Surface Protections
Corrosion
Types of Corrosion
Introduction
To classify the different types of corrosion, they are characterised by their visual indicators and the type of degradation they cause.
- Uniform Corrosion.
- Pitting Corrosion(pitting corrosion).
- Crevice Corrosion.
- Filiform Corrosion.
- Fretting Corrosion (fretting corrosion)
- Galvanic Corrosion.
- Intergranular Corrosion.
- Exfoliating Corrosion or lamellar.
- Stress Corrosion.
- Microbiological Corrosion.
Uniform Corrosion
Uniform Corrosion
It is probably the most common type of corrosion.
It results from a direct chemical attack on a metallic surface that occurs uniformly across the entire exposed surface. The metal gradually thins and eventually fails.
Uniform Corrosion
On a polished surface, this type of corrosion first manifests as a general dulling or surface attack and, if the attack is allowed to continue, the surface becomes rough and may take on a frosted appearance.
Pitting Corrosion (Pitting corrosion)
Pitting Corrosion
This type of corrosion is generally caused by:
- local loss of the protective coating,
- the permanent presence or stagnation of a corrosive agent,
- significant surface roughness (poor machining),
- heterogeneity in the alloy due to heat or mechanical treatment.
This corrosion particularly affects aluminium alloys of the 2000 and 7000 series and stainless steels.
Pitting corrosion must be treated promptly as it can be the starting point for intergranular corrosion.
Pitting Corrosion
Pitting corrosion is a form of localised attack resulting in small holes in the metal. Pits are usually first noticeable as a white, dust-like powdery deposit that stains the surface. When the deposit is cleaned away, tiny pits or holes can be seen in the surface.
Crevice Corrosion
Crevice Corrosion
This type of corrosion can occur in any cavity where a stagnant solution has accumulated. Crevices are generally found on joint surfaces, lap joints, and under bolt or rivet heads. Crevice corrosion occurs because the local environment differs significantly from the general environment.
Crevice Corrosion
Consequently, the surfaces of metal parts, even of the same metal, react differently and corrosion occurs inside the crevice. This type of corrosion can also occur when a surface is covered by a foreign material.
Filiform Corrosion
Filiform Corrosion
This type of corrosion is generally caused by:
- The ingress of water or oxygen through the paint coating (never truly watertight),
- The ingress of water or oxygen through the paint at a fastener hole,
- Poor surface preparation before painting.
This corrosion progresses rapidly (0.4 mm per day) in warm and humid conditions.
Filiform Corrosion
Filiform corrosion (filiform corrosion) is characterised by the formation of fine filaments on metals protected by a relatively impermeable organic coating (varnish or polyurethane paint). Over time, these filaments join together and form blisters. When these blisters are scraped, a white powder escapes. This corrosion attacks the cladding but rarely the core itself. It generally initiates at the edge of a fastener.
Fretting Corrosion
Fretting Corrosion
This damage results from micro-movements between two surfaces, at least one of which is metallic. These particles oxidise and increase the abrasive effect between the two surfaces.
Fretting Corrosion
Friction damages the protective layers, promoting a brownish oxidation. It is also possible that this type of corrosion causes fatigue cracking.
Conception YA Group Pour LAUAK ESKOLA – Tous droits réservés.
Galvanic Corrosion
Galvanic Corrosion
This type of corrosion is caused by an electrical transfer between metals in contact or connected, where the most electronegative one is destroyed. It is generally recognisable by an accumulation of corrosion deposits at the junction between the metals. For galvanic corrosion to occur, the assembly must have: Different materials, An electrolyte (water, liquid, ...), An electrical connection between the materials.
Galvanic Corrosion
Galvanic corrosion is characterised by the attack of a metal in contact with or connected to another metal of a different nature. It acts as a battery effect when these two metals are wetted by an electrolyte (moisture, various liquids).
Corrosion Corrosion
Intergranular Corrosion
It is a common form of corrosion resulting from an internal electrochemical mechanism.
In the case of intergranular or intercrystalline corrosion, the cause essentially comes from a defect during heat treatment. This type of corrosion occurs mainly in heat-treated alloys, most often in the 2000 and 7000 series.
Intergranular Corrosion
Intergranular or intercrystalline corrosion is characterised by an attack inside the metal at the grain boundaries. This corrosion is only detectable once well advanced, by the appearance of a blistered area in the affected zone or delamination (exfoliation corrosion).
Exfoliation Corrosion
Exfoliation Corrosion
Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted by the expansion force of corrosion products forming at grain boundaries.
Aluminium alloys containing elements such as copper or the zinc-magnesium-copper group are the most susceptible to this form of corrosion.
Exfoliation Corrosion
Lifting or swelling is visible evidence of exfoliation corrosion. Exfoliation corrosion occurs frequently on extruded sections, as grain density is generally lower than in rolled forms.
Stress Corrosion
Stress Corrosion
The simultaneous application of a stress force (internal or applied) and a corrosive environment can cause stress corrosion cracking. Stress corrosion cracking is intergranular or transgranular cracking of a metal caused by the combined effects.
Stress Corrosion
Residual or internal stresses result from forming, machining, assembly, or heat treatment. They are unsuspectable and can result in the sudden failure of part of the component or assembly. A part subjected to the combined action of stresses and corrosion (of any type) can fail at a stress level lower than that for which it was designed.
Microbial Corrosion
Microbial Corrosion
This type of corrosion is caused by the acid secretions of micro-organisms that feed on sealants and residues suspended in kerosene. Deterioration of the protective layer can give rise to stress corrosion and fatigue corrosion.
Microbial Corrosion
A large number of micro-organisms such as bacteria, fungi and algae cause this type of corrosion. These micro-organisms can generally occur - in areas where fluids are contaminated, - in hot and humid climates.
Micro-organisms or fungi, combined with moisture, increase the electrochemical effects.
Microbial growth appears as a viscous, brownish substance that can coat fuel system components and may eventually cause clogging of engine filters or pipes.
05-01-02-Types_of_Corrosion.pptx
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Created on June 23, 2026
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Transcript
Surface Protections
Corrosion
Types of Corrosion
Introduction
To classify the different types of corrosion, they are characterised by their visual indicators and the type of degradation they cause.
Uniform Corrosion
Uniform Corrosion
It is probably the most common type of corrosion.
It results from a direct chemical attack on a metallic surface that occurs uniformly across the entire exposed surface. The metal gradually thins and eventually fails.
Uniform Corrosion
On a polished surface, this type of corrosion first manifests as a general dulling or surface attack and, if the attack is allowed to continue, the surface becomes rough and may take on a frosted appearance.
Pitting Corrosion (Pitting corrosion)
Pitting Corrosion
This type of corrosion is generally caused by:
This corrosion particularly affects aluminium alloys of the 2000 and 7000 series and stainless steels.
Pitting corrosion must be treated promptly as it can be the starting point for intergranular corrosion.
Pitting Corrosion
Pitting corrosion is a form of localised attack resulting in small holes in the metal. Pits are usually first noticeable as a white, dust-like powdery deposit that stains the surface. When the deposit is cleaned away, tiny pits or holes can be seen in the surface.
Crevice Corrosion
Crevice Corrosion
This type of corrosion can occur in any cavity where a stagnant solution has accumulated. Crevices are generally found on joint surfaces, lap joints, and under bolt or rivet heads. Crevice corrosion occurs because the local environment differs significantly from the general environment.
Crevice Corrosion
Consequently, the surfaces of metal parts, even of the same metal, react differently and corrosion occurs inside the crevice. This type of corrosion can also occur when a surface is covered by a foreign material.
Filiform Corrosion
Filiform Corrosion
This type of corrosion is generally caused by:
- The ingress of water or oxygen through the paint coating (never truly watertight),
- The ingress of water or oxygen through the paint at a fastener hole,
- Poor surface preparation before painting.
This corrosion progresses rapidly (0.4 mm per day) in warm and humid conditions.Filiform Corrosion
Filiform corrosion (filiform corrosion) is characterised by the formation of fine filaments on metals protected by a relatively impermeable organic coating (varnish or polyurethane paint). Over time, these filaments join together and form blisters. When these blisters are scraped, a white powder escapes. This corrosion attacks the cladding but rarely the core itself. It generally initiates at the edge of a fastener.
Fretting Corrosion
Fretting Corrosion
This damage results from micro-movements between two surfaces, at least one of which is metallic. These particles oxidise and increase the abrasive effect between the two surfaces.
Fretting Corrosion
Friction damages the protective layers, promoting a brownish oxidation. It is also possible that this type of corrosion causes fatigue cracking.
Conception YA Group Pour LAUAK ESKOLA – Tous droits réservés.
Galvanic Corrosion
Galvanic Corrosion
This type of corrosion is caused by an electrical transfer between metals in contact or connected, where the most electronegative one is destroyed. It is generally recognisable by an accumulation of corrosion deposits at the junction between the metals. For galvanic corrosion to occur, the assembly must have: Different materials, An electrolyte (water, liquid, ...), An electrical connection between the materials.
Galvanic Corrosion
Galvanic corrosion is characterised by the attack of a metal in contact with or connected to another metal of a different nature. It acts as a battery effect when these two metals are wetted by an electrolyte (moisture, various liquids).
Corrosion Corrosion
Intergranular Corrosion
It is a common form of corrosion resulting from an internal electrochemical mechanism.
In the case of intergranular or intercrystalline corrosion, the cause essentially comes from a defect during heat treatment. This type of corrosion occurs mainly in heat-treated alloys, most often in the 2000 and 7000 series.
Intergranular Corrosion
Intergranular or intercrystalline corrosion is characterised by an attack inside the metal at the grain boundaries. This corrosion is only detectable once well advanced, by the appearance of a blistered area in the affected zone or delamination (exfoliation corrosion).
Exfoliation Corrosion
Exfoliation Corrosion
Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted by the expansion force of corrosion products forming at grain boundaries.
Aluminium alloys containing elements such as copper or the zinc-magnesium-copper group are the most susceptible to this form of corrosion.
Exfoliation Corrosion
Lifting or swelling is visible evidence of exfoliation corrosion. Exfoliation corrosion occurs frequently on extruded sections, as grain density is generally lower than in rolled forms.
Stress Corrosion
Stress Corrosion
The simultaneous application of a stress force (internal or applied) and a corrosive environment can cause stress corrosion cracking. Stress corrosion cracking is intergranular or transgranular cracking of a metal caused by the combined effects.
Stress Corrosion
Residual or internal stresses result from forming, machining, assembly, or heat treatment. They are unsuspectable and can result in the sudden failure of part of the component or assembly. A part subjected to the combined action of stresses and corrosion (of any type) can fail at a stress level lower than that for which it was designed.
Microbial Corrosion
Microbial Corrosion
This type of corrosion is caused by the acid secretions of micro-organisms that feed on sealants and residues suspended in kerosene. Deterioration of the protective layer can give rise to stress corrosion and fatigue corrosion.
Microbial Corrosion
A large number of micro-organisms such as bacteria, fungi and algae cause this type of corrosion. These micro-organisms can generally occur - in areas where fluids are contaminated, - in hot and humid climates.
Micro-organisms or fungi, combined with moisture, increase the electrochemical effects.
Microbial growth appears as a viscous, brownish substance that can coat fuel system components and may eventually cause clogging of engine filters or pipes.