Hydrogen is normally found in a diatomic state, H² molecules, composed of two hydrogen atoms. In a diatomic state, molecules will not permeate the thin diaphragm, with a typical thickness of 75μm. However, if the hydrogen splits into two H+ atoms, it can permeate the diaphragm. This occurs because H+ ions are smaller than the space between the molecules of the diaphragm material.
Once they have permeated the diaphragm, H+ ions can reunite into H² molecules that become trapped inside the Diaphragm Seal. The H² will then dissolve into the Diaphragm Seal fill fluid and, over time, cause the Diaphragm Seal fill fluid to become saturated, and hydrogen bubbles to appear. The presence of the hydrogen gas bubbles can alter the performance of the transmitter with Diaphragm Seals and thus the measurement to fail.
When the process pressure drops, the trapped H² ions are not able to permeate back through the Diaphragm Seal and remain in the system. With the H² molecules still in the system, the pressure within the Diaphragm Seal can remain as high as the original process pressure. This will cause the diaphragm to bulge, which leads to a zero reading and span shift, and reduces the performance of the Diaphragm Seal system.
Gold is one of the noblest materials. As such, providing a thin layer of gold to a base metal will provide protection against hydrogen permeation. To protect the Diaphragm Seal against the permeation of H+ ions through the diaphragm foil, Badotherm offers a layer of 25µm or even 40µm a thickness of gold.
Due to the intermediate layer of nickel, the temperature limit of Gold Coating is normally 280°C. Above this limit a normal Gold Coating will be damaged or even detached from the diaphragm. Together with TNO Delft, an independent Dutch Research Organisation, Badotherm developed a new procedure to attach the gold directly to the diaphragm without any intermediate layer. With this new type of procedure the temperature limit on Gold Coating is increased to 400°C.
Whereas a layer of 25 µm is often considered the market standard and offers sufficient protection, the 40 µm thickness offers increased security at extreme conditions. The 40µm Gold Coating thickness was engineered and designed specifically for the Hycon project for one of the largest refineries in the world, located in The Netherlands. Because the operating temperatures (320°C) and pressures (180 bar) in this project were extreme, additional tests on various thicknesses were executed by TNO. The result was that only a 40µm thickness offered the required additional warrantees for the non-porosity for H+ ions.
As stated before, gold is one of the noblest materials available. The Gold Coating protects the diaphragm from the permeation of even the smallest molecules; it is also very suitable for increasing the chemical resistance of the Diaphragm Seal materials.
• To protect against hydrogen permeation (HP) only the diaphragm is coated.
• To ensure chemical resistance (CR), the Gold Coating is extended to the outside diameter of the gasket surface.
Gold coating in combination with 316(L) material has a temperature limit up to 400°C. In combination with Monel or Hastelloy C276 the temperature limit is 275°C.
The table presents the technical specifications of the Gold Coating for Diaphragm Seals. Gold coating is suggested in applications where atomic hydrogen is present. However, it is not necessary for all hydrogen service applications. Every application is different and the materials should be selected carefully. It remains the end user responsibility to select the appropriate materials suitable for the process conditions.