corrosion lecture
TRANSCRIPT
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SSCC : SULPHIDE STRESS CORROSION
CRACKING
IGC : INTERGRANULAR CORROSION
TWO IMPORTANT CORROSIONS TYPES
ENCOUNTERED BY US ARE COVERED:
BROADLY CLASSIFIED AS :
LOW TEMPERATURE CORROSION
---- CORROSION AT ROOM
TEMPERATURE AND BELOW
HIGH TEMPERATURE CORROSION
---- ELEVATED TEMPERATURE
CORROSION INCLUDING
THAT BY MOLTEN METALS
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MEDIA
ONE OF THE MOST IMPORTANT
FACTORS.
PROCESS FLUIDS
CONTAMINANTS
SOLVENTS
CATALYSTS ETC.
HIC --
HYDROGEN INDUCED CRACKING
SSCC ---
A CRACKING PROCESS THAT
REQUIRES SIMULTANEOUS ACTION
OF A CORRODENT AND SUSTAINED
TENSILE STRESS.
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WATER :
FUNCTIONS AS ELECTROLYTE
HYDROLYSES INORGANIC SALTS
COMBINATION OF AIR & WATER
IS DANGEROUS ---
AIR SUPPLIES O2
WATER ACTS AS ELECTROLYTE
SOURCES :
ALMOST EVERY PART OF PROCESS
MEDIA :
EXCLUDED :
FAILURES DUE TO REDUCED SECTION AS A
RESULT OF CORROSION
FAILURES DUE TO INTER - CRYSTALLINE OR
TRANS - CRYSTALLINE CORROSION
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H2S SOUR WATER
COMMON SOURCES :
CRUDE OIL
DECOMPOSITION OF ORGANIC
COMPOUNDS OF SULPHUR
PROCESS WATER CARRYING H2S
WITH NH3 HCN ETC.
H2S REACTS WITH STEEL TO GIVE
A BLACK SULPHIDE FILM
C.S. HAS GOOD RESISTANCE TO
AQUEOUS SULPHIDE CORROSION
SOUR WATER
COMMON SOURCES
PROCESS WATER CARRYING H2S
WITH NH3 HCN ETC
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AQUEOUS H2S
C.S. AND LAS ARE USED
AQUEOUS H2S + NH3 ETC
CONDITION MORE SEVERE
C.S. & LAS NOT USED
DEPENDING ON SEVERITY
S.S OR TITANIUM USED
HIC NACE STANDARD TM - 02 84
SSCC NACE STANDARD TM - 01 77
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HIC TESTING :
SPECIMEN SIZE :
20 X 20 X 100 mm
SOLUTION :
A -- SODIUM CHLORIDE +
ACETIC ACID ( pH 2.7 )
SATURATED WITH H2S
B -- SYNTHETIC SEA WATER
( AS PER ASTM D 1141 )
pH 8.1 - 83
SATURATED WITH H2S
TEST DURATION :
96 HOURS ( 4 DAYS )
FINAL pH
SOLUTION A -- 4.0
SOLUTION B -- 4.8 - 5.4
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TEST RESULT :
THREE SECTIONS OBSERVED
BLISTERS SHALL BE NIL
CSR , CLR AND CTR SHALL
BE WITHIN SPECIFIED
LIMITS
CSR =100 [ (a x b) / ( W x T) ]
CLR =100 [ a / ( W x T) ]
CTR =100 [ b / ( W x T) ]
a = CRACK LENGTH
b = CRACK AREA THICKNESS
W = SECTION WIDTH
T = TEST SPECIMEN THICKNESS
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SSCC TESTING :
SPECIMEN SIZE :
ROUND BAR TENSILE SPECIMEN
GAUGE DIA. 6.35 mm
GAUGE LENGTH 25.4 mm
( OVERALL LENGTH 200 mm)
SOLUTION :
A -- SODIUM CHLORIDE +
ACETIC ACID ( pH 2.6 - 2.8 )
SATURATED WITH H2S
B -- SODIUM CHLORIDE +
SODIUM ACETATE +
ACETIC ACID ( pH 3.4 - 3.6 )
SATURATED WITH H2S
TEST DURATION :
720 HOURS ( 30 DAYS )
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FINAL pH
SOLUTION A -- 4.0
SOLUTION B -- 4.0
TEST LOAD :
GENERALLY LOADED TO
72% OF STANDARD METAL
YIELD STRENGTH.
THIS MAY BE CHANGED BY
CUSTOMER
TEST IS DEEMED PASSED IF
SPECIMEN DOES NOT BREAK
EXAMINATION AT 10 X DOES
NOT REVEAL CRACKS
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SUSCEPTIBLE MATERIALS :
STAINLESS STEELS
PROCESS :
CORROSION RESISTANCE OF
STAINLESS STEELS IS DERIVED
FROM
Cr + O2 --------> Cr2O3
FROM
METAL
FROM
ATMOSPHERE
Cr2O3 FORMS A TENACIOUS FILM
ON THE SURFACE.
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MINIMUM 12 % Cr IS NEEDED
TO MAKE STEEL STAINLESS
WHEN S.S. PASSES THROUGH THE
TEMPERATURE RANGE OF 425O C
TO 850O C WE HAVE :
C + Cr -----> Cr23C6
THE REACTION OCCURS AT
GRAIN BOUNDARIES
THUS THERE IS A BAND AT THE
GRAIN BOUNDARIES WHERE
Cr IS LESS THAN 12%
THIS AREA IS NO MORE
STAINLESS
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THE PROCESS FLUIDS AND
CONTAMINANTS CAN ATTACK
IN THIS AREA.
THERE ARE A LOT OF PROCESSES
OF MANUFACTURING THAT TAKE
SS IN THIS RANGE
EG. WELDING ; CUTTING ; HOT/WARM
FORMING
SERVICE TEMPERATURE CAN ALSO
BE IN THIS RANGE
THUS THERE IS A NEED TO TEST
THE SUSCEPTABILITY TO IGC
THIS NEED IS GOVERNED BY
SERVICE CONDITIONS
APPLICABLE STD. ASTM A262
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THERE ARE 5 PRACTICES
PRACTICE SOLUTION APPLICATION
A OXALIC ACID RAPID
ETCH TEST IDENTIFICATION
OF MATERIALS
WHICH WILL FAIL
B FERRIC SULPHATE DETECTS IGC BUT
+ SULPHURIC ACID NOT SIGMA PHASE
SUSCEPTABILITY
C 65 % NITRIC ACID DETECTS IGC BUT
AND SIGMA PHASE
SUSCEPTABILITY
E Cu +CuSO4 + DETECTS IGC BUT
16 % H2SO4 NOT SIGMA PHASE
SUSCEPTABILITY
F Cu +CuSO4 + DETECTS IGC BUT
50 % H2SO4 NOT SIGMA PHASE
SUSCEPTABILITY
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PRACTICE TEST CONDITIONS
A ETCHING AND EXAMIN AT
250 x AND 500 x
B BOILING SOLUTION
120 HOURS
C BOILING SOLUTION
240 HOURS
E BOILING SOLUTION
24 HOURS
F BOILING SOLUTION
120 HOUR
NORMALLY PRACTICE B ; C AND E
ARE USED
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IN PRACTICE B AND C WEIGHT
LOSS IS MEASURED.
THE CORROSION RATE IS GIVEN
BY THE WEIGHT LOSS
THIS IS NOT THE CORROSION
RATE IN ACTUAL SERVICE.
THE RESULT IS A RELATIVE
MEASURE
IN PRACTICE E SPECIMEN IS
BENT THROUGH 180O OVER A
FORMER OF 1T RADIUS.
THE BENT SURFACE IS EXAMINED
AT 100 x AND 250 x
TEST IS PASSED IF THERE ARE NO
CRACKS. CRACKS FROM EDGE ,
WRINKLES AND ORANGE PEEL
EFFECT ARE DISREGARDED.
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PRACTICE B EVALUATION
Millimeter Per Month = 7290 x W
A x d x t
W = Weight Loss in gms.
A = Total Area of the Sample in cm2
t = Time of Exposure in hours
d = Density of sample in gms / cc
Millimeter Per Month x 0.04 = Inches Per Month
Millimeter Per Month x 0.47 = Inches Per Year
Millimeter Per Month x 12 = Millimeter Per Year
Millimeter Per Month x 472 = mpy
Millimeter Per Month x Density / 3 =
Milligram Per Square Decimeter Per Day
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PRACTICE C EVALUATION
3440880 (W)
( A ) ( d ) ( t )MPY =
A= Total Specimen area (cm2 )W= Weight loss ( gms)
d= Density of sample ( gms / cm3 )t = Time of exposure ( hrs)
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PRACTICE E
EVALUATION
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FAILED MARGINAL
( DOUBTFUL )
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PASSED
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STEP DUAL
DITCH
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As Received
After exposure
After bending
PARENT METAL
100 x
100 x
100 x
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HAZ
Before exposure
After exposure
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WELD
Before exposure
After exposure
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PASSED
FAILED
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