Intergranular corrosion of metal fasteners

Intergranular Corrosion This is where the grain boundaries of the material structure are more susceptible to corrosion. So what does this mean? Every material is made up of grains, surrounded by grain boundaries, that are tightly packed together to form a metal or alloy.

Think of the material as a brick wall, the particles of the material are the bricks, and the grain boundaries are the mortar. Intergranular corrosion occurs in the mortar (intergrain boundaries) or along the edges of the mortar, while the particles themselves (or bricks, for example) are largely unaffected.

Intergranular corrosion is localized corrosion that affects grain boundaries rather than grains. Even if corrosion occurs only at grain boundaries, it can still cause the material to decompose. Think about our brick walls. If you break the mortar, the wall will collapse, even though the bricks themselves are still intact.

Intergranular corrosion is usually caused by:

1. Grain boundary impurities

2. Too much concentration of an alloying element

3. Loss of alloying elements in grain boundary region

Corrosion fatigue of metal fasteners

Fatigue cracking This type of corrosion occurs when a material is subjected to cyclic stress in a corrosive environment. Altering the stress is thought to cause the passive protective film to rupture, thereby causing the underlying material to occur. Remember that often rapidly fluctuating stresses can be well below the tensile strength of the material. Similar to stress corrosion cracking, corrosion fatigue is caused by cyclic stress whereas stress corrosion is caused by constant stress.

Think of it as a shoe. Cyclic stress occurs when you walk (versus continuous stress while standing - SCC). This altered stress can cause cracks or breaks in this protective sole, such as a passive layer.

Stress corrosion cracking has many branch cracks, and fatigue cracks are relatively few

Crevice corrosion of metal fasteners

Crevice corrosion is an intense localized corrosion that occurs in crevices or gaps between two connected services. It can occur in crevices or gaps between two metals or metallic and non-metallic materials. Outside this area, the material is fully resistant to crevice corrosion corrosive environments, but when the corrosive solution is trapped or stagnant in this gap, chemical changes can occur in the solution, resulting in a very corrosive microenvironment.

Galvanic corrosion

Also known as two-metal corrosion, this occurs when two dissimilar metals or alloys come into contact with each other and are immersed in a corrosive or simply conductive solution such as seawater. When these dissimilar metals come into contact with each other in this environment, an electrical current is created between them, resulting in galvanic corrosion - the less inert materials corrode badly and the better inert materials less corrosion.

With regard to galvanic corrosion of fasteners, good inertness of the material is very important for fasteners because galvanic corrosion can rapidly corrode the threads of the screw. The effects of galvanic corrosion can be minimized if the mass of the low inert metal is significantly increased. Like a screw into a large device. Galvanic effects are distributed over large mass devices rather than concentrated on relatively small threads. Galvanic corrosion can also be avoided by electrically isolating fasteners with non-conductive polymers or ceramics. The following table provides a better understanding of the risk of galvanic corrosion between different metals.