99 Diseases of Pressure Equipment: Hydrogen Bake-out

As noted in the discussion on delayed cracking, when the steel contains hydrogen as a result of service exposure (or corrosion, or high temperature - high pressure hydrogen processing) then a hydrogen bake out may be needed to avoid cracking problems during or after welding. However, not every attempt in outgassing meets with success (i.e., no cold cracking). In theory, you simply heat the steel to an elevated temperature and allow time for the hydrogen to diffuse out of the steel, leaving it hydrogen free and weldable. Several key issues all play a role in the success of the hydrogen bake out, including: (1) the amount of hydrogen charged in the steel from the process exposure or corrosion, (2) the amount of hydrogen traps (e.g., interfaces at inclusions and precipitates, and voids at hydrogen blisters or hydrogen induced cracks, HIC), (3) the solubility and diffusivity of hydrogen in the alloy, (4) the steel thickness, (5) the alloy’s susceptibility to hydrogen embrittlement, and (6) whether or not the equipment is weld overlaid/clad with a corrosion resistant alloy or bare.

Typically the need to perform a hydrogen bake out happens during a maintenance turnaround and so the need for the bake out is often challenged to try and save time and cost. However, the time and cost to remove cracks from delayed cracking and attempting multiple repairs can more than offset the time/cost of the hydrogen bake out effort. A good practice is to do a “bead-on-plate” test on the surface to be welded. Simply weld a bead, using the welding procedure intended for the repair or alteration, about six inches long. Allow the bead to cool to ambient temperature and then wait 2 to 4 hours. Inspect for cracking with wet fluorescent magnetic particle testing (WFMT). If cracking occurs, then a bake out procedure is needed. If cracking does not occur, welding without hydrogen cracking may be possible. From the author’s experience, the time spent on a hydrogen bake out is often cheap insurance to mitigate problems associated with hydrogen cracking during and after weld repairs, especially if signs of hydrogen activity (hydrogen blistering, HIC damage or wet H2S related cracking) are present in the vessel. The time and temperature needed to bake out the hydrogen to a level for a successful weld repair is controversial and when discussing with a metallurgist and practitioners in the field, you will find varying degrees of opinion. A common “general rule of thumb” is 600°F (316°C) for four hours or at least 1 hour per inch of thickness. Some practitioners have spreadsheets with calculations based on steel thickness and hydrogen solubility, etc. When these moderate temperature bake outs have not been successful, often a bake out for about 1 hour (or less) at the alloy’s PWHT temperature will help.

When performing a bake out, another question is whether to bake out before removing the cracks/weld preparation, or after weld preparation, or both. Typically only the bake out after weld preparation is needed and should prove to be the most effective, as the weld preparation will allow the full surface to be welded to be more readily outgassed. In steel heavily charged with hydrogen, cracking can also occur while grinding, especially if the crack removal process is too aggressive. In these cases a hydrogen bake out may be needed before weld preparation. If the equipment is weld overlaid or clad with a corrosion resistant alloy, then the overlay or cladding in the area of the repair may need to be removed. The overlay/cladding will typically have a higher concentration of hydrogen than the base metal and during the bake out procedure hydrogen can diffuse from the area of higher concentration (overlay) to the area of lower concentration (base metal). This can negate the success of the hydrogen bake out.

Before attempting repair welding, be aware of whether the steel may contain hydrogen from its service exposure and to seek advice from competent materials or inspection engineers who have experienced weld repairs in similar equipment. When welding, follow guidelines for avoiding delayed hydrogen cracking. Many types of in-service cracking have been attributed to what was in reality hydrogen cold cracking that may have been there since initial fabrication or occurred during previous weld repairs. When that happens, it’s sometimes very difficult to sort out what may be in-service environmental cracking from hydrogen cold cracking, and hence be very difficult to determine what to do about it since the strategy for operation and repairs to equipment that is exposed to in-service environmental cracking could be very different from what you might do to handle some previously existing hydrogen cold cracking.

Do your field service personnel know how to repair equipment that might contain hydrogen within the steel that could become the initiation site of some other of the 99 Diseases?