Oxides in Caustic

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10_02.jpg
Cross section of the oxide layer formed in aerated 0,5 n NaOH. The inner layer is magnetite with many cracks and on the outside hematite (alpha -Fe2O3) platelets. (foto KEMA, ref 10).
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Detail of picture 3561. In the magnetite layer clear perlite islands are still present. (foto KEMA, ref 10).
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Pitting with the hematite caps was found on C-steel exposed in NaOH (pH=10) at 250 gd C (40 atm) in oxygen containing solution. The experiment was under electrochemical controll at a high corrosion potential. (foto KEMA, ref 10).
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Corrosion scale, often found in boilers. Pitting with a cap of hematite plateles and presence of Copper particles on the oxide layer. In laboratory experiments the pitting was caused by corrosion experiments in high oxygen containing solutions. (foto KEMA, ref 10).
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Magnetite formed on a perlite area in 0.01 n NaOH at 250 gd C during 17 hours. (foto KEMA, ref 10).
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Very minor magnetite formation on a perlite area in NaOH (ph=9.0) at 250 gd C during 17 hours. The perlite area was marked by means of Vickers hardness indents. (foto KEMA, ref 10).
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MnS inclusions are solved in 0.01 n NaOH and near the inclusion places magnetite is not present. (foto KEMA, ref 10).
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SEM-picture of the alpha-Fe203 crystals, formed in aerated 0,5 n NaOH. See Figure 3008. (foto KEMA, ref 10).
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Oxidation of pure iron in oxygen free NaOH (pH 12.5) at 310 gr C and 100 atm. A break-away is present in the oxidation curve, indicating cracing of the oxide. (ref 10)
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SEM picture of the magnetite grown in the NaOH after 50 days. After the break-away point cracks are formed in the adherent magnetite. (foto KEMA, ref 10).
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Second example of a cracked surface at an oxidation time of 50 days. (foto KEMA, ref 10).
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Cross sections of the magnetite layers on 3 steels: 15Mo3, 14Mn4 and C-35. (Foto KEMA, ref 23).
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23_01a.jpg
"Weight losses after a 4-day exposure to NaOH under non-deaerated conditions at 250°C (experiment 1 ) (ref 23)."
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Oxide layers formed on C-steel after a 31-day exposure to NaOH (A) and LiOH (B) in 10-molar solutions under non-deaerated conditions at 310 ° C. In LiOH (at such high concentration) a very open non protective scale is formed. (foto KEMA, ref 23, 61).
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23_fig10a.jpg
Oxidation of C steel during 4 days in increasing caustic concentration. (ref 23).
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SEM picture of a fracture surface of a corrosion scale. On the steel surface a porous topotactical magnetite layer and on the outer surface a epitactical layer of coarse magnetite octaeders have been formed. Caustic environment onder bopiler conditions. (foto KEMA, ref 11, 61).
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2509.jpg
SEM picture of a post-break-away sample exposed in deaerated 0.03 NaOH at 310°C. (foto KEMA, ref 8).
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2510.jpg
Blistered oxide formed in 0,5 n NaOH at 360 °C in an autoclave test. (foto KEMA, ref 8).