9.6.2 HSC Q - Feedbak

2001

c) You needed to answer this Q. You should have described the effect of increasing C% but also said why Mn, Si & Cu are added to steel in small amounts. You should have detailed what the Cr, Ni and Mo are for in stainless steel and what the Ti (and Nb…etc) are used for in HSLA steel. Tis would be a very good Q to practise.

2003

aii) You must note the properties of the passivating layer on passivating metals. For active metals you must say that they react with air/water to corrode but their corrosion product is often porous and non-tightly bound (eg rust)  = NO BARRIER to further corrosion.

2005

bii) You would get metal displacement between Mg and Fe ions – detail reactions & explain why in terms of defining metal displacement and relating to Fe and Mg reactivity. Also – you would get rusting of the nail – there is no electrical contact between the Mg and Fe so no galvanic (sacrificial anode) cell can form – thus detail the rusting equations. This is a question to redo!

2008

b) If you start describing an experiment we didn’t do you risk accidentally missing something that a student who DID the experiment you described would not = putting you at a disadvantage. We put iron and stainless steel in test tubes of equal conc salt water, same temp, pressure …. We weighed before and after 20 days. Visually observed (first appearance of rust and final extent of rust – qualitative comparison) and weighed mass of samples once corrosion had been wiped off. Stainless = no corrosion or mass loss. Iron – lots of orange/brown corrosion and small mass loss. Was it reliable – NO – but you can still demonstrate your understanding of reliability by noting that and defining reliability OR you can say these values were compared to the results of the previous year of HSC Chemistry classes and were found to be consist.

2010

c) There was  bit of a fuss that year because despite the fact that the Q says they are steels, #3 is clearly cast iron. You should pick up on this (ie anything > 2.1wt%C is no longer steel) and specify its uses in gates/cookware/hotplates etc – hard strong good heat retention. You need to explain what happens to the properties of steel when C is increases (ie compare properties and applications of #1 and #2 – NB you should explain why corrosion rates increase in terms of cathode sites). For Steel #4 you need to explain what both the Cr and the Ni do.