Corrosive environments

Corrosion and corrosive environments:

In most chemical environments, metals are exposed to corrosion, for example, they are corroded in contact with water, moisture, acids, bases, salts, oils, gases and other chemicals.

In general, compared to organic materials, minerals are more corrosive. For instance, in the oil, gas and petrochemical industries corrosion is often caused by mineral substances such as chlorine, sulfur, etc. rather than organic substances such as, petroleum, diesel, and oil.

Corrosion reacts in different environments, which will be explained briefly as follows:

1) Acidic environment

2) Neutral environment

3) Alkaline environment

4) other environments

Corrosion in acidic environments:

Among the common ways to produce hydrogen in the laboratory, one is to place zinc in a dilute acidic solution such as hydrochloric acid or sulfuric acid. In this way, through occurring a rapid reaction, zinc metal is attacked (dissolution of zinc metal) and hydrogen is released as a gas:

ZnCl₂ + H₂(g)=> Zn + 2HCl

In the same way, zinc metal reacts with sulfuric acid as follows and the result is zinc sulfate (salt) and hydrogen gas:

ZnSO₄ + H₂(g) => Zn + H₂SO₄

It is true for other metals as well, which are corroded or dissolved by acids, and the result is a soluble salt and hydrogen gas:

FeCl₂ + H₂(g) => Fe + 2HCl

2AlCl₃ + 3H₂(g) => 2Al + 6HCl

The two reactions mentioned above show that both iron and aluminum metals are corroded by hydrochloric acid.

Corrosion in neutral and alkaline solutions:

Corrosion reactions mostly take place in environments such as freshwater, sea water, saline and alkaline solutions, of course, to carry out such reactions, the presence of oxygen is necessary, which is usually provided from the air in aquatic environments. As an example, rusting of iron in a humid environment can be mentioned, which actually takes place in the reaction below: 

Anodic reaction:   Fe→Fe²⁺+2e^–

Since the desired environment is in contact with the atmosphere, it contains dissolved oxygen. Water and sea water are almost neutral, so the cathodic reaction will be:

Cathodic reaction:  O₂+ 2H₂O 4e^–→4OH^–

With due attention to the fact that sodium and chlorine ions of salt solutions do not take part in the reaction, the whole reaction is obtained from the sum of both anodic and cathodic reactions:

2Fe+2H₂O+O₂→2Fe²⁺+4OH^–→2Fe(OH)₂

Precipitates of ferrous hydroxide are separated from the solution. This compound cannot be stable in solutions containing oxygen and the result is ferric oxide salts.

2Fe(OH)₂+H₂O+1/2 O₂→2Fe(OH)₃

The obtained ferric hydroxide loses its water and turns into a rust and brownish-red color:

2Fe(OH)₃→Fe₂O₃+3H₂O

Also, zinc metal reacts in the presence of oxygen and humid environment as follows:

2Zn+2H₂O+O₂→2Zn(OH)₂

Zn(OH)₂→ZnO+H₂O

Zinc oxide is obtained from a white precipitate that can be seen on galvanized buckets, rainwater channels, etc.

Note: Compared to soft water, hard water usually has less corrosive properties, the reason is the tendency of hard water to precipitate and form a protective layer of calcium carbonate.

Corrosion in other environments:

Corrosion may also take place in non-acidic and oxygen-free environments, for example, the reaction of zinc metal in chloroferric and copper sulfate environments can be mentioned:

ZnCI₂ + 2FeCl₂ =>Zn + 2FeCl₃

ZnSO₄ + Cu => Zn + CuSO₄