[Faunastia]

EDIT: Self-solved. Obviously it has to be the 300sec interval for each calculation. And ideally, corrosion speed had to be calculated real-time each second or even ms, to be as accurate as possible. Im just keeping this here, in case anyone ever wondered the same thing once.

Hello. I do know how Electroplating works, but there is one task I dont really understand how to solve it, since the teachers did not discuss it yet, but still want to see the task done.

Basically, it was about electroplating/galvanic corrosion. There is Magnesium and Iron.

We weighed the Magnesium in dry state before putting it in the liquid together with the Iron. (It was 100ml of NaCL of 0,1Mol liquid, mixed with water)

We weighed the Magnasium after 5, 10, 15min... etc. (Basically, every 5min) and noted down the mass. And with a multimeter, we got the amperage at these times.

We did not add extra amperage, but made use of the electrochemical potential of elements.

Now the task is to compare the weighed mass with the theoretical calculated mass of magnesium.

I understand, that the weighed mass of Magnesium is 'fake', because magnesium is in a wet state when taken out every 5min, to weigh the mass.

So, I do see logic in calculating the mass and comparing weighed mass with calculated mass.

My issue is: I dont exactly know if I am doing it wrong. To  be able to get the mass, I calculated the corrosion speed. The corrosion speed is always different at given time, since the amperage also naturally changes.

Now, I can basically multiplay corrosion speed with the time delta t. (V * t) to get the mass delta m.

But what time do I need to take? Do I basically use 300, 600, 900, 1200 etc... (since it was in 5min steps) or do I uase only 300sec? Because amperage changes itself, so when it was 130mA at 300sec, then it is, 150mA at 900sec. My head says: I cant use 900sec to get the mass for 150mA, because until 300 sec, it was only 130mA, and it reached 150mA only at 900sec (which is actually not true either, because reaching 150mA was only at 900sec, but not even during the whole 300sec from 600-900sec.)

So, how exactly do I need to handle this, to stay fair and close enough to the real amperage changes, to get the mass changes?

So, do I use 300sec every time for each calculation, or do I actually use 300, 600, 900sec... etc

[Borek]

Do you know what integration is?

To calculate change in the mass you basically you need the total charge, which is an integral of current over time.

[Faunastia]

Do you know what integration is?

To calculate change in the mass you basically you need the total charge, which is an integral of current over time.

Thanks for your advice. I probably expressed myself confusing. By now, I solved the issue. It was about finding out the theoretical calculated mass change every 300sec.

Unfortunately, we did not measure the current. I meant it literally, that we only had the timestamps, the weighed masses and the amperage. The teacher told us to work only with these numbers and that we shouldnt pay attention to the current.

(Im sorry, I couldnt find out how to add mathematical formulars.)

The solution I came up with, was the following: V = (I * M) / (z * F)
I = Amperage,
M= g/mol of Magnesium,
z= valence electrone number of Magnesium
F = Farady constant
V = corrosion speed

After calculating the corrosion speed at each timestamp, I can calculate each mass-loss at each time stamp, by using the formular: V * (delta) t= (delta) m

The new issue was basically: do I use 300sec as "t" for every calculation, or 300, 600, 900, 1200 etc... After thinking about it a bit, it was logical to use 300sec, since the amperage changes every 300sec, and I cant (for example) use 1200sec for the calculation at timestamp 1200sec, since at 900sec the amperage was also different. So, I can only roughly say between 900 and 1200 sec the amperage of timestamp 1200sec was present (basically for 300sec long, which is actually not true either, since amperage changes every second or even every ms, but we only measured it every 300sec).

This way, I can get the theoretical mass-loss every 300sec and compare it with the manually weighed mass loss.

[Borek]

Unfortunately, we did not measure the current. I meant it literally, that we only had the timestamps, the weighed masses and the amperage. The teacher told us to work only with these numbers and that we shouldnt pay attention to the current.

I am afraid what you wrote doesn't make much sense. Amperage IS the current, you can't use it and not use it at the same time.

From your description you are reinventing the wheel - you are doing numerical integration of the current over time assuming it was constant for every 300 sec. That's generally OK, although you would probably get a slightly better result assuming linear change between the data points (google for trapezoidal rule).

[Enthalpy]

Magnesium and electroplating? That's looking for worries.

Could you tell which are the base materials and what the deposited layer shall be?

A few companies claim to deposit protective layers on magnesium and comment "difficult". They use special cleaning processes and deposit a first layer (nickel) before depositing the chosen material.

[Borek]

Magnesium and electroplating? That's looking for worries.

Could you tell which are the base materials and what the deposited layer shall be?

Have you read the whole post? The actual question is not about deposition, but about corrosion speed, so about dissolving magnesium.

[Enthalpy]

Mea maxima culpa!

Magnesium corrodes spontaneously in water. It doesn't have to produce any current for that. So measuring the current doesn't represent the corrosion speed.

Mg/Fe batteries exist that use seawater as an electrolyte. But their are known for their strong self-discharge despite using Mg of high purity. Only bulky electrodes and fast use makes sense with them.

If the experiment's Mg was an alloy, no chance.

[Faunastia]

I am afraid what you wrote doesn't make much sense. Amperage IS the current, you can't use it and not use it at the same time.

My bad, you are right. I mixed up voltage with current all the time. (Thinking Volt was current, for some reason).

I think I partially understand now, what you are trying to say. Since the current is not linear (it is jumping up and down) I have to calculate multiple partial charges for every 300sec time range.
With the partial charges, I can calculate the number of electrones and thus the total mass of these electrones. This way, I have the mass loss values for every 300sec. Is that right?

Yeah, your method sounds less complicated than the one our teacher wanted.

And just by the way: By now the teacher approved the method I used. Looks like they wanted it the way I did it. (In case anyone is interested, if the solution I tried was right or wrong)

---

EDIT: Again, thanks everyone for your comments. They made me understand the experiment better. I think this experiment was supposed to be inaccurate. Since, as some comments say, there are alot of other factors influencing the experiment, like the one that Magnesium corrodes inside of water spontaneously, making the mass calculation based on current only inaccurate etc. I guess it was just an experiment to find out these factors, when comparing manually weighed number with the (close to the) mathematical mass loss, even if the mathematical mass loss was strictly not accurate enough.

EDIT 2: The title of my question was maybe a little bit misleading. We basically made use of the electroplating/galvanic idea, to cause magnesium to corrode faster. The whole experiment was made of different partial experiments under the very same title, including the magnesium corrosion experiment. Of course this doesnt mean that only Magnesium is a good element for electroplating.