All About Environmental Cracking in Nickel-Based Alloys
Factors such as high chloride concentration, acidic conditions and the presence of oxidizing species can make nickel-based alloys more susceptible to environmentally assisted cracking (EAC).
Nickel (Ni)-based alloys are used in highly corrosive environments and often where other metals—such as stainless steels—have insufficient corrosion resistance. Since Ni-based alloys tend to be more corrosion-resistant than stainless steels, they often replace stainless steels where chlorides are present and, combined with minimal residual stresses, can cause Chloride Stress Corrosion Cracking (SCC) of those alloys. (For more on this topic, see: Chloride Stress Corrosion Cracking of Austenitic Stainless Steel.)
The alloy of choice seems to be alloy C276, which is practically immune to SCC.
Plus, many believe Ni-based alloys are also resistant to environmentally assisted cracking (EAC). Unfortunately, however, there are some specific environments—combined with certain microstructural changes—where these alloys can also be susceptible to EAC. (Note that the tensile stress required can be either applied or residual.)
This article will highlight the environments within which various classes of Ni-based alloys are susceptible to EAC. Such cracking is not always very common; but if these environments are potentially present, evaluation of potential cracking via testing—for example U-bend, C-ring or slow strain rate evaluations—is highly recommended. Distinguishing between SCC and hydrogen embrittlement will not be done in this article.
Environmentally Assisted Cracking in Ni-Based Alloys: The Basics
For aqueous halide systems, a combination of conditions may promote the susceptibility of Ni-based alloys to EAC. These include:
- Temperatures above 205 degrees Celsius.
- High chloride concentration.
- Acidic conditions.
- Presence of oxidizing species.
- Presence of hydrogen sulfide (H2S).
- High tensile stress.
There are two main classifications of Ni-based alloys: heat-resistant and corrosion-resistant. And the latter category consists of three basic types:
- Ni-Mo alloys.
- Ni-Cr-Mo alloys.
- Ni-Cr-Fe-(Mo) alloys.
Novel Techniques to Assess Environmentally Assisted Cracking in Ni-based Superalloys
Although slow strain rate testing—also known as dwell fatigue testing—can provide a measure of an alloy’s susceptibility to EAC, it doesn't always provide sufficient information on the cracking mechanism.
In principle, techniques can be used to examine fatigue specimens' crack-tip regions. Possible methods include transmission electron microscopy coupled with Energy Dispersive X-ray Spectroscopy (EDXA), nano-scale secondary ion mass spectrometry (nano-SIMS) and atom probe tomography—all of which have been used to study this problem.
These studies have shown that, often, the environmental degradation is occurring very locally at the sub-micron scale. However, environmental damage's effect on local mechanical properties in the region ahead of the crack-tip has not been studied so far. The use of micro-mechanical testing techniques can now be applied to assess mechanical behavior at the sub-micron scale. Using these tests, we can do site-specific measurements at a sub-micron length scale; and this can play a critical role in better understanding crack mechanisms.
Environmentally Assisted Cracking in Ni-Mo Alloys
The most common Ni-Mo alloys are Alloy B, Alloy B2 and Alloy B3. These alloys have excellent corrosion resistance in non-oxidizing and reducing acidic environments and have been shown to be resistant to Cl-SCC, such as in boiling magnesium chloride (MgCl2) solutions.
Alloy B2—and to some extent Alloy B3—when heated to between 550 and 850 degrees Celsius, lose ductility from the solid-state formation of an ordered intermetallic phase such as Ni4Mo. Such phases can occur in the heat-affected zone (HAZ) during welding. Slow strain rate studies have demonstrated these alloys' susceptibility to cracking in reducing acidic conditions when heat treated to 570 degrees Celsius or conditions typical of welding.
The extent of cracking was attributed to be associated with the formation of the intermetallic phase and the subsequent hydrogen (H2) embrittlement. This study might explain the observed intergranular cracking in Alloy B2's HAZ, exposed to organic solvents containing traces of sulfuric acid (H2SO4) and transgranular cracking in the presence of hydrogen iodide (HI).
The chemistry of the cathodic and anodic solutions near welds may be the critical factor for EAC. Composition of Alloy B3 delays the aging reaction and allows it to be used in the as-welded condition, which may reduce the potential of EAC.
Environmentally Assisted Cracking in Ni-Cr-Mo Alloys
Ni-Cr-Mo alloys are the most versatile Ni-based alloys because of the inclusion of molybdenum (Mo)—which can increase corrosion resistance under reducing conditions— and the presence of chromium (Cr)—which provides greater corrosion resistance under oxidizing conditions.
Hastelloy C was the first alloy of this group and was the basis for the development of many alloys including Alloys C276, C4, C22, C-2000, 625, 5923hMo and 686. When these alloys are aged at temperatures higher than 600 degrees Celsius, precipitation of tetrahedrally closed packed phases can occur, which can lower their ductility. The time it takes for each alloy to transform through these phases varies; for example, alloy C4 has higher resistance to such microstructural changes than alloy C276. EAC susceptibility may also be increased by cold work followed by a low temperature treatment. Thus, these alloys may have susceptibility to EAC in environments containing H2S.
It has also been reported that Alloys C276 and 625 can suffer intergranular cracking when exposed to various aqueous solutions near the critical point of water. Crack growth extension tests in acidic brine to simulate nuclear waste for Alloys C4, -22 and 625 seem to be associated with time—which, for such aggressive and critical environments, must include longer term testing.
For wet-hot hydrogen fluoride (HF)—and depending on temperature and HF concentration—these alloys can be susceptible to EAC. Alloys containing tungsten appear to be the most affected.
High levels of Mo in these alloys appears to be detrimental in hot caustic environments, with Mo and Cr dealloying. Such a mechanism may promote transgranular cracking in Alloy C276. However, the susceptibility may also be a function of the testing conditions.
Alloy C22 is susceptible to EAC in environments containing chloride and bicarbonate (HCO3) or carbonate at elevated temperature and under anodic potentials. The loss of Cr from the dissolution by HCO3-in the protective oxide film may be the source of the susceptibility.
Environmentally Assisted Cracking in Ni-Cr-Fe-(Mo) Alloys
In particular, Alloys 600 and 690 have been found to suffer EAC in pure water and caustic with susceptibility to cracking strongly depending on temperature, level of tensile stress, presence of H2 gas, solution pH and electrochemical potential. Metallurgical factors that impact cracking include the presence of minor or impurity elements, the extent of cold work and heat treatment for the formation and location of carbides. Alloy 690, having higher Cr content, has greater resistance to cracking than Alloy 600 in these environments; but can still crack.
It has been suggested that inward diffusion of oxygen at grain boundaries can result in the intergranular oxidation of Cr, where the intergranular oxidation embrittlement is a precursor for the subsequent cracking. Alloy 800 is also susceptible to EAC under these conditions; but the mechanism is different. At 300 degrees Celsius and a pH greater than 10, iron (Fe) and chromium dealloying can occur and lead to a film-induced cleavage mechanism. The presence of lead (Pb) or sulfate anions can enhance the degradation of Alloy 800 in these environments.
Alloy 825 is more resistant to Cl-SCC than the austenitic stainless steels; however it is still susceptible. Alloys 800 and 825, when heated to between 400 and 800 degrees Celsius, will sensitize—which is Cr-carbides precipitating in the grain boundaries. If process conditions are such for a sulfide scale to form on the metal surface, these alloys are susceptible to polythionic acid stress corrosion cracking. (For more on this topic, see: Polythionic Acid Stress Corrosion Cracking of Austenitic Stainless Steel.)
Crack Morphology in Ni-Based Alloys
EAC crack morphology for Ni-based alloys may be transgranular (through the grain), intergranular (along grain boundaries) or mixed mode, with branched secondary cracking depending on environmental conditions such as temperature, presence of process impurities, process chemistry and microstructural variations. However, these cracks existing does not automatically mean EAC is the failure mechanism, since other mechanisms such as stress relaxation cracking propagate by an intergranular mode. (For more on this topic, see: Stress Relaxation Cracking, a Forgotten Phenomenon.)
A detailed failure analysis, including a thorough evaluation of process conditions and possible testing for EAC, may be required to properly identify the exact failure mode.