General Corrosion

Uniform corrosion, also known as general corrosion, is the most common form of corrosion, which contributes most to the annual global cost of corrosion. In conventional applications this form of corrosion can usually be recognized on steels, copper alloys and silver jewelry when they are covered with corrosion products.

As depicted in figure 1, uniform corrosion results in a homogeneous deterioration of the entire metal surface, that is exposed to the corrosive environment. This initially causes a change in the visual appearance of the surface, at which point the impact on the mechanical properties of the material are negligible. If the corrosion process is allowed to continue, then it leads to gradual loss of weight and decrease in dimensions. Since this form of corrosion occurs uniformly in a given environment, the lifetime of a material can be estimated by carrying out corrosion tests in order to measure the rate of corrosion. For that purpose, the corrosion test has to be carried out in conditions similar to the real application environment. 

Figure 1. Photo of a uniformly corroded steel saw.

In the case of steels in outdoor conditions with regular exposure to moisture and oxygen, uniform corrosion is caused by randomly distributed anodic and cathodic areas on the surface, that migrate as the process takes place (Fig. 2). These regions can be created by the difference of electrolyte concentration in the environment, variance in the layer thickness of corrosion products, the local microstructure of the alloy and other factors. Corrosion of the material takes place in anodic regions, which is promoted by nearby cathodic areas due to simultaneously occurring electrochemical processes:

Anodic reaction:
Me(s) -> Men+(aq) + ne-

Cathodic reactions:
O2(g) +2H2O +4e- -> 4OH-(aq)
or
2H+(aq) + 2e- -> H2(g)

In the case of some alloys, the anodic and cathodic areas migrate on the surface over time, resulting in an uniform etching of the original material.

Figure 2. Illustration of randomly distributed and migrating anodic and cathodic areas on the surface of a metal during uniform corrosion.

Depending on the environment the metal may dissolve or form a layer of corrosion products. The formation of corrosion products depends on its the chemical stability in the environment, porosity, adhesion and physical durability in the case of mechanical interactions. Those factors play a crucial role in the progression of corrosion as well as the possible transition from uniform corrosion to other forms of corrosion. For instance, figure 3 depicts the surface of a steel container, which initially corroded uniformly but now also shows numerous sites of pitting corrosion.

Figure 3. Photo of the surface of a corroded steel container, depicting uniform corrosion with numerous sites of pitting corrosion.

Another example is aluminium, which is highly reactive and quickly oxidizes when a fresh metal is exposed to oxygen in air, forming a thin aluminium oxide layer. The latter is naturally a good diffusion barrier and well adhered to the metal, which prevents further oxidation of the metal already at a very low thickness (few nanometers). In contrast, the corrosion of iron and some steels produces a fluffy porous layer of corrosion products, which provides little protection for the metal against the environment and the corrosion process continues.

Uniform corrosion may also take place on other materials such as ceramics and polymers but there it is often described with different terminology. For instance, ceramics can be uniformly etched in highly alkaline or acidic solutions while polymers can degrade uniformly over time when exposed to sunlight. Alternatively, polymers can also be dissolved in some organic solvents (e.g. polyethylene in acetone), which can be considered a uniform deterioration of the material.

The rate of corrosion depends directly on environment, which is helpful in predicting the lifetime of certain metals in the given application. Based on corrosivity, the atmospheric environments can be divided into classes as described below;

C1 environment has very low corrosivity as it is an interior atmosphere with clean air in heated rooms without any water condensation. Practical examples would be offices and living rooms.

C2 environment has low corrosivity. It includes interior atmospheres of unheated or poorly heated rooms that have some water condensation. Examples include sports halls and exterior atmospheres that have minimal pollution for example rural areas.

C3 environment has medium corrosivity and contains considerable amount of pollutants. This class includes exterior urban and industrial environments with sulfur dioxide in air as well as coastal areas with higher levels of moisture and salinity. In the case of interior conditions, this class can be applied in the case of higher levels of humidity or pollution, which is common in swimming pools, chemical plants and unheated buildings close to sea.

C4 environment has high corrosivity and describes atmospheres with moderate pollution and/or salinity. It includes industrial and costal exterior atmospheres and interior atmospheres with high humidity and pollution.

C5-I (industrial) environment has very high corrosivity. The environment is common in industrial areas with very high humidity and aggressive polluted air. This class includes buildings with nearly permanent condensation and high pollution.

C5-M (marine) environment has very high corrosivity. This environment is common in coastal areas with high salinity and humidity with nearly permanent condensation.

CX (offshore) environment has extremely high corrosivity. This environment is common in offshore, industrial, sub-tropical and tropical areas that have extremely high humidity and chemically aggressive atmosphere. It also includes industrial buildings with similar internal conditions.

Captain Corrosion Resources and Services

Bibliography

J.R. Davis, Corrosion Understanding the Basics, 2000 ASM International.

S.A. Bradford, Corrosion Control Second Edition,

G. Wang, J.S. Spencer, D.L. Olson, B. Mishra, Handbook of Environmental Degradation of Materials second Edition, 2012 Elsevier

S. Virtanen, Tribocorrosion of Passive Metals and Coatings, Corrosion and passivity of metals and coatings, 2011

ISO 12944-2 Paints and varnishes – Corrosion protection of steel structures by prodective paint systems Part 2: Classification of environments, 1998

ISO 12944-9 Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 9: Protective paint systems and laboratory performance test methods for offshore and related structures, 2018