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  • 1.  Fluoride in Concentrated Sulfuric Acid

    Posted 04-21-2022 05:20 PM
    Hi:

    I am looking for any data on the effect of low concentrations of fluoride ion in concentrated  sulfuric acid (93% and up) on stainless steels.  I have all the data - and first hand experience- on the effects on high silicon austenitic SS (it's bad) but I am looking for similar data on other stainless steels. Ideally I'd like data on 316L, superduplex and superaustenitic.

    I'm interested in low concentrations of F-  below 20 ppm.

    Thanks.

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    Steve Clarke
    SJC Materials Engineering
    Vancouver BC
    604 417 9126 (M)
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  • 2.  RE: Fluoride in Concentrated Sulfuric Acid

    Posted 04-22-2022 12:25 PM
    Hello Steve,
    There is not much data in low pH-fluoride chemistry which can be attributed to incompatibility of (commonly-used) glass reactor/beakers in a lab set-up and the safety risks associated with fluoride. Below are some of the articles which you may find interesting,

    (1) Materials for Hydrofluoric Acid Service in the New Millennium - Jennings (NACE CORROSION 2021). Some of the cross-references are relevant to this topic.
    (2) The Effect of Velocity on Corrosion in H2SO4 + HF Mixtures - Sridhar (NACE CORROSION 1990). I remember seeing corrosion data by Sridhar on 316L in 93% sulfuric acid + HF (probably 1% or higher) at different temperatures. You can plan to reach out to Haynes International.
    (3) Corrosion Behavior of 904L in Hydrofluoric Acid - Zou (RSC Advances, 2018) - A pdf copy of this article is available online at no cost
    (4) Recent progress on the corrosion behavior of metallic materials in HF solution - Dai (Corrosion Rev., 2021) - A pdf copy of this article is available online at no cost

    Additional comments:
    (1) What is the temperature range of your interest?
    (2) Flow rate can play a big role, with or without fluoride (lots of good information on this topic in MS-1)
    (3) There are some literature that suggests fluoride ion act as an inhibitor in an acidic chloride solution. However, 20ppm seems too low to provide an inhibition.
    (4) Fluoride (like, chloride) can lead to localized corrosion attack to 316L in an acidic solution at an elevated temperature. I've seen rapid pitting corrosion attack (in headspace) in an acidic solution containing fluoride (no chloride) in stressed region. However, the process condition was complex and other parameters would have played a role.

    Thanks
    Ajit

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    Ajit Mishra
    Materials Engineering Consultant
    Corteva Agriscience
    Midland MI
    7654162879
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  • 3.  RE: Fluoride in Concentrated Sulfuric Acid

    Posted 04-25-2022 04:50 AM

     

    Hello

    For F below 20 ppm

    • 316 Loverall, has a reliable corrosion performance (In fertilizer plant: 316 L found uses in the packed "drying" tower in which that air downstream flow contains less than 20 PPM fluoride compound (mainly HF comes from phosphoric acid manufacturing).

     

    • Although I did not have a numerical value of corrosion rate (10 mm thick plate) but there's no any form excessive corrosion for more than 3 years (24 000 hours) of continuous functioning no excessive noticeable corrosion detected by the naked eye.
    • Molybdenum bearing stainless steel (316L, 317L, 904L..) offer better corrosion performance in halides containing (fluorine, Chlorine, Iodine..) environment.

     

    • Silicon grade austentic stainless steel [5%] alloy used in sulfuric acid manufacturing equipments like (S30601 , S38815 , S32615 , S30600 ) contain less than 1% (weight ) Molybdenum which could explain their poor corrosion performance in halide containing environment.
    • Once we had a sulfuric acid cooler (tube-shell heat exchanger: sea water to cools sulfuric acid) failed dramatically due high chloride rate as the OEM claims. However for another fertilizer plant (ambient environment contain HF)  the SANDVIK SX (S32615) alloy used for all sulfuric acid piping & reactor (packed tower) for more than 10 years without any excessive corrosion . So can you tell us which silicon containing stainless steel you worked on it.

     

    Regards



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    Elhazel Aymen
    GCT
    Fertilizer
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