Shipwrecks and Salvages (Chemistry)

4 Essays, Hello Stannies

Process information from secondary sources to outline and analyse the impact of the work of Galvani, Volta, Davy and Faraday in understanding electron transfer reactions.

Electron transfer reactions are reactions that involve the movement of electrons between different elements or atoms. These reactions are what make up oxidation-reduction reactions. Oxidation is the removal of electrons from a substance and reduction is the gain of electrons by a substance. Electron transfer is a key concept of electrolysis and the chemistry of rust. There are four main people who have contributed to our understanding of electron transfer; Luigi Galvani, Alessandro Volta, Humphry Davy and Michael Faraday.

Galvani was an Italian physicist who investigated static charges and their effects on muscles and nerves in the 1780’s. According to; he experimented with static electricity on frogs legs and documented how they contracted when coming into contact with a charge. He made another discovery, that when the spinal chords of frogs were attached to a metal railing by a copper hook, the frog’s muscles would twitch. This led him to further experiments where he used different metals touching together to make the muscles move. He decided that muscles contained a fluid called ‘animal electricity’ which moved between an animals muscles and nerves to make muscles move. He was the first to recognise that there was a movement of electricity that caused muscles to move and this lead the way for further discoveries in electron transfer by his peers.

One of these peers was Volta, also an Italian physicist from about the same time as Galvani. The ‘Chemistry Contexts 2’ textbooks says that Volta disagreed with the conclusion that Galvani had made about his frog experiments, believing that there was no animal electricity generated in tissue, but instead electricity was being transferred from the metals to the muscle. To demonstrate this, he created the Voltaic Pile in the 1800’s, a DC battery made of two alternating metals, separated by cloth soaked in a salt solution. When two wires were attached to the ends of the pile, a spark could be produced when they were touched together. This proved that electricity could be generated without tissue and Volta incorrectly deduced that the two metals touching was the source of the movement of electricity. It was left to other scientists to correct his mistake.

Davy worked with the Voltaic piles created by Volta in his experiments relating to electricity. He constructed the largest battery ever made at the time (the late 1800’s) to conduct his experiments with molten salts. He was able to produce the first samples of potassium, sodium, magnesium and calcium with the battery. He made the correct deduction that the Voltaic pile worked by chemical changes in the electrolytes between the metals and not just the metals themselves. His deductions influenced the work of Faraday.

Faraday worked with Davy as his assistant and was influenced by his work. In the 1830’s he measured the amount of oxygen given off from an electrode and related this to the charge in the electrolytic cell. He formulated two laws for electrolysis, the first being that the amount of displacement is proportional to the current used and the second relating to the masses of solids and gasses formed when electrolytic cells are connected in series. These laws allowed other scientists after Faraday to develop theories about how electricity was distributed and what it was made of. Scientists were able to deduce that electricity moved because of electrons as a result of the work of Faraday and scientists before him.

Identify data, gather and process information from first-hand or secondary sources to trace historical developments in the choice of materials used in the construction of ocean-going vessels with a focus on the materials used.

The materials used in boats has a very long history that only began to change very recently with the advancement of metal extraction technology and increasing knowledge of metallic properties and corrosion.

From very early on in the history of humans, boats have been made. Before technology to extract metals was available, boats were only made out of dug out trees, animal skins and logs as these were the strongest and most seaworthy materials available.

As the technology advanced, so did the materials used in ships. According to the CSU HSC Online website; many early civilisations such as the Egyptians, Greeks, Vikings, Romans and Europeans built boats made of wooden planks fastened together with metals. Wood was still the predominant material in a ship as metal technology was still not efficient enough to make. Each “Age” of metal brought about new tools that the wood for boats could be shaped with. The beginning of the Copper Age 3000 BC, for example, allowed for copper tools that could shape wood in boats. It also allowed different metals to be used to fasten the wood together. The Iron Age from around 750 BC allowed Iron to be used in shaping the wooden planks for a ship.

Different sheets of metals would be fixed to the wood hull such as lead and copper. These sheets were used to prevent the wood rotting from the sea water and in the open air. The hardness of the metals also provided strength and reinforcement for the ship. These sheets could be fixed to the ship using metals for strength as well. Copper was used when it was found that iron fastenings were oxidised easily according to the following equation:

Fe(s) --> Fe2+(aq) + 2e-

With advancements in metallurgy, alloys such as bronze were developed to give more strength to the ship while not corroding as easily.

The Chemistry Contexts 2 Textbook says that wooden ships were the dominant form of sea transport until technology could advance as far as making entire ships out of metal. In 1843 the first ship made entirely of metal was built: the SS Great Britain was made out of wrought iron. This allowed for great strength when travelling in the sea.

Technology was forced to increase at a great rate with the coming of the American Civil War of 1861. In this war, ships had a wooden ‘skeleton’ which was plated in thick iron; designed to withstand heavy gunfire from other ships. Around this time was the first major use of sacrificial anodes to prevent iron corrosion on the ships.

More recent sea vessels are built from iron and iron alloys such as steel that provide more strength and don’t corrode as easily. Iron is used a lot due to the large amounts still available for use which are cheap and easily changed to suit the builders needs.

Gather and process information to identify applications of cathodic protection, and use available evidence to identify the reasons for their use and the chemistry involved.

In a galvanic cell, the cathode is always reduced, meaning it gains electrons. The anode loses electrons and its atoms change into ions, which go into solution. It ‘corrodes’ Changing a metal anode into a cathode to protect it from corrosion is called ‘cathodic protection.’ There are several ways of cathodically protecting a metal. They include the use of sacrificial anodes, impressed currents and galvanisation.

The Chemistry Contexts 2 Textbooks says that sacrificial anodes are blocks of metal that are attached to a less reactive metal, for example, a block of zinc is attached to the outside of a ship’s iron hull. The zinc block slowly corrodes in the sea atmosphere while the iron remains largely unchanged. This is demonstrated in the following equations of the anode and cathode:

Zn(s) -> Zn2+(aq) + 2e-
2H2O(l) -> 4H+ + O2 + 4e-

In this case, the block attached is more reactive than the ship so it becomes the anode. The zinc loses electrons and goes into solution while water is reduced at the iron cathode (there are no iron ions to be reduced). The sea acts and an electrolyte in this situation. Sacrificial anodes are used to stop the corrosion of metal objects such as ships.

Impressed current is the process of running a current (DC) through a metal, electrolyte and another metal to prevent the process of corrosion. The anode to be oxidised is attached to the positive terminal and is usually an inert metal. Water at this anode is oxidised and the electrons are given to the cathode which is attached to the negative terminal. This is the use of an electrolytic cell instead of a galvanic cell reaction as in the sacrificial anode. Inert metals are used for the anode so that it does not have to be replaced. This kind of protection can be used on sea jetties and ships. It is expensive but lasts much longer so is suitable for underground pipes and other objects that are hard to maintain.

Galvanisation involves coating the metal to be protected in a more active, and usually passivating, metal. Usually galvanised iron is steel coated in zinc. When first coated, the zinc reacts with the air and forms a protective zinc carbonate coating. This coating protects the zinc and therefore the steel from further corrosion. If for some reason the coating of zinc on the steel is broken, the zinc will corrode preferentially to the steel, as it is more reactive. It becomes a large sacrificial anode. This kind of protection is useful on roofing and water pipes. The protective coating on the metal increase the lifetime of the metal being used and is much cheaper than the other methods of protection.

Gather information from secondary sources to compare conservation and restoration techniques applied in two Australian maritime archaeological projects.

Restoration and conservation are important in keeping Australia’s maritime history alive and on display. Objects found in the sea after historic events must be kept in good conditions so they can survive for future generations to see.

Captain Cook of the HMB Endeavour ordered canons to be thrown overboard after the ship ran aground. After almost 200 years, the canons were found by researchers. They were heavily concreted with coral. To restore the cannon to its former state, it was soaked in sea water with formalin to kill bacteria residing in it. Concretions were manually removed from the surface and the shaft of the cannon was drilled out. It was placed in a sodium hydroxide solution and made the cathode of an electrolytic cell, removing chlorides from the iron. The solution was regularly replaced over a few weeks. It was then continually washed with a low concentration solution of chromate ions to remove remaining ions and form a protective chromic oxide layer on the iron.

Preservation of the canon involved keeping it dipped in molten wax over several days. This ensured that no air bubbles were trapped with the cannon which would restart corrosion. The wax layer keeps out any outside effects that could deteriorate the state of the canon.

The anchors from the ship Vernon were made in 1839 and have been heavily used since then, but they were not lost at sea. They did not need electrolytic treatment as they were in fairly good condition and it would destroy the timber on the anchors. The anchors were, however, blasted to remove protective paint and then coated in a temporary protective material. Years later they were blasted and then garnet polished. The iron was painted with zinc epoxy and the timber soaked in zinc napthenate to preserve them for viewing outdoors. They are displayed on aluminium mesh that allows water drainage and, in the event of a break in the coating, will corrode preferentially to the anchors.

The two artefacts both come from different contexts and so were treated differently. The canon endured years of concretion and damage from the ocean environment while the anchors were kept in relatively good shape. The canon required delicate but vigourous physical work to remove concretions while the anchors were blasted with sand. Having spent a considerably longer time in the ocean, the canon had to be treated for salt penetration and corrosion. The anchors where cleaned and then preserved straight away as they were in good condition and the damage can tell the story of the anchors better than if they were damage free. Both are regularly checked for abnormalities, the cannon is checked for signs of corrosion everyday and the anchors are checked for corrosion and vandalism as they are displayed outdoors.