I was cleaning my battery terminals the other day by scrubbing some backing soda on the corrosion around my battery terminals. It creates a foamy, blue liquid. What is the actual chemical reaction taking place? What am I creating?
Well, battery corrosion is caused by the reaction… um, what?
WA(educated)G:
Assuming that the battery terminals are made out of metal “X”, you have a reaction going on to produce the corrosion. X + O[sub]2[/sub] -> X[sub]a[/sub]O[sub]b[/sub] (This is unbalanced, of course, because I don’t know the particular metal that the terminals are made out of.) So the baking soda would react with the metal oxide to make O[sub]2[/sub] and the metal bicarbonate.
Assuming that doesn’t work, it may be a reaction with the battery terminal and some leaking H[sub]2[/sub]SO[sub]4[/sub] (battery acid) and the terminal, again metal “X”. Then, the blue foam you’d see would be the NaHCO[sub]3[/sub] (baking soda) and some excess sulfuric acid reacting to make H[sub]2[/sub]0 and some CO[sub]2[/sub], along with some ions that wouldn’t leave solution. I’m guessing the foam that you saw would have been some dissolved corrosion in the H[sub]2[/sub]0, with the CO[sub]2[/sub] making foam.
All of this is scientific WAGing, but I toned down the sciency language, before all of you chemists jump down my throat. 
Meant to add… if anyone can tell me what causes the corrosion in this case, that would make the OP much easier to answer.
Moving this from IMHO to General Questions.
Battery posts (the top-mount, clamp-on kind) are made of lead.
Blue… gotta be some copper in there somewhere, wouldn’t you say? Maybe deposited from the battery cable connectors, rather than present in the battery terminals themselves (which in my experience are usually made from lead).
Assuming we’re talking about a car battery, that is…
All of the references I looked up says that batter corrosion is a result of the condensation of sulphuric acid vapor on the area around the battery. The terminal posts are lead so I would think the corrosion is lead sulphate. That around the electrical leads is probably copper sulphate. Baking soda is sodium bicarbonate. Having gone that far I’ve exhausted my chemical knowledge and maybe even exceeded it. Maybe this start will cause an actual chemist to come on board and make things clear as to the chemical reactions.
This was for a car battery. Around the positive terminal I can usually find a white, powdery substance. It does appear that the terminal is made with lead and the connector is made with copper.
I think the blue stuff is (or contains) probably copper carbonate, which is pale blue in colour; I’m not sure how this is being formed though. Are you cleaning the terminals with the copper connectors still in place?
Blue could also indicate copper sulphate (CuSO4) from a reaction between the copper and sulphuric acid. In most batteries with the newer “side-mount” terminals, there is a copper stud that has the threads to which the wires are attached with a nut; this could be the source of the copper, even if the wires are disconnected from the battery before cleaning.
Cu + H2SO4 = CuSO4+H2
CuSO4 + 2NaHCO3 = H2O + CO2 + CuCO3 + Na2SO4
Something like this? I can’t get the numbers subscripted, but you get the idea.