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Hello everyone:
I'm new to this plating game, and while I've been having success with other Caswell kits (anodizing and powder coating), this one has me completely frustrated. I'm already a month behind in my attempts to zinc plate, and need to get on with my project. This post is going to be long, and somewhat complicated. I've tried to organize it to make sense, but that's not one of my strong points. I hope you can bear with me.
I got Caswells smallest CopyCad & Zinc kit. Everything is set up exactly to Caswells instruction from the Plating Manual. Part prep and cleaning is not an issue. I've tried more than a dozen tests, and have eliminated that as a cause.
Problem: Plating is substandard due to too little voltage & too much current.
Specifics:
1) using Caswell kit supplied 1.7v, 300mA power brick.
2) test piece is 2" x 3" piece of 18ga. mild steel sheet (12 sq.-in. total, supposedly exact dimensions for Caswell power brick).
3) measured voltage is 0.4v, measured current is 800+ mA. (specs call for 1.5v, 300mA @ 25mA/in.)
4) the part plates, but very little evidence of gas "fizzing", and plate is dark, uneven, and discolored w/streaks & patches.
I reported this to Caswell via PRS, and they suspected bad electrolyte, so the sent me a new bottle. That's GREAT customer support, thanks Caswell!
However, even with the new electrolyte, I get nearly the same results: measured voltage is 0.5v, measured current is 740 mA.
I'm back in contact with Caswell about it, and hope to get it figured out, but meanwhile, I've been trying different things, and have spent a lot of time on the net investigating it. Unfortunately, I've not found anything specific addressing my issues, but have learned a lot about electroplating in general, and some technical info about zinc plating (a lot of it seems to be proprietary).
Things I've tried:
In messing about with it, I found that by lifting the part out of the electrolyte, voltage would increase. At about 0.7-0.8v., the part would clearly begin to "fizz" as expected (moderately). At this point, there's only about 2-3 sq.-in. still immersed in the electrolyte, and current is down to @ 600mA. The part plates much better this way, giving a fairly even finish w/out the discoloration. But, the current density is WAY UP with respect to specification, and parts coverage is equivalent to about a single 1/4" bolt at a time!
Pulling the part out so that only the tip of one corner is immersed (@ 1/4"), would produce @ 1.1v, but the part is fizzing away furiously! Obviously not correct.
Suspecting a defective power supply, I bench tested it with various resistive loads. It is consistent with its specification: with no load it measures 4.85v, 1 ohm load produced 0.72v = 0.72A, 5.2 ohms gave 1.7v = .33A, 15 ohm gave 2.6v @ .17A. It's clearly operating up to spec.
So I tried beefier supplies:
1) a 7.5v, 600mA brick: voltage was still drawn down to <= 0.4 v. The brick died at @ 5 minutes, I suspect the rectifier diode let go due to excessive current.
2) a 12v 12 AH gel-cell battery: voltage drawn down to <= 0.4v.
3) same battery w/in-line precision current limiter set to 350mA (an LM117 circuit). Voltage was down to 0.2-0.225 v.
4) a CC/CV lab supply set to 300 mA. voltage came in @ 0.2v. Increasing current limit allowed voltage to rise, but once again, by the time it was up to 1.2 v, it was drawing 1.2 A!
After these tests, a few thing became obvious to me:
1) V = IR, and there's STILL not a damn thing I can do about it (I have know this for years, but that doesn't stop me from trying to get around it on occasion)!
2) resistance (or conductivity) of the electrolyte is non-linear WRT immersed surface area. That really complicates troubleshooting by simple math.
3) the electrolyte has COMPLETE control over plating bath resistance, and the only way to bring voltage & current to spec is to reduce electrolyte conductivity.
With this in mind I began searching the net for more info. Here's what I've found:
Regarding electrolytes:
1) there are 3 zinc plating chemical processes: Cyanide zinc, Alkaline non-cyanide zinc, and Acid zinc. Acid zinc uses either potassium chloride or ammonium chloride as the primary bath ingredient, some w/ zinc chloride as an added metallic element, and all w/proprietary ingredients known as "carriers" and "wetting agents".
It appears the Caswell kit is of the Acid zinc variety, since the MSDS lists it as 15%/85% (by weight) zinc chloride/ammonium chloride.
Ammonium chloride is said to be more forgiving, providing a buffering action, producing acceptable plating over a wider current density range. I presume that's why Caswell chose it for us DIYers. Potassium chloride requires the addition of Boric acid to provide the buffering.
2) the chloride content of the electrolyte bath determines it's conductivity. Chloride ions provide the electrical conduit to transport zinc atoms from anode to cathode. More chloride = more conductive, and vice-versa (up to a point).
I more or less confirmed this by test. I set up a bucket of plain tap water. It produced 2.6v @ 150mA. Clearly not conductive enough.
3) industry standards vary, but generally call for between 20-40 grams/liter of zinc chloride, and 120-200 grams/liter of ammonium chloride.
Caswells electrolyte falls right in this range percentage wise, but there's nothing that says how much chloride (by weight) is actually present, so I can't compare the mixed bath w/industry standards.
4) pH: in general industry calls for 4.5-5.5. My electrolyte is sitting at 5.5. Adjustment usually calls for small additions of hydrochloric acid (decrease) or ammonium hydroxide (increase).
Regarding power supplies:
1) industry standards vary here too, and there are 2 mechanical processes used: barrel plating (used for loose items, like nuts, bolts, washers etc.), and rack plating (items that can be fixed in position).
2) the voltage and current density specs are different for these 2 processes: barrel plating specs generally fall in the range of 6-15 ASF (Amps/sq.-ft., = 40-100mA/sq.-in.) at 6-8v. Rack plating in generally calls for 20-40 ASF (140-275mA/sq.-in.) at 3 v.
Caswells kit would SEEM to be a rack system (items fixed hanging in a bucket) but their V-I specs split the 2 processes, with voltage conforming more to rack plating standards, and current being more in line w/ barrel plating. However, both are slightly lower than respective industry standards. And the results I'm geting are WAY out of line.
In closing:
I've come to my own conclusions, which I hope someone can verify or correct, and get some input as to what I might do to rectify this problem.
1) my electrolyte is WAY too conductive. No amount of external electrical correction is going to fix this or provide a workaround.
IMHO, the "robber" and "rheostat" techniques are not applicable in this case. Neither can provide a boost in voltage, only a reduction in loop current, or a further reduction in anode-cathode voltage.
2) one way to reduce electrolyte conductivity would be to dilute it with more distilled water until electrical parameters fall into line. However, this would consequently dilute the ammonium and zinc content, and I don't know how ultimately important their contribution is to the final result (since the ammonium acts as a buffer, too little might cause its own problems). At $33/gallon for electrolyte, I'm loath to experiment w/out some confirmation.
3) does pH affect conductivity? I have found no information about this.
4) are there any chemical additives that could be used to reduce conductivity (ie, reduce chlorides) without adversely affecting ammonium and zinc content?
5) It's clear in my tests, that until voltage hits 0.7-0.8v., there is no appreciable "fizzing" of the part, so from this observation, and industry specs, it would seem that voltage is more critical than current density (40-300mA/in. is a wide window). Is this correct?
6) What is a usable range of current densities for the Caswell kit, assuming I can get voltage to fall in line? (Caswell says 1-3 v, but only specifies the absolute current of 25mA/in)
7) Does anyone have actual measured voltages and currents for their successful Caswell kits?
Thanks for any help you can give!
I'm new to this plating game, and while I've been having success with other Caswell kits (anodizing and powder coating), this one has me completely frustrated. I'm already a month behind in my attempts to zinc plate, and need to get on with my project. This post is going to be long, and somewhat complicated. I've tried to organize it to make sense, but that's not one of my strong points. I hope you can bear with me.
I got Caswells smallest CopyCad & Zinc kit. Everything is set up exactly to Caswells instruction from the Plating Manual. Part prep and cleaning is not an issue. I've tried more than a dozen tests, and have eliminated that as a cause.
Problem: Plating is substandard due to too little voltage & too much current.
Specifics:
1) using Caswell kit supplied 1.7v, 300mA power brick.
2) test piece is 2" x 3" piece of 18ga. mild steel sheet (12 sq.-in. total, supposedly exact dimensions for Caswell power brick).
3) measured voltage is 0.4v, measured current is 800+ mA. (specs call for 1.5v, 300mA @ 25mA/in.)
4) the part plates, but very little evidence of gas "fizzing", and plate is dark, uneven, and discolored w/streaks & patches.
I reported this to Caswell via PRS, and they suspected bad electrolyte, so the sent me a new bottle. That's GREAT customer support, thanks Caswell!
However, even with the new electrolyte, I get nearly the same results: measured voltage is 0.5v, measured current is 740 mA.
I'm back in contact with Caswell about it, and hope to get it figured out, but meanwhile, I've been trying different things, and have spent a lot of time on the net investigating it. Unfortunately, I've not found anything specific addressing my issues, but have learned a lot about electroplating in general, and some technical info about zinc plating (a lot of it seems to be proprietary).
Things I've tried:
In messing about with it, I found that by lifting the part out of the electrolyte, voltage would increase. At about 0.7-0.8v., the part would clearly begin to "fizz" as expected (moderately). At this point, there's only about 2-3 sq.-in. still immersed in the electrolyte, and current is down to @ 600mA. The part plates much better this way, giving a fairly even finish w/out the discoloration. But, the current density is WAY UP with respect to specification, and parts coverage is equivalent to about a single 1/4" bolt at a time!
Pulling the part out so that only the tip of one corner is immersed (@ 1/4"), would produce @ 1.1v, but the part is fizzing away furiously! Obviously not correct.
Suspecting a defective power supply, I bench tested it with various resistive loads. It is consistent with its specification: with no load it measures 4.85v, 1 ohm load produced 0.72v = 0.72A, 5.2 ohms gave 1.7v = .33A, 15 ohm gave 2.6v @ .17A. It's clearly operating up to spec.
So I tried beefier supplies:
1) a 7.5v, 600mA brick: voltage was still drawn down to <= 0.4 v. The brick died at @ 5 minutes, I suspect the rectifier diode let go due to excessive current.
2) a 12v 12 AH gel-cell battery: voltage drawn down to <= 0.4v.
3) same battery w/in-line precision current limiter set to 350mA (an LM117 circuit). Voltage was down to 0.2-0.225 v.
4) a CC/CV lab supply set to 300 mA. voltage came in @ 0.2v. Increasing current limit allowed voltage to rise, but once again, by the time it was up to 1.2 v, it was drawing 1.2 A!
After these tests, a few thing became obvious to me:
1) V = IR, and there's STILL not a damn thing I can do about it (I have know this for years, but that doesn't stop me from trying to get around it on occasion)!
2) resistance (or conductivity) of the electrolyte is non-linear WRT immersed surface area. That really complicates troubleshooting by simple math.
3) the electrolyte has COMPLETE control over plating bath resistance, and the only way to bring voltage & current to spec is to reduce electrolyte conductivity.
With this in mind I began searching the net for more info. Here's what I've found:
Regarding electrolytes:
1) there are 3 zinc plating chemical processes: Cyanide zinc, Alkaline non-cyanide zinc, and Acid zinc. Acid zinc uses either potassium chloride or ammonium chloride as the primary bath ingredient, some w/ zinc chloride as an added metallic element, and all w/proprietary ingredients known as "carriers" and "wetting agents".
It appears the Caswell kit is of the Acid zinc variety, since the MSDS lists it as 15%/85% (by weight) zinc chloride/ammonium chloride.
Ammonium chloride is said to be more forgiving, providing a buffering action, producing acceptable plating over a wider current density range. I presume that's why Caswell chose it for us DIYers. Potassium chloride requires the addition of Boric acid to provide the buffering.
2) the chloride content of the electrolyte bath determines it's conductivity. Chloride ions provide the electrical conduit to transport zinc atoms from anode to cathode. More chloride = more conductive, and vice-versa (up to a point).
I more or less confirmed this by test. I set up a bucket of plain tap water. It produced 2.6v @ 150mA. Clearly not conductive enough.
3) industry standards vary, but generally call for between 20-40 grams/liter of zinc chloride, and 120-200 grams/liter of ammonium chloride.
Caswells electrolyte falls right in this range percentage wise, but there's nothing that says how much chloride (by weight) is actually present, so I can't compare the mixed bath w/industry standards.
4) pH: in general industry calls for 4.5-5.5. My electrolyte is sitting at 5.5. Adjustment usually calls for small additions of hydrochloric acid (decrease) or ammonium hydroxide (increase).
Regarding power supplies:
1) industry standards vary here too, and there are 2 mechanical processes used: barrel plating (used for loose items, like nuts, bolts, washers etc.), and rack plating (items that can be fixed in position).
2) the voltage and current density specs are different for these 2 processes: barrel plating specs generally fall in the range of 6-15 ASF (Amps/sq.-ft., = 40-100mA/sq.-in.) at 6-8v. Rack plating in generally calls for 20-40 ASF (140-275mA/sq.-in.) at 3 v.
Caswells kit would SEEM to be a rack system (items fixed hanging in a bucket) but their V-I specs split the 2 processes, with voltage conforming more to rack plating standards, and current being more in line w/ barrel plating. However, both are slightly lower than respective industry standards. And the results I'm geting are WAY out of line.
In closing:
I've come to my own conclusions, which I hope someone can verify or correct, and get some input as to what I might do to rectify this problem.
1) my electrolyte is WAY too conductive. No amount of external electrical correction is going to fix this or provide a workaround.
IMHO, the "robber" and "rheostat" techniques are not applicable in this case. Neither can provide a boost in voltage, only a reduction in loop current, or a further reduction in anode-cathode voltage.
2) one way to reduce electrolyte conductivity would be to dilute it with more distilled water until electrical parameters fall into line. However, this would consequently dilute the ammonium and zinc content, and I don't know how ultimately important their contribution is to the final result (since the ammonium acts as a buffer, too little might cause its own problems). At $33/gallon for electrolyte, I'm loath to experiment w/out some confirmation.
3) does pH affect conductivity? I have found no information about this.
4) are there any chemical additives that could be used to reduce conductivity (ie, reduce chlorides) without adversely affecting ammonium and zinc content?
5) It's clear in my tests, that until voltage hits 0.7-0.8v., there is no appreciable "fizzing" of the part, so from this observation, and industry specs, it would seem that voltage is more critical than current density (40-300mA/in. is a wide window). Is this correct?
6) What is a usable range of current densities for the Caswell kit, assuming I can get voltage to fall in line? (Caswell says 1-3 v, but only specifies the absolute current of 25mA/in)
7) Does anyone have actual measured voltages and currents for their successful Caswell kits?
Thanks for any help you can give!
month summer) and in the winter temps (55 degrees or so) the only difference I see is a "slower to plate" issue, so sometimes I put the heater in to raise the temp to about 80 degrees.
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