Why do metals corrode

According to this definition, the term corrosion can be applied to all materials, including non-metals. But in practice, the word corrosion is mainly used in conjunction with metallic materials. Why do metals corrode? Apart from gold, platinum and a few others, metals do not occur in the nature in their pure form. They are normally chemically bound to other substances in ores, such as sulphides, oxides, etc.

Energy must be expended e. Pure metals contain more bound energy, representing a higher energy state than that found in the nature as sulphides or oxides. Energy state of metal in various forms. As all material in the universe strives to return to its lowest energy state, pure metals also strive to revert to their lowest energy state which they had as sulphides or oxides.

One of the ways in which metals can revert to a low energy level is by corrosion. The products of corrosion of metals are often sulphides or oxides. Chemical corrosion can be seen as oxidation and occurs by the action of dry gases, often at high temperatures. Electrochemical corrosion on the other hand takes place by electrode reactions, often in humid environments, i. This layer is built up by chemical corrosion with the oxygen in the air.

At very high temperatures, the reaction with the oxygen in the air can continue without restraint and the metal will rapidly be transformed into an oxide.

Oxidation of metal in different temperatures. At room temperature the reaction stops when the layer is thin. These thin layers of oxide can protect the metal against continued attack, e.

These layers of oxide may be more or less durable in water, for instance. We know that plain carbon steel corrodes faster in water than stainless steel. The difference depends on the composition and the penetrability of their respectively oxide layers. The following description of the corrosion phenomenon will only deal with electrochemical corrosion, i.

How do metals corrode in liquids? Let us illustrate this, using a corrosion phenomenon called bimetal corrosion or galvanic corrosion. The bimetal corrosion cell can e. The electrolyte contains dissolved oxygen from the air and dissolved salt. If a lamp is connected between the steel plate and the copper plate, it will light up. This indicates that current is flowing between the metal plates. The copper will be the positive electrode and the steel will be the negative electrode.

The current flows through the lamp from the copper plate to the steel plate. The driving force of the current is the difference in electrical potential between the copper and the steel. The circuit must be closed and current will consequently flow in the liquid electrolyte from the steel plate to the copper plate. The flow of current takes place by the positively charged iron atoms iron ions leaving the steel plate and the steel plate corrodes.

The corroding metal surface is called the anode. Oxygen and water are consumed at the surface of the copper plate and hydroxyl ions OH- , which are negatively charged, are formed. The negative hydroxyl ions "neutralize" the positively charged iron atoms. The iron and hydroxyl ions form ferrous hydroxide rust. In the corrosion cell described above, the copper metal is called the cathode. Both metal plates are referred to as electrodes and the definition of the anode and the cathode are given below.

Anode : Electrode from which positive current flows into an electrolyte. T he driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic form.

To reach this metallic state from their occurrence in nature in the form of various compounds ores , it is necessary for them to absorb and store up the energy required to release the metals from their original compounds for later return by corrosion.

The amount of energy required and stored varies from metal to metal. It is relatively high for metals such as magnesium, aluminum, and iron, and relatively low for metals such as copper, silver, and gold. Table 1 lists a few metals in order of diminishing amounts of energy required to convert them from their oxides to metal. A typical cycle is illustrated by iron. The most common iron ore, hematite, is an oxide of iron Fe 2 O 3.

The most common product of the corrosion of iron— rust—has a similar chemical composition. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration. All metals can corrode. Some, like pure iron, corrode quickly.

Stainless steel , however, which combines iron and other alloys, is slower to corrode and is therefore used more frequently. All small group of metals, called the Noble Metals, are much less reactive than others. As a result, they corrode rarely. They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not surprisingly, are often very valuable. They include rhodium, palladium, silver, platinum, and gold. There are many different reasons for metal corrosion.

Some can be avoided by adding alloys to a pure metal. Others can be prevented by a careful combination of metals or management of the metal's environment. Si-COAT Anti-Corrosion Protective Coatings can be applied to a wide range of applications, such as structural steel, bridges, machinery and equipment, areas with heavy corrosion, tank exteriors, metal roofs, cladding, and more.

Si-COAT AC protective coatings are ideally applied to where the necessary coverage is essential and maximum protection, adhesion, elasticity, and longevity are required. Posted on Monday, September 12, in Blog. What Causes Corrosion? What Is Corrosion? How Corrosion Occurs Corrosion is an electrochemical reaction that appears in several forms, such as chemical corrosion and atmospheric corrosion, the latter of which is the most common form.