April 03, 2020, 08:37:00 PM
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Topic: We're unable to electroplate chromium to zirconium and we don't know why...  (Read 157 times)

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Offline uninc4life2010

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Hello,

I'm a student conducting an electroplating experiment for a senior design project.  We are attempting to plate chromium onto the inner diameter surface of a zirconium alloy tube.  We have been unsuccessful in plating any chromium to the surface of our zirconium tubes or to sections of the tubing that have been cut in half.  I'll detail our experimental setup below:


Plating solution:

283 g/L CrO3
2.83 g/L H2SO4


Tubing

Zircaloy-2 (Zirconium alloy)


Anode Material

Titanium Wire (0.8 mm in diameter)

Current Density Applied

20-30 Amps/square decimeter (in relation to the surface area of the tube we're plating.)

Procedure:

A segment of zirconium tubing was cut in half down the middle and etched in a 0.4 M solution of perchloric acid for 20 minutes.

Chromium plating solution of the above concentration was prepared in a beaker and brought to a temperature of 150 degrees Fahrenheit.  The solution was not agitated.  A section of 0.8 mm titanium wire was placed in the beaker and connected to the positive end of a power supply unit. The half section of etched zirconium tubing was placed in the solution about one inch from the titanium and connected to the negative lead of the power supply.  A current density of around 20-30 Amps/square decimeter was applied to the anode in solution and the reaction was allowed to proceed for about 20 minutes.  Vigorous bubbling was noticed on the both the anode titanium wire and the zirconium cathode. After 20 minutes, the power source was turned off, the hot plate was turned off, and the solution was allowed to cool. 

The zirconium tube was rinsed in deionized water and placed under a microscope.  Optical images showed little visible difference between etched zirconium and the zirconium sample that was in the plating solution with current applied for 20 minutes.  A similar procedure was performed  with a full tubing segment and a titanium anode strung through the center of the tube held in place with plastic endcaps that insulated the tubing body from the centralized anode. Similar results were observed under the optical microscope. 


Does anyone know why we are not getting a chromium plating on the surface of the tube?  My feeling is that we could be using too thin of an anode, using the wrong material for the anode, using too much/too little current, not agitating the solution, or some combination of all of these things.

My main hypothesis is that the titanium anode is too thin and has too much resistance compared to something like a lead anode.  This is causing the anode to get too hot in the bath, possible causing localized boiling of the solution, and causing a thin film of bubbles to envelope the anode and possible the cathode.  Also, since the solution is not being agitated, it is possible that there is localized depletion of the chromium ions in solution. 

Any ideas here?  I feel like there is a very simple explanation for why this isn't working.  I very much appreciate any suggestions.   

Offline chenbeier

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Probably the surface is still not clean enough, so do you have any pretreatments with Cleaner/ conditioner before plating?

Offline Enthalpy

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Section of the Ti wire: the resistance of the lukewarm full length is nearly 1Ω/m. 30A (how much do you have?) would dissipate 900W/m where the wire conducts the full current. That's not huge for a fully immersed wire. Difficult to compute, but you might be near the bubbling limit. Try the same current through you wire in clear water, observe. However, if bubbles stopped the current flowing, you'd notice it at the ammeter.

If you use a current like 30A in a 0.1m long Ti wire (what are your figures?), the voltage drop of 1V (for a current that decreases linearly with the position on the wire) is too big and may create parasitic electrochemical reactions.

The current density in the solution is big near the wire. Plating tries to avoid that. What is the voltage drop in you setup?

Depletion of the solution: let's imagine that you put 30A for 20mn. That would deposit 3g of Cr. You have maybe 100g in the solution. But close to the Ti wire, or within the full-section tube (dimensions?), the solution will be depleted if not stirred. However, you'd notice even a thin layer of chromium, much brighter than zirconium.

Maybe zirconium gets corroded or oxidized in warm acid solution, before any significant layer of chromium deposits on it?

Did you find literature confirming that zirconium can be chrome-plated that way? Does it need a nickel layer first? How does the voltage vary over time at your setup, admitting that the current is constant?

My suggestion is to experiment with variants.
  • Connect the wire at both ends if not already done.
  • Put several Ti wires in parallel, hold separated from an other by twisted Ti wire.
  • Try to deposit Cr on Cu or Ni.
  • Replace the Ti wire by Cu (your solution will be polluted after that).
Please be careful with the chromium compounds. They are seriously dangerous
https://en.wikipedia.org/wiki/Chromium_trioxide#Safety
and my gut feeling is that you have a limited knowledge margin over this particular setup.

Your titanium wire may also catch fire brutally if it conducts the current and emerges in the air. Small amount, but burning titanium ignites surrounding materials very effectively. Eliminate anything flammable to 1m distance at least.

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