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Liquid-Air Interface Corrosion Studies at SRS

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					                                                SRNL-STI-2010-00695




        Liquid-Air Interface Corrosion Studies at SRS
             EM Waste Processing Technical Exchange 2010

                                  Philip E. Zapp
                                  Savannah River National
                                  Laboratory
                                  November 17, 2010




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        Introduction
         Pitting Corrosion in Carbon Steel Waste Tanks
         Pitting Induced by Dilute Waste Solutions
              Dilution for feed preparation for vitrification; waste removal
         Focus: Pitting Susceptibility at Liquid-Air Interface
              Service Experience
              Hydroxide Depletion Model




           Type III tanks under constr.           Tank with ventilation & cooling equip.
                      Liquid-Air Interface Corrosion Studies at SRS      SRNL-STI-2010-00695       2




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        Background
    Pitting in carbon steel at and above
    liquid-air interface in dilute wastes                 Partial Immersion Coupons in
    ([Nitrate] < 1 M)                                     Simulated Dilute Waste

    ≥ 1 M hydroxide prevents pitting at
    interface; < 1 M hydroxide, pitting
    susceptibility
    Hydroxide concentration or pH of dilute
    waste decreases through reaction with                             Liquid
    absorbed CO2                                                      Level

    Hobbs-Wallace model predicts rapid pH
    decrease at interface and film above
                                                                               0.3 M Hydroxide
    meniscus, relative to bulk waste’s pH
                                                         Corrosive                Inhibited
    Nitrite additions prevent pitting under
    low pH condition (pH <12)




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        Liquid-Air Interface Environment
        Meniscus at Tank Wall Differs from Bulk Liquid
    Hypothesis: Low pH, High O2 Anodic Area and
 High pH, Low O2 Cathodic Area
        Different Electrochemical Potentials?

        “Galvanic Corrosion” w/out Dissimilar Metals                               Anodic
                                                                                   area,
                                                           CO2
                                                                                   corroded
                                                                     O2

                                    Liquid                                           Tank
                                    Surface                                          Wall
                                                           NO3-
                                                                                  Cathodic
                                                                      OH-         area, not
                                                                                  corroded
                                                           NO2-

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        Liquid-Air Interface Corrosion as Galvanic Corrosion

         •   Steel Specimens Exposed to Dilute Salt Solutions with Different pH
         •   Different pHs yield Different Electrochemical Potentials
         •   Specimens Connected through Zero Resistance Ammeter
         •   Ionic Current through Salt Solution Bridge to Complete Circuit




                                                                         A537 Carbon
                                                                         Steel Disk




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                                    Replicate Electrochemical Potentials vs. Time: 0.5 M Nitrate, 0.1
                                    M Nitrite at 40 °C


                                                      “Meniscus” Environment
                                                      Meniscus Solution Open Circuit Potentials
                                                                                                                                                                     “Bulk” Environment
                                                                                                                                                                      Bulk Solution Open Circuit Potentials

                           0.000                                                                                                          0.000


                           -0.050                                                           07m2lc                                        -0.050                                                      07B2Lc
                                                                                            07M2b                                                                                                     07B2La

                                               pH 9
                                                                                            07M2La                                                                                                    07B2L

                                                                                                                                                              pH 13
                           -0.100                                                                                                         -0.100
Potentialvs. Ag/AgCl (V)




                                                                                                              Potential vs. Ag/AgCl (V)
                           -0.150                                                                                                         -0.150


                           -0.200                                                                                                         -0.200


                           -0.250                                                                                                         -0.250


                           -0.300                                                                                                         -0.300


                           -0.350                                                                                                         -0.350


                           -0.400                                                                                                         -0.400
                                    0   1000   2000       3000       4000      5000       6000       7000   8000                                   0   1000   2000        3000     4000      5000     6000     7000       8000
                                                                   Time (s)                                                                                                       Time (s)



                                                      Varying potentials in same (repolished) specimens
                                                      exposed to identical solutions → No consistent
                                                      galvanic effect.

                                                                  Liquid-Air Interface Corrosion Studies at SRS                                                                  SRNL-STI-2010-00695                  6




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        Replicate ZRA Current vs. Time Measurements


                              5.0
                                                                                    Current 072L
                              4.0                                                   Current 072La
                                                                                    Current 072Lb                Positive current
                              3.0                                                                                indicates
                                                                                                                 corrosion of the
                              2.0
                                                                                                                 meniscus
        Current (microamps)




                              1.0                                                                                specimen;
                                                                                                                 negative current
                              0.0
                                                                                                                 indicates
                              -1.0                                                                               corrosion of the
                                                                                                                 bulk specimen
                              -2.0

                              -3.0

                              -4.0

                              -5.0
                                     0   10000     20000   30000   40000    50000     60000     70000   80000
                                                               Time (seconds)
                                             Current polarity inconsistent and changeable

                                                 Liquid-Air Interface Corrosion Studies at SRS           SRNL-STI-2010-00695       7




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        Liquid-Air interface corrosion as a
        “galvanic” action based on pH
        difference not demonstrated.



        Next approach to LAIC issue: Measure
        electrochemical potential at meniscus
        relative to bulk liquid.




           Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695       8




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        LAI Electrochemical Potential Measurements
        0.11-inch diameter A537 specimens
        electro-discharge machined from
        flat coupons
        4 specimens mounted in epoxy
        Measure potential above, at, and
        below LAI versus saturated calomel
        reference electrode                       ASTM A537 Cl 1
        Simple dilute waste simulants (1)
        0.5 M NO3- and 0.1 M NO2- (2) 0.05
        M NO3- and 0.01 M NO2-
        Two pH values: 8.5 and 11.7
        buffered with bicarbonate/carbonate




                      Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695       9




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        Measurement Equipment




                  Potentiostat with
                  8-channel
                  multiplexer



                 Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695      10




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        Specimen configuration and solution level


                                    H position
Solution level                      2 position
                                    1 position
                                    L position




                 Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695      11




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        Electrochemical Potential Measurement vs. Time: No Significant
        Differences Among Positions


               0.5 M Nitrate        Room Temp. 28 °C
               0.1 M Nitrite        No aeration
               pH 11.8              High bicarbonate




                                                                      Red = H
                                                   H position
                                 Solution          2 position         Green = 2
                                 level             1 position
                                                   L position         Purple = 1
                                                                      Blue = L




                      Liquid-Air Interface Corrosion Studies at SRS    SRNL-STI-2010-00695      12




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        Electrochemical Potential Measurement vs. Time: Replicate,
        repolished




                                                                      Red = H
              0.5 M Nitrate       Room Temp. 26 °C
                                                                      Green = 2
              0.1 M Nitrite       No aeration
                                                                      Purple = 1
              pH 11.8             High bicarbonate
                                                                      Blue = L




                      Liquid-Air Interface Corrosion Studies at SRS     SRNL-STI-2010-00695      13




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   Electrochemical Potential Measurement vs. Time: Elevated
   Temperature




            0.5 M Nitrate       50 °C                             Red = H, off scale high

            0.1 M Nitrite       Aerated                           Green = 2
                                                                  Purple = 1
            pH 11.8             High bicarbonate
                                                                  Blue = L




                  Liquid-Air Interface Corrosion Studies at SRS      SRNL-STI-2010-00695       14




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        Electrochemical Potential Measurement vs. Time: Decreased
        Buffer




                                                                      Red = H
              0.5 M Nitrate       50 °C
                                                                      Green = 2
              0.1 M Nitrite       Aerated
                                                                      Purple = 1
              pH 11.5             Low bicarbonate
                                                                      Blue = L




                      Liquid-Air Interface Corrosion Studies at SRS     SRNL-STI-2010-00695      15




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    Electrochemical Potential Measurement vs. Time: Dilute
    Nitrate/Nitrite




                                                                  Red = H
             0.05 M Nitrate     Room Temp. 26 °C
                                                                  Green = 2
             0.01 M Nitrite     Aerated                           Purple = 1
             pH 9.0             High bicarbonate                  Blue = L




                  Liquid-Air Interface Corrosion Studies at SRS    SRNL-STI-2010-00695      16




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    Electrochemical Potential Measurement vs. Time: Dilute
    Nitrate/Nitrite with Low Buffer




                                                                     Red = H
            0.05 M Nitrate        Room Temp. 26 °C
                                                                     Green = 2
            0.01 M Nitrite        Aerated
                                                                     Purple = 1
            pH 8.7                Low bicarbonate                    Blue = L




                     Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695      17




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        Summary of Potential Measurements

         Meniscus potentials similar to fully immersed (bulk) potentials
         Conductive film above meniscus observed on some specimens
         Little variation in potentials with position in less dilute solutions
         More variation in potentials with position in more dilute solutions




                     Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695      18




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        Conclusions

         Meniscus electrochemical potentials, as measured, not
         significantly different from bulk potentials
         No consistent effect of pH variation on potentials
         Aqueous electrolyte film above meniscus
         Macro-scale potential measurements appear unproductive
         LAIC due to microscopic separation of anodes (sites of oxidation
         reaction) and cathodes (sites of reduction reaction)




                    Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695      19




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        Q and A

    Contact information: philip.zapp@srnl.doe.gov




                  Liquid-Air Interface Corrosion Studies at SRS   SRNL-STI-2010-00695      20




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