Electrolysis of Salt Water Using Steel Electrodes

[Uber-Dan]

Hello all,

I have been working on an assignment based on the electrolysis of salt water using steel electrodes, and I have hit a research block. I am finding it hard to find out what happens to the steel electrodes during the reaction. I know that a small portion of the water separates into H2 and 2OH- at the cathode, that the chlorine ions turn into a gas at the anode, and that some sodium hydroxide is produced (please correct me if any of this is wrong or if I have missed anything), but I am not sure what happens to the steel. I’m assuming that it dissolves from the anode and gets deposited on the cathode as rust, but I can’t find any research to support this (or really any other) hypothesis. If anyone could help me find the correct reactions in relation to the steel or even the other reactions, that would be greatly appreciated.

Regards, Dan

[chenbeier]

The Chloride will react with the metal of the steel. For example iron react like this.

3 Cl^- + Fe => FeCl₃ + 3 e^- 

Chlorine will be not developed.

Similar things happen to Ni, Mn, Co, and others. 

On cathode side the hydrogen will be developed 2 H⁺ + 2 e^- => H₂

Sodium and hydroxide will be in solution. If the solution is styred the Hydroxide reacts with the metal chlorides. For example FeCl₃ + 3 OH^- => Fe(OH)₃ + 3 Cl^-

 

[Uber-Dan]

Thanks for the response. My only problem is that when I conducted the experiment, there was a small amount of chlorine gas produced. I'm guessing this is because the iron wasn't pure, and had carbon (and quite possibly other impurities) added to make it into steel. What do you think?

[chenbeier]

Maybe you have a inert steel, then chlorine will be developed.  Did you observe also change of colour around the anode?

[John Cuthber]

5 hours ago, chenbeier said:

Maybe you have a inert steel, then chlorine will be developed

If you have something which is inert to wet chlorine, then you do not have steel.

[Uber-Dan]

If it helps at all, the electrodes were stainless steel (I think that adds in chromium or something)

Also, as for the colours, because of the method we had to follow (because it is an assignment), we could only have the power on for around 5 seconds, and even then we could only use 6 volts and it had to be done in a test tube. This was to try and protect the school-owned electrodes from unnecesary harm, but due to that, measuring the colours was a bit difficult. Some of my friends who are doing the same assignment, however, aparently left their’s on too long and the top part of the water near the electrodes (unsure if it was around the anode or the cathode), turned a yellow-ish colour.  Hope that helps.

[Uber-Dan]

Here is my attempt at summarising the experiment based on the above responses and extra research. So, correct me where I go wrong, the equations go as follows:

At the cathode:

2H₂O(l) + 2e^-—> H₂(g) + 2OH^-(aq)

The hydroxide is then attracted to the anode and moves there.

At the anode:

2Cl^-(aq) —> 2Cl₂(g) + 2e^- (Only where it reacts with an alloying element)

3Cl^-(g) + Fe(s) —> FeCl₃(aq) + 3e^- (More common than the straight chlorine reaction)

FeCl₃(aq) + 3OH^-(aq) —> FeOH₃(s) + 3Cl^-(aq)

Fe₃(s) + 6OH^-(aq) —> Fe₂O₃(s) + 3H₂O(l) (I am almost 100% sure this is wrong, but I would like to know if anything similar to this happens, and if so, what is the exact reaction?)

In the solution in general:

Na⁺(aq) + OH^-(aq) —> NaOH(aq)

 

This is likely riddled with issues and is also probably missing some parts, so I would appreciate it if you could point them out.

 

 

[chenbeier]

You are right that you are wrong.

It should be 2 Fe + 6 OH- => Fe2O3 + 3 H2O + 6 e-

And also this can happen Fe(OH)3 =>  FeOOH + H2O

Edited August 4, 2019 by chenbeier

[Uber-Dan]

But apart from those two, do the other ones I mentioned work? 

Edited August 4, 2019 by Uber-Dan

[chenbeier]

Yes , but Chlorine will be developed only on inert anodes or carbon. Steel or iron will be dissolved as already mentioned.

[chenbeier]

I dont think WCl4 is existing in water. It get hydrolysis to HCl and Tungsten oxides.