Forms of Corrosion

Forms of Corrosion

There are eight [1] classified forms of corrosion, these are:

  1. Uniform attack (or general corrosion)
  2. Crevice corrosion
  3. Pitting
  4. Stress-corrosion cracking
  5. Galvanic corrosion (two metal corrosion)
  6. Intergranular corrosion
  7. Selective leaching (dealloying)
  8. Erosion corrosion

 ref: 1. M. G. Fontana and N. D. Greene, “Corrosion Engineering”, Chapter 3, McGraw-Hill, New York (1978).

1. Uniform Attack

This is arguably the most common form, at least by common sight, and it is when the metal is attacked relatively evenly over the surface, due to the distribution of the electrochemical cells. Generally there is nothing on the surface or in the metal structure that would give preference to corrosion to one area over another. The corrosion causes the metal to thin,  breaking off in flakes and eventually leading to complete destruction if no countermeasures are taken.

2. Crevice Corrosion

This form of corrosion is one of three localised forms, the others being Pitting and Stress-corrosion cracking, and it differs from uniform corrosion in that the electrochemical cells are fixed in place. The consequence is such that corrosion takes place along defined paths on the metal surface. This could be because the right conditions (temperature, pH and oxygen concentration) are concentrated at a specific point, or that a protective film on the surface of the metal is much thinner or non-existent at a point. A good example is where a potion of the metal is shielded from the environment, like a screw thread or a surface coating that has a crack or defect, the remaining metal is exposed.

The effect of partial shielding of the metal creates a difference on the oxygen concentration: the shielded areas are of lower concentration compared to the exposed areas. This creates a difference in the reduction (redox electrochemical cells) of dissolved oxygen and the limiting ability of an electric current for that reduction (limiting current density is inversely proportional to the thickness of the dissolved oxygen layer) to take place. Because of this restriction in the geometry of the crevice, the electrolyte becomes acidic and other ions become concentrated. In sea water applications, chloride ions with in the electrolyte become sufficiently concentrated to the point where an highly aggressive attack starts to break down the protective film on the metal. It may take years to initiate this corrosion, but once started the process rapidly accelerates in this acidic environment.

3. Pitting

In this form of localised attack, the protective surface film on the metal surface has been damaged or broken at a specific spot and the base metal is exposed. A common occurrence of this type of corrosion is usually due to the ability of chloride ions, or any halide ions, to penetrate the protective surface film, then form complexes which causes film thinning and the eventual dissolution of the film.

4. Stress-corrosion Cracking

The effect of this form of corrosion is the combination of metal under mechanical stress and the corrosive environment. Stress-corrosion cracking (or mechanically assisted corrosion) can occur in the metalwork structures under high loads or high stresses, like bridge structures, supporting beams and cables or even turbine blades in aircraft engines. This localised form of corrosion, much like crevice and pitting, starts at a specific spot, but aided by mechanical stresses, the spot develops cracks which effectively reduces the cross sectional area of the metal under load. As you can imagine, reducing the cross sectional area of the metal under load, there comes a point where the metal cannot withstand the load and the cracks rapidly propagate, causing complete failure.

5. Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar are placed in physical or electrical contact. In an ionic environment, like sea water, the two metals set up a electric potential across the ionic bridge and corrosion takes place on the metal that acts as an anode (negative charge).

6. Intergranular Corrosion

Intergranular corrosion is the attack that occurs in the regions around the grain boundaries in an alloy

7. Selective Leaching

Selective leaching is the selective removal of one element in an alloy by corrosion

8. Erosion Corrosion

Erosion corrosion is the attack of a metal by the movement of a corrosion liquid against the surface. The degree of corrosion is tied to the flow rate of the liquid and the turbulent nature. Naturally this is common in pipe work and other similar installations.