KEY POINTS
* If you are talking
about barrier protection you must say it protects the iron from corrosion as it
is a barrier preventing oxidant/electrolyte (oxygen/water) coating the steel,
thus preventing corrosion.
* If you are talking
about cathodic protection – you MUST say that there is a galvanic (or electrolytic - for impressed current) cell
established between the iron and the anode (eg Zn for sacrificial anodes) where the iron is the
cathode thus protected from oxidation/corrosion.
*If you’re talking
about galvanising you must say BOTH of the above for the unscratched/scratched
conditions
*If you are talking
about Noble coatings you must say that if scratched the galvanic cell produced
accelerates corrosion.
*If you are asked to
discuss the use of materials over time – it is a Q about the history of
shipping materials
*If you are asked to
explain why steel is used in ships you have to talk about its useful properties
(low $ and high strength:weight ratio) AND about the fact that its corrosion
can be prevented with a range of strategies:…
2001
bii) You must
note that the sacrificial anode forms a galvanic cell when in contact with the
hull(and electrolyte). Ie you can’t just say that the hull is the ‘cathode’, it
is ‘the cathode of the galvanic cell established when in contact with the
sacrificial anode and electrolyte’. Also – while it is ‘obvious’, don’t
overlook the fact that you actually need to say ‘they prevent
corrosion/oxidation of the hull’ –but they DON’T protect because the cause the
hull to be reduced (ie the iron doesn’t turn into negative iron ions) – the
hull becomes the site of reduction – so oxidation cannot occur there (nd any
iron irons that have formed will be reduced back into iron).
diii) Define
terms – accuracy is how close measurements are to the actual value (ie the
measuring equipment and measuring technique used to take each reading).
Validity is whether an experiment measured what was intended (ie did it
actually address the aim/hypothesis) – this is determined by control of
variables (all but IV and DV are constant) and ensuring accuracy and
reliability. Reliability is whether the result are repeatable – this is done by
repeating the experiment and checking for consistency. So to improve accuracy
you need more sensitive measuring apparatus or a more accurate technique. For
reliability you must repeat the experiment – but this is where you can talk
about control of variables too as if there is no control of variables there
will be variation in results.. See here.
2005
c) For BIG mark Q in the electives (6-8 marks)
you will always need to include equations. For this Q the equations should
have related to corrosion protection and rusting. Eg sacrificial anode
equations, impressed current equations etc. For barriers you must say they prevent
contact between metal and oxygen/water (ie oxidant/electrolyte) thus preventing
corrosion. For scratched noble, scratched galvanised, sacrificial anodes
explanations you MSUT say that a galvanic cell is established between them and
the base metal. This was a DISCUSS question so you MUST mention disadvantages
of all corrosion protection techniques as well as advantages. Eg stainless =
expensive, paints (*even rustmaster) do not work if scratched and need
maintenance, sacrificial anodes need replacing, impressed current needs electrolyte
and constant power supply. Surface alloying more expensive than paint and cant
protect deep scratches (and stainless surface can be attacked by Cl ions just
like normal stainless steel). Replacement metals often too expensive for whole
hull and/r inferior mechanical properties.
2006
diii) For
cathodic protection you must specify that there is a voltage applied so that electrons
are supplied to the structure (ie iron) to make it a cathode (of an electrolytic
cell) thus preventing oxidation/corrosion. Also – the tower is NOT in an
electrolyte thus sacrificial anodes and impressed current will not be the best
choice. It would be ideal to describe a method suitable for the tower and one
for the pylons.
2007
b) A lot of
people misinterpreted this Q to just be about ‘factors that increase corrosion’
In this Q it as important to note that corrosion involves a redox reaction with
the metal being oxidised and thus the warm, oxygen rich, salty water where
ships travel is ideal for corrosion BUT because this question specified the
choice of meals over time so this was a clue (“over time”) that it was about
the history of shipping metals = you
need to mention the use if Cu in early ships, Fe Nails, Fe hulls, steel hulls,
modern steel hulls with low impurities and Mn, Si added etc. Then modern alloying eg stainless and alloys that are used as
fittings etc on ships (brass, bronze, Cu-Ni, Al) and sacrificial anode
materials. ie for each development say their +/- focussing on marine factors –
eg Cu biocidal against marine organisms but heavy/soft. Fe strong and easily
shaped but rusted. Modern alloys corrosion resistant but $$ of softer then
steel = used for specialised applications. One thing I strongly recommend
focussing on – we know Fe has a problem with rust so we choose REACTIVE metals
such as Mg / Zn to act as sacrificial anodes – this is ideal to discuss as
these are i) metals (so answering the Q), ii) they overcome a limitation of
steel in the marine environment, ii) you can use equation and talk about how the
ocean provides the electrolyte etc.
2008
ai) As
above for 2006 diii, you have to say that there is a galvanic cell setup
between the Fe and sacrificial anode. And because it specified equations use
the Zn oxidation equation and NB that any iron ions are reduced back to Fe on
the surface (then put the Fe reduction equation), Note this set-up could not be
impressed current as there is no external power supply.
2009
aii) You
can talk about the aluminium passivating layer ‘coating’ but should focus on
plastic and Sn coatings of steel, NB tin is unreactive and non-toxic, and will
not corrode in acidic solutions (ie betroot) – but sets up a galvanic cell
between itself and iron if scratched where Fe is the anode and thus rusts
rapidly. Plastic is a barrier to oxygen and electrolyte contacting the steel
when intact but when scratched…
b) When adding a Zn coating, the
Zn corrodes slowly because it is passivating, but be careful saying ‘the steel
is given a passivating layer’ because unlike stainless steel, the passivating
layer forms on the Zn coating NOT on the steel itself. You need to say that
galvanising provides a barrier to oxidant/electrolyte thus corrosion wont occur
when intact AND must say that a galvanic cell is produced when the layer is scratched
with Fe as the cathode of the galvanic cell thus cannot corrode
2011
di) The
samples should have been washed and lightly sanded to remove any corrosion
product that could affect the results. They could have been weighed. They
should have been placed in separate samples in case the ions produced by their
oxidation influenced the corrosion of the other samples. Ideally they would
have the same size / shape and be stress free. Note: There will be no
‘sacrificial anode effects’ because the samples are not in electrical contact
so there will be no galvanic acceleration/slowing of corrosion – HOWEVER if Cu ions (for example) were produced they
could cause metal displacement reactions with Fe, Zn etc
di) This
is an ASSESS = INTRO (Define validity), PROS (same electrolyte, temperature
& time for each) COMS = he samples were all different size, shape (and
stress levels different) – uncontrolled variables means the validity is low
(cannot be sure of alloy material or stress/size/shape influenced the result). Qualitative
photographic method is not as accurate as quantitative methods. Thus validity
low.
2012
e) Brain
dump – must introduce corrosion, the history of boating from wood – wood/metal,
iron, steel, modern steel (and composition). Then need to say advantages of
steel to outweigh poor corrosion resistance because of protection systems that
can be applied – replacements (for specific jobs) barriers, galvanised,
cathodic protection. But because it is an EVALUATE – you needed to end with a judgement
on ‘the continued use of steel’ ie Steel use is justified because of its
superior strength, low cost and because its corrosion can be prevented with a
range of protection systems’
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