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 (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. Regular inspections are necessary, despite this cracking is often not found visually, therefore it is also necessary to setup a replacement cycle.
Screws that are resistant to stress corrosion cracking
Earlier, we introduced a ferritic type stainless steel that is resistant to stress corrosion cracking. We offered 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 its particularly excellent resistance to chloride corrosion, it has many uses in chemical plants, marine instruments and more.
Keep this in mind when designing components and structures that are used on the coast.
That's all for today!
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