Hydrogen damage and HIC risk assessment in sour systems
Hydrogen induced cracking (HIC) of mild and carbon steel depends upon the steel being susceptible to HIC, due for example to a banded microstructure and MnS inclusions. It is also contingent upon a high hydrogen activity in the steel. This high hydrogen activity can arise when steel is cooled from high temperature high pressure hydrogen service, or high temperature acid corrosive service, because hydrogen solubility in steel decreases markedly with decreased temperature. Or it can be caused by corrosion in the presence of hydrogen promoters such as sour gas. In the case of sour corrosion, it is possible to correlate flux with hydrogen crack risk.
Traditionally, assessment of hydrogen damage risk arising from hydrogen flux in sour systems was the chief application for flux monitoring. Over the years damage has been dramatically decreased by the development and deployment of sour service steels and chemical corrosion inhibitors. However, it persists in locations where inhibitor costs are critical, such as in gas gathering lines. Here routine spot flux measurements using Hydrosteel 6000 should be carried out in areas of most likely condensation, such as low spots. An effective flux measurement regime to assess hydrogen crack risk has recently been published (S.Al-Sulaiman, A.Al-Mithin, A.Al-Shamari, M.Islam, S.S.Prakash, ‘Assessing the possibility of hydrogen damage in crude oil processing equipment’, Corrosion 2010, Paper 10176, Conference series, NACE, Houston, 2010).
In other locations, such as in gas plants, methane reactors, gas oil separation plants, hydrodesulfurization and desulfutrization units, constant monitoring of hydrogen flux with Hydrosteel 7000 TL may ensure that effective process and chemical control of sour corrosion prevents blistering of critical equipment on a continuous basis.
The sulfur is present in part as mercaptans and dissolved H2S, which release sour gas in distillation units (crude distillation units, CDU, and vacuum distillation units, VDU) . The gas is released from the top of the distillation columns, into overheads, together with water and acids such a HCl which may dissolve scale which otherwise protects steel from sustained corrosion. Sour corrosion may also arise in downstream equipment in which the sour water condensers out, such as overhead condensers, coolers and fin-fans. At such locations or Hydrosteel 7000 TL may be viable for identifying episodes of severe corrosion and their effective control. Spot measurements using Hydrosteel 6000 with AT-S probes and continuous measurements using Hydrosteel 7000 have been used in defining the extent and severity of an issue, particularly at locations where corrosion can be managed by better process or chemical control (F.Al-Faqeer, F.Addington,‘Hydrogen permeation application to crude unit overhead corrosion monitoring’, Corrosion 2008 , Paper 08548, NACE Conference Series, NACE, Houston 2008).