Burn marks on zinc plated item

[rcx132]

Slowly starting restoring my trusty Vitara. It's stipped down and I'm cleaning up the bits ready for painting. I should start a build threat at some point.

I started zinc plating some of the brackets, and terrible results. This is today's attempt, on one side there is a dull area in the centre implying no plating there and on the other side there's two serious burn marks. I've generally been getting such results recently, odd because I did my gearbox flange when I first got the kit and it came out perfect.

The part is dull as I haven't added brightener and the part has a rough finish.






My procedure went like this:

1. Sand blast part
2. Phosphoric acid dip to remove old zinc, until stopped bubbling
3. Pressure wash
4. Cold caustic soda 5 min
5. Pressure wash
6. Acid pickle 1 min
7. Pressure wash
8. Into zinc-nickel electroplating tank with water agitation, temp 22 C
9. Wait 10 minutes, check part, see burn marks, give up.
10. Rinse

I read somewhere on this forum of someone else saying that irregular parts gave them bad results.

I calculated the surface area and set the current to 0.07A per sq. inch, a bit low I know, as I wanted to be sure I wasn't getting bad results because of high current.

 

[Mr shiney]

That size would need around .9amp. You also don't have enough brightener in the mix. It would need around 40min to plate that.

You also need to wire wheel the surface to get a good bright clean finish before you clean but after the sin removal.

I would hang that with two wires so it's just below the surface and don't bother with agitation.

I can get mine like chrome.

 

[rcx132]

That size would need around .9amp.

I think I had 1.2A. Is the current that critical?

You also don't have enough brightener in the mix.

I was told the brightener is only for aesthetic purposes so I didn't top it up.

You also need to wire wheel the surface to get a good bright clean finish before you clean but after the sin removal.

You mean after the ZINC removal?

I would hang that with two wires so it's just below the surface and don't bother with agitation.

Why no agitation? And why just below the surface?

I will try again in a couple of days.

 

[rcx132]

The rough finish is because I had the parts grit blasted at a huge plant. Now the parts are all rough. The parts then sat in my garden for two months so needed re-blasting.

 

[Gareth J]

Any reason not to pay to get the bits hot dipped? Bracketry like that would be fine and the cost would be almost entirely whatever they chose to add for doing such a small part. The firm I send stuff away with charge ~50p per kg (that's the weight of the item to be dipped) but fiddly bits like that might attract an annoyance addition.

I was surprised recently as I sent away a pair of tractor wings as an (expensive) experiment. They came back almost fine, the only ripples were on the small portion that was flat, the rest of the "seashell" shape was absolutely satisfactory despite only being made of steel around (or maybe just over) 1mm thick. They did not quench them in order to minimise distortion. Brackets like yours would be very sturdy.

 

[rcx132]

I originally had all the bits professionally zinc plated... the bumpers, diff guards, control arms, everything. About half the parts leaked pre-processing fluids out the seams/cavities/welds and spoilt a lot of the plating. I saw that my friends parts, done at another plant, had the same problem. I spoke to the independent specialists at finishing.com and they told me it's unavoidable as the parts are processed quickly and without much attention.

Hot dipping has the problem that apparently paint won't stick to it. I considered it for the chassis as it would coat and protect the inside of the chassis as well as the outside. But for the zinc to stick to the inside, the chassis needs professional cleaning because there's no other way of stripping the paint off inside. That, with transport, and galvanising, and fuel costs, comes to almost £1000 so in the end decided to do the old fashioned method of wax and probes to do the chassis inner and Jotun epoxy paint on the outside. Plus hot dip has risk of distorting the chassis.

Home zinc plating should be good for small parts. It has some advantages:

1. The plants only plate to 10 microns. At home you can do three times thicker ツ
2. The plants use zinc plating, at home you can do zinc-nickel, which my book and Wikipedia say is 5 times more resistant
3. At home you will spend more time getting pre-processing fluids out of every seam (I use a pressure washer) so way less chance of the plating being spoilt.

Also the plants I went to all have a £40 minimum charge, so not great if you just want to get some bolts done

 

[Mr shiney]

The current is critical. Too high and it will burn as it's laying on to fast.

You need to regularly top up the brightener, without it you get a dull finish.

Remove the zinc either by blasting or acid then treat the surface by polishing, wire wheel etc. What you see before you plate is what you get after the plate so if the surface is rough and dull it's going to stay that way.

Agitation just stirs up the crap in the bath and effects the finish. If you look in the bottom you will see sediment. You don't want to disturb that. If you set up the current and hang it correctly at the top

 

[rcx132]

The current is critical. Too high and it will burn as it's laying on to fast.

You need to regularly top up the brightener, without it you get a dull finish.

Remove the zinc either by blasting or acid then treat the surface by polishing, wire wheel etc. What you see before you plate is what you get after the plate so if the surface is rough and dull it's going to stay that way.

Agitation just stirs up the crap in the bath and effects the finish. If you look in the bottom you will see sediment. You don't want to disturb that. If you set up the current and hang it correctly at the top

Ok, only I measured the parts surface area to set the right current. The calculation said I needed 1.6A so I have it 1.0A. I'm not fussed about the dull finish. I will try without the water agitation and will regularly rotate the part.

 

[Mr shiney]

Your don't need to touch. It just hang it and leave it!

 

[Gareth J]

You can paint on top of galvanise. Different paint manufacturers recommend different primer products but it's usually an acid etch of some sort.

Anyway, sounds like you've thought it through, it was just what sprang to mind when I saw the object in the picture you posted

 

[rcx132]

I got an email from Gateros, where I bought the plating kit from. They reckon the part must have got too close to the anode and got burnt. I think that my water agitation tube which blows air bubbles must have pushed the part. As for my bad results on previous parts they think it's either dirty tanks or a problem in the parts cleaning process. They suggested test plating a piece of new sanded copper to isolate if the problem is with cleaning or the tanks.

Hopefully I'll have time to try again soon.

 

[steve_s]

Its trial and error to get it right the parts need to be surgical clean. The smother the surface of the part will give a better finish almost like chrome. I find that moving the part around in the electrolyte gives me the best finish and not using the bubbler gatros advise.
Im just using an old old battery charger and the gatros spring resistor thing, don't get hung up on calculated the surface area I found it was suck it and see.
It frustrated the life out of me for a fair bit but got it in the end

try try try try try try again good luck

 

[rcx132]

Thanks Steve. I'm keen to get this sorted so will keep trying. Often I buy special bolts which are only available unplated/blackened, so I need a way to plate them without paying the £40 minimum charge at the plants. Also big suspension bolts rust after only a year or two of being replaced, home plating should last much longer.

I bought a textbook on plating, it's a boring read so I'm only skim reading it, but a few interesting things:

1. The water agitation is actually to knock off the hydrogen bubbles that form on the part. The textbook says that the parts can be periodically shaken instead.
2. Irregular parts plate unevenly but you can adjust for this by positioning your anodes carefully. I probably can't be bothered to do this
3. The passivisation with the highest corrosion resistance is the olive drab not the yellow like most people think! The drab is about 30% more resistant! Apparently people struggle applying the yellow so maybe I should switch to the drab.
4. Caustic soda and the pickle are only a FINAL stage of cleaning. The parts are supposed to be cleaning using other methods first.

So a lot of people are basically saying cleaning is most important and the textbook says the same, so that's maybe part of why I'm getting bad results.

I've bought myself a 3 rpm motor for £7 to automatically rotate my parts as it apparently helps and I don't think it will be hard to set up.

 

[steve_s]

Yep clean clean clean and clean again !

I clean my parts in a blast cabinet to get all the crud off then de-grease off to the brick acid for a soak then caustic soda, wash in clean water then in the plating tub.
I normal hang stuff as close to the surface as possible and when Im set up I time the plating to the length of a record playing on the radio give or take.
I did all the odd nuts and bolts and some brackets on my BSA A65 4 years ago other then going dull still fine.
The clear and yellow passivate work ok for me but not had any joy with the green.
I did have an issue at one point stuff was not plating correctly I ended up mixing some new electrolyte as I must have contaminated it some how ...... so clean clean clean clean

Heres a few bits I did on a BMW GS1150