Corrosion in fertilizer industry – 1.2 Elemental sulphur attack

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  •  Elemental sulphur attack is NOT sulphidation or hydrogen sulphide corrosion.
  • It is an aqueous corrosion phenomenon.
  • It considers two modes – 
    1. acidification of sulphur – formation of sulphuric acid
2.    direct cathodic reduction of sulphur with anodic dissolution of iron

  • The lowering of pH is the main source of corrosion in both the methods.
  • The phenomenon is temperature dependent. It increases with increase in temperature and becomes particularly severe above the melting point of sulphur (~112.8 degree Celsius).
  • Hydrogen sulphide present in the petroleum may aggravate the sulphur attack by enhancing uniform pitting corrosion.
  • Monoethylene glycol is used to prevent condensate formation ans may be present in traces in the feedstock petroleum. This enhances the sulphur attack in the form of uniform corrosion, and crevice corrosion.

References: 
    • Fang, Haitao, Brown, Bruce, Young, David, and Srdjan Nešic. “Investigation Of Elemental Sulfur Corrosion Mechanisms.” Paper presented at the CORROSION 2011, Houston, Texas, March 2011.
    • Yoon, Yuhchae, Srinivasan, Sridhar, Yap, Kwei-Meng, and Russell D. Kane. “Elemental Sulfur and Speciation in High Pressure High Temperatures Oil and Gas Well Environments: Their Role in Stress Corrosion Cracking of Corrosion Resistant Alloys.” Paper presented at the CORROSION 2017, New Orleans, Louisiana, USA, March 2017.
    • Yaakob, Nurul & Singer, M. & Young, David. (2015). Elemental sulfur corrosion of carbon steel in the presence of sulfur solvent and monoethylene glycol. NACE – International Corrosion Conference Series. 2015. 

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        Important terms in API certifications – CRV & IOW – II

        CRV can be understood by using a simple example of milk kept for boiling in a pot on a stove.

        The motive, i.e. the reliability of this process has two main factors –
        1) The milk should boil.
        2) It should remain the vessel and not overflow.

        There are three critical variables here –

            • Temperature 

        Why is this a critical variable? 
        Answer:
        The melting point of milk is 92 degrees C. That is the optimum level.
        If temperature is too low (30 degrees C), the milk may not boil at all.
        If the temperature is too high (200 degrees C), it may burn quickly.

            • Vessel dimension = volume of milk is 1 ltr

        Why is this a critical variable? 
        Answer: 
        Vessel volume < 1 ltr, milk will not fit in it in the first at all.
        Vessel volume = 1 ltr, milk will fit, but will overflow on boiling
        Vessel volume >1 ltr, milk will fit and not overflow.

            • Time

        Why is this variable critical?
        If the other two variable are at the optimum level, then – 
        Too less time (~ 1 min), insufficient for milk to reach boiling point
        Too much time (~ 2 hours), milk will boil and condense, taste will change
        Optimum time (~ 10 min), milk will boil, not overflow, and the heating will be stopped at the correct time.
        Thus, maintaining the three CRVs will help to initiate and complete the process of boiling milk perfectly.

        So, whenever one has to decide the CRV for larger processes, the important questions to ask are –

            • WHY IS THIS VARIABLE CRITICAL? 
            • WHAT WILL HAPPEN IF IT DEVIATES? 
            • HOW BAD WILL THE DAMAGE BE?

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        Reference: “Development of Corrosion Control Document Database System in Crude Distillation Unit”, Junghwan Kim, Wonsub Lim, Younghee Lee, Seungnam Kim, Sang-Rok Park, Sun-Kyu Suh, and Il Moon, Industrial & Engineering Chemistry Research 2011 50 (13), 8272-8277
        DOI: 10.1021/ie101871a

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        https://corrospective.com/

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