The stress-corrosion cracking of screws is a phenomenon where corrosion-resistant materials, such as stainless steel, become cracked and damaged. It typically occurs in alloys, such as stainless steel, and not in pure metals.
The corrosion also tends to occur in an environment where stress is applied to the components.
There are two possible types of stress:
- Externally applied tensile stress
- Residual stress during welding or machining
In general, this phenomenon occurs when these three factors are present:
1. Materials (chemical composition) — typically occurs in combinations such as stainless steel and chloride, brass and ammonia, carbon steel and hydrochloric acid, and strong acids or strong alkalies facilitate corrosion.
2. Mechanics (tensile stress)
3. Environment (dissolved oxygen / chloride ions)
Stress-corrosion cracking only occurs when all three of these factors are present. This is why it’s important to select materials while taking the environment into consideration.
For example, with stainless steel (SS), austenite type: SS303, SS304, etc. is more prone to corrosion when compared with the less ferritic type: SS403, etc. However, the austenitic ultra-low carbon steel SS316L, etc. with reduced carbon content is resistant against stress-corrosion cracking.
It is also necessary to take preventative measures, such as coming up with designs and machining methods that reduce stress as well as remove chloride.
This means regular, high-quality inspections are necessary, even if cracking is not found visually. It’s also necessary to setup a replacement cycle.
NBK, a specialty manufacturer of mechanical components, has introduced a ferritic-type stainless steel that is resistant to stress corrosion cracking. The manufacturer offers a duplex stainless steel screw that mixes austenite and ferrite stainless, thereby bestowing on it the advantages of high-strength and corrosion-resistance.
With its resilience to stress-corrosion fractures and excellent resistance to chloride corrosion, it has many uses in chemical plants, marine instruments, and more.