Shipwrecks, corrosion and conservation Flashcards
1
Q
what is the most common salt in seawater?
A
NaCl
2
Q
what is, on avergae, the salnity of sea water?
A
3.5% - 4.0% w/w
3
Q
what can sea water be called in a chemical sense?
A
a massive complex solution of different electrolytes and decayed organic matter resulting from the teeming of life in the sea
4
Q
what are the two sources of minerals in the ocean?
A
- leaching by rainwater( and other water courses) from terrestrial environments
- hydrothermal vents ( or mid-ocean ridges)
5
Q
describe the leeching by rainwater as a source of ions in seawater
A
the earth contains a significant amount of minerals and salts which are soluble in soils, rocks and other land masses
rivers, streams and underground water courses emptying into seas and oceans penetrate rock and soils on its way, leaching out the minerals contained in these land masses and moving them to the sea
6
Q
what effect does acidic solutions have on the leaching of minerals in soils and what causes this acidic solution?
A
acidic solutions hasten the process.
acidic solutions can form from decaying organic matter and and acidic rain
7
Q
what does the composition of the ions found in rainwater depend upon?
A
it depends on the composition or structure of the landmass over which it flows
8
Q
what examples of ions enter the sea through the leaching by rainwater?
A
sodium, potassium, calcium, sulfate, hydrogen carbonates and silicates just to name a few
9
Q
what is a mid ocean ridge?
A
an underwater mountain ridge which forms where two tectonic plates meet underneath the earths crust
10
Q
what happens at a mid-ocean ridges?
A
these ridges spread apart and moles material cools. as the rock cools, it cracks, allowing sea water to sea and fill these cracks where the temperature is roughly around 350-400 degrees Celsius and is also under great pressure
the water is then forced back into the ocean at high temp and pressure out of other cracks in the rock
this superheated water dissolves the surrounding minerals from the rock via leaching, rising into the ocean and forming hydrothermal vents
11
Q
what happens to some ions as they come into contact with the cold water?
A
these vents are sources of sulphides of copper, zinc and iron which form insoluble deposits on the ocean floor when in contact with the cold water
12
Q
who was luigi Galvani?
A
he was a professor of anatomy at Bologna
13
Q
what experiments did Galvani perform that contributed to increasing understanding of electron transfer reactions and what were his results?
A
performed experiments where freshly killed frogs legs would become in contact with the ends of two dissimilar metals
Galvani observed the legs contract when placed in a circuit with the two plates
14
Q
from his experiment with the frogs legs, what did Galvani incorrectly conclude?
A
he incorrectly stated that the frog contained some sort of animal electricity which had been causing the twitching. he regarded their activation as generated by an electrical fluid that is carried to the muscles by the nerves
15
Q
what was Galvani unaware of in his experiments?
A
that two different metals in contact could generate an electric current
16
Q
what was significant about galvani’s work?
A
was the first to consider this field of science, inspiring other individuals including Volta etc to further investigate the nature of this animal electricity
17
Q
who was Alessandro Volta?
A
a professor of physics at the university of Pavia
18
Q
how did Volta lead on with galvani’s work regarding electron transfer reactions?
A
he showed that it was not muscles but the two different metals that generated the electric current in his frogs legs
he was so stimulated that he developed the voltaic pile. this is a pile of alternating discs of two different metals. between the discs are pieces of cardboard soaked in brine. wires are connected to the top and bottom plates.
19
Q
what was significant about volts work with building the first battery?
A
- previously, electricity was generated using electrostatic machines but, for the first time, a continuous and reliable source of electricity was available
- provided the world with a different way of producing electricity that was entirely portable
- this first battery was demonstrated to the institut de France in Napoleons presence. it caused considerable excitement throughout Europe and soon the voltaic pile made its way into all scientific labs. spread like wild fire
20
Q
what did Volta incorrectly think in his understanding of electron transfer reactions?
A
Volta incorrectly thought it was the contacts between the two metals that produced an electric current
21
Q
how did sir humphrey davy contribute to our understanding of electron transfer reactions?
A
-by performing many experiments with volta’s voltaic pile, he was correcly able to recongise that the electric curent from the pile was due to a chemical reaction and not the contacts between the two dissimilar metals
-he further realised that these chemical reactions forming the electric current were decomposition reactions
he used electrolysis to decompose water into its two elements, hydrogen and oxygen, thereby showing water is a compound
- by using electrolysis Davy was the first person to prepare sampels of potassium and sodium metals. he passed electricity through molten salts and made samples of caclium, magnesium, strontium and barium
- humphrey invented a process to desalinate water and was able to show how ships could be protected from corrsion by attaching zinc plates to them to act as sacrificial anodes
22
Q
how did michael faraday contribute to electron transfer reactions?
A
faraday was davy’s assistant, and extended much of his ideas:
- faraday carefully measured the amount of electricity necessary during electrolysis and compared it to the amount of substance produced. this allowec him to propose two major laws of electrolysis
1) the amount of substance produced or dissolved at an electrode during electrolysis depends on the quantity of electricity passing through the electrolyte
2) the amount of substance, in moles, produced or dissolved at an electrode by passing the same amount of electriicty are ubversely proportional to the charges on their ions.
23
Q
what was faradays major contributions to electron transfer reactions?
A
the devised that measurements could be made to predict the amount of substance produced or dissolved and link that to the quantity of electricity
24
Q
what is the definition of corrosion?
A
the oxidation of mtals and alloys due to chemicals in the environment which causes mass deterioration fo materials
25
what is the definition of an active metal?
a reactive meal that readily oxidies in air through a spontaneous electrochemical reaction releasing energy
26
what speeds up corrosion?
industrial air pollution, including acidic gases such as sulfur dioxide greatly speeds up corrosion
27
what happens when active metals corrode?
when active metals corrode, they form a non-adherent, flaky, porous oxide layer. this exposes fresh metal underneath the oxide layer allowing the corrosion process to be repeated
28
what is an example of an active metal that forms a non-adherent oxide layer?
iron is an active metal that forms an a layer of hydrated iron(III) oxide which provides litle protection and allows for rapid corrosion
29
what is a passivating metal?
reactive meals that spontaneously form a strongly adhereing coating that protects the metal from any further corrosion.
30
whats special about a passivating metals oxide layer?
this coating, often an oxide, is impermeable to oxygen or water, thus preventing the metal from reacting with any of these substances and corroding
31
what is an example of a passivating metal?
aluminium is a more reactive metal than iron, yet iron resuts away while aluminium does not.
the aluminium oxide layer adheres to the surface as a non porous, inert layer preventing further oxidation. if this layer is removed, the layer quickly reforms to prevent any further corrosion
32
what material was used before steel and iron in shipbuilding and when was it replaced?
towards the end of the 19th century wood gave way to iron and steel in ship building. technology had by then, progressed to make steel readily available at competitive prices. it was first produced in the 19th century and steel production is now one of the chief world industries
33
what properties of iron and steel make them useful in shipbuilding?
- good mechanical strength
- relatvively hard
- it can be rolled into sheets and pressed into desired shapes
- welding is also an important feature that has allows it to become so versatile, not only for ship building
34
what sucks about iron used in shipbuilding?
its high densitiy and vulnerability to corrosion limit its use in shipbuilding, considering water is nearby
35
what conditions promote the rusting of iron?
- acidic conditions promote the dissolution of iron
- electrolyes in the water assist in the process by increasing the conductivity of the solution. this provides a conduting path between the anode and cathode sites
- impurities in the iron assist in the process by initiating ceel action. an electrochemical cell is set up where iron becomes the anode and is oxideised, while the other metal (or carbon or silicon) becomes the cathode, where oxygen is reduced in the film of water
- stresses in the iron lattice weaken it so that the iron atoms are not strongly held. these atoms form ions more readily. stresses occur where nails or wire are bent or at tips and shapr edges
- contact with less active metals, such as copper or tin, provides surfacews where oxygen reduction can occur. the electrons are conducted from the oxidising iron, as this metal is moreactive than those it is in contact with
- when iron undergoes oxidation, parts of the iron surface that sre most exposed to oxygen beomce cathodic and so are protected from corrosion. areas of limited oxygen contact become anodic and so corrode.vthis differential aeration principle explains why riveted steel plates corrode in crevices between the plates and not on the surface.
36
whats the first step in the rusting of iron?
iron is oxidised
Fe --> Fe^+2 +2e
37
what is the second step in the rusting of iron?
oxygen dissolves in water and is reduced:
O2 + 2H2O + 4e --> 4OH^-
38
what is the third step in the rusting of iron?
iron(II) hydroxide precipitates: Fe^+2 + 2OH --> Fe(OH)2
39
what is the fourth and final step in the rusting of iron?
the iron(II) hydroxide oxidises to hydrated iron(III) oxide
Fe(OH)2 + O2 --> 2Fe2O3.H2O
40
What is the overal reaction of the rusting of iron?
4Fe + 3O2 +2H2O --> 2Fe2O3.H2O
41
what are the ways in which a metal hull may be protected?
- using corrosion resistant metals
- development of surface alloys
- new paints
42
what are carbon steels and what are the four main types?
alloys of iron with up to 1.7% Carbon and small amounts of Maganese, phosphorous, sulfur and silicon
low carbon steels
medium carbon steel
high carbon steel
cast iron
43
whats the composition of low carbon steels, what are its properties and uses?
0. 07-0.25% C
features: inexpensive, soft, malleable, easily cold worked
uses: vehicle bodies, sheet roofing and nails
44
whats the composition of medium carbon steels, what are its properties and uses?
0. 25-0.6% C
features: wear resistant, hard and strong. amnedable to welding and tooling, structural uses
uses: rails and rail products, gears, crankshafts, reinforcing, axles
45
whats the composition of high carbon steels, what are its features and uses?
0. 85-1.2% C
features: very hard and brittle, not very malleable, strong, wear resistant
uses: cutting tools, railway lines, drill bits, knives, hammers, axes
46
whats the composition of cast iron steels, what are its features and what are its uses?
2. 5-3.8 C , 1.1 Si and 1 Mn
features: easy to cast but brittle, hard
uses: pistons, cylinders, anchors, chains
47
the hardness and tensile strength of steel increases as....
the carbon content increases, up to 0.85%
48
the ductility and malleability of steel decreases with....
increasing carbon content
49
why are paints used to protect from corrosion?
because they place a physical barrier between the material and oxygen and water
50
what are some examlples of paints used to protect materials from corrosion?
organic paints
| polyurethans and epoxy resins
51
what are flaws in using paints for protection?
overtime, eventual flaws develop in the paint that allow the ravaes of rusting to take place
52
why are materials protected from corrosion using other metals?
because it also produces a physical barrier
for example, tin cans are really steel cans with a thin tin coating. the tin oxidies to form a protective and stable oxide layer. as long as the tin layer is not stratched the steel will be protected
53
what are sacrficial anodes?
sacrifical anodes are more active metals that are coated on materials to stop them from corroding.
for example, galvanised iron is iron with a layer of zinc, zinc oxidises in prefernce to the iron, thereby acting as a sacrificial anode. the zinc oxide reacts with water and cabonate in the environment to form an impervious layer of zinc carbonate zinc hydroxide
54
how are surface alloys made and how do they protect from corrosion?
typically, chromium or nickel ions bombard a steel surface at high temps. lazers or focussed electron beams rapidly melt this suface, allowing the ions to bond to the steel and forming a harder and more corrosion resistant surface.
the steel surface then forms a passivaintg layer as the chromium or nickel reacts with oxygen
55
what are the pros of surface alloys?
they are environmentally friendly and cost-effective method of protecting metals such as steel from corrosion
56
what new water based polymer paint have chemists made and how does it work?
rustmaster pro, the paint cures in air forming an impenetrable barrier to both air and water vapour
the paint forms and interlayer between the metal being protected and the polymer coating, this interlayer grows into the neighbouring polymer layer preventing the curcial movement of ions between the anodic and cathodic areas on the surface of the metal and so rusting cannot occur
only releases only about 1% of volatile solvents into the environment compared to conventional paints
57
what is cathodic protection?
The protection of a metal from corrosion by making it the cathode in a galvanic cell.
58
what is a sacrificial anode?
A metal that oxidises preferentially to protect another metal from corrosion, thus providing cathodic protection.
59
what happens when an artefact is dried and it has just been in a solution containing dissolved chlorides and sulfates and why is this bad?
because an artefact is impregnated with salt solutions, these solutions will become more concentrated as it dries out. eventually a saturate solution is produced and any further evaporation will cause salt crystals to form. these crystals can form anywhere within the artfect and grow larger with a slower evap rate. the salt crystals can damage the artefact by placing pressure on the structure, enlarging cracks and pushing it out of shape.
sometimes concentrated salts can initiate chemical reactions with the artefact thereby further damaging it.
salts in metals objects can hasten corrosion
60
define conservation
the minmum treatment needed to stabalise an object amd tp prevent is suffering further damage
61
define restoration
involves conservatin treatment plus the repair or replacement of missing or damaged sections
62
whats the difference between preventive and interventive conservation?
preventive involves involving altering the environment to reduce the factors which contribute to the degrdation of objects
interventive involves cleaning, removal of corrosion and application of a protective coating
63
how is electrolysis used to restore iron artefacts?
artefact is made the cathode while stainless silver mesh is made the anode connected to a power supply
hydroxide ions at the anode is oxidised to form water, oxygen and 4 electrons
at the cathode, corrosion products such as insoluble iron hydroxide are reduced and form solid iron and hydroxide ions. hydroxide ions then migrate to the anode
64
what is desalination and how does it work?
the process through which salts are removed from an artefact. the artefact is placed into a dilute alkaline solution that is regularly changed over extended periods of time. the solution may consist of NaOH, Na2CO3 OR NaHCO3.
over time the anions diffuse into the artefact replacing the chloride ions, which diffuse out of the object. the solution is regarly replaced as salts leach out as to maintain a steep concentration gradient for diffusion
65
for iron artefacts, why are they soaked in alkaline solutions during preservation?
because iron corrodes quicker in acidic solutions, so it only seems right to slow the corrosion process
66
describe the treatment for the vernon anchors outside the marintime museum in terms of:
```
physical treatment
electrolytic treatment
final treatment
display
maintaining
```
physical treatment:
iron surface blasted with copper slag to remove the outer corrosion products and the remains of protective paint.
then garnet polished, this revealed previously unnoticed inscriptions on the anchor
electrolytic treatment:
not chosen for two reasons
1. such treatment would destroy the timber
stocks or require their removal which
would cause unnecessary damage
2. It was considered that the anchors ere
in sufficiently good condition not to
require electrolytic treatment to
preserve the wrought iron
Final Treatment:
coated with a zinc epoxy paint, timber stocks are submerged into a saturated solution of zinc napthetate
Display:
the anchors are placed outside in the elements exposed to rain, wind, hail, the sun etc
placed on a mounting system made from aluminium mesh, not solid metal chosen as to allow water to drain away from the anchors. aluminium chosen based on its position of the activity series relative to the iron acting as a sacrificial anode
Maintaining:
regularly inspected for deterioration and damage from vandals
also hosed down with freshwayer to reduce salt build up which occurs close to the sea, and of course salt build up is bad based on structural damage
67
describe the treatment of the endeavour cannon which was captain cooks cannon that he dropped from his ship to reduce weight in terms of:
```
physical treatment
electrolytic treatment
final treatment
display
maintaining
```
physical treatment:
- first placed in a solution of sea water with 10% formaline to kill any bacteria present
- hammers were used to remove hard coral from the surface of the cannon
- core drilling of the cannon bores recovered wadding, cannon balls and teh residue of powder charge from some of the cannon
electrolytic treatment:
- first placed in a 2% sodium hydroxide solution for a week during which time gas evolved from the surface which was found to be 80% hydrogen
- using the cast iron as the cathode and mild steel anodes( including a bar which inserted into the cannon bore for good current distribution), electrolytic treatment in 2% sodium hydroxide solution was maintained for many weeks. current density kept at around 10 amps per square metre. baths were changed regularly and the cannon washed reguarly with freshwater at each change. this continued until the concentration of chloride ions was 20ppm
Final treatment:
- prolonged washing as carried out to remove the remaining chloride and hydroxide
- then immersed in molten crystalline wax which was kept at 135 degrees for 5 days until no gas bubbles were evident. this is reversible
- allowing the wax to cool to just above melting point (80 degrees) before remving the cannon from the wax ensured maximum penetration of wax
display and monitoring:
- one of the cannons is found indoors in the australian national marintime musuem, and therefore it is not exposed to the elemtens. hence monitoring isnt really important
68
whats does a conservator have to know about an artefact before starting any treatment?
knowing what metal is in question
69
whats another important consideration conservators must make when treating artefacts?
give an example
although one conservation treatment may effectively stabilize one type of metal it may damage another
for example, 2% sodium hydroxide solution is used to draw out Cl- ions for iron
but aluminium artefacts would be eaten away via the alkaline conditions
70
what can the corrosion products present on an artefact tell a conservator? give an example
whether or not preventive or interventive treatment is required for the artefact
for example, if the conservator found carbonates, preventive treatment could be used
however, if a copper chloride corrosion layer was found the conservator would be concerned about the presence of these highly reactive corrosion products and would use interventive treatments such as desalination, mechanical removal of the corrosion products
71
what are corrosion inhibitors?
A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy.
72
what is an electrolytic cell?
an electrochemical cell which undergoes a redox reaction when electrical energy is applied
73
what are the usual parts of an electrolytic cell?
- DC power source
- electrical wiring
- two electrodes
- an electrolyte solution
74
in an electrolytic cell, the cathode is connected to the....
negative terminal of the power supply
75
in an electrolytic cell, the anode is connected to the....
positive terminal of the power supply
76
whats the process in predicting reactions which occur during electrolysis?
for the cathode:
1) identify the oxidants that can be reduced
2) in the table of standard electrode potentials locate the reduction potentials of the possible oxidants
3) choose the oxidant with the most positive reduction potential. this species will undergo reduction
4) write the reduction half equation and reduction potential
for the anode:
1) identify the reducants that can be oxidised
2) in the table of standard electrode potentials locate the oxidation potentials of the possible reductants
3) choose the reductant with the most posivie oxidation potential. this species will be oxidised
4) write the oxidation half equation and oxidation potential
77
what factors affect the rate of electrolysis and how?
- applied potential difference( higher v = higher rate)
- electrode seperation ( closer = faster )
- surface area of the electrodes (more area = faster )
- conc of the electrolyte (higher conc = faster )
78
for gases, how does solublity in water change with temp?
increased temp means less solubility
79
for gases, how does pressure affect solubility?
increased pressure gives us more solubiltiy
80
for salts. how does solubility change with temp?
decreased temp gives us greater solubilty
81
for salts, how does pressure affect solubility?
pressure has no affect for salts
82
so, theoretically, oxygen levels as you go deeper into the ocean should...
increase, but this is not the case
83
what factors affect the levels of DO at depth?
diffusion of oxygen from air into the water at the surface takes years to get to near the ocean floor
- although oxygen is produced by photosynthetic oraganism such as phytoplankton, this only occurs near th top 100m as light cannot penetrate deeply enough
- oxygen is consumed by other organims in the water such as fish and anearobic bacteria
- at depth greater than 1000m, there is an increase in DO due to deep ocean currents which bring cold oxygen rich water from the polar regions into the bottom layers of the ocean water
84
so, theoretically, how would corrosion be affected when at great depths?
corrosion at depth would be expected to occur very slowly due to
1) the rate of reaction being slowed down based on low temps through the collision theory
2) low DO levels
REMEBER: corrosion flourishes in conditions of high oxygen concentration and concentrated elecroylte solution
85
if corrosion at depth would be expected to be very slow, than why was their great corrosion of the titanic at depth?
through bacterial corrosion, due to the action of sulfate reducing bacteria as they release energy from food by reducing sulfate into sulfide
86
what does bacterial corrosion form through the action of sulfate reducing bacteria?
produces reddish-brown corrosion products known as rusticles. these often contain a mix of oxides and hydroxides, as well as traces of bacteria and calcium carbonate
87
trace the historical development of different materials used in the construction of ships, focusing on the materials used
- Metals were first used in ship-building by Vikings on their long boats. They used wrought iron and tough, durable alloy bronze.
- Some early ships used lead sheathing but was replaced with copper as it protected wood from attack by marine boring worms.
-Around 1500 AD, iron nails were developed to connect wooden planks together. Allowing for larger boats with stronger hulls.
By the early 1800s, Iron was widely available and ships were built of wooden planks over iron frames.
- By the late 1800s, most ships were made completely of iron. Although iron rusts readily, these early iron ships needed to be dry docked, to have rust removed. Iron ships could be build longer, safer and with more room inside.
- Warships began to be constructed of the iron alloy steel as it was stronger, lighter and could withstand shells and cannonballs fired at them.
- 20th century – progressive improvements in steel alloys to incorporate aluminium, chromium, titanium, zinc and nickel.
- Aluminium – not alloyed with iron in ships, used in yachts and dinghies due to its lightness and strength allowing for improvements in economy and speed.
- Chromium – used in surface alloying. A laser is used to bind a protective chromium coat to surface layers of steel ships giving them a great deal of corrosion protection to the steel hull.
- Zinc – used as sacrificial anodes on the surface of large ships, oxidises preferentially to iron, protecting iron from corroding.
- Stainless Steel – contains 20% chromium and 10% nickel, used in modern ships as railings and in the kitchen sinks and dining utensils, as it is corrosion resistant, hard and shiny.
- Brass – consists of 65% copper and 35% zinc, used to make ship’s fittings like door knobs and light fittings, as it resists corrosion and polishes well.
- Bronze – consists of 90% copper and 10% tin, used to construct ship’s bells and anchors due to its hardness and resistance to wear and is easily casted.
88
how is electrolysis used to restore copper artefacts?
surface layers of insoluble copper (I) chloride and copper (II) hydroxychloride, (Cu(OH)Cl form on copper artefacts as a result of action of marine organisms
copper and copper alloys are usually cleaned by chemical stripping, dissolving surface deposits with a chemical solution:
-5-10% Citric acid containing 1-2% thiourea as a corrosion inhibiter.
-Object is submerged for several days, allowing surface objects to be dissolved and exposing bare metal.
-Such stripped objects still have residual chloride, thus is soaked in sodium hydrogen carbonate solution or pure water.
-Electrolysis can also be used to remove chloride.
89
how is electrolysis used to restore lead artefacts?
It is poisonous thus very little concretion grows on lead objects.
Some chloride penetrates into micro-pores in the lead
- Restored using dilute HCl removing carbonate concretions.
- Soaked in EDTA solution buffered at about pH 10 – 11. Dissolves insoluble lead compounds as it forms a strong complex with Pb2+ ions.
- Leaching with water or electrolysis is used to remove residual chloride
- If ELECTROLYSIS is used, sodium carbonate solution is used as lead dissolves in hydroxide solution
- Then Coated with a suitable lacquer or wax preventing further corrosion.
