In the previous article (Corrosion basics #1, What is corrosion?) we have seen how corrosion can impact the life of a structure and how it evolves with environmental conditions. In this new article, we propose to give you an overview of the main types of corrosion.

1-UNIFORM CORROSION

It is the most classic form of corrosion and is the easiest to spot. As its name indicates, this corrosion attacks a metal surface in a relatively uniform manner. The surface degrades more or less rapidly depending on its environment and the appearance of iron oxide (i.e. red rust) can be observed. The impact of corrosion can be quantified by measuring the decrease in thickness per year (μm/year) and the average values per material being known, it is possible to calculate the life expectancy of a component.

 Example of uniform corrosion

2-PITTING CORROSION

It is a localized corrosion that leads to the creation of small cavities on the surface of a material. The corrosion process occurs when the passive layer* degrades in a localized manner. Holes can corrode very quickly in depth, while the rest of the surface remains intact. Passive metals and alloys such as aluminum, titanium and stainless steels are subject to this type of corrosion once the thin oxide layer on their surface is locally degraded.

*a passive layer is a thin protective layer on material surface. See the article "The Basics of Corrosion #4", Focus on Stainless Steel

This type of corrosion is most often observed in very aggressive, high temperature, high chloride ion environments (such as indoor swimming pools). Unlike uniform corrosion, this type of corrosion is more difficult to detect, because the holes formed are usually no larger than a pinhead. The quantity of material consumed in depth is difficult to quantify, which does not allow to predict the life span of the attacked part. For this reason, the choice of a suitable protection must be made at the design stage.

Example of pitting corrosion on stainless steel

Phases of pitting corrosion on stainless steel

3-STRESS CORROSION CRACKING, SCC

This form of corrosion results from the combined action of tensile stress and a corrosive environment. This type of phenomenon can lead to the sudden brittle failure of a ductile metal* subjected to a stress level below its elastic limit. All that is required is for a small cavity to be formed on the surface of the metal to develop into a crack due to the level of stress (residual or applied) in the material (stresses are highest at the crack front). Once the crack exists, the reaction is self-accelerating, as new active zones (not protected from corrosion) appear. These zones are then corroded which weakens the metal and leads to the propagation of the crack and thus the appearance of other active surfaces which corrode. The crack propagates continuously and the material in the crack corrodes rapidly until mechanical failure occurs. This makes SCC a dangerous phenomenon, because despite a stress level below the elastic limit, the component can be destroyed.

*A ductile material is defined by its ability to deform plastically (without breaking) once the elastic stress is exceeded. Conversely, a brittle material fractures as soon as the elastic limit is exceeded.

Generally, SCC occurs only with alloyed metals, it is known for example that some grades of stainless steel can be subject to this kind of corrosion when they are in "aggressive" environments such as indoor swimming pools (high chloride content). In this type of configuration, stainless steel grades with high corrosion resistance must be used (with a molybdenum content higher than 6% - see the article Corrosion Basics #4, Focus on Stainless Steel). It is therefore important to provide upstream anti-corrosion protection adapted to the environment in contact and/or to ensure a very low level of stress.

Stress corrosion cracking of an assembly after 4 years in an indoor pool

4-CREVICE CORROSION 

This is localised corrosion that occurs at a crevice or the gap between two or more joining metals. It is pitting corrosion that occurs specifically in the low flow region of a crevice. This type of attack is usually associated with small volumes of stagnant solution caused by holes and crevices under bolt and rivet heads, among other things. Degradation occurs due to the difference in concentration of constituents, primarily oxygen, in the surfaces involved. Since the crevice has a restricted oxygen concentration, it becomes an anode and thus the preferred region for metal dissolution.

Being very localised, crevice corrosion can be extremely rapid. In some cases, this type of corrosion is a danger that can be avoided by choosing the right materials upstream, by adapting the joints (by opting for welding for example) and by designing an assembly that is not very prone to crevice formation.

Illustration of crevice corrosion 

5-GALVANIC CORROSION

This reaction occurs when two dissimilar metals are in contact (or there is an electrically conductive path between them). The metal that is less resistant to corrosion (the less "noble" one*) becomes the anode and corrodes more quickly. The metal that is more resistant to corrosion becomes a cathode and its corrosion slows down. This type of corrosion causes the degradation of only one of the metals in contact. The more noble metal is protected. This phenomenon, called cathodic protection, is sometimes used voluntarily to protect elements while sacrificing others. This method is particularly used to protect buried or immersed metallic structures (such as the shell of ship for example).

*A metal is noble when it resists corrosion and oxidation (gold, silver, platinum... are noble metals). Here it is to compare the resistance to corrosion between two metals.

Example of galvanic corrosion in an assembly, here the washer is the weakest corrosion resistant element

The impact of galvanic corrosion on life span depends on the materials involved. Below is a table showing the impact of this corrosion for various combinations of materials under outdoor atmospheric conditions.
  

Impact of galvanic corrosion on fastener lifetime

To learn more about corrosion, please visit our AskHilti platform:

- Read our article on the basics of corrosion #1, What is corrosion?

- Read our article on the basics of corrosion #3, Our anti-corrosion solutions

- Read our article on the basics of corrosion #4, Focus on Stainless Steel

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