Stress Corrosion Cracking – What Is it and How Do You Protect Against it?
If you run a power plant, stress corrosion cracking could cause catastrophic damage. What is stress corrosion cracking? What causes stress corrosion cracking? How can you find stress corrosion cracking before it causes a serious failure? How do you prevent stress corrosion cracking?
From the experts at Allied Power Group (APG), here’s what you need to know about stress corrosion cracking. Contact us today to learn more about how we can help if you choose us as your partner in the power industry.
What Is Stress Corrosion Cracking (SCC)?
Stress corrosion cracking (SCC) is a type of corrosion caused by the combined influence of tensile stress and a corrosive environment. SCC excludes transcrystalline or intercrystalline corrosion, which can disintegrate an alloy without stress, as well as corrosion-reduced sections that fail by fast fracture. However, SCC may occur in combination with hydrogen embrittlement.
Crack growth rates can be very fast, depending on the conditions, and propagation can be transgranular (where the fracture forms through the grains of a material rather than along the boundaries), intergranular (where the fracture forms along the grain boundaries of a material), or a combination of both. Since stress corrosion cracking may be microscopic, it can cause catastrophic failures before being detected.
What Causes Stress Corrosion Cracking?
Stress corrosion results from the simultaneous action of 3 components: tensile stress, a specific chemical species (environment), and a susceptible mineral. For example, stainless steels are susceptible to chlorides, mild steels are susceptible to alkalis, and copper and its alloys are susceptible to ammonia compounds.
There is no single mechanism for SCC. Some of the most popular models include:
- Embrittlement model: Hydrogen embrittlement is a significant mechanism of stress corrosion cracking for steels and other alloys, like titanium. Hydrogen atoms diffuse to the tip of the crack and make the metal brittle.
- Pre-existing active path model: Stress corrosion cracking follows a pre-existing path, like grain boundaries where compounds and intermetallics are formed.
- Film rupture model: Stress locally ruptures passive film, setting up an active-passive cell, while newly formed passive film ruptures again under stress in a repetitive cycle.
- Adsorption model: A specific chemical species adsorbs on the crack surface, lowering the fracture stress.
Common Types of Stress Corrosion Cracking
The most common types of stress corrosion cracking include:
- Caustic embrittlement of carbon steels
- Ammonia stress corrosion cracking of carbon steels
- Chlorine stress corrosion cracking of austenitic stainless steels
- H2S stress cracking of carbon steel and low-alloy Cr—Mo steels
Where Does Stress Corrosion Cracking Occur?
While stress corrosion cracking can happen anywhere there is the possibility for a concentration of a contaminant and a source of tensile stress, the most common locations for stress corrosion cracking include:
- Butt welds
- Welded attachments in straight tubing
- Swages
- Lower bends in assemblies where condensate can collect
- Cold-formed bends
However, if the ferritic tubing has received an appropriate stress-relieving heat treatment or if the austenitic stainless tubing has been properly annealed, stress corrosion cracking shouldn’t occur because the tensile stress component won’t be enough to initiate damage.
How to Find Stress Corrosion Cracking
Whether you’ve found some stress corrosion cracking and want to search for more or you’re doing preventative maintenance, there are a few ways you can search for stress corrosion cracking, including:
- Eddy-current testing
- Wet fluorescent magnetic-particle testing
- Fluorescent dye-penetrant testing
- Ultrasonic testing
- Radiography
- Visual examination
- Enhanced hydro test
It’s important to note that stress corrosion cracking may not appear visible to the naked eye, so component failure can come without warning if you don’t actively look for SCC.
Can You Fix Stress Corrosion Cracking?
Once stress corrosion cracking has started, there is little to nothing you can do to fix the existing damage. Your best bet is typically going to be to determine and address what caused the SCC in the first place, then replace the affected part. If you don’t figure out what caused the stress corrosion cracking to begin with, the new part will likely fail in the same potentially catastrophic way.
Industries Commonly Affected by Stress Corrosion Cracking
The industries most commonly affected by SCC include:
- Nuclear power creation
- Chemical processing and refinement
- Medical and pharmaceutical
- Food processing
- Petroleum and oil extraction and processing
How to Prevent Stress Corrosion Cracking
Stress corrosion cracking avoidance is best implemented during the design phase by choosing the right designs and materials to avoid the conditions that cause SCC. Heat treatments and metal coatings may help manage SCC risks for parts already in operation, however.
The best ways to prevent stress corrosion cracking are to:
- Control the electrochemical potential of the alloy and/or the operating temperature
- Use materials known not to crack in the specific environment
- Introduce compressive stress by shot-peening
- Control stress level (load or residual) and hardness
- Avoid the chemical species that cause SCC
Make Allied Power Group (APG) Your Power Partner
Allied Power Group (APG) was founded in 2005 with the goal of building the most synergistic, efficient, and comprehensive independent aftermarket solutions organization in the power generation industry. We provide the most comprehensive field, repair, and critical maintenance services so we can maximize the efficiency, reliability, and availability of your power-generating assets.
APG always strives to: change the game by constantly challenging the status quo; own the outcome by transparently communicating expectations; lead with service by building sustainable relationships; and elevate others by actively looking for opportunities to empower others.
Whether you need component, rotor, or fuel nozzle repair; plant or field services; part life extensions; heat rate improvements; reliability and availability or output and capacity upgrades; capital assets and consumables; or custom solutions, APG can help. We also specialize in comprehensive maintenance solutions for industrial gas turbines and related turbomachinery.
Learn more about why we should be your power partner and get in touch with us by clicking here, emailing sales@alliedpg.com, or calling us at 281-444-3535 or 888-830-3535. Let us know how we can help you today. We’re conveniently located in Houston, Texas.