Most people associate corrosion with air and moisture (water). However, in some cases the stresses (residual stress or applied stress) acting on a material can play a major role in a unique type of corrosion known as stress corrosion cracking (SCC).
In this article we will take a closer look at the mechanics of stress corrosion cracking to gain a deeper understanding of the influence of stress.
What is Stress Corrosion Cracking (SCC)?
Stress corrosion cracking (SCC) is cracking of a metal due to the combined action of corrosion and tensile stress. SCC is known for its deceptive nature because corrosion damage is not always obvious from a visual inspection. While little to no metal loss occurs during SCC, the mechanical strength of the material is significantly reduced. This can often lead to sudden failure in usually ductile metals.
This form of stress-induced corrosion is known to be the cause of several catastrophic industrial disasters, such as:
- Rupture of high-pressure gas transmission pipes
- Explosion of boilers
- Destruction of power stations and oil refineries
Therefore, it is essential for design engineers to understand the relationship between stress and corrosion in SCC. (For more about SCC of pipelines, read What Causes Stress Corrosion Cracking In Pipelines?)
How is Stress Corrosion Cracking Formed?
Active Path Dissolution
Active path dissolution is accelerated corrosion along a vulnerable path (such as the grain boundaries of a metal), while the bulk of the material remains unaffected. For example, in austenitic stainless steels, heat treatment can result in the depletion of chromium carbides near the grain boundaries. The reduction in chromium content makes it more difficult for the grain boundaries to be passivated. As a result, these boundaries represent areas of weakness where intergranular corrosion and microcracking can occur.
When stress is applied, these cracks open, and accelerated corrosion is observed at the crack tips. SCC caused by active path dissolution is governed by the corrosion rate at the crack tip. (Trying to measure corrosion rates? Be sure to read Corrosion Rate Conversion: Simple Ways to Convert Data Between Common Corrosion Units.)
Hydrogen atoms can easily fit in the crystalline structure of metals due to their small size. This allows hydrogen, to some extent, to dissolve in metals.
Hydrogen atoms are attracted to regions under tensile stress, where the metal is dilated. As such, it concentrates in areas where cracks are likely to form. Dissolved hydrogen can promote the fracture of the material by either making cleavage easier or by assisting in the development of intense plastic deformation. Consequently, the metal becomes brittle, making it prone to cracking under stress.
What Materials are Susceptible to Stress Corrosion Cracking?
Very specific conditions must exist for SCC to occur. Below, we will list the most common metal-environment combinations known to encourage SCC.
Brass in ammonia-containing environments
Many studies show that brass crack failures can occur in moist conditions where ammonia, oxygen and water are present. This phenomenon was first noticed by the British Army in India when they observed cracking in their brass cartridges during the rainy season (the ammonia originated from decaying organic material). Its prevalence in the rainy season gave rise to the name season cracking.
High strength steels subjected to hydrogen embrittlement
Steels are affected by hydrogen-rich environments as evidenced by:
- Hydrogen-related corrosion fatigue crack growth
- Hydrogen-induced cracking
Hydrogen embrittlement under static loads is mainly observed in high strength steels and is a function of:
- The concentration of hydrogen in the steel
- The applied tensile stress
- The stress concentration, composition and microstructure of the steel
Hydrogen embrittlement in high strength steels is not likely to occur at yield strengths below 600 MPa (87 ksi). However, at yield strengths above 1000 MPa (145 ksi), embrittlement can be an issue. (Learn more about in An Introduction to Hydrogen Embrittlement.) Hydrogen is typically introduced into high strength steels during welding, pickling, electroplating and exposure to hydrogen gases.
The Role of Stress in Corrosion Failure
As mentioned previously, stress is a significant factor in SCC. However, not all stresses will cause corrosion failure. A minimum threshold tensile stress value must be achieved for SCC to occur. Once this threshold stress is reached, the weakened grain boundaries open up, causing miniature cracks to form in the material.
One of the most significant contributors to SCC is stress concentration. Stress concentrations occur in areas where there is a sudden change in the shape of the material. Design details, such as corners, notches, welds, sharp changes in cross-section, etc., can disrupt the flow of stress, causing it to rise abruptly. These locations represent areas where the local stresses may exceed the minimum threshold, even though the overall stress in the material may be significantly less.
Welding, heat treatments and cold deformations can also introduce residual stresses that may initiate SCC.
How to Control Stress Corrosion Cracking?
The key to controlling SCC is to understand how and why metals corrode from this stress-related phenomenon. The first line of defense in controlling SCC is proper material selection. By choosing materials that are not susceptible to SCC in their operating environment, future stress-related problems can be avoided.
However, aggressive environments such as those containing high temperature water will cause SCC in most metals. In this case, proper engineering controls should be put in place to minimize adverse effects on the material.
Since stress is needed for SCC, it stands to reason that controlling the applied or residual stress can go a long way in preventing this type of corrosion.
For example, stress relief can be done in carbon steels by subjecting it to annealing. Surface treatments, such as shot peening or grit blasting, can also introduce compressive stresses that can counter the effects of damaging residual tensile stress.
With regards to applied stress, it is best to avoid design choices (e.g., sharp corners, holes, notches, etc.) that may introduce stress concentrations. (More on this topic can be found in Stress Concentration: Top 4 Tips to Reduce Stress Concentration in Machine Parts.)
Stress cannot cause corrosion on its own. However, in the presence of intergranular corrosion caused by active path dissolution or hydrogen embrittlement, tensile stresses may open cracks and weaken the metallic structure. By understanding the role stress plays in SCC, engineers can take the necessary precautions to ensure the safety and longevity of their designs.