What percentage reduction in fracture toughness has been reported in gaseous hydrogen compared to air for pipeline steel?
The answers to the question, “What percentage reduction in fracture toughness has been recorded in gaseous hydrogen compared to air for pipeline steel?” shows a clear majority of you have responded with a 50% reduction, with just about over a quarter of you saying it is higher than this, at an 80% reduction, and just under a quarter of you saying it is lower, at 10%.
The good news is that you are all right! The whole question of how gaseous hydrogen affects material properties is a really hot topic and the subject of a lot of interest in the industry at the moment. Unfortunately, as with so many simple questions, the answer can be very complex. The effect of hydrogen on material properties in general, and fracture toughness in particular, is dependent on a variety of different factors, which all need to be understood.
As a starting point, it needs to be recognized that pretty much every mechanism of hydrogen damage requires the hydrogen to be atomic (or ionic) in form and present in the metallic lattice of the pipe wall. Gaseous hydrogen is molecular, and therefore it is sometimes assumed that the embrittlement effect of atomic or ionic hydrogen can be ignored. Unfortunately, molecular hydrogen can and does dissociate at a bare metal surface, as described in Sieverts’ law, leading to atomic hydrogen being absorbed into the pipe wall. The more hydrogen is absorbed, the greater the effect on material properties, including fracture toughness. The amount of absorbed hydrogen is dependent on various factors, including the partial pressure (not just the blend percentage) of hydrogen and the cleanliness of the pipe surface.
Besides the amount of hydrogen present, the second area that can affect susceptibility to hydrogen damage is the pipeline steel microstructure. Note that this is not the same as pipeline grade, although grade is often used as a proxy for microstructure, and existing codes such as B31.12 generally restrict the use of grades higher than X52. Nevertheless, there is a lot of evidence available showing that higher grade materials can be resistant to hydrogen damage, for example in the related field of sour service, where “sour-rated” X65 and X70 steels are now relatively common. At the opposite end of the scale, “dirty” steels with large numbers of oxide or sulfide inclusions – typically more often found in vintage pipelines – are thought to be more prone to hydrogen embrittlement.
Finally, the interaction between testing parameters and hydrogen needs to be understood. When talking about fracture toughness, various different test types, specimen geometries and test measures are commonly used. At a basic level, Charpy impact testing is often used as a proxy for fracture toughness, with the Charpy energy being converted into a “true” fracture toughness by means of various codal calculations. Alternatively, a “true” fracture mechanics test can be performed, although there are more variations present here with different test standards (e.g. ASTM E1820), test piece geometries (e.g. SENT) and parameters (e.g. CTOD) reported. The difference in fracture toughness can vary depending on the test type and measure used. For example, it is generally understood that hydrogen embrittlement is dependent on time – and hence on the strain rate. The limited published data available implies that there is little, if any, difference in Charpy impact energy in gaseous hydrogen compared to air, but there is a larger difference in CTOD.
The overall result of all these different factors is that it can be very difficult for operators of existing pipelines – who may have in-air Charpy test data if they are lucky – to quantify what the effect of gaseous hydrogen will be on the material properties, and thus the integrity, of their pipelines. Depending on which paper you read in the literature, reductions in “fracture toughness” of 10%, 50% and 80% have all been reported in gaseous hydrogen compared to air. Indeed, one paper has even reported an increase in fracture toughness in hydrogen compared to air. This makes a fundamental understanding of the pipeline (materials, presence of any pre-existing crack-like defects, and operating pressures and loading) key to any safe and successful conversion. For more information, please visit our dedicated hydrogen page.