Insititue of Paper Science and Technology Central Files CORROSION CONTROL ANNUAL RESEARCH REVIEW HANDOUT BOOK

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1 Insititue of Paper Science and Technology Central Files CORROSION CONTROL ANNUAL RESEARCH REVIEW HANDOUT BOOK J

2 CORROSION CONTROL HANDOUT BOOK April 2, 1991

3 CONTENTS PROJECT 3309 FUNDAMENTALS OF CORROSION CONTROL IN PAPER MILLS 1 PROJECT ELECTROCHEMICAL STUDIES IN WHITE WATER 41 PROJECT 3628 RECOVERY BOILER FIRESIDE CORROSION 69

4 PROJECT 3309 FUNDAMENTALS OF CORROSION CONTROL IN PAPER MILLS -1-

25 Optical micrograph of the surface of CA-15 showing pits. Note the one on the fracture surface. -23-

26 Scanning Election Micrograph of CA-15 showing Depth of pit on fracture surface. -24-

27 Scanning Electron Micrograph depicting multiple fatigue crack initiation sites in Alloy A

37 Table 1. Threshold Stress Intensities for A75 and A86 for 1000 mg/i cr at a ph of 3.5 and Temperature of 50 C. Alloy R-ratio AK. (ksi in A A A A

38 Table 2. Threshold Stress Intensities for A75 and A86 for 200mg/1 Cr, 500mg/l S0 4 2' and 50mg/l S 2 03 ' at a ph of 3.5 and Temperature of 50 C. Alloy A75 A86 R-ratio AK,_ (ksi in)

39 Scanning Electron Micrograph of the Fracture Surface of A

43 PROJECT ELECTROCHEMICAL STUDIES IN WHITE WATER -41-

44 ELECTROCHEMICAL STUDIES IN WHITE WATER Project leader: Tero Hakkarainen (visiting scientist from The Technical Research Centre of Finland, VTT) -43-

45 OBJECTIVE To relate the steel composition and microstructure to the susceptibility to pitting & crevice corrosion. * of cast suction roll duplex stainless steels * in white water with thiosulfate * by electrochemical experiments and * microstructural observations -44-

46 BACKGROUND * Laboratory corrosion fatigue tests vs. practical experience (suction roll stainless steels) * Possible role of localized attack (pitting & crevice corrosion) * Effects of thiosulfate on austenitic stainless steels -45-

47 TEST MATERIALS Alloy %Cr %Ni %Mo %Cu Alloys 75 and 86 slowly cooled Alloy 63 water quenched -46-

48 MICROSTRUCTURES Alloy 75 * Approximately 60% austenite * Dual appearance: - coarse dendrites - areas of fine distribution * Small colonies presumably containing sigma phase Alloy 86 * Very similar to Alloy 75 Alloy 63 * Homogeneous structure - no finely distributed austenite - no "sigma phase colonies" -47-

49 Microstructure of Alloy 75. Optical micrographs. a) General view, loox. b) Detail showing a "sigma phase" colony, looox. -48-

50 Microstructure of Alloy 86. Optical micrographs. a) General view, 100x. b) Detail showing "sigma phase" colonies, 1000x. -49-

51 Microstructure of Alloy 63. Optical micrograph. 100x. -50-

52 ENVIRONMENTS * Basic white water: sulfate chloride thiosulfate mg/l mol/l C, ph = 3.5 * Only one or two of the components present * Less chloride or/and thiosulfate * More (less) sulfate -51-

53 TEST PROCEDURES Cyclic potentiodynamic "pitting tests" mv/s (12 mv/min) - reversal of scan direction at 1 ma Potentiostatic tests - basic white water - increasing chloride & thiosulfate contents Specimens: wet ground or polished & etched -52-

54 200 mg/1 chloride 500 mg/l sulfate 50 mg/l thiosulfate 75 = Alloy 75( 86 = Alloy : 63 = Alloy 63/

59 2

61 A

62 Pitting attack of Alloy 75 in the basic white water after 10 minutes at -200 mv(sce). Specimen surface polished and etched prior to exposure. Pits covered with corrosion products. Optical micrographs. a) 200x, b) 500x. -61-

63 Pitting attack of Alloy 86 in the basic white water after 55 minutes at -200 mv(sce). Specimen surface polished and etched prior to exposure. Pits covered with corrosion products. Optical micrograph. 500x. -62-

64 Pitting attack of Alloy 75 in the basic white water after 10 minutes at -200 mv(sce). Specimen surface polished and etched prior to exposure and again after the exposure. Optical micrograph, 400x. -63-

65 CONCLUSIONS Alloy 75 is susceptible to thiosulfate induced pitting in white water, Alloy 86 is marginally susceptible, Alloy 63 is practically immune. Initiation of attack at colonies containing sigma phase and austenite. The colonies probably do not otherwise impair the corrosion resistance or the mechanical properties of the alloys. (Continued...) -64-

66 CONCLUSIONS (Cont'd) For thiosulfate attack to occur, both the electrode potential and the relative content of the thiosulfate ions have to be within certain ranges. For rapid pit growth (Alloy 75), the presence of chloride ions is required. If thiosulfate ions are present,, localized attack induced by chloride ions at high potentials may continue at low potentials in Alloys 75 and 86, but probably not in Alloy 63. (Continued...) -65-

67 CONCLUSIONS (Cont'd) Copper (Alloy 86) strongly inhibits and molybdenum (Alloy 63) eliminates the thiosulfate pitting of duplex stainless steel (Alloy 75) in white water. The differences in service performance (corrosion fatigue cracking) between Alloys 63 and 75 are not explained by differences in the susceptibilities to localized attack in white water. -66-

68 FURTHER WORK Scanning electron microscopy - relation between attack and microstructure - role of copper Transmission electron microscopy - identification of phases in colonies "Avesta cell" - threshold concentrations -67-

69 PROJECT 3628 RECOVERY BOILER FIRESIDE CORROSION -69-

70 Project Objective TO UNDERSTAND THE CAUSES OF CORROSION ON THE FIRE-SIDE OF THE WATER WALL TUBES IN THE LOWER FURNACE OF KRAFT RECOVERY BOILERS AS BASIS FOR DEVISING METHODS OF REDUCING CORROSION DAMAGE. -71-

71 Task 1., Port Corrosion Objectives 1. IDENTIFY THE CORROSIVE AGENT(S) and determine the effect of: 2. ENVIRONMENTAL FACTORS 3. STEEL COMPOSITION 4. GALVANIC CONTACT -72-

72 RECOVERY BOILER CORROSION TASK 1 PORT CORROSION TASK 2 GAS/SMELT REACTIONS TASK 3 CONTINUOUS KINETICS TASK 4 LOCAL OXYGEN AND SULFUR ACTIVITIES Overall Project Scope -73-

73 Task 1., Port Corrosion Experimental Approach Electrochemical tests in molten NaOH electrode holder gas outlet outlet gas inlet thermocouple potentiostat CR W ZRA Electrodes -74-

74 Polarization curves for carbon steel and stainless steel obtained simultaneously in molten NaOH under N 2 at 320 oc. -75-

75 UndSr N Polarization curves for carbon steel and stainless steel obtained simultaneously in molten NaOH under N 2 at

76 Electrode potential, mv/pt Polarization curves for carbon steel and stainless steel obtained simultaneously in molten NaOH exposed to air for 2.5 hours. -77-

77 Electrode potentioa, mv/pt Polarization curves for carbon steel and stainless steel obtained simultaneously in molten NaOH exposed to air for 21 hours. -78-

78 Task 1., Current Port Corrosion Results STAINLESS STEEL CORRODES PREFERENTIALLY IN NaOH MELT UNDER N2 AND AIR WHEN IN CONTACT WITH CARBON STEEL. -79-

79 Task 1., Port Corrosion Experimental Approach (cont.) Field testing with port probes CARTRIDGE HEATER T-CONTROLLER -80-

80 Task 1., Port Corrosion Future Work 1. TESTS WITH C02 IN THE GAS OVER NaOH MELT 2. TESTS WITH H 2 0 IN THE GAS OVER NaOH MELT 3. TESTS WITH VARIOUS SS GRADES 4. FIELD TESTING -81-

81 C

82 Task 2. Gas / Smelt Reactions Experimental Approach gas outlet thermocouple H2S+N2 02+C02+H20+N2 N2 Horizontal tube furnace for corrosion tests with coupons in gas and gas + smelt powder environments -83-

83 1200 NO KCI Pyro NaOH Sulfit NaCI S Thio Smelt Additive (5%) -84-

84 1200

85 Task 2., Gas/Smelt Reactions Initial and Equilibrium Gas i n it.: 10%C02+2.0%H %H2S + 1%02 eq300: 10%C02+2.9%H %S %H2S+400 ppm S8+90 ppms2 eq400: 10%C02+2.8%H %SO2+0.24%H2S+660 ppm S2+3 ppms8 at equilibrium: also traces of COS, H 2, CO, SO 3, and 02 Main reaction: H 2 S + 02 => H /2 SO 2 + 1/2 Sx -86-

86 Task 2., Gas/Smelt Reactions Reactions between gas and smelt init. : after 5h: 8.0% Na2S, 12% Na2SO4, and 80 % Na2CO3, 0.2% Na2S, 16% Na2SO4, and 40% Na2C03 Thermodynamically feasible reactions: Na2S + C02 + H20 => H2S + Na2C03 Na2S + 2C02 => Na2C03+ 1/2S2 + CO Na2CO3 + S02 + 1/2 02 => CO2 + Na 2 SO 4-87-

87 Task 2., Gas / Smelt Reactions Results and Conclusions 1. TEST GAS TENDS TO GENERATE Sx, SO2, AND H THE GAS REACTS WITH SMELT AND MAY RELEASE H 2 S AND CO 2 3. SULFUR, THIOSULFATE, SULFITE OR PYROSULFATE ADDED TO SMELTAGGRAVATE THE CONDITION 4. CHLORIDE AND HYDROXIDE ADDITION INCREASE CORROSION 5. H20 SEEMS TO RETARD CORROSION -88-

88 Task 2., Gas/Smelt Reactions Future Work STUDY OF: 1. H 2 0-EFFECT ON CORROSION RATE 2. THE EFFECT OF S0 2 AND C02 ON SMELT -8,9-

89 Task 3., Continuous Kineticts Objectives 1. CORROSION RATE DEPENDENCY WITH TIME 2. SCALE MORPHOLOGY -90-

90 800 Equation of line w = (In t) Slope = 4500/t (mpy) RECOVERY BOILER CORROSION Carbon Steel Coupons (C1018) No Smelt Present 63 Mpy 188 Mpy Instantaneous Corrosion Rate Equation of line w = (n t) Slope = 310/t (mpy) Instantaneous Corrosion Rates Calculated from Equation of Line. -91-

91 800 RECOVERY BOILER CORROSION Carbon Steel Coupons (C1018) Smelt Present Time (hours) Instantenous Corrosion Rates Calculated from Equation of Line. Average Values Given in Table -92-

92 5,000 RECOVERY BOILER STUDIES Carbon Steel Corrosion Coupons (C1018) No Smelt Present Time (hours) Front Coupon Back Coupon Average Value of Corrosion Rate versus Time -93-

93 Micrograph of cross section of corrosion scale formed on carbon steel during 24 hour at 440 C. Magnified 500x. -94-

94 - Equation of line RECOVERY BOILER CORROSION Stainless Steel Coupons (304) No Smelt Present

95 Equation of line y = (In t) Slope: 485/t (mpy) RECOVERY BOILER CORROSION Stainless Steel Coupons (304) Smelt Present

96 500 RECOVERY BOILER STUDIES Stainless Steel Corrosion Coupons (304) No Smelt Present Time (hours) Front Coupon Back Coupon Average Value of Corrosion Rate versus Time -97-

97 400 RECOVERY BOILER STUDIES Carbon Steel Corrosion Coupons (440 C) 80% N 2, 20% CO Time (hours) 100 Instaneous Corrsion Rates Calculated from Equation of Line and Given by Upper Values. Average Corrosion Rate Shown by Straight Line. -98-

98 Task 3. Continuous Kinetics Summary of Current Results: 1. CORROSION OBEYS IN THE EARLY STAGE A LOGARITMIC RATE LAW AND AFTER ABOUT ONE DAY BECOMES NEARLY LINEAR (< 500 HR) % C02 RESPONDS TO CORROSION SIMILARLY THAN THE TEST GAS 3. CORROSION SCALE MULTILAYERED WITH TWO DIFFERENTLY STUCTURED LAYERS 4. CHEMICAL PASSIVATION APPEARED INEFFECTIVE -99-

99 Task 3., Future Continuous Kinetics work 1. LONG TERM TESTS WITH THERMOBALANCE 2. TESTS WITH FIXED Po 2 AND PS2-100-

100 Task 4, Local 02 and S 2 Activities Objectives DEVELOP CAPABILITY TO MEASURE LOCAL Po 2 AND PS2 FOR DETERMINATION OF CORROSION CONDITIONS AT METAL SURFACE UNDER SMELT LAYER

101 Task 4., Local 02 and S2 Activities Experimental Approach 1. DEFINE LOCAL CONDITIONS 2. CONSTRUCTION OF 0 2 -PROBE 3. DEVELOP S 2 -PROBE 4. CONTINUOUS MEASUREMENTS UNDER SYNTHETIC SMELT 5. SAME FOR FIELD PROBES -102-

102 liquid sulphur at 300 & 400 C 0-5

103 Task 4., Local 02 and S 2 activities Progress and Conclusions PROBE UNDER CONSTRUCTION 2. TEST CONDITIONS CLOSE TO OXIDE/SULFIDE BOUNDARY LINE 3. HIGH TEMPERATURE FAVORS OXIDE OVER SULFIDE -104-