Evaluating Metals

Assessing Metal Performance by Testing

When designing a component for use in a hydrogen environment, testing (i.e., direct exposure of the material to hydrogen) and analysis ideally would be done to assure performance of the material at planned operating conditions as well as in worst-case conditions.

One way of assessing a material’s performance is by testing it in hydrogen gas following standardized practices such as described in ANSI/CSA CHMC 1, Test Methods For Evaluating Material Compatibility In Compressed Hydrogen Applications – Metals. Material properties evaluated are ductility as measured by reduction of area, stress amplitude vs number of cycles fatigue characteristics, fracture toughness, threshold stress intensity factor, and fatigue crack growth rate (da/dN) vs stress intensity factor range (ΔK).

In limited circumstances, decisions on material compatibility can be based directly on results from materials testing in hydrogen gas.

• For example, according to ANSI/CSA CHMC 1, austenitic stainless steels and aluminum alloys with relative reduction of area (RRA) values ≥ 0.9 are compatible with hydrogen independent of stress magnitude
• The RRA is the ratio of the reduction of area measured in hydrogen gas and the reduction of area measured in an inert environment
• The reduction of area in hydrogen must be measured from either smooth or notched tensile specimens at the gas pressure and temperature representative of the application

In most cases, results from materials testing in hydrogen gas must be coupled with component design analysis to inform material compatibility decisions. For example, there is no material compatibility criterion that can be applied to the fatigue crack growth rate (da/dN) vs stress intensity factor range (ΔK) data in the plot below. Rather, these data must be coupled with fracture mechanics analysis of the component to assess material compatibility.

Hydrogen compatibility must be established for the material, mechanical, and environmental variables relevant to the specific hydrogen gas containment component.

Materials testing data or service experience should not be extrapolated. For example, the hydrogen compatibility of 316 stainless steel in system components operating at -50 °C (-58 °F) should not be based on materials testing performed at room temperature.

Assessing Metal Performance by Service Experience

Another way of assessing material performance is by evaluating the history of components in operation. Some successful applications:

• Fuel Tanks - aluminum or polymer lined composites for hydrogen pressure up to 87.5 MPa (~1,2691 psi)
• System Components (e.g., piping) - austenitic stainless steel with specified minimum tensile strength less than 520 MPa (~75,420 psi) for hydrogen pressure up to 100 MPa (~14,504 psi)
• Storage Tanks – seamless low-alloy ferritic steel with tensile strength less than 950 MPa (~13,7786 psi) for hydrogen gas pressure up to 42 MPa (~6,092 psi) and infrequent pressure cycling
• Pipelines - C-Mn ferritic steel with specified minimum tensile strength less than 480 MPa (~69,618 psi) for hydrogen pressure up to 21 MPa (~3,046 psi) and moderate pressure cycling amplitude