Oxidation / corrosion Flashcards
oxidation vs corrosion
oxidation requires oxygen
corrosion occurs in various forms, by chemical reaction of materials with environment (usually ACID / SALT)
the most common cause of corrosion is oxidation - oxidation is a subset of corrosion
oxidation of iron
iron oxidises –> iron(II) ions, +water –> hydrated form of ion(III) oxide = rust
why is gold stable in its pure state (doesn’t oxidise)?
its oxidation every is endothermic (ie. energy must be provided for it to oxidise) so it does not tarnish under normal conditions
define weld decay
severe corrosion attack along grain boundaries in welds in stainless steel
why does weld decay occur?
as the weld cools, C in stainless steel “soaks up” the chromium –> chromium carbide ppts
insufficient chromium at welds to provided complete chromium oxide cover
requirements for galvanic corrosion to occur
two different metals in direct contact
immersed in common electrolyte solution (eg. seawater)
explain galvanic corrosion in terms of cathode/anode
the two metals form an anode (+ve) and cathode (-ve)
electrons travel from anode to cathode, so the anode corrodes preferentially, protecting the cathode
what is galvanised steel?
zinc-plated steel - zinc more reactive, corrodes preferentially
other ways to prevent corrosion
eliminate the cause (eg. harmful chemicals)
correct choice of material (marine grade or inert)
surface condition (rough pitted surfaces corrode more easily)
corrosion inhibitors (chemicals that react with corrosive substances before they react with the material)
physical barriers (coating)
oxidation of copper
relatively inert, takes a long time to oxidise
green “patina” forms, does not affect the structural integrity of the metal
oxidation of brass
brass = Cu + Zn
exposed to O2 forms patina = barrier to further oxidation
BUT corrodes by “dezincification” - the amount of Zn in alloy diminishes, weakens structural integrity
oxidation of bronze
bronze = Cu + Sn
highly corrosion and oxidation resistant - tarnish forms which protects surface
bronze disease - when exposed to salts (chlorides and ammonia) diminishes structural integrity
