D3b.1 Overview of materials suitable for an ammonia transport infrastructure

Ammonia has potential as energy carrier for hydrogen transport for the green economy. As such the expectation is that there will be a substantial scale-up in its use. However, there are substantial risks involved. Apart from the comparatively low risk of flammability, the most significant danger is toxicity. This makes it important to be aware of any possible issues when using ammonia in pipelines, especially in cases that they might pass through high density population areas. This report discusses the material compatibility of the current natural gas pipelines with ammonia.

The most significant problem for steel in anhydrous ammonia is stress corrosion cracking. Fortunately, there are steps that can be taken to reduce this effect. These are limiting the strength and weld hardness of piping steel, since softer steels are less susceptible to stress corrosion cracking due to ammonia. Additionally, small quantities of water (0.2-0.5 wt.%) added to the ammonia will inhibit stress corrosion cracking, combined with avoiding the presence of oxygen.
Different sources and standards agree that in order to avoid ammonia stress corrosion cracking the yield strength should be less than 360 MPa. Maximum values for weld hardness differ – the highest limit is 225 HV, which is still quite low. However, these values may be too strict since they do not take the effect of limiting the oxygen content and adding water to the ammonia as used in commercial pipeline ammonia into account.

Alloys commonly used in ammonia pipelines typically have a yield strength of around 400 MPa, which compares well to some mild steels used for natural gas pipelines specified in OSW-01-N, like ASTM A333 grade 6, L245 NE/ME and L290 ME/NE, which have specified yield strength between 240 and 440 MPa. However, some medium strength steels are also used for natural gas pipelines, and would probably be less suitable for use in ammonia as the risk to stress corrosion cracking will increase. Additionally, the weld requirements for anhydrous ammonia pipelines could not be established during this study and therefore cannot be compared to those used for natural gas pipelines. Further investigation is recommended to determine the weld requirements.

When using natural gas pipeline material specification OSW-01-N for future ammonia pipelines, care should be taken to specify that only the low-yield strength pipeline steels should be used.

Since historically stress corrosion due to ammonia has been controlled with the relatively simple measures of limiting the strength and weld hardness of the piping steel, and by adding small quantities of water to the ammonia, the matter is no longer seen as urgent and there has been comparatively little research performed on interactions between ammonia and steel. For this reason there is no data to be found on the effect of anhydrous ammonia on other properties, like fatigue rates and fracture toughness, which are known to be effected by the same environment as stress corrosion cracking.

It must be pointed out that at present most ammonia pipelines are located in areas of low population density. The use of ammonia as an energy carrier would most likely place it in areas of higher density, which would necessarily have an effect on risk assessment. This makes the amount of unknowns – the safe limit for weld hardness and yield strength in commercial ammonia, and the effect on fatigue and fracture toughness - more of an issue.

Note that this report addresses only material compatibility and selection. System requirements and design, risk assessment and maintenance and the like are not within the scope of this study.