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Thread: Home anodising

  1. #1
    Join Date
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    Default Home anodising

    As part of the restoration of an Oz designed 1960's kit built guitar amp I needed to organise a new front panel to replace the original which was in very poor shape, the original was silk screened but back in the 60's engraved panels were available so I thought to try and engrave a replacement. The first attempt was a fail - an error in choosing the tool path resulted in a few of the letters being engraved way too deep, the second attempt was better and possibly could be used - to finish it off I thought of plain anodising then filling the engraving with black paint, just incase I changed my mind and decided on a colour scheme, I picked up 3 packets of Rit clothing dye while I was in Spotlight with my wife.
    After watching a few youtube videos I made a start one morning - clean the panel - dunk in caustic soda solution for a few minutes - wash with rain water - into the anodising tank (1 in 10 sulphuric acid/water mix) which had a lead (old roof flashing) cathode/negative - the positive connected to the amplifier panel, power came from a 6/12v battery charger max 8 amps. The youtube videos said bubbles were an indicator that things were working - battery charger indicated 6 amps @12V - there were plenty of bubbles but mostly from the 'stainless' bolt and washers I had used to connect an Al connection strip to the panel - pulling it out I saw that the washers were in fact plated brass, I managed to fashion a new connection from just a 2mm Al weld rod and tried again - plenty of bubbles from the lead cathode but nothing from the anode(front panel) - bad connection ? I didn't think so - the charger was still showing 6 amps. I let it go for 5 min - still no bubbles from the anode so I thought something is wrong with the anode, maybe not clean enough - rather than continue I shut it down and put the front panel to one side and grabbed a section of the first panel with the failed engraving, a quick scrub with steel wool and into the tank, a smaller surface area so the charger was now showing 2 amps but still no bubbles from the anode - I thought let it go for around 1.5 - 2 hrs and see what happens. A quick wash with rain water, the section of panel looked no different - I couldn't tell if it had worked or not, I though not because of the lack of bubbles from the anode.
    As a test I mixed up approx a tea spoon of Rit dye into a 500ml jar of warm water and dunked the panel in and left it for around 10 min, I pulled it out to find a nice red wine colour had penetrated the section of Al, 15 min in a saucepan of boiling water 'fixed' the colour - not bad from something I though had failed.


    For those that are interested the sulphuric acid mix was 1 to 10 of water - the acid is 98% conc, a thick oily liquid - one needs to be careful with this - ALWAYS add acid to the water - the reason for this is that a great deal of heat is generated when mixing the two - the acid is denser than water and sinks so any heat generated is well below the surface, if water is added to acid the water being lighter will sit on top and the heat may cause the water to boil which results in conc acid being thrown off and onto the operator(you).


    The current density required from the power supply is between 700 and 900 amps per squ foot per minute - that is with 1 squ foot of anodising, a current of 800 amps will result in the anodising taking place in 1 minute - I first calculated my front panel would take 45 mins @ 5 amps, this was incorrect as I had forgotten the panel had a front as well as a back so time should be 90 min so possibly it was just as well I put that aside and went ahead with a test instead.


    I had read conflicting reports about the suitability of clothing dye, but thought it was worth a try, I also have black and blue so I'll do a few test pieces before trying the real thing.

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    Screenshot 2022-07-06 233249.jpg 20220713_201431.jpg

  2. #2
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    Quote Originally Posted by familyguy View Post
    For those that are interested the sulphuric acid mix was 1 to 10 of water - the acid is 98% conc, a thick oily liquid - one needs to be careful with this - ALWAYS add acid to the water - the reason for this is that a great deal of heat is generated when mixing the two
    Yep. It’s scary how hot it gets. I recommend adding half the acid to the water and waiting for it to cool down before adding the rest.
    Chris

  3. #3
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    Sodium bisulphate is supposed to be a good (safer) alternative to sulphuric acid.

    Anyone tried it?

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by russ57 View Post
    Sodium bisulphate is supposed to be a good (safer) alternative to sulphuric acid.

    Anyone tried it?
    Haven't tried it but I've seen plenty of you tube vids demonstrating success with it and if Was going to do it that's what I would use.

    My BIL gave me half a litre of sulphuric acid and I was going to use it for anodising but it turned out to be more dilute than I thought so I neutralised it with sodium carbonate and flushed it down the sink. I already have too many noxious things in my shed to worry about more st

  5. #5
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    If one doesn't feel comfortable using a process then that is fine each to his own - I have an almost full winchester of the acid and couldn't see any reason not to use it and go out and buy some sodium bisulphate, in addition the 10% acid solution is a good pickling agent to clean up copper and brass parts after silver soldering - as with any part of our hobby/work be it turning grinding etc one needs to be aware and act responsibly, in the majority of cases injury in the workshop is the injured person's own doing - loose clothing, only one hand on the angle grinder, thongs on the feet, no safety glasses etc etc etc. we are probably all guilty of not taking proper care at one time or another.


    I had another go at the Al front panel, it seemed that my efforts the day before did result in anodising taking place as it was a bit of an effort to get a good electrical connection onto the panel, - once this was fixed I let it go for around 120 min then rinsed off - didn't really look much different, then boiled in my home made steel tank on the kitchen stove - it spans 2 burners - to fill the engraving with black I first tried a very fine brush wiping away with a cotton bud anything that went over the lines - very time consuming and didn't really work well, I ended up flooding the engraving with black water based paint and wiping the excess off with paper towel wrapped around a cork sanding block, once most of the excess paint had been wiped off I let the paint dry and then went over the panel with a paper towel moistened with acetone, while it looks ok it isn't great - I used an old piece of Al and all of the nicks and dents showed up.


    I did a test piece to see how the Rit black would turn out - a piece of Al approx 65mm x 120mm - anodising in 10% sulphuric - 2.5 amps for 90min - rinse then into the dye, about 1 small teaspoon into 500ml of warm water for around 10 min then boil for approx 10min to 'fix' the colour - result was better than expected and it is indeed black - perhaps those that report that clothes dye does not work used a different anodising method, given that it turned out ok I'll probably have another go at the front panel. This time though I'll use a fresh piece of Al - anodise and dye black then I'll do the engraving so it will be silver text on a black background. I know there are those on the forum that make their own enclosures and it is always a bit of a headache on how to finish off and give it a decent look, I need to make a front panel to hold my CNC off line controller - It will have various switches and controls that need to be labelled so I'll probably anodise and then engrave.

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  6. #6
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    Having the sulphuric acid on hand is a great reason to use it...

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