6 Tests to Assess Intergranular Corrosion Using ASTM A262
ASTM A262 tests can help evaluate whether an austenitic stainless steel is susceptible to intergranular corrosion. However, they do not provide the maximum acceptable corrosion rates -- these require correlating test results with service experience.
Intergranular corrosion of austenitic stainless steels selectively attacks the vicinity of the grain boundaries.
To evaluate, in a relatively short period of time, whether an austenitic stainless steel metallurgy might be susceptible to intergranular attack, the American Society for Testing and Materials (ASTM) has developed several standard tests that are referred to in the ASTM A262 Standard Practice for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels.
Note that these practices do not simulate any intended service environments but are considered by the industry to determine an alloy's susceptibility to intergranular attack. Any results from these practices do not indicate potential issues from non-intergranular forms of corrosion. Rather, ASTM A262 is designed to assess whether a material will sensitize for a treatment of 650 to 675 degrees Celsius for about one hour. Specific parameters can be decided between the material producer and purchaser.
This article aims to outline the practices delineated in ASTM A262 and determine when to use each one. Here are six tests to assess intergranular corrosion using ASTM A262:
1. Practice A
One of the common approaches to examining an austenitic stainless steel's microstructure is to etch the polished section using electrolytic oxalic acid. This etchant preferentially attacks carbides and can be used to evaluate the presence of a sensitized microstructure.
Practice A requires a polished section to be electrolytically etched with 10 percent oxalic acid at a current density of 1A/cm2 for one and a half minutes. The resulting microstructure is then examined under the optical metallograph and evaluated for the presence of steps or ditches. A step structure between grains with no ditching is considered to be free of grain boundary carbides. A dual structure, which has some ditches at grain boundaries but no grain that is completely surrounded, indicates some sensitization. A ditched structure where grains are completely surrounded by ditches is defined as a sensitized microstructure. Practice A provides images for comparison to the sample of interest. The step structure may not be apparent on some Molybdenum (Mo)-containing austenitic stainless steels.
Since Practice A is a rapid test, it is often used for screening for sensitization before employing one of the more time-consuming practices and can be used for both wrought and cast alloys. On the other hand, Practice A is very commonly used for evaluating the extent of sensitization when conducting a failure analysis. (For more on failure analyses, read: How to Conduct a Corrosion-Related Failure Analysis.)
2. Practice B: The Streicher Test
Practice B, also referred to as the Streicher test, uses weight loss to detect sensitization as a result of carbide precipitation and intergranular corrosion associated with sigma phase.
This practice is applicable to cast Mo-alloys such as CF-3M and CF-8M containing sigma phase but not necessarily to wrought Mo-alloys such as 316 or 317. The procedure involves immersing the sample in a boiling solution of ferric sulfate-50 percent sulfuric acid for 120 hours. The resulting weight loss is converted to a corrosion rate that is compared to a reference that is of the same alloy grade and is expected to be resistant to intergranular attack in service.
Because of this practice's quantification, it is often used as part of a mill certification to demonstrate that, during production, sensitization was not produced. Shorter times deviate from the practice but have been used to confirm the ditched structure from Practice A.
3. Practice C: The Huey Test
The boiling HNO3 test in Practice C is commonly referred to as the Huey test.
The Huey test involves subjecting the samples to boiling for five consecutive periods of 48 hours with a fresh solution for each period. Weight loss is measured after each boiling period and then converted to a corrosion rate.
This practice is suitable for detecting Cr-deficient regions from carbide and various intermetallic precipitates. It can also be used for applications where the material is exposed to strong oxidizing agents.
Practice C has limited applicability because of lengthy testing time, variability of Cr+6 buildup in each time period, sensitivity to surface preparation and attack of Laves and chi phases.
4. Practice D
Practice D , which includes hydrofluoric acid (HF) with the HNO3, was developed to differentiate between the effects of Cr-carbides and sigma phase in Mo-containing alloys since this solution does not attack sigma phase.
It consists of two two-hour periods in a 10-percent HNO3, three-percent HF solution at 70 degrees Celsius; but it is no longer included in ASTM A262.
5. Practice E: The Strauss Test
The Strauss test was the earliest test used for detecting susceptibility to intergranular corrosion in sensitized stainless steel alloys. It's a pass-fail test.
Initially, the Strauss test became the basis for ASTM A393, which is now inactive, where the sample was exposed to a copper sulfate and sulfuric acid solution for 72 hours and then bent 180 degrees. This practice was considered too mild and was modified to include metallic copper, which accelerates the attack.
With Practice E, the sample is exposed to a boiling solution of copper shot, copper sulfate and 16 percent sulfuric acid for 15 hours. After the exposure, the sample is bent at 180 degrees. The fracture surface is then examined under optical microscopy at low magnification. If fissures or cracks appear, the presence of intergranular attack is confirmed.
When an evaluation is questionable, a metallographic analysis at the outer radius of a longitudinal section of the bent sample is to be prepared and examined for intergranular attack at a magnification of 100 to 250 times. This practice is often incorporated in a mill certification that the alloy has been properly heat treated.
6. Practice F
Practice F is similar to Practice E, but the concentration of the boiling copper sulfate and sulfuric acid solution is higher; and the metallic copper is not in contact with the sample.
Instead, the metallic copper generates cuprous ions that deposit on the sample surface and lowers the corrosion potential. Weight loss after 120 hours is used to determine the corrosion rate.
What is Intergranular Corrosion?
Also known as "weld decay," intergranular corrosion (IGC) attacks grain boundaries, causing damage to a metal at a molecular level. Cracking and grain loss can occur subsequently , leading to reduced structural integrity, less ability to withstand pressure and further corrosion advancement. IGC can lead to catastrophic failure of piping and stainless steel components, including, for example, valves used for flow controls in process industries.
Unlike many other forms of stainless steel corrosion, intergranular corrosion:
- Normally occurs at a microscopic level.
- In most cases, impacts the metal's basic structure without showing a sign of attack on the surface.
- Requires specific conditions to exist.
In some cases, the damage intergranular corrosion causes is reversible.
How Does Intergranular Corrosion Occur?
IGC's propagation is the result of the corrosive agent attacking the chromium (Cr) depletion associated near the grain boundaries when certain metals and alloys reach temperatures ranging between 425 and 870 degrees Celsius. These temperatures are mostly experienced during welding, heat treatment or operation in high-temperature environments.
The most common cause of the chromium depleted zone is Cr-carbide precipitation adjacent to the boundary, which is referred to as sensitization. In other words, chromium present in the alloy reacts with carbon to form chromium carbide near the grain boundaries. This carbide formation results in the boundaries to convert into anodic cells. The grain interiors then act as cathodic cells and the intergranular corrosion starts.
Chromium carbides (M23C6) can be precipitated in the temperature range of about 400 to 800 degrees Celsius. The alloy, its C composition, and the time for which it was at that temperature will determine the extent of sensitization. This Cr-depleted zone is then susceptible to corrosive attack such as with polythionic acid stress corrosion cracking (For more on polythionic acid stress corrosion cracking, read: Polythionic Acid Stress Corrosion Cracking of Austenitic Stainless Steel.)
Other phases -- such as sigma (Cr-Fe) and chi (Cr-Mo-Fe) -- can also cause Cr-depleted zones adjacent to grain boundaries. Thus, determining an alloy's susceptibility to sensitization is an important parameter in the selection of an alloy for a specific application, as is whether that alloy has been properly heat-treated and is not, at the time of construction, susceptible to intergranular corrosion.
How to Prevent Intergranular Corrosion
Material choice is is important to reduce the risk of IGC and ensure long and safe performance of pipes and components. In particular, low-carbon alloys with very low carbon percentages -- i.e., below 0.03 percent -- are preferred. These allows are often designated with an "L" -- such as 304L and 316L and will ensure insufficient carbon is available for carbide formation.
If low-carbon alloys are not appropriate for the intended application, alloys with added titanium or niobium may also be used. However, these are susceptible to "knifeline attack (KLA)," a specialized form of intergranular corrosion which occurs when carbon interacts with the titanium or niobium instead of the chromium. Heat treatments can often resolve KLA.
Solution-annealing, also known as "quench-annealing" or "solution-quenching," may also be used for austenitic stainless steel -- particularly in cast austenitic stainless steel valve bodies. The process involves heating the metal to a temperature between 1,060 and 1,120 degrees Celsius and then water quenching, thus rapidly solidifying the grain and structure. However, solution-annealing is not effective for assemblies and piping wherein subsequent welding is used. (For more on quenching, read: How Quenching Improves the Performance of Metals.)
Comparison between the practices has shown good correlation.
However, ASTM A262 does not provide the maximum acceptable corrosion rates/weight loss for stainless steel alloys that have suitable resistance to intergranular attack. Instead, ASTM A262 leaves the purchaser and producer to agree upon an acceptable rate.
The criteria upon which an acceptable corrosion rate should be based requires correlating these tests with service experience. Requiring ASTM A262 as part of a mill certification is good practice and is particularly important for the H-grades of austenitic stainless steel.
ASTM G-28 can be used to evaluate susceptibility to intergranular attack for wrought nickel-based alloys and stainless steels with higher nickel and chromium content. (For more on nickel-rich alloys, read: All About Environmental Cracking in Nickel-Based Alloys.)
Written by Steven Bradley | Principal Consultant, Bradley Consulting Services
Steven Bradley is currently the principal consultant at Bradley Consulting Services having retired after 44 years from UOP-Honeywell as Senior Research Fellow.
Steven is also membership chair of the ASM Failure Analysis Society and is a registered Professional Engineer in the state of Illinois. He has presented and authored/co-authored over 60 technical papers involving failure analyses of complex systems, materials characterization and advanced electron microscopy of materials and catalysts and holds 16 patents. Steven completed his BSSE and PhD in Materials Science and Engineering from Northwestern University.
In addition to being a member of ASM and ASTM, he is also a member of NACE and ACS.