Y12 HSC Chemistry Shipwrecks and Corrosion R. Slider Protection of Steel Y12 HSC Chemistry Shipwrecks and Corrosion R. Slider

Steel pits forming rust Tin Plating A tin can is made of steel with a thin layer of tin plated on the surfaces. What happens if the tin coating is scratched to expose the steel? Refer to the standard reductions table. Answer: Tin (-0.14 V) is less reactive than iron (-0.44V), so the iron will preferentially oxidise and rust away. In fact, the presence of the tin accelerates this process. 2H2O+O2+4e-  4OH- tin steel Electrolyte film Steel pits forming rust Fe  Fe2+ + 2e- electrons

Steel covered by layer of Zn(OH)2 Galvanising with zinc Galvanised steel is produced by dipping steel into molten zinc forming a chemical barrier. This is commonly used in roofing materials and boating, especially trailers What happens when the galvanised surface is scratched, exposing the steel beneath? 2H2O+O2+4e-  4OH- zinc steel Electrolyte film Steel covered by layer of Zn(OH)2 Zn  Zn2+ + 2e- electrons Answer: Zn is more reactive and forms Zn ions which react with the hydroxide to form zinc hydroxide. This product loses water to form a protective coating of zinc oxide over the exposed steel. (ZnCO3 can also be formed with rxn of CO2.)

Protection of ship hulls Historical protection Pb or Cu sheets over wooden ships Tar or other petroleum products as coatings Varnishes from plants or oils More recent protections Corrosion resistant metals/alloys Paints that provide protection by: Prevention of exposure to electrolytes, oxygen or water Containing more active metals such as zinc Release biocides such as Cu or Sn which inhibit marine organism growth (however this can cause environmental concerns) Cathodic protection (e.g. sacrificial anodes)

Why rusting? Corrosion occurs due to localised anode and cathode sites within a metal. These sites often form due to slight differences in properties of the surface of a metal such as steel. Cathodic sites can be created due to cracks or small impurities such as carbon atoms. Steel structure - + Current flow (e-) through electrolyte Corrosion pit Anode Cathode A ship in seawater is immersed in an electrolyte. Anodic and Cathodic sites are formed: Anode: Fe  Fe2+ + 2e- Cathode: 02 + 2H2O + 4e-  4OH- Recall, rust is formed by the reaction of iron ions with hydroxide (see previous notes)

Cathodic Protection There are two basic types of cathodic protection: Sacrificial Anodes Impressed Current

Sacrificial Anodes A more active metal is placed in electrical contact with the metal structure to be protected. The more active metal becomes the anode (corrodes) The structure is protected by becoming the cathode by gaining electrons. For iron: Fe2+ +2e-  Fe The sacrificial anode must have the right shape and surface area to discharge enough current to protect the structure and enough weight to last the desired lifetime when discharging this current. The example to the left shows Al being used as a sacrificial anode (providing e-). Note it’s position on the Standard Reduction Potentials Table relative to iron. Useful metals are: Aluminium Zinc Magnesium

Impressed Current The protected structure is again turned into the cathode, but this time by a low voltage DC source. The structure is connected to the negative terminal of the power supply. The anode is usually inert and discharges the impressed current (e.g. Pt) The circuit is completed by the electrolyte; in the case of ships, the electrolyte is sea water. In the case of sea water, the oxidation reaction will be: 2Cl-  Cl2 + 2e-

Questions A steel hot water tank contains a long Mg rod as a sacrificial anode. Write a half reaction for the oxidation Write a half reaction for the reduction The negative terminal of a car battery is connected by a thick conductor to the metal frame. When the ignition is turned on, electrons flow from the negative terminal through the car frame, to the starter motor and back to the positive terminal of the battery. The voltage is at too low a voltage to enter a person’s body. Why does the electron flow from the battery through the car body go back to the battery instead of going to the Earth? Electrons flow in an external circuit from the negative terminal to the positive through the car. What is flowing inside the battery? Explain how negative grounding rather than positive grounding (+ve terminal connected to the body of the car) may reduce the corrosion of the metal car body.

Answers a)Mg  Mg2+ + 2e- b) 02 + 2H2O + 4e-  4OH- a) Rubber tyres prevent electron flow to Earth. The low voltage (12V) is not high enough to force current through a persons body to the Earth when they touch the car and the Earth. b) Anions and cations flow in the electrolyte solution of the battery c) Negative grounding supplies e- directly to the metal car body. These e- could reduce any newly formed iron ions back to iron atoms, reducing the corrosion of the car. Positive grounding would have the opposite effect and increase the level of corrosion