TECH 101 Zinc Electroplating for Corrosion Resistance

[BDCarrillo]

The anode is what ends up being consumed and plating the item. The bath must be friendly to both the anode and cathode, and electrically conductive.

Simply put, current travels from the anode to cathode it takes bits of metal with it (on the atomic scale).

An electroplating rig can also be used to clean parts by reversing the charge and using a sacrificial cathode. Used to clean gun barrels like this.

 

[laserjock]

Great DIY thank you so much for sharing this. I am going to use you Zn plating recipe seems like a good one from the look of your parts and low toxicity. I also like the configuration of your anodes seems like a good simple way to get even deposition. Could you if you don't mind please provide your recipe for the Ni plating bath? And a couple of questions Do you pickle the parts in muriatic first? any other prep tips? And a picture of how you wired the variac. Very good of you to take the time to do this tutorial.

When I was an undergrad the chemistry club president(me) made stoichiometric Hydrogen and Oxygen balloons for a career fair demo, the first one I lit off knocked 50years of dust off the ceiling tile in the auditorium. Most people left in a big hurry....They added some new rules to the chemistry demonstration handbook on that one. The President of the University was SERIOUSLY MAD, I thought he was going to have the big one right there. We beat a hasty retreat. Cant hurt to run a fan in the room just in case

I have a somewhat similar story that ended in new rules relating to demos for grade school kids. Something about sodium metal and water and a big pop.... It's all a little fuzzy now. [emoji6]

I had not been pickling my parts for the zinc stuff. You can. We did do it for the nickel parts. Most of the time after sandblasting I don't typically feel it's necessary. It won't hurt anything though and if you have some stubborn rust pits it will help clean them up.

On power supplies, here's a couple bits of advice. I think I quoted a current density above. If you are doing small parts, you can use a wall wort supply for power. Some of them are good for a couple amps. If your variac is a real old school variac, you can use it to regulate a battery charger. Wire the output of the battery charger into the input of the variac and the output to you parts. I used an outlet for the connection point so as not to have to cut off the standard wall plug. This method works but it's pretty taxing on the battery charger so I wouldn't use a charger you care about and the dumber it is the better. Smart chargers may think they are shorted or the battery is charged and shut off.

The problem if you are going to do large parts is you need a lot of current but you want very low voltage. There are ways to design the system to minimize the voltage. More anode surface area is good. It cuts down on the resistance so it takes less voltage to push the current. The battery charger will get you around 6-8 amps if you attenuate the voltage. That really will do most things.

The third option if you happen to have a really large transformer around.., say s 5kVA like this:

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Because everyone has one right?? WARNING!!!!! We are now dealing with serious current. It can be dangerous. If you don't know what you are doing.. Don't do this.

So a high current and low voltage power supply is hard to come by or expensive. There are other routes to it but I'm not an electrical expert so I'm not going to do a huge tutorial but some reading on repurposing microwave oven transformers.

It's a lot easier to regulate the AC side than the DC side. It's much lower current. In this case I'm feeding that giant step down transformer with a variac. It steps the 120 down to 30v max. By varying the input voltage, the output is also varied. But it's still AC. Need to convert it to DC. To do that you need a rectifier. Search bridge rectifier if you want to know how to build one or how it works. I picked up a single phase 100 amp rectifier with heat sink from Amazon (it's the black box with the wires hooked to it). It gets hot and needs to be cooled. Yes rectified DC is "dirty" but you aren't running a computer here. I highly recommend an ammeter and a volt meter to watch what's going on. At these currents you probably will need a shunt for the ammeter. HF jumper cables are a good source of big wire and clamps. Remember you have to size things appropriately. It's going to get hot if you are pushing a bunch of current through the system. The bath will get warm too.

Hopefully that answered a few questions.

 

[marmot]

IDIOT, You want a pure anode to avoid impurities in your plating, zinc II ions are what plates your Cathode the reason you use a soluble zn salt is to speed up the process by "seeding" the solution with Zn ions, otherwise it would take a long time to get enough zn in solution to plate your part,so again you want as pure znso4 as you can get. The bath needs to be acidic to prevent the ZnII from crashing out as zinc hydroxide. Anode rivets erode by the same process as the anode itself and by oxidation from the acidic solution. Hope this helps a bit I'll let laserjock answer the rest. Cheers

 

[laserjock]

Okay. BD is correct. The purpose of the zinc sulfate is to preload the bath with zinc. The parts don't care where the zinc comes from. If it comes from the sulfate, it's Zn+2. If it comes from the anode, it starts as Zn 0 (zero oxidation state) and turns into Zn +2 when you put current through the system. In general, the anode is consumed in the process when plating so putting steel rivets in things will be eat up. The aluminum will work in the case of zinc because it has a higher work function than zinc (the zinc oxidizes first) so it doesn't dissolve. In the case of other processes, like the nickel, you can't put much of anything metallic on the anode side besides the nickel. We used titanium welding filler rod for the wires to hold the anodes for the nickel plating because it has a higher work function than the nickel.

Brighteners. Search for zinc plating brighteners. Sugars are common ones but saccharine is also pretty common. The way to get the parts shiny is really to start with a smooth part, strike it multiple times with a burnishing step in between. If you want it shiny, it may rewire several trips through the bath. What you will find is that the parts literally come out "fury". You buff it down to make it smooth and put more metal down so you have enough to polish.

On the zinc sulfate, the purity is important. You aren't so much worried about the zinc content as the impurities. I think one of the impurities tend to be nasty like arsenic. It's really a small percentage but less other metal impurities is better.

I also recommend putting a piece of scrap metal in and running it a while just to pull out whatever metal impurities are in the bath.

 

[laserjock]

IDIOT, You want a pure anode to avoid impurities in your plating, zinc II ions are what plates your Cathode the reason you use a soluble zn salt is to speed up the process by "seeding" the solution with Zn ions, otherwise it would take a long time to get enough zn in solution to plate your part,so again you want as pure znso4 as you can get. The bath needs to be acidic to prevent the ZnII from crashing out as zinc hydroxide. Anode rivets erode by the same process as the anode itself and by oxidation from the acidic solution. Hope this helps a bit I'll let laserjock answer the rest. Cheers

Beat me to it. [emoji6]

 

[IDIoit]

concept grasped!
thank you!

but you still didnt answer how you check the pH

 

[marmot]

So I am trying to grasp why you need that big step down transformer? Variacs are variable voltage transformers so you should be able to turn em down to near 0 volts then just use a potentiometer(variable resistor) to regulate the amperage, Then rectify to dc correct? I am not overly skilled in circuit design and I have all the stuff except the transformer....thanks.

 

[laserjock]

The point of the step down transformer is more current. In a transformer, power is conserved (almost) so lets say you have 10 amps you can apply at 120 volts. That's 1200 watts of power. If you step that down to 30 volts, that's 40 amps. If you just rectifiy the variac, you are stuck at whatever the current output of the variac is. That might be enough depending on what you are doing but I knew for that hub, it wasn't going to be nearly enough. Make sense?

Edit: It also give you finer control because you are able to use much more of the full scale fo the variac (typically 120 divisions or 140 divisions or 100 depending on what it is) that's only moving the knob a tiny bit as apposed to being able to use most of the scale.

 

[laserjock]

concept grasped!
thank you!

but you still didnt answer how you check the pH

What, you don't have your own pH meter on the work bench??? Rookie.

You can use pH paper but I typically don't. That's the beauty of the salts and the acetic acid. It pretty much sets the pH for you because being a weak acid, acetic acid will form a buffer with a salt of itself. Yes you can adjust the buffer pH with a strong acid but if you don't add any "strong acid" (if you don't know the definition of strong acid, look it up if you care) you will pretty much settle in at around 3.4 if memory serves. The solution does not need to be extremely acidic. It needs to be somewhat acidic to keep things from precipiating. A lot of metal salts are only soluble at acidic pH.

Now to actually answer your question. You can use regular old pH paper that is pretty similar looking to the coolant test strips but not exactly the same.

 

[IDIoit]

What, you don't have your own pH meter on the work bench??? Rookie.

You can use pH paper but I typically don't. That's the beauty of the salts and the acetic acid. It pretty much sets the pH for you because being a weak acid, acetic acid will form a buffer with a salt of itself. Yes you can adjust the buffer pH with a strong acid but if you don't add any "strong acid" (if you don't know the definition of strong acid, look it up if you care) you will pretty much settle in at around 3.4 if memory serves. The solution does not need to be extremely acidic. It needs to be somewhat acidic to keep things from precipiating. A lot of metal salts are only soluble at acidic pH.

Now to actually answer your question. You can use regular old pH paper that is pretty similar looking to the coolant test strips but not exactly the same.

" you cant handle that ish on strong acid man"- Slater
I use a refractometer on my coolants, other than that, I dont check the pH on my coolants.
I used to have a chemical pH tester, but It went with my HPS light set up lol
test strips, gotcha

I am more of a rookie than ill ever know!!!

 

[marmot]

Cool use of the transformer got it. Did a bit of reading on hydrogen embrittlement and it seems it is only a problem in systems that use strong acids....seems like in your acetate buffered zn bath it would be a non issue. Just for your ease of mind. Using HCL, HNO3 or H2SO4 as a pickling acid can lead to embrittlement issues as well.

 

[laserjock]

Yep. It's also only really a problem with hardened steels as best I can tell. Cast pieces and low grade fasteners, seems it shouldn't matter.

 

[marmot]

I am going to buy a power supply of the flea bay like this. Should do for most parts I'd mess with....http://www.ebay.com/itm/DC-Power-Supply-30V-10A-110V-Precision-Variable-Digital-Adjustable-w-Clip-Cable-/181622646079?pt=LH_DefaultDomain_0&hash=item2a498da53f

 

[laserjock]

Yep. That will definitely do most of the zinc stuff easily. The nickel likes a little more current density. Really though, it shouldn't care that much either.... it will just take longer.