REACTOR CHEMISTRY AND CORROSION Section 1: Introduction to Corrosion By D.H. Lister & W.G. Cook Department of Chemical Engineering University of New Brunswick

Did you know? Internationally, 1 tonne of steel turns into rust every 90 seconds. The energy required to make 1 tonne of steel is approximately equal to the energy an average family consumes over three months. Of every tonne of steel from the world’s production, approximately 50% is required to replace rusted steel. Source: Zinc Today, newsletter of the American Zinc Association (Spring 1997)

What is corrosion? Usually, it is the reaction of a metal with its environment: e.g. rusting of iron: 4 Fe + 3 O2 + 2 H2O → 4 FeOOH (lepidocrocite) BUT: at high temperature the corrosion of iron can produce: 3 Fe + 4 H2O → Fe3O4 (magnetite) + 4 H2 Sometimes, non-metals are said to corrode: e.g. degradation of graphite moderator in a CO2 cooled nuclear reactor C + CO2 → 2 CO (may be inhibited by addition of CH4)

Discussion points: Other examples of metal corrosion? Do buildings corrode (brick, stone, concrete)? What about erosion? - is it a corrosion phenomenon? Can corrosion be desirable? Other types of corrosion (e.g., plastic, wood, etc.)?

what are the metal oxidation states? Why does metal corrode? Energy is released as material proceeds towards “natural” state i.e. the thermodynamically stable state. Thus, rusting of iron is a reversion towards the (hydrated - usually) ore; e.g., if lepidocrocite is dehydrated, we get: 2Fe OOH → Fe2O3 (haematite) + H2O Corrosion, especially of metals, is usually an oxidative process i.e. the metal loses electrons: e.g., 4 Cu + O2 → 2 Cu2O Zn + 2HCl → ZnCl2 + H2 what are the metal oxidation states? what are the ion oxidation states?

Chemistry “refresher” … the Periodic Table important elements for construction materials in the nuclear industry

The rows on the periodic chart are periods. The columns are groups. Elements in the same group have similar chemical properties. e.g. Group 8 are all gases and are relatively inert, hence they are called the NOBEL GASES.

Some other groups also have special names: Groups 1B – 8B are called the transition metals

Valency and Ions When atoms lose or gain electrons, they become Ions. Cations are positive and are formed by elements on the left side of the periodic chart. Anions are negative and are formed by elements on the right side of the periodic chart. Valence describes the possible number of electrons an ion will pick up or donate. The valence state a particular compound is in is also called its oxidation state.

The Group “A” elements are classified as having a single oxidation state (usually); The transition metals can have multiple oxidation states:

Balancing Chemical Equations The Law of Conservation of mass applies to chemical (and electrochemical) reactions … i.e. matter cannot be destroyed, only changed from one from to another. Take our corrosion of iron from before: 4 Fe + 3 O2 + 2 H2O → 4 FeOOH Here, we make sure we have the same number of each species on each side of the balanced equation (4 iron, 8 oxygen and 4 hydrogen)

What about the magnetite formation reaction? 3 Fe + 4 H2O → Fe3O4 + 4 H2 3 iron, 8 hydrogen and 4 oxygen atoms are on each side. What’s missing here? Oxidation Numbers

Corrosion reactions are actually electrochemical reactions that proceed through the transfer of charge (electrons). Consider the oxidation state of the atoms in the previous corrosion reaction. Fe (metal) oxidation state = 0 other possible oxidation states = +2 and +3 O oxidation state = -2 (always) Therefore the 3 positive iron atoms must combine to balance the 8 negative charges of the oxygen atoms … how?

Fe(2+)Fe2(3+)O4(2-) So in order for this reaction to proceed, 8 electrons had to be released from the iron. The release of electrons is called oxidation. For the system to remain balanced, the electrons need to be consumed – this is done in a reduction reaction. The combination of reduction and oxidation reactions is called a redox reaction. What is the reduction reaction in this example?

We typically break redox reactions into their two separate reactions that may be added together to obtain the overall reaction. Thus: 3 Fe → Fe2+ + 2 Fe3+ + 8 e- oxidation 4 H2O + Fe2+ + 2 Fe3+ + 8 e- → Fe3O4 + H2 reduction What we generally find is several intermediate steps may occur with the basic reaction being: Fe → Fe2+ + 2 e- 2 H+ + 2 e- → H2

Zn + HCl → H2 + ZnCl (Zn+ + Cl-) In general, for metal M corroding: M → Mz+ + z e- (oxidation) z e- + Z/4 O2 → Z/2 O2- (reduction) M + Z/4 O2 → MOZ/2 (combined) For example, the corrosion of zinc in hydrochloric acid is the net result of two electrochemical reactions: Zn → Zn2+ + 2 e- 2 HCl + 2e- → H2 + 2 Cl- (2 H+ + 2 e- → H2) Zn + HCl → H2 + ZnCl (Zn+ + Cl-) Question: which is the reduced species?