Understanding Metal Corrosion: Types, Causes, and Control
Corrosion is the destructive attack of a material by chemical or electrochemical reaction with its environment. When etching is accompanied by physical damage, then there is corrosion-erosion, or corrosive wear by friction.
- It is a natural process; there is a transformation of the material to a more stable compound, which is an oxide.
- Plastic or wood do not corrode; they can crack or break down.
- Corrosion of iron leads to oxidation.
Two Kinds of Corrosion
- Dry: When an attack occurs via a chemical reaction, without the interference of electric current.
- Wet: Where there is an electrochemical nature, meaning there is no electric current within the corrosive environment.
Chemical corrosion occurs when a material is dissolved in a liquid medium until the corrosive material is consumed or the liquid is saturated. Electrochemical corrosion occurs when certain metals, upon reaching a high point of valence electrons with other metals, tend to capture and produce corrosion-free areas.
Electro-chemical corrosion is a process in which a metal reacts with its environment to form an oxide or another compound. The cell is composed of an anode, a cathode, and an electrolyte.
Electrochemical Corrosion
The most accepted definition of electrochemical corrosion is the passage of electrons and ions from one phase to another boundary to be a phenomenon. Electrode, meaning material transformations with fundamental cooperation, active or passive, of a macroscopic electric field.
Corrosion Problems
There is a deterioration in the kinds of metals, eventually affecting them. If untreated, it leads to their destruction, which implies huge economic and production losses.
Types of Corrosion
General or Uniform Corrosion
Thinning occurs with uniform loss of metal as a product of regular superficial reaction. This is subdivided into:
Atmospheric Corrosion
It produces more material damage. The severity increases with the presence of salt, sulfur compounds, and other pollutants. Atmospheric corrosion is divided according to the present environments:
- Industrial: Contains sulfur compounds, nitrous acid, and other agents that can promote metal corrosion.
- Marine: Characterized by the presence of hydrochloric acid.
- Rural: The least severe kind of atmospheric corrosion, characterized by low levels of acidic and other aggressive species.
Factors influencing atmospheric corrosion are temperature, the presence of contaminants in the environment, and humidity.
Galvanic Corrosion
A form of accelerated corrosion that can occur when dissimilar metals are joined electrically in the presence of an electrolyte.
Liquid-Metal Corrosion
Degradation of metals in the presence of some liquid metals such as zinc, mercury, and cadmium.
High-Temperature Corrosion
Known as fogging or scaling. Generally, this kind of corrosion depends directly on the temperature.
Ways to avoid this kind of corrosion:
- High thermodynamic stability, as far as possible, to generate other products for different reactions.
- Low vapor pressure, so that the products generated are solid gases and do not mix with the environment.
Localized Corrosion
The loss of metal occurs in discrete or localized areas. It is subdivided into:
Crack or Crevice Corrosion
It is produced in small cavities or voids formed by contact between a piece of metal similar or different from the first, or more usually with a non-metallic element.
Ways to prevent crevice corrosion:
- Redesign the equipment or part concerned to eliminate cracks.
- Close the fissures with non-absorbent materials or incorporate a barrier to prevent moisture.
- Prevent or remove the formation of solids on the metal’s surface.
Pitting Corrosion
Occurs in areas with lower generalized corrosion, and the anodic process produces small chips that affect the body. Within this, there is:
- Friction or Fretting Corrosion: Occurs with small movements between two contacting substances.
- Cavitation Corrosion: Is produced by the formation and collapse of bubbles near the metal surface.
- Selective Corrosion: It is selective because it acts only on noble metals such as Copper or Silver-Copper-Gold.
Microbiological Corrosion
Biological organisms are the sole cause of the fault or act as accelerators of the localized corrosion process. It usually occurs in aqueous media where the metals are submerged or floating.
Corrosion in Industry and its Processes
This is the area most affected by corrosion. There are two kinds of economic losses:
- Direct: Affect occurs immediately after the attack. The most important are the cost of repairs, replacement of worn equipment, and preventive measures.
- Indirect: All stemming from failures due to corrosion, such as stopping production.
In 1971, the Hoar Report showed how costs could be reduced in each country.
Corrosion Control
Cathodic Protection
Occurs when a material is forced to be the cathode of a cell, adhering to a metal that corrodes more easily.
Anodic Protection
The metal is coated with a thin oxide layer so that it does not corrode.
Selection of Materials
- For non-oxidizing and reducing conditions, such as acids and aqueous solutions, nickel-chromium alloys are used.
- For oxidizing conditions, lead alloys are used.
- For highly oxidizing conditions, Titanium is used.
- Ceramic elements have good resistance to corrosion and high temperatures but are brittle.
Summary
- Corrosion should be seen as a fact that reveals the natural process by which metals return to their original condition, leading to the deterioration of the metal. It is this process that leads to research and approaches that allow extending the life of materials subjected to this process.
- In cathodic protection, many factors come into play and must be taken into account when designing the system.
- This work confirms that fighting and controlling corrosion is an art within the area of maintenance, and it is quite large, given the myriad of conditions that are under the metal-forming equipment, tools, and especially pipes.