You may often hear about "stress corrosion cracking" of screws, etc. in accidents.
This is a phenomenon where corrosion-resistant materials such as stainless steel are cracked. Here, we will explain its causes and countermeasures.
In what kind of environment does "stress corrosion cracking" occur?
"Stress corrosion cracking" is known to occur frequently to components near coasts and underwater due to chloride ions, but it also occurs due to exposure to rainwater that contains oxides. You also need to be careful with components in chemicals.
Caused by three overlapping conditions
In other words, it occurs when the three factors "materials (chemical composition)", "mechanics (tensile stress)" and "environment (dissolved oxygen / chloride ions)" are present.
For example, combinations such as stainless steel and chloride, brass and ammonia, carbon steel and hydrochloric acid, and strong acids or strong alkalies facilitate corrosion.
There are two possible types of stress: externally applied tensile stress and residual stress during welding or machining.
Corrosion can be prevented and predicted with sufficient countermeasures!
"Stress corrosion cracking" occurs when all three factors are present. In other words, it does not occur when these factors are not met. That's why it's important to select the materials while taking the environment into consideration.
For example, with stainless steel, corrosion is more prone in the austenite type (SUS303, SUS304, etc.) and less in the ferritic type (SUS403, etc.). However, the austenitic ultra-low carbon steel (SUS316L, etc.) with reduced carbon content is resistant against stress corrosion cracking.
It is also necessary to take measures such as coming up with designs and machining methods that reduce stress as well as removing chloride. Regular inspections are necessary, but cracking is often not found visually, so it is also necessary to set a replacement cycle.
Screws that are resistant to stress corrosion cracking
Earlier, we introduced the ferritic type that's resistant to stress corrosion cracking. We offer a "duplex stainless steel screw" that mixes the two structures of the austenite and ferrite phases, bestowing on it the advantages of both the strength and corrosion resistance.
With its resilience to stress corrosion fractures and especially excellent resistance to chloride corrosion, it finds wide use in chemical plants and marine instruments among other areas.
Keep this in mind when designing components and structures that are used on the coast.
That's all for today!