Corrosion is a natural process where metals deteriorate due to reactions with environmental elements like moisture, oxygen, or chemicals. For screws, corrosion can weaken their structural integrity, shorten their lifespan, and impair functionality, particularly in harsh conditions. By understanding the types of corrosion and how they affect different metals, you can select the right screw material for your project.
Here’s a breakdown of the various types of corrosion affecting metal screws, such as rust, galvanic corrosion, and stress corrosion cracking, and how different metals react in different environments.
1. General Corrosion
Also known as rust, general corrosion occurs when a metal’s entire surface is exposed to a corrosive environment, leading to uniform degradation. This is common in carbon steel and iron, which rust when exposed to oxygen and moisture.
Key Points:
- Carbon steel screws are highly susceptible to rust.
- Protective coatings, like zinc plating, are used to prevent rust.
Prevention: Apply protective coatings like zinc plating to create a barrier that minimises exposure to moisture and oxygen.
2. Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals come into contact in the presence of an electrolyte (e.g. water). The more reactive metal acts as the anode and corrodes faster, while the other metal (the cathode) is protected. This is common when mixed-metal fasteners are used, such as aluminium screws with steel surfaces.
Key Points:
- Aluminium screws are vulnerable when paired with more noble metals like stainless steel.
- The galvanic series helps determine which metal will corrode first.
Prevention: Use insulating washers or screws made from compatible materials to prevent direct contact between dissimilar metals.
3. Crevice Corrosion
Crevice corrosion happens in confined spaces like under washers or screw threads where oxygen is limited, allowing corrosive agents to accumulate. Even corrosion resistant metals like stainless steel can suffer from this type of corrosion in low-oxygen environments.
Key Points:
- Stainless steel screws may corrode in moisture-rich gaps or crevices.
- Crevices trap moisture and corrosive substances, accelerating corrosion.
Prevention: Use designs that minimise crevices, or seal gaps with appropriate materials to prevent corrosion agents from gathering.
4. Pitting Corrosion
Pitting corrosion is a localised form of corrosion leading to deep pits or holes in the metal. It affects metals like stainless steel and aluminium, particularly when their protective oxide layers are damaged, exposing the metal to corrosive elements such as chlorides.
Key Points:
- Common in stainless steel and aluminium, especially in salty environments.
- Pits can cause significant damage even if the rest of the metal seems intact.
Prevention: Use barrier coatings (paints, epoxies) to protect the metal from corrosive environments.
5. Stress Corrosion Cracking (SCC)
SCC occurs when metal screws are subjected to tensile stress in a corrosive environment. It can lead to cracks that eventually cause the screws to fail. This is common in brass and stainless steel screws exposed to corrosive chemicals like chlorides or ammonia.
Key Points:
- Brass screws can suffer SCC in ammonia rich environments, while stainless steel can crack in the presence of chlorides.
- Reducing tensile stress during installation can mitigate SCC.
Prevention: Use screws suited to the environment and reduce tensile stress during installation.
6. Intergranular Corrosion
Intergranular corrosion occurs along the grain boundaries of metals, particularly after improper heat treatments. Stainless steel screws are vulnerable if they undergo improper welding or heat treatment, which depletes chromium and leaves certain areas exposed to corrosion.
Key Points:
- Occurs in improperly treated stainless steel screws.
- Low carbon stainless steel (e.g. 304 or 316) reduces susceptibility.
Prevention: Use low-carbon stainless steel or ensure proper post-weld treatments to avoid grain boundary weaknesses.
7. Fretting Corrosion
Fretting corrosion happens when two surfaces rub against each other in the presence of corrosive agents. It’s common in high vibration environments like machinery, where screws may wear down over time.
Key Points:
- Frequent in steel screws used in vibrating machinery.
- Lubricants or anti-corrosion coatings can reduce friction and prevent fretting.
Prevention: Use lubricants, anti-corrosion coatings, or hardened metals like titanium to resist wear and friction.
By understanding these corrosion mechanisms, you can select screws that will provide long-term durability and reliability in your projects.
Our team of experts are ready to assist you in choosing the most suitable security screws for your specific requirements. We’re here to help with the secrets of security screw thread types.
Contact us today on 1800 776 565.
Secure Your Assets with Sentinel Group Security Screws.
Choose Sentinel Group Security & Customised Fastening Solutions.
