Shipwrecks, corrosion and conservation Flashcards
what is the most common salt in seawater?
NaCl
what is, on avergae, the salnity of sea water?
3.5% - 4.0% w/w
what can sea water be called in a chemical sense?
a massive complex solution of different electrolytes and decayed organic matter resulting from the teeming of life in the sea
what are the two sources of minerals in the ocean?
- leaching by rainwater( and other water courses) from terrestrial environments
- hydrothermal vents ( or mid-ocean ridges)
describe the leeching by rainwater as a source of ions in seawater
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
what effect does acidic solutions have on the leaching of minerals in soils and what causes this acidic solution?
acidic solutions hasten the process.
acidic solutions can form from decaying organic matter and and acidic rain
what does the composition of the ions found in rainwater depend upon?
it depends on the composition or structure of the landmass over which it flows
what examples of ions enter the sea through the leaching by rainwater?
sodium, potassium, calcium, sulfate, hydrogen carbonates and silicates just to name a few
what is a mid ocean ridge?
an underwater mountain ridge which forms where two tectonic plates meet underneath the earths crust
what happens at a mid-ocean ridges?
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
what happens to some ions as they come into contact with the cold water?
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
who was luigi Galvani?
he was a professor of anatomy at Bologna
what experiments did Galvani perform that contributed to increasing understanding of electron transfer reactions and what were his results?
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
from his experiment with the frogs legs, what did Galvani incorrectly conclude?
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
what was Galvani unaware of in his experiments?
that two different metals in contact could generate an electric current
what was significant about galvani’s work?
was the first to consider this field of science, inspiring other individuals including Volta etc to further investigate the nature of this animal electricity
who was Alessandro Volta?
a professor of physics at the university of Pavia
how did Volta lead on with galvani’s work regarding electron transfer reactions?
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.
what was significant about volts work with building the first battery?
- 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
what did Volta incorrectly think in his understanding of electron transfer reactions?
Volta incorrectly thought it was the contacts between the two metals that produced an electric current
how did sir humphrey davy contribute to our understanding of electron transfer reactions?
-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
how did michael faraday contribute to electron transfer reactions?
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.
what was faradays major contributions to electron transfer reactions?
the devised that measurements could be made to predict the amount of substance produced or dissolved and link that to the quantity of electricity
what is the definition of corrosion?
the oxidation of mtals and alloys due to chemicals in the environment which causes mass deterioration fo materials
what is the definition of an active metal?
a reactive meal that readily oxidies in air through a spontaneous electrochemical reaction releasing energy
what speeds up corrosion?
industrial air pollution, including acidic gases such as sulfur dioxide greatly speeds up corrosion
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
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
what is a passivating metal?
reactive meals that spontaneously form a strongly adhereing coating that protects the metal from any further corrosion.
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
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
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
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
what sucks about iron used in shipbuilding?
its high densitiy and vulnerability to corrosion limit its use in shipbuilding, considering water is nearby
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.