Discussion on hydrogen permeation

Commonly found in transmitter diaphragms, using gold-plated diaphragms or altering the position of the part of the diaphragm that comes into contact with the liquid.

API 551 specifies the following conditions: Take a look to see what those conditions are……

Hydrogen permeation is the process by which hydrogen passes through structural materials and is released. Hydrogen molecules are small in size, allowing them to penetrate through the gaps in a material’s structure; this phenomenon is particularly evident under high temperature and pressure conditions. It increases the difficulty of storing hydrogen and poses safety risks, but it can serve as a method for the slow release of hydrogen. It is one of the key technical challenges that must be addressed in the practical application phase of hydrogen energy systems. Therefore, hydrogen permeation occurs in any situation where hydrogen molecules are present; it happens at any temperature and pressure, with only differences in the speed at which it occurs.

— wet hydrogen service; — hydrogen in corrosive environments; — hydrogen partial pressure ≥621 kPa (90 PSIA); — for a transmitter temperature ≥43 °C (110 °F) when any hydrogen is present.

Hydrogen permeation is a challenge for diaphragm-based devices. Hydrogen ions (i.e., protons) are formed by electrochemical reactions between different metals or by corrosion on the surface of membranes. Due to their small size, hydrogen ions can migrate through the metal membrane. Once on the other side, it will recombine to form a diatomic molecule, increasing in size and no longer able to pass through the membrane. Instead, it will get trapped in the filling fluid. The problem is obvious: when the process pressure drops below the vapor pressure of hydrogen, this causes the diaphragm to expand and hydrogen molecules to escape. Gilding the stainless steel diaphragm seal helps to control this issue. It reduces the permeability of the membrane. Stainless steel is the material least affected and is the preferred base material. On the other hand, tantalum is prone to hydrogen embrittlement and is not suitable for use. The following conditions should be taken into account during gilding:

1Q019: Is it necessary to gold-plate the diaphragm used for measuring hydrogen pressure? A: Refer to 3.6.6 in API 551, which specifies four situations in which gold plating of the pressure transmitter diaphragm should be considered: 1. In environments with wet hydrogen； 2. Hydrogen in corrosive environments ； 3. Hydrogen partial pressure ≥ 621 kPa ； 4. The sensor temperature in a hydrogen environment is ≥43°C. 3.6.6 Hydrogen permeation is a challenge for diaphragm devices. Hydrogen ions (i.e., protons) are formed by electrochemical reactions between different metals or by surface corrosion at diaphragms. Due to its small size, hydrogen ions migrate through the metal membrane. Once on the other side of the membrane, it recombines to form a diatomic molecule that can no longer pass through the membrane. Instead, it will get trapped in the filling fluid. This problem arises when the process pressure is lower than the hydrogen vapor pressure, causing the diaphragm to expand. At this point, the transmitter output freezes or drops to zero. Plating the stainless steel diaphragm seal with gold helps to control this issue. It reduces the permeability of the membrane. Stainless steel is the material least affected and is the preferred base material. On the other hand, tantalum is prone to hydrogen embrittlement and should not be used. Plating should be considered under the following conditions: wet hydrogen service ； Hydrogen in corrosive environments ； Hydrogen partial pressure ≥ 621 kPa (90 PSIA) ； When any hydrogen is present, the transmitter temperature is ≥43°C (110°F). To reduce pressure losses during transmission and prevent corrosion by the measured medium, the diaphragms of pressure transmitters are typically made from thin-walled metal materials with certain elasticity and corrosion resistance (with a thickness ranging from 40 to 80 μm; the thickness is less than 0.1 mm, and this value varies slightly among different manufacturers). The materials used for these diaphragms include 316L stainless steel, HC, Ta, Ti, and others. Hydrogen embrittlement can easily occur in environments with high temperature and pressure along with high hydrogen concentrations, leading to a decline in the toughness of the measuring diaphragm and a loss of its elasticity, as well as the formation of bulges or cracks. Over time, hydrogen molecules can even penetrate through the measuring diaphragm and enter the conductive fluid containing silicone oil. Bubbles increase the losses during pressure transmission; they also interfere directly with the measurement performance of pressure transmitters, causing drift in their readings, unstable outputs, measurement errors, or fluctuations in pressure measurement values. In severe cases, this can even damage the instruments and lead to accidents. (PS: Types of hydrogen corrosion: hydrogen blistering, hydrogen embrittlement, hydrogen erosion.) ）