Unlike many forms of stainless steel corrosion, intergranular corrosion occurs at a microscopic level — impacting the very structure of the metal without always showing a sign of attack on the surface.
Fortunately, an intergranular attack requires specific circumstances to occur and, in some cases, the damage is reversible.
This guide will discuss what intergranular corrosion is, when it occurs, and best practices to avoid it or minimize the damage caused by an intergranular attack.
What is Intergranular Corrosion?
Stainless steel, like many other metals and alloys, is composed of a crystallite structure.
This means the metal itself is composed of grains of various sizes. Where these grains meet are called the grain boundaries.
Also known as weld decay, intergranular corrosion attacks these boundaries, causing damage to the metal at a molecular level.
Cracking and grain loss can occur, leading to reduced structural integrity, ability to withstand pressure, and further encouraging additional corrosion.
Much like stress corrosion cracking, this can occur with little to no visual indication of a corrosive attack.
As such, not accounting for intergranular corrosion risks can lead to catastrophic failure of impacted piping processes and stainless steel components.
What Causes Intergranular Corrosion?
Intergranular corrosion occurs when certain metals and alloys reach temperatures between 425°C and 870°C (887°F to 1598°F.)
These temperatures are most common during welding, heat treatment, or operation in high-temperature environments.
When the metal endures such extreme temperatures, it changes at a structural level.
Chromium present in the alloy reacts with carbon to create chromium carbide near the grain boundaries.
This carbide formation essentially converts the boundaries into anodic cells. The grain interiors then function as cathodic cells, and the intergranular corrosion begins.
Preventing and Repairing Intergranular Corrosion Damage
Material choice is an essential element of risk mitigation and ensures long-lasting safety and performance of pipes and components.
When looking for stainless steels with excellent intergranular corrosion resistance, consider low-carbon alloys (often designated with an L.).
When choosing low-carbon alloys, options with carbon percentages below 0.03 percent will ensure that insufficient carbon is available for carbide formation.
If low-carbon alloys are not appropriate for your intended use, alloys with added titanium or niobium also provide exceptional intergranular corrosion resistance.
However, alloys with added titanium or niobium are susceptible to a specialized form of intergranular corrosion called a knife-line attack.
This occurs when the carbon interacts with the titanium or niobium instead of the chromium.
Fortunately, heat treatments can often resolve the issue and return the metal’s structure to a near-original state.
In some cases, solution-annealing (also known as quench-annealing or solution-quenching) is an effective means of reversing intergranular corrosion damage in austenitic stainless steels.
The process involves heating the metal to a temperature between 1,060°C and 1,120°C (1940°F and 2048°F.)
Once heated, the metal is water quenched, rapidly cooling it and solidifying the grain and structure.
Unfortunately, the method is less than ideal for treating large assemblies. Also, it does nothing to protect pipes or other components from damage later should welding or repairs occur.
ASTM International offers standards to help to determine the susceptibility of a given alloy or component to intergranular attack as follows:
- ASTM A262 – 15: Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels
- ASTM A763 – 15: Standard Practices for Detecting Susceptibility to Intergranular Attack in Ferritic Stainless Steels
- Intergranular corrosion (also known as weld decay) affects stainless steel at the structural level and may not show visible signs of damage until corrosion has significantly progressed.
- Welding, improper heat treatments, and exposure to temperatures between 425°C and 870°C (887°F to 1598°F) are required to initiate an intergranular attack.
- Choosing low-carbon alloys or alloys with added titanium or niobium can help to increase intergranular corrosion resistance.
- When choosing a low-carbon alloy, options with 0.03 percent carbon or lower are recommended.
- While solution-annealing and heat treatments can offer options to reverse or mitigate intergranular corrosion damage, they often do not protect against future damage. They may have limited success in reversing corrosion depending on the severity of the attack.
With an extensive selection of piping, tubing, flanges, fittings, and other stainless steel products, Unified Alloys has the expertise and selection to help industries across Canada and North America with piping solutions big and small. With more than four decades of experience, our sales analysts are ready to assist you with options tailored to the needs of your industry. Call today to get started.