The less active metal acts as thecathodic site and the iron as the anodic site in the galvanic cell. These lessactive metals provide large surface areas on which reduction of oxygen canoccur.

Some other metals, e.g. A high percentage of chromium and some nickel


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plus smaller amounts of molybdenum, titanium, or cobalt, can be added tosteel to increase its corrosion resistance.

First-hand investigation1 to compare the rate of corrosion of iron

and an identified form of steel:

Procedure:Steel used: stainless steelEquipment: samples of cast iron and stainless steel, salt water 2.5% (w/w)

1. Place a sample of iron and a sample of stainless that are identical in sizeinto separate identical containers that are open to air.2. Set up anther set of containers that are the same as in step 1.3. To one set of containers, add the same amount of tap water and to the

other set, add the same amount of salt water, so that all the metal samplesare covered.4. Leave all containers to stand for a few weeks- observe them regularly forsigns of corrosion, using a scale from 0-4 to indicate the extent of corrosion,e.g brown color from rust, at each observation.5. Compare the amout of corrosion that has ocurred in each container.6. Repeat the whole experipment several times.

Independent variable: cast iron or stainless steelDepenent variable: rate of corrosion

First-hand investigation2 to compare the rate of corrosion of ironAim: to look at the effects of air, distilled water and salt water on the rate of corrosion, you could do the follwoing:1. Clean 4 identical iron nails with steel wool to remove any corrosion.2. Set up 4 identical test tubes, labelled 1-4, and add a nail to each testtube.3. Cover the nail in test tube 1 with 150ml distilled water.4. Cover the nail in test tube 2 with 150ml distilled water that has been

boiled to remove dissolved gases. Add a layer of oil to the water surface.5. Add about 2 cm anhydrous calcium chloride to test tube 3 to absorb anymoisture. Stopper the test tube with a rubber stopper.6. Cover the nail in test tube 4 with 150ml of a distilled water and saltmixture.7. Obsever the nails each day for 5 days to observe the extent of corrosion,.8. Record the extent of corrosion on a scale of 1-5 (1= none, 5=mostcorosion)

The rusting of iron can be reduced in a marine environment by painting theiron to exclude contact with the air or salt water.


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Magnesium is more reactive than iron, so magnesium will displace iron ionfrom soltuion. The magnesium will be oxidised to magneisum ion and theiron ion will be reduced to iron metal that will form as clumps on themagnesium and fall to the bottom of the beaker:

Oxidation : Mg (s) Mg2+ (g) + 2e -

Reduction Fe2+ (l) + 2e - Fe (s)

The iron nail will also begin rust, and the iron metal will oxidise and theoxygen dissooved in the water will be reduced: (rusting) because of thepresence of water and oxygen.

This can be further oxidised to rust.


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3. Electrolyte cells invovle oxidation-reduction reactions.

Electrolysis is the decomposition of a substance by passing electric currentthrough it. Reactions that occur during electrolysis are NOT spontaneous,

they are forced to occur by applying a voltage.

In electrolytic cells,the cathode is negatively charged. Reduction occurs at the cathode.The anode is positively charged. Oxidation occurs at the anode.

In galvanic cell,the cathode is positively charged. Reduction occurs at the cathode.The anode is negatively charged. Oxidation occurs at the anode.

The two electodes are ususally placed into the same electrolyte andconnected by an external circuit. A source of electricity must be connectedinto the external circuit so a voltage can be applied. Electrolysis may becarried out using inert electrodes or else reactive eletrodes can be used.Inert eletrodues, such as platinum or carbon, conduct the current, but donot react. When using reactive eletrodes, the anode itself may be oxidised.Inert electrodes are used when electrolysis of the actual electrolyte, asgraphite rod are conductors of current but are inert, and so therefore do nottake part in any chemical reactions in an electrolytic cell.

TYPE1 ELECTROLYSIS OF MOLTEN SODIUM CHLORIDE:1. The electrolysis of molten sodium chloride can be carried out in thelaboratory by placing inert electrodes in molten sodium chloride andconnecting them to a power supply.

Oxidation : 2Cl - (l) Cl2 (g) + 2e -

Reduction Na + (l) + e - Na (s)

TYPE2 ELECTROLYSIS OF AQUEOUS solution:1. In the electrolysis of aqueous sodium chloride using inert electrodes

If the concentration of sodium chloride is very lowOxidation : 2H 2O (l) O2(g) + 4H + (l) +4e - OXYGENReduction: Na + (aq) + e - Na (s) METAL CRYSTAL

If the concentration of sodium chloride is very highOxidation : 2Cl - (l) Cl2 (g) + 2e - CHLORINE GASReduction: 2H 2O (l) +2e - H2(g) + 2OH - (aq) HYDROGEN


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2. In the electrolysis of aqueous KBr solution using platium electrodesOxidation : 2Br - (aq) Br2 (aq) + 2e -

Reduction: 2H 2O (l) +2e - H2(g) + 2OH - (aq) HYDROGEN

K is too stable to be reduced, so water is reduced instead.

3. In the electrolysis of aqueous dilute copper sulfate solution using graphiterods. Bubbles of gas from and a coating builds up on the electrodes.Oxidation : 2H 2O (l) O2(g) + 4H + (l) +4e -

Reduction: Cu 2+ (aq) +2e - Cu (s)

3. In the electrolysis of aqueous siver nitrate solution using platinum.Bubbles of gas from and a coating builds up on the electrodes.Oxidation : 2H 2 O (l) O2(g) + 4H + (l) +4e -

Reduction: Ag+ (aq) +e - Ag(s)

4. In the electolysis of a dilute solution of potassium sulfate using graphiteelectrodes.


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+Oxidation : 2H 2O (l) O2(g) + 4H + (l) +4e - turning litmus red-Reduction: 2H 2 O (l) +2e - H2(g) + 2OH - (aq) turning litmus blue

5. In the electrolysis of solution containing nickel ions with inert electrodeand brass key being coated with nickelOxidation : 2H 2 O (l) O2(g) + 4H + (l) +4e -

Reduction: Ni2+ (aq) +2e - Ni(s)

6. In the electrolysis of aqueous solution of potassium chloride.Oxidation : 2Cl - (aq) Cl2 (g) + 2e -

Reduction: 2H 2O (l) +2e - H2(g) + 2OH - (aq)

TYPE3 TWO METALS 1. Iron and Copper:As a spontaneous cell, the iron would oxidise, and the copper(II) ions wouldreduce. As an electrolytic cell, therefore, the relevant half-equations wouldbe:Oxidation: Cu (s) Cu 2+ (aq) +2e -

Reduction: Fe 2+ (aq) +2e - Fe (s)

TYPE 4 REFINING OF METALS:Pure copper is obtained by using an electrlytic cell with impure copper as the

anode and either steel or pure copper as the cathode. The electrolyte iscopper sulfate solution.Oxidation: Cu (s) Cu 2+ (aq) +2e -

Reduction: Cu 2+ (aq) +2e - Cu (s)Impurities fall to the bottom of the cell.Pure copper forms on the cathode.

Factors affecting electrolytic cells:

1. Nature of the electrodes: Reactions can be carried out using inertelectrodes that simply act as conductors or elecrodes may be used that will


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oxidise in preference to the electrolyte.

In the electrolytic refining of copper, blister copper is used as the anode andthe cathode is pure copper. The blister copper electrode oxidised, while

layers of pure copper build up around the cathode. The reactions are:Oxidation: Cu (s) Cu 2+ (aq) +2e -

Reduction: Cu 2+ (aq) +2e - Cu (s)

However, if inert platinum electrodes are used, the anode reaction becomes:Oxidation : 2H 2 O (l) O2(g) + 4H + (l) +4e -

2. Nature of the Electrolyte: Molten electrolytes with inert electrodesyield only one product at each electrode. For example, the electrolysis of magnesium chloride yields magnesium metal and chlorine gas.

Oxidation : 2Cl - (l) Cl2 (g) + 2e -

Reduction Mg2+ (l) + 2e - Mg(s)

However, electrolytes that consist of aqueous solutions may result inwater being reacted at the electrodes in preference to other alternatives. Forexample, an aqueous solution of magnesium chloride would yield hydrogengas at the cathode rather than magnesium.Oxidation : 2H 2O (l) O2(g) + 4H + (l) +4e -

Reduction: 2H 2O (l) +2e - H2(g) + 2OH - (aq)

3. Concentration of the Electrolyte: Standard cell potentials aremeasured using 1 mol electrolyte solution. Varying the electrolyteconcentration can also affect the rate of reaction.

4. Other factors: Other factors such as temperature, the size of theapplied voltage and the area of electrode immersed affect the rate andproducts of an electrolytic cell. Higher voltage, increased surface area of theelectodes used. A decreased distance between the electodes.

First Hand Investigation- The factors that affect the rate of anelectrolysis reaction:Safety Precauion:The electrolysis was done in a fume cupboard to aviod any inhalation of toxicgases and the room was also well-ventilated as an added precaution.Safety goggles were worn to protect the eyes from any splashes of chemicals.Latex gloves were worn to aviod contact of the skin with any chemicals.A lab coat was worn to protect skin and clothing, and correct footwear toprotect feet.


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Only small quantities of the reactants were used to minmise waste.The leftover chemical wastes were stored in labelled bottles in a fumecupboard for appropriate disposal later on by the lab attendant.Glass items may be broken if dropped and cut a person.

Chemicals may be harmful to people and the environment and so need to bedisposed of correctly after the experiment.

Procedures to overcome these risks:Handle all glassware with care. Wear protective gloves when disposing of broken glass. Wrap broken glass thickly with paper to avoid being cut.Wear safety goggles, gloves, covered footwear and protective clothing whenperforming this electrolysis experiment and perform it in a fume cupboardwith a well-ventilated lab to avoid any contact with chemicals.Return all chemicals to the lab assistant for correct disposal.

Procedure:Aim: To test the effect of different concentrations of KI solutions on the rateof electrolysis.

Method:1. A glass of U-tube was set up containing 0.5 mol KI solution.2. Inert graphite electrodes were inserted on both sides into the KI solutionand connected to a 2V power source.3. The rate at which hydrogen gas bubbles formed on the cathode wasobserved over 5 minutes as this was directly related to reaction rate- andwas scored on a scale of 1-54. Steps 1-3 were repeated using 1 mol KI solution, 2 mol, etc5. Other variables were kept constant, e.g. Same electrodes in sameposition, same voltage, same amount of time.

POSSIBLE CONCLUSION:We electrolysed a copper sulfate solution with copper electrodes. As thevoltage increased, we recorded an increase in the current on an ammeter, an

increase in the rate of reaction (seen by the increase in the mass of copperformed per unit time), and sometimes different products were formed.

Conclusion: the higher the voltage, the higher the current and the greaterthe rate of formation of the products, although sometimes it caused adifferent electrolysis process to occur.

Other factors: which affect conductance and hence current include: theconcentration of ions, the surface area of the electrodes, the distancebetween electrodes.


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4.Iron and steel corrode quickly in a marine environment andmust be protected.

Trace historical developments in the choice of materials used in the

construction of ocean-going vessels with a focus on the metal used:

As ocean-going vessels have changed through history, other metals, alloysor combinations of metals were developed and better understood for theconstruction.

All aquatic environments cause metals to corrode as they contain water andoxygen. Oceans are an electrolyte solution and accelerate corrosion morethan fresh water, due to the ions they contain (e.g. Na, Cl, Mg, Ca, K and Br)

From ancient times, wood was the main material used to build vesselsalthough some primitive societies used animal skin and woven goats hair. Byabout 300 BC, the Romans found that copper nails used in wooden ships didnot corrode as much as iron nails and that iron and copper should not beused together as two dissimilar metals would corrode if in contact. By 1700s,the British realized that copper rather than iron sheeting corroded less andso used copper sheeting to prevent bio fouling on ship's hulls.

By the mid-19 th century, vessels were made of sheet iron, and then steel wasused in earlier ships because it has advantages over timber and it corrodedless than iron. It is relatively hard, has good mechanical strength, can berolled into sheets and can press into desired shapes.

In mid-20 th century, steels with lower levels of carbon (0.2%) and sulfurwere used in ship hulls as they were more corrosion resistant and overcamethe problem of brittleness due to carbon. However, it still had small amountsof sulfur and phosphorous which made the steel brittle and subject to stresscracking, so ships were not very durable.

After World War II, steel alloys with Mn were even tougher and so hullsimproved again and structural steel was used inside ships for girders andcolumns as it was stronger. The development of bronze alloys that were veryresistant to corrosion led to their use for bronze bells and propellers on shipsinstead of copper sheeting.

Since the 1950s, aluminum alloys have been used in ships superstructuresand fittings because of their lightness and strength and also their resistanceto corrosion in seawater which allowed for improvements in economy andspeed. Stainless steel has a great corrosion resistance and so is now used inmodern ships for railings, kitchen sinks, etc. However it is not used for ship's


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Uses: ship's hull such as Titanic, underground pipelines

Type 2 Impressed electric current:A small voltage is applied through an electical circuit attached to the steel

structure and to an inert electode. The steel strcture is attached to thenegative terminal of the voltage source. The steel structure is given a supplyof electrons and becomes a cathode. The inert electode acts as an andoe.Water completes the circuit. Water or oxygen is reduced at the cathode andwater is oxidised at the anode.

Advantages/Disadvantages: Costs more to set up but little maintenanceis required. Not polluting. Useful for inaccessible underground pipes andtanks.

Uses: High-tech modern naval vessels have built-in impressed currentsystems.

Cathodic protection only works in a wet environment and not in theatmosphere. It is relatively inexpensive compared to other methods and ismore reliable for preventing the corrosion of iron poles underwater as paintsdo not work well in a wet environment. Anodes get used up so they needmonitoring and replacing. As sacrificial anode can only protect a certainamount of iron, so larger poles need more sacrificial anodes.

Protection from Corrosion in the Marine Environment:

Physical Barriers: keeps oxygen and water away from the iron.TYPE 1 Galvanising:Galvanising invovles coating the iron with a thin layer of zinc alloy. Thecoating acts as a physical barrier preventing oxygen and water fromreaching the iron. Since zinc is a passivating metal, it forms a protective coatof zinc oxide on its own surface when expose to air/oxygen. If a scratch doesoccur and the iron is exposed, the zinc, being a more active metal than the

iron, becomes the sacrificial anode and corrodes preferentially. This protectsthe cathodic iron from rusting: this occurs because zinc is a strongerreductant than iron, so zinc is oxidised in preference to the iron.

Physical barriers: 2Zn (s) + O 2 (aq) 2ZnO (s)Anode: Zn (s) Zn 2+ (AQ) + 2e -

Cathode: Fe 2+ (aq) + 2e - Fe (s)

It is used outside the home on water tanks and fencing and buildingconstruction. Atmosphere!


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TYPE 2 Painting:A water-based polymer paint such as Rustaster Pro would provide aphysical barrier (impervious layer) that neither oxygen nor water couldpenetrate and not contacting the underlying metal, so would be the best

option for coating the iron tower as well as the exposed parts of the ironpylons as this tolerates salts and some acids, remains flexible and resistscorrosion in harsh conditions. However, it is affected by UV and socontinually requires a top coat of paint when used outdoors as on the oil rigto ensure that the protection is maintained. The interlayer prevents themovement of ions so no galvanic cell can be set up. It is effective andenvironmentally friendly. If scratches occur, rusting can occur beneath thepain surface/covering and go undetected.

Corrosion-resistant alloys-SHIPS ONLY Use of alloys: alloy often improve the properties of pure metals.

Bronze alloys are very resistant to corrosion and used for bells andpropellers in ships.

Brass has an attractive lustre and its resistance to corrosion led to manyship fittings.

Stainless steel is expensive but very resistant to corrosion and is used inmodern ships for railings, kitchen sinks, cooking utensils and cutlery.

Surface alloy: was created by bombarding mild steel with chromium andnickel at high temperature. This resulted in a stainless steel-like surface thatacted as an impermeable passivating surface layer (chromium oxide) thatresisted corrosion. The development of surface alloys has led to acost-effective and environmentally friendly method of ship hull protection.This is bound strongly into the surface layers of the ship hulls. Chromiumbound in this way is not affected by chloride ions from the sea water anddoes not pollute the environment.

Corrosion resistant metals:2009 HSC: The owners have decided to replace the guttering.

They have steel screws and a choice of aluminium or copper guttering. Justify the course of action the owners should take,based on chemical principles.

The aluminium would be a better choice than copper because a metal suchas aluminium is a passivating metal, whereas copper is not. Hence thealuminium will form a layer of aluminium oxide on its surface. Thealuminium oxide layer is thin, non-porous and impermeable to water andoxygen. Therefore it will act as a protective barrier to prevent corrosionoccurring. If scratches occur, the aluminium is also more reactive than steel,so the aluminium will act as a sacridicial anode and corrode preferentially to


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the iron in the steel and thus protect the steel screws from rusting:

4Al (s) + 3O 2 (aq) 2Al2O3(s)Anode: Al (s) Al3+ (aq) + 3e -

Cathode: Fe2+

(aq) + 2e-

Fe (s)

First hand investigation-Compare the corrosion rates, in a suitableelectrolyte, of a variety of metals, including names modern alloys toidentify those best suited for use in marine vessels.

Procedure:Place a weighted piece of each metal or alloy such ad mild steel into series of test tubes. Add dilute hydrochloric acid to each test tube. Allow the reactionto continue for the same time (e.g 1 hour). Remove the pieces of remaining

metal with plastic tweezers. Wash and dry the metal pieces. Then reweighthem, the greater the loss of mass, the greater and faster the reaction of corrosion.

Accuracy:Make sure the metal pieces are thoroughly washed with water, andcompletely dry before weighing them.

Allow metals to react for different lengths of time. Some metals may corrodefast initially then slow down or stop, others may take time to start corroding.

Reliability:Use the same concentration hydrochloric acid for each metal sample andmeasure equal volumes of the acid for each test tube.Repeat the experiment several times, to ensure than the results are thesame for the same experimental conditions.


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Aluminium cans are used for drinks, while tin-plate alloy steel cans are used

for packaging foods.

Aluminium cans are protected externally by the formation of an unreactiveoxide layer (Al 2O3 ) and has an internal polymer coating to protect thealumium from corrosion with acids. Alloy steel cans are coated with a layerof tin inside them that corrodes less readily than the alloy steel, as tin is aweaker reductant than the alloy steel. Also, the tin does not react readilywith any food acids nor brine.

Both types of coating act as a physical barrier to help prevent oxygen, water

and salts coming into contact with the metal, thus protecting the can fromcorrosion. However, the polymer coating is inert, so neither oxygen, wateror brine can penetrate the can and so it is more effective in preventingcorrosion.

SHIPS ONLY : A thin layer of a metal, e.g tin, can be used to coat iron/steelto protect against corrosion and will "scarificially" corrode preferentially tothe iron/steel. However, if the tin coating is scratched, rust can start.

First-hand investigation-compare the effectiveness of differentprotections used to coat a metal such as iron and prevent corrosion.


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i) Sodium chloride is the electrolyte solutionii)

Blue: anode reaction at point and head of the nailFe(s)---->Fe2+ + 2e-

Pink: cathode reaction in middle of nail Rusting2H2O+ O2 + 2e-----> 4OH-


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iii) Dish C- Magnesium is a much more active metal than iron and is inelectrical contact with the iron so that a galvanic cell is established. Themagnesium acts as a sacrificial anode- it reduces any irons ions formed anddissolved in preference to the iron- thus the magnesium protect the iron.


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5. When a ship sinks, the rate of decay and corrosion may bedependent on the final depth of the wreck

Temperature decreases with depth

Pressure increases with depthSolubility of gases decreases as temperature increasesSolubility of salts varies with temperature but pressure has little effect As the pressure of the gas above water increases, the solubility of gasesincreases.

First hand investigation- Compare and describe the rate of corrosionof materials in different: oxygen concentrations, temperatures andsalt concentrations.

Procedure:1.Clean 3 identical mild steel nails with steel wool to remove any coating.2.Set up 3 identical beakers, labelled 1-3 and add a nail to each beaker.3.To beaker1, add 150 ml water that has been boiled to remove oxygen.(This water was kept in a sealed jar with no air space until at roomtemperature)4.To beaker2, add 150 ml tap water and an aquarium aerator to add oxygen.5.To beaker 3, add 150 ml tap water.6.Add a layer of oil to the water surface in each beaker.7.Observe the nails each day for 5 days. Record the extent of corrosion on ascale of 1-5, 1=none, 5=most corrosion.

Result:The nail in Beaker2 had the most red-brown rust on it with lots of rustyprecipitate in the water- score 5. The nail in Beaker3 had the second mostred-brown rust and some rusty and some rusty precipitate in the water-4.The nail in beaker1 had very little red-brown rust and no precipitate in thewater- score 2.


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The temperature of water decreases with depth, so the submerged ship is inwater at a lower temperature. The submerged ship is exposed to lessoxygen than the ship wrecked on the island as oxygen solubility decreaseswith depth as the temperature decreases with depth. The rate of a chemicalreaction decreases as the temperature decreases and it also decreases witha lower oxygen concentration. Thus the rate of corrosion is greater at highertemperatures and in a higher concentration of oxygen as found in the shipwreck on the island. High temperature due to tropical land, moisture,oxygen and water from the air.

Predict the rate of corrosion of a metal wreck at great depths in theoceans and give reasons for the prediction made:

Metal objects are corroded by oxidising agents.

Low concentration of dissolved oxygen Dissolved oxygen is theoxidising agent commonly found in the seawater. Oxygen concentration inseawater falls with depth due mainly to less photosynthetic organisms anddissolved oxygen has been used up by aerobic organisms. This would lead toless corrosion as oxygen is necessary for the formation of metal oxides.


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Low temperatureTemperature falls with ocean depth generally and it slower chemicalreactions as the interacting particles have lower kinetic energies. Thedecreasing temperature has affected to the oxygen where its solubilitydecreases and hence a lower oxygen concentration. Thus the rate of corrosion is greater at higher temperatures and in a higher concentration of oxygen. Particles move slower. In contrast, in a higher temperature, the ions

and particles collide more frequently therefore accelerate the rate of corrosion.


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High dissolved salt concentrationHigh salinity levels (NaCl) may promote corrosion but the ocean is already

highly saline and so there should be little difference due to small increases insalt concentrations. It may be higher at some depths. This facilitates anycorrosion reactions that are occuring. Iron rusts more in salty water than infresh water. The presence of sodium and chloride ions in the water filmprovides a conducting path between the anodic and cathodic sites on thesurface of the iron.


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6. Predictions of slow corrosion at great depths wereapparently incorrect.It was previously thought that they only factors responsible for corrosionwere oxygen concentrations, temperature and pH. So earlier theories about

corrosion at great ocean depths postulated that corrosion rates were fairlyslow. This was based on the assumption that there was only a smallconcentration of oxygen, a low temperature and a close to neutral pH atgreat depths. Hence it as thought that wrecks such as the Titanic would bein very good condition however the evidences from the Titanic changedscientists' ideas by showing the predictions of slow corrosion at great depthswere apparently incorrect.

The discovery of the Titanic extensively covered in small rusticles and flakesof rust has changed earlier theories. These rusticles was found to consist of

various iron-rich oxides and several species of anaerobic bacteria. Thecurrent theory is that these anaerobic bacteria cause most of the corrosionby reducing sulfate ions to hydrogen sulfide ions:

Sulfate reducing bacteria microenvironment :

SO 4 2- (aq) +H 2O (l)+ 8e - HS - (aq) +9OH - (aq)They use electrons from the oxidation of iron on sea wrecks to obtain theelectrons for this reaction.

4Fe (s) 4Fe 2+ (aq) +8e -

The Fe 2+ then forms insoluble FeS and Fe(OH) 2 which This results in rust asfingers of reddish-brown growth called rusticles hanging from the wreck.Also, as a result of their chemistry, these bacteria lower the pH of thesea-water in their environment which increases the rate of corrosion. Thus abetter explanation of the corrosion of shipwrecks that lie at great depth hasemerged.

Acidic microenvironment : H + is present at depth due to sulfate reducingbacteria, as well as carbon dioxide reacting with water:

CO 2(aq) +H 2O (l) H2CO 3(aq)

H2CO 3(aq) 2H + (aq) +CO 3 2- (aq)This ares of increased acidity would result in an increase rate of corrosion, asthe acid reacts with non-passivating metals (as these metals do not have aprotective oxide layer), and so can accelerate the formation of rust, e.g fromthe iron on the shipwreck:

Fe (s) +2H + (aq) Fe 2+ (aq) +H 2(g)


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Carbon dioxide levels increase with depth, and up to a limit this decreasespH (increases hydrogen ion). This will increase the extent and rate of corrosion.

First-hand investifation- compare and describe the rate of corrosion of metals in different acidic and neutral solutions.

Procedure: We used 6 different metals (copper, zinc, brass, iron, silver,magnesium) which were all cut into 5mm square sheets. We obtained foursolutions- HCl at pH2, 4 and 6 and distilled water with pH7.

After cleaning with sandpaper, a square of each metal was placed in each of the four solutions. (24-test tubes in all), so that each piece of metal wascovered completely.

The metals were examined immediately, after two hours, after 1 day, andadter 5 days. The extent of corrosion was judged visually-we looked foreither the metal disappearing, or bubbles on the metal surface (hydrogengas), or a powedery appearance (evidence of corrosion) on the metal.

Conclusion: For iron, we found the lower the pH, the faster the corrosion.At pH2, iron started to corrode (bubbles were observed) within a fewminutes. At pH7, evidence of corrosion developed after 5 days.

Magnesium and zinc also followed this pattern. Magnesium, being reactive,corroded quickly at pH2 and 4. Even at pH6, bubbles were evident withinseconds. At pH7, magnesium showed visual evidence of corroion after 1 day.Zinc also corrode fastest at pH2. No corrosion was evident over 5 days atpH7.

Brass, copper and silver however showed no evidence of corrosion over fivedays at any pH (although some people saw evidence of bubbles on the brassat pH2).

While most ships are made of steel, which is mainly iron, there would beother metals in the fittings or in the cargo, as well as many other materials.Our data was only for the 6 metals tested. This data showed that, for iron,magnesium and zinc, the more acidic the environment is , the faster thanrate of corrosion.

Since the brass, copper and silver did not corrode, the data from thesemetals does not support the hypothesis.

Risk: HCl solution is acidic and is harmful if it gets onto the skin or is


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splashed into the eyes.

Why shipwrecks at great depth experience accelerated corrosion?

SUMMARY: Our experimental results show that the rate of corrosion isgreater in more acidic conditions. Shipwrecks at great depths are in acidicmicroenvironments due to sulfate-reducing bacteria that reduce sulfate ionsto sulfide (which is acidic) and various other anaerobic bacteria. Acidicconditions also result from the high levels of carbon dioxide, which arepresent. Such acidic conditions accelerate corrosion.


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7. Salvage, conservation and restoration of objects fromwrecks requires careful planning and understanding of thebehaviour of chemicals

Range of chemical procedures can be used to clean, preserve andstabilise artefacts recovered from shipwrecks:

Copper artefacts (including those made from copper alloys, such as brassand bronze) are recovered as follows:

Mechanical cleaning may be used to remove any encrustations if they arepresent. It is not always needed as copper is poisonous to marine growth.Dental tools are used for this. It should only be carried out if it can be donewithout damaging the artefact.

Chemcial stripping is the next stage. The artefact is submerged in dilutecitric acid for several days, to dissolve any surface carbonate deposits andexpose the bare metal: 2H + (aq) +CO 3 2- (aq) H2 O (l) +CO 2(g) . Exposure tothe air can causes surface chlorides to oxidise to produce acids, so thioureais often added to the citric acid to act as a corrosion inhibitor.Then artefact isthen washed in an alkaline solutions of Na 2CO 3 or just pure water to removeany residual chloride. This effectively stabilises it rather than trying toremove it.

If there are still any signs of corrosion, electrolysis may be necessary. If so,an electrolyte of Na 2 CO 3 solution is used and the artefact is made thecathode, where copper compounds, such as CuO, CuOHCl and CuCl arereduced to copper metal. Chloride ions are released and migrate to theanode where they may be oxidised.

oxidation: 2Cl - (aq) Cl2(g) + 2e -

Reduction: CuCl (s) +e - Cu (s) +Cl -

The artefact must then be washed thoroughly with water to remove anyresidual chemicals, alcohol rinses and dried. It may then be coated with aclear protective acrylic lacquer to help prevent further corrosion.

Both the chemical and electrolytic methods may destroy markings, etchings,etc of interest to archaeologist, as they remove the accumulated corrosionlayers. So care must be taken.


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Removing salt from an artefact: Washing

The use of electrolysis as a means of cleaning and stabilising copperartefacts:

Electrolysis is the decomposition of a substance by passing electric currentthrough it. Two electrodes are immersed in an electrolyte. At the cathode,reduction occurs and at the anode, oxidation occurs. This process is used incleaning and restoring marine artefacts.

Salts can be leached from metal objects however chloride salts areparticularly hard to remove, as they are often present as insoluble hydroxychlorides trapped in the iron oxide deposits and can cause further corrosionif left. Electrolysis may be necessary. If there are still any signs of corrosion,

electrolysis may be necessary. If so, an electrolyte of Na 2CO 3 solution is usedand the artefact is made the cathode, where copper compounds, such asCuO, CuOHCl and CuCl are reduced to copper metal. Chloride ions arereleased and migrate to the anode where they may be oxidised.

Oxidation: 2Cl - jin (aq) Cl2(g) + 2e -

Reduction: CuCl (s) +e - Cu (s) +Cl -

Electrolysis therefore can be used effectively to restore many artefactswithout the risks of scratching or abrading them associated with usingphysical or chemical methods of cleaning. Electrolysis can still destroymarks of interest to archaeologists so care must still be taken before usingit .


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I) Wax is non-polar and reples water. A wax coathing provides an attractivelustre while protecting the object from further corrosion as it provides alayer than is impervious to oxidants such as oxygen and water. Thisprotective layer can later be removed if neccessary. It acts as like a physicalbarrier.

ii) The cannon was placed in an NaOH solution for a week. The NaOH keptthe objects moist, thus preventing the formation of salt crystals, which could

cause damage. These conditions inhibited further corrosion as they helpedan oxide layer to form on the iron (a passivating layer). The NaOH solutionhelped to leach out impregnated salts such as chlorides, the OH - took theplace of Cl - , and the Cl - were then removed by rinising. The sodiumhydroixde solution is replaced regularly to keep the concentation gradient of chloride ions high so that they continue to diffuse out of the cannon. Thisensured that the solutions surrounding them did not become acidic as thiswoud have accelerated corrosion.


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iii) Some salts, e.g chloride salts are hard to remove from fine pores andcracks and can cause futher corrosion if left. Electrolysis can remove thesesalts. The metal object to be restored is made the cathode and a stainlesssteel anode is used with a dilute NaOH solution as an electolyte.

At the cathode: Fe 2+ (aq) +2e - Fe (s) and anions such as Cl - and OH - in thecorrosion compounds on the surface of the artefact are released and migratetowards the anode. Their migration out of the iron and into solution meansthan can be removed.

At the anode, water is oxidised: 2H 2O (l) O2(g) + 4H + (l) +4e -

Electrolysis can therefore be used to restore many artefacts without the riskof scratching or abrading them that is associated with using physical or

chemical methods of cleaning. Electrolysis can still destory marks of interestto archaeglogists, so care must still be taken before using it.

Wooden Chest: the wood in the barrel is porous and will have been

impregnated by salts such as chlorides and sulfates. The timber will befragile due to the missing cellulose that has been consumed by bacteria,fungi and woodworms and replaced by seawater or salts. The metal hoopsand nails would be either iron or copper. Any iron will be heavily rusted dueto oxidation and will be encrusted in marine organisms. Any copper will becorroded with some chlorides and carbonates, but much less corrode thaniron as it is more resistant to oxidation. Encrustations of CaCO3 from marineorganisms will be more limited because copper is toxic to marine organisms.Iron:

Iron (anode) Fe (s) Fe 2+ (aq) +2e -


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Oxygen in the water (cathode): O 2 (g) + 2H 2 O (l) + 4e - 4OH - (aq)

Restoring the chest : conserving wooden artefacts is time consuming andinvolves several procedures. It needs to be washed with cold fresh water to

remove salt, mud and other debris. Then it should be soaked in water toremove soluble ions such as chlorides and sulfates, and the water changedregularly. The salt water and cavities in the wood should then be replaced bysoaking it in an inert wax or oil. E.g polyethylene glycol. Finally, the object isvery slowly air-dried to remove residual water and kept in a cool, humidenvironment. If necessary, it is coated with a higher melting point wax.

If the lock is made of iron, its concretions can be removed firstly with dentaltools and then citric acid. If it can be removed from the chest, electrolysis indilute NaOH can be used to restore the lock by removing chlorides and

reducing oxides. It should then be coated with a clear protective acryliclacquer to help prevent further corrosion.

Copper coins: the concretions from marine encrusting organisms will notbe as heavy as on other metals, as copper ions are poisonous to marineorganisms. These surface layers will contain chlorides of copper and calciumcarbonate.

Lead cannon balls: Lead is poisonous to marine organisms, so there will belittle concretion on the lead balls. Corrosion will result in surface coatings of lead carbonate, oxides, sulfates, sulfides and chlorides.

Restoring: first they need to be immersed into 10% HCl to remove anyconcretions and carbonates and then soaked in EDTA solution to removeinsoluble lead compounds, and then thoroughly washed with water. Anyresidual chlorides can be removed with electrolysis. They should then becoated with a clear protective acrylic lacquer to help prevent furthercorrosion.


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I) The process of electrolysis is used. The metal object to be restored ismade the cathode, the stainless steel electrode acts as an anode and theNaOH solution is the electrolyte. Iron(II) compounds such as FeCl 2 and FeScan be electrolytically reduced at the cathode to Fe(s)

At the cathode: Fe 2+ (aq) +2e - Fe (s) and anions such as Cl - and OH - in thecorrosion compounds on the surface of the artefact are released and migratetowards the anode. Their migration out of the iron and into solution meansthan can be removed.

At the anode, water is oxidised: 2H 2O (l) O2(g) + 4H + (l) +4e -

ii) to become silver-plated, the nickel spoon should be fully immersed in anelectrolyte containing silver ions. The nickel can then be electroplated usingelectrolysis. The nickel spoon needs to be made the cathode and a silverelecetrode used as the anode. The two electordes need to be connected to a

DC power supply.


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Australian maritime archaelogocialThe steam engine from SS Xantho took nearly 20 years to complete, whilethe cannon from HMB Endeavour took less than a year. Initially, the steamengine had an aluminium anode attached to it at the wreck to slow the

rusting before moving it. The cannon did not require this. It was transportedin seawater, then left fo soak in a seawater solution with 10% formalin to killany bacteria for a short time.

After recovery, the steam engine was placed in a tank of NaHCO 3 and NaOHfor 9 years, whereas the cannon was placed in an NaOH solution for a week.The NaOH kept the objects moist, thus preventing the formation of saltcrystals, which could cause damage. These conditions inhibited furthercorrosion as they helped an oxide layer to form on the iron (a passivatinglayer). The NaOH solution helped to leach out impregnated salts such as

chlorides, the OH - took the place of Cl - , and the Cl - were then removed byrinising. This ensured that the solutions surrounding them did not becomeacidic as this woud have accelerated corrosion.

Hammers, chisels and an oxyacetylene flame technique were used toremove encrustations on the steam engine, whereas only hammers wererequired for the cannon and it took a much shorter time to do.

Both the cannon and the steam engine were subjected to electrolyticreduction to futher remove chloride contamination and convert rust back. Astainless stell anode was used with a dilute NaOH solution as an electrolyte.At the cathode: Fe 2+ (aq) +2e - Fe (s) and the Cl and OH were released. Afterelectrolytic treatment, prolonged washing was carried out, with frequentchanges to remove the remaining chloride and hydroxide. At this stage,Xantho and its parts had to undergo further deconcretion, whereas thecannon did not.

Once the surfaces were stabilised, all internal and external surfaces weredried. A coating of wax was then applied to protect both objects from further

oxidation as it provided a layer that was impervious to oxidants such asoxygen and water.

option shipwreck

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