Corrosion of Magnesium: Reaction & Description

[Priyank]

1. In my Homework, the teacher gave me the project to collect the sample of corroded magnesium (pure metal, and not an alloy) and write a detailed description about the process related to it.

2. Although I collected the corroded sample, yet I am unable to write its description. What I know about about other metals like iron, copper and silver is that a layer of oxides or some compounds get developed on the surface of the metals due to their reaction with Oxygen and other compounds present in air. But about Magnesium, I am unaware with compounds in the atmosphere it reacts during corrosion.3. In my opinion, I think that magnesium develops a layer of Basic Magnesium Carbonate on it when left in damp air, but I am not sure about it. If I am true, please mention the reaction during the corrosion process and a brief description about it. If I am wrong, please rectify me.

 

[Borek]

You say you have collected a sample. Was it covered with any corrosion products, or just pitted?

 

[Priyank]

Not pitted, but just its surface had been defaced to some extent. Something greyish has been deposited on its surface...

 

[Borek]

surface had been defaced to some extent. Something greyish has been deposited

Isn't it the description of the collected sample?

I doubt the corrosion product is a simple substance (simple as in homogeneous), more likely it is a mixture. Basic carbonate (there are several, with different formulas) sounds like a reasonable guess. You need to find an explanation as to why it wasn't white though.

 

[Priyank]

Then it must be Basic Carbonate of Mg only...

 

[Priyank]

Isn't it the description of the collected sample?

I doubt the corrosion product is a simple substance (simple as in homogeneous), more likely it is a mixture. Basic carbonate (there are several, with different formulas) sounds like a reasonable guess. You need to find an explanation as to why it wasn't white though.

White though?? You mean a white layer must have been deposited??

 

[Borek]

What is the color of the basic magnesium carbonate?

 

[Priyank]

The color of basic Mg Carbonate is pinkish-grey... as far as my knowledge is concerned.

 

[Borek]

Pink hue is most likely because of impurities.

In general, salts of the block s elements are colorless (which makes them white in when not in the form of large crystals), unless the anion is colored.

 

[Priyank]

So we arrive at at the conclusion that the layer on the surface of Magnesium (corroded) is due to the Basic Magnesium Carbonate formed.
But, what could be the reactions during the corrosion process? Please help me in this case.

 

[Borek]

You have Mg, atmospheric oxygen, water and carbon dioxide, just choose one of the possible formulas of the basic carbonate and try to balance.

Alternatively, you may assume the first step is the oxidation of Mg by the oxygen, followed by the reaction of MgO with water and carbon dioxide. Same thing overall, just split into stages.

 

[Priyank]

1. 2Mg+O2 -----> 2MgO
2. MgO + H2O----------> Mg(OH)2
Now, the third reaction is going to be :
3. Mg(OH)2 + CO2 -----------> MgCO3.Mg(OH)2
Please check if the third reaction is correct or not.

 

[Borek]

It is not balanced, so technically it is incorrect no matter what. However, have you checked any source for the formula? Wikipedia lists several hydrates and no anhydrous form at all.

 

[Priyank]

I searched for the correct formula on few of my textbooks, and I saw one example of Basic Copper Carbonate as CuSO4.Cu(OH)2 . And on certain sites too, their was anhydrous form of Basic Carbonates. Can you tell me the correct one?

 

[Borek]

There is no "correct one", I told you much earlier - there are many possible, similar compounds. Please check the wikipedia article on the magnesium carbonate for a list of some of them.

My bet is that when the Mg corrodes in the air what is produced is a mixture of compounds, so you won't be able to give one answer. You can at best choose one of the formulas and give its synthesis reaction as an example.

 

[Priyank]

Thank You! Your advice helped me a lot...