Positive and negative poles

tonybarebones

tonybarebones

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Can anyone please explain why it is that the positive terminal on a lead acid battery becomes brittle, hard and discoloured and also why the positive on untinned copper wire turns a very dark colour, sometimes even black? Regards Tony. /forums/images/graemlins/confused.gif
 
The crusty stuff on a positive battery terminal, and the black stuff on positive copper wires are (mainly) the oxides of the metals. They react with the oxygen in the atmosphere more than usual simply because they are positively charged.
The only way to inhibit the oxidation is to shield them from the air by a thin impermeable layer. Vaseline (before assembling the connection) is the usually recommended stuff, and it does work. ACF50 works well too.
 
[ QUOTE ]
They react with the oxygen in the atmosphere more than usual simply because they are positively charged.

[/ QUOTE ] I'd be interested in a more detailed explanation of that please. Why are normally uncharged oxygen molecules attracted to the positive terminal or cable?
 
I do not know the scientific answer but years ago cars changed from pos earth to neg earth and corrosion was given as one of the reasons for the change.
 
Try to keep it simple! Non /forums/images/graemlins/tongue.gif-spontaneous redox reactions can occur if a voltage potential is applied between two or more elements. For electrical components, we will study the effects of salt water (NaCl (aq)) on copper wire. For this reaction, there are several chemical elements involved. Salt water is a solution of sodium ions (Na+) and chloride ions (Cl-) in water (H2O). For simplicity, the sodium ions (Na+) will not be used in the calculation. The other elements involved are oxygen (O2) and copper (Cu). For all of these chemical elements, they can be written as 3 half reactions.

2Cl-(aq) ® Cl2(g) + 2e- -1.36 vdc
Cu(s) ® Cu+2(aq) + 2e- -0.34 vdc
O2(g) + 2 H2O(l) + 4e- ® 4 OH-(aq) +0.40 vdc

When you add up all of the reactions, you get

2 Cl-(aq) + 8 Cu(s) + O2(g) + 2 H2O(l) ® 4 Cu2OH(s) + Cl2(g) -1.30 vdc

This equation states that when copper is in the presence of chlorine, water, oxygen, and 1.30 vdc of electrical potential, the copper becomes oxidized into copperhydroxide. The solid copper wire is eaten away (reduced) and copperhydroxide is produced (oxidized). A reaction will also occur without the presence of chlorine; but chlorine, a small atom and an excellent oxidizer, greatly accelerates the corrosion process.
 
To be a smartarse and to extend the discussion /forums/images/graemlins/smile.gif copper hydroxide is blue or blue green. The dark colour the original poster refers to is probably copper oxide formed when moist copper hydroxide decomposes.

Can anyone confirm, disagree, throw a tantrum over, etc? /forums/images/graemlins/smile.gif
 
I quite agree. Remember also the dark colour can be caused by over heating.
 
the usual reason is that your battery terminal seal to the battery itself is leaking and allowing acid in vapour form to attack the copper wire.
 
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