Analysis of the Main Causes of Metal Corrosion

Metal corrosion is a widespread and complex form of material degradation that is related not only to the intrinsic properties of the metal itself, but also to the external environment and the manufacturing process. Below, we provide a systematic analysis of the causes of metal corrosion from three perspectives.

I. Internal Factors Contributing to Metal Corrosion

From a thermodynamic perspective, the metal itself is in a higher energy state compared to its corrosion products, representing an unstable condition. Consequently, it has a natural tendency to transform into lower-energy corrosion products (such as oxides and hydroxides). Therefore, metal corrosion is a spontaneous and irreversible natural phenomenon.

In industrial applications, most metals are multi-component alloys. Due to their complex microstructure, there are physical, chemical and electrochemical inhomogeneities between different components. Coupled with potential issues such as impurities, uneven stress distribution and mechanical deformation, these internal structural defects readily form micro-cells upon contact with external media (such as moisture or salt spray), thereby inducing chemical or electrochemical corrosion.

II. External Factors Affecting Metal Corrosion

Relative Humidity

The higher the air humidity, the more readily a water film forms on the metal surface, providing a medium for oxygen penetration and electrochemical reactions. When relative humidity exceeds a certain critical value (for example, approximately 70% for steel), the corrosion rate rises sharply.

Temperature

In dry environments, corrosion is unlikely to occur even at high temperatures. However, when humidity exceeds the critical value, the accelerating effect of temperature on corrosion becomes very pronounced. Generally speaking, for every 10°C increase in temperature, the corrosion rate of metals approximately doubles; this explains why equipment in humid tropical regions or during the rainy season is more prone to rusting.

Oxygen

Oxygen is an indispensable component in the rusting process of metals. Typical reaction processes include:

Fe + H₂O → Fe(OH)₂

Fe(OH)₂ + H₂O + O₂ → Fe(OH)₂

Fe + H₂O + O₂ → Fe(OH)₂

It is evident, therefore, that rust cannot form without the combined action of water and oxygen. Oxygen constitutes approximately 20% of the air by volume and is highly permeable.

Other Atmospheric Pollutants

If the air contains pollutants such as salt spray, sulphur dioxide, hydrogen sulphide or dust, the rate of corrosion will be significantly accelerated. Consequently, there are marked differences in the corrosive environment across different regions—for instance, corrosion is generally more severe in urban areas than in rural areas, in industrial zones than in residential areas, in coastal regions than in inland areas, and in areas with high dust concentrations than in clean environments.

III. Corrosion Triggers During Processing

During the production and use of metals, certain process operations may also become sources of corrosion.

For example:

1. Existing rust on the surface of raw materials has not been thoroughly removed;

2. Residual cutting fluids or lubricants from machining processes;

3. Incomplete neutralisation or rinsing following acid pickling;

4. Oxidation or peeling of the carbon layer during heat treatment;

5. Residual salts not fully removed during cleaning;

6. Operators’ hand perspiration remaining on metal surfaces during assembly or handling;

7. Improper stress-relief or hydrogen embrittlement annealing processes;

8. Failure to apply timely rust prevention between processes, or inappropriate storage measures.

The above factors may all act as ‘trigger points’ for corrosion, leading to rusting issues in metals even before they are put into service.