Ship Wrecks Question (1 Viewer)

[Constip8edSkunk]

A bit confused bout this aspect of corrosion of iron/steel

Macmillan explains corrosion of ships' hull by differential aeration as the set up of a galvanic cell where the anode is the site of least concentration and cathodic site as those with maximum oxygen concentration

Earlier, it explained that the cathode are sites where impurities such as carbon is present, where they act as passive electrodes and anodes are where stress points and cracks are found.

How do these two statements relate together?

Which takes preference?

Are one the result of the other?

 

[ND]

I've never read that oxygen concentration determined the site of oxidation/reduction. Only that the cathode likes to be at impurities, and the anode at points of damage. I'd say the latter plays a dominant role (considering that the [O2] wasn't mentioned in either CC or surfing).

 

[inasero]

A bit confused bout this aspect of corrosion of iron/steel

Macmillan explains corrosion of ships' hull by differential aeration as the set up of a galvanic cell where the anode is the site of least concentration and cathodic site as those with maximum oxygen concentration

Earlier, it explained that the cathode are sites where impurities such as carbon is present, where they act as passive electrodes and anodes are where stress points and cracks are found.

How do these two statements relate together?

Which takes preference?

Are one the result of the other?

in response to your query...the above two statements are both correct and the syllabus does not require you to consider a system in which both of the aforementioned factors a re influencing the corrosion rate. All students are required to know are how these two variables affect corrosion in different systems. Anyhows, it would be difficult to give you a qualitative conclusion as to which of the factors influences corrosion more, because we are not sure the extent to which oxygen conc varies between anodic and cathodic sites; nor are we certain the extent to which impurities are present in the metal lattice structure.

 

[Constip8edSkunk]

hmmm ok then.... cheers

hehe the guy from the HSC advice line was like "hmmm, that's ...eh.... a very interesting question. <insert extended monologue repeating what macmillan said> Good luck!"

 

[Dangar]

I would have said that the impurities or strain on the iron would take preference because they are an extremely important factor in the causing of rust... like that monolith of iron in india somewhere that hasn't rusted because it's so pure...it wouldn't matter if the oxygen concentrations surrounding it were different because differential aeration just promotes rust. However without impurities or strains rust practically doesn't occur.

Just a guess... and excuse my grammar

 

[inasero]

hehe the guy from the HSC advice line was like "hmmm, that's ...eh.... a very interesting question. <insert extended monologue repeating what macmillan said> Good luck!"

which is why you post on boredofstudies instead...where i can answer all of your confounded questions!

 

[phenol]

JH,

Two different mechanisms here

for the hull differential aeration thing, it is the peeled off paint which causes difference in oxygen concentration. The place of high oxygen concentration gets a higher reduction potential and vice versa. So where there is low oxygen concentration, iron will be oxidised.

For the stress thingy lattice distortion corrosion,

the carbon in the lattice distorts the iron lattice and causes irregular distribution of electron density. The places of high electron density would obviously be anodic (wants to lose those excess electrons). These places are like cracks and stress points and hence will be attacked first. Also cracks -> low oxygen concentration and hence differential aeration would make these places even more vulnerable to attack.
ALSO, cracks = larger surface area = rate goes up.

 

[Constip8edSkunk]

u legend, cheers!