0................................................................ Corrosion

Corrosion

Corrosion is the deterioration of a material by reaction with its environment. Corrosion causes the following Problem such as:

The most common corrosion is the electrochemical oxidation of metals in reaction with an oxidant such as oxygen in the air. Rusting, the formation of iron oxide is a well-known example of electrochemical attract. This type o damage produces oxides or salts of the original metal. Corrosion can occur in materials other than metals such as ceramics or polymers too.

Corrosion control is achieved by recognizing and understanding corrosion mechanisms, by using corrosion-resistant materials and designs, and by using protective systems, devices, and treatments.

Corrosion mechanism

The environment is the main factor in any corrosion situation. The definition and characteristics of this variable can be quite complex. One can use thermodynamics, e.g., Pourbaix or E-pH diagrams, to evaluate the theoretical activity of a given metal or alloy provided.

In our societies, water is used for a wide variety of purposes in our life and industries, such as heat exchange and waste transport. Since steels and other iron-based alloys are the metallic materials most commonly exposed to water, aqueous corrosion will be discussed with a special focus on the reactions of iron (Fe) with water (H2O).

Anodic Reaction:

Fe = Fe++ + 2e- or
4Fe = 4Fe++ + 8e¯

This process is referred to as electrochemical corrosion. The reaction can only occur if there is a suitable electron acceptor to combine with the electrons released by the iron atom. Seawater contains dissolved atmospheric oxygen which readily serves this purpose. The oxygen is electrochemically reduced to hydroxyl ions in the cathodic reaction.

Cathodic Reaction:

O2 + 2H2O +4e =4OH- or
2O2 + 4H2O + 8e = 8OH¯

The heterogeneous character of steel allows for some sites to favour the anodic reaction and for others to favour the cathodic reaction).

The ferrous ions and hydroxyl- ions formed combine together to produce ferrous hydroxide:

4Fe++ + 8OH¯= 4Fe(OH)2

The ferrous hydroxide formed reacts with more oxygen to form hydrated ferric oxide, the familiar reddish brown rust.

4Fe(OH)2 + O2 =2Fe2O3 .H2O

Rust in theory, all of these reactions occur simultaneously and can be simply represented as:

4Fe + 3O2 + 2H2O = 2Fe2O3.H2O

In reality, the process is not so simple. Corrosion, and the rate at which corrosion occurs in seawater for example is depended of many factors, such as oxygen concentration, pH, water temperature and the presence of dissolved salts ( chloride, sulphate, and sulphites).

There are many different types of corrosion such as pitting corrosion and bacterial corrosion, both of which are particularly relevant for the marine industry (these topics will come later).

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