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electroplating strangeness


SmokingSkillz

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hi all,

I have recently taken an interest in chemistry and am currently learning the basics. So any advice/answers from you folks would be great as an alien subject can be a bit confusing when starting out.

 

I have noticed two strange occurences when electroplating a ten pence with copper and when using old stainless steel cutlry for electrodes.

 

I first noticed that there is a pattern to the way stainless cutlery corrodes when used as an anode(hopefully my mother wont), I used a jam jar with 2 parallel holes cut in the lid through which i put 2 identical knives, insulated where in contact with the lid.

The face of the knife, used as an anode, that faced the cathode, has corroded most, with definate lines running from the tip to the wards the handle, with the tip totally corroded. I can see why one side will corrode faster than the other, but havent a clue why the corrosion has formed these lines, it looks like it has been sanded in one direction.

 

The other odd thing i have come across was when i electroplated a 10p piece with copper.

I first added a pinch of salt to water and used a copper wire for electrodes. This made a load of light blue precipitate (copper chloride)?

 

Besides the blue precipitate a white paint-like residue was deposited on the anode (sodium hydroxide)? and a mysterious black stuff on the cathode that rubs off (Copper Oxide)? . Does Na(OH) and/or CuO account for the lack of oxygen gas produced.

 

please correct if i am wrong or there is a better way of writing the reactions, i think the reactions were:

 

2H0 -> 2H2 + 02

Na + Cl- -> Na(OH) + Cl

Cu + Cl -> CuCl

 

I connected the positive clip to a graphite electrode, then clipped the negative onto a 10p coin and partialy submerged it in the copper solution. The coin turned from silver colour to the colour of corroded copper. When i removed the coin from the clip i noticed that the tiny area of the coin held by the teeth was clean copper coloured and the rest dull'ish green, so i moved the clip so the coin was gripped in the centre, I expected again only the part of the coin in contact with the clip to turn a nice copper colour. What happened is the entire area of the coin that was between the jaws of the clip (not touching it) turned a bright copper colour while the rest, not under cover (for want of a better word)of the copper clip turned greeny-grey. I had hoped i would get a perfect copper coat evenly spread over the whole coin. Any ideas why this was observed, how i can improve the plating method to avoid corroded copper.

 

Kind regard chr1s

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wow!, way too many questions all in one go for me, but all good ones and all have answers be sure :)

now I`m stuck as to where to start :)

 

the lines you`ll see will be part of the manfacturing process, also grease (even from fingers ot drying towels) will significanly effect the electrode. it seem you know why one side gets effected greater than the other already, electricity likes to take the shortest path, and so the imediate face presented will be effected the most :)

 

re: the 10P, I`de be very surprised it the copper coating even stuck to it, a simple wipe with a finger should remove it easily. for a 10P I`de de-grease it 1`st (tipes thinner, 1.1.1 Trichloroethylene) and then dip it in some dillute nitric acid for a few secs, then in a very weak copper sulphate soln, no electricity needed, it will bond anyway as a result of displacement reactions :)

you`ll not get sodium Hydroxide (only for nano seconds anyway as there`s side reactions to remove it as it forms), copper Chloride is possible but in this case copper Hydrocxide is the more likely :)

 

the white stuff I would suggest a contaminant, as sodium hydroxide isn`t white either :)

 

the "black stuff" would indeed be copper oxide and more than likely Chrome and Iron oxides (from the steel composition).

 

there is no ONE WAY to improve the plating operation, it all depends on the materials used. PCR is a good electro plating method, but since it`s copper here, most metals will take a coating or plate without the need for juice :)

the 2 main factors are this, slow and dilute makes a better coat than strong and fast and 2, it`s VITAL to make sure the surface to be coated is 100% clean!

 

hope that helps a little, sorry if I didn`t address ALL your points, but there were rather a lot in one go :)

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lol, thanks and sorry for bombarding you with random questions, ready for a nother round? :)

 

ok, so i think i was getting at wether the lines were due to the molecular structure of the metal, some effect of electromagnetism or electron flow. There is also the fact that more corrosion occured at the tip of the knife than towards the hilt (i will try a setup where the tips are definatly further apart than the handles, i am guessing that the corrosion will occur at the other end of the blade as the tips will be further apart).

 

I tried to wipe the 10p to get the corroded part looking like the rest which had no efffect; the copper did undoubtably stick to the coin it wont sctatch off. The 10p is 75% copper and 25% nickel. Any ideas why the plating is only shiny behind the the copper plated jaws of the electrode clip? this is really mysterious. I'd like to understand the difference and be able make a perfectly coated shiny copper coin or a perfectly coated green-grey coin depending on my mood. Ah could it be chlorine related maybe the tarnished part is CuCl, another thought is perhaps the tarnish has some sodium too? whih brings me round to yet another question: where has the sodium/hydroxide gone? When i was removing the copper from the wire no bubbing occured at the anode so i thought CuCl and Na(OH) was being made from the salt. When i was plating the coin the anode was bubbling and i could detect a vauge smell of chlorine, I guess this is the chlorine being relesed from CuCl made earlier (shit there will also be unbroken NaCl that may release chlorine as the copper has gone home). Arrrrgggh!

 

The diplacement reaction sounds interesting although i've no nitric acid to hand. Is the nitric acid an active part of the reaction or does it just help the copper to displace the nickel?

 

The white stuff was only noticable after about 30 mins of electrolysis and the copper is electrical grade, so i am guessing it is from the tap water. flouride, calcium or something like that.

 

lol i think that was longer than my 1st post!

 

I like the fact that (my) theory and practice yeild different results; it's fun working out why (it's would also be nice for something to happen as expected sometimes though).

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Chlorine will be liberated and your NaCl will become NaClO3 along with other side reactions including the production NaOCl (toilet bleach) and HCl (but that doesn`t last long as it gets converted).

 

the nitric acid is basicly to make sure the coin is 100% clean and give a better surface for the copper to adhere to.

 

it`ll have nothing to do with Fluorine or the calcium, they are in such minute quantities as to be wholey ingnorable.

 

keep your voltage LOW, 2 volts will be more than enough, higer voltages will cause pitting and spongey plates.

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