SSCC : SULPHIDE STRESS CORROSION CRACKINGIGC : INTERGRANULAR CORROSION

TWO IMPORTANT CORROSIONS TYPESENCOUNTERED 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

MEDIA

ONE OF THE MOST IMPORTANTFACTORS.

PROCESS FLUIDS CONTAMINANTS SOLVENTS CATALYSTS ETC.

HIC -- HYDROGEN INDUCED CRACKING

SSCC --- A CRACKING PROCESS THAT REQUIRES SIMULTANEOUS ACTIONOF A CORRODENT AND SUSTAINEDTENSILE STRESS.

WATER :

FUNCTIONS AS ELECTROLYTEHYDROLYSES INORGANIC SALTS

COMBINATION OF AIR & WATERIS 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

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

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

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

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 LENGTHb = CRACK AREA THICKNESSW = SECTION WIDTHT = TEST SPECIMEN THICKNESS

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 )

FINAL pH SOLUTION A -- 4.0 SOLUTION B -- 4.0

TEST LOAD :

GENERALLY LOADED TO 72% OF STANDARD METALYIELD STRENGTH.

THIS MAY BE CHANGED BYCUSTOMER

TEST IS DEEMED PASSED IF

SPECIMEN DOES NOT BREAK

EXAMINATION AT 10 X DOES NOT REVEAL CRACKS

SUSCEPTIBLE MATERIALS : STAINLESS STEELS

PROCESS :

CORROSION RESISTANCE OF STAINLESS STEELS IS DERIVED FROM

Cr + O2 --------> Cr2O3

FROMMETAL

FROMATMOSPHERE

Cr2O3 FORMS A TENACIOUS FILM

ON THE SURFACE.

MINIMUM 12 % Cr IS NEEDEDTO MAKE STEEL STAINLESS

WHEN S.S. PASSES THROUGH THETEMPERATURE RANGE OF 425O C TO 850O C WE HAVE : C + Cr -----> Cr23C6

THE REACTION OCCURS ATGRAIN BOUNDARIES

THUS THERE IS A BAND AT THEGRAIN BOUNDARIES WHERE

Cr IS LESS THAN 12%

THIS AREA IS NO MORE STAINLESS

THE PROCESS FLUIDS AND CONTAMINANTS CAN ATTACKIN THIS AREA.

THERE ARE A LOT OF PROCESSESOF MANUFACTURING THAT TAKESS IN THIS RANGE

EG. WELDING ; CUTTING ; HOT/WARM FORMING

SERVICE TEMPERATURE CAN ALSOBE 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

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

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

IN PRACTICE B AND C WEIGHTLOSS IS MEASURED.THE CORROSION RATE IS GIVENBY THE WEIGHT LOSS

THIS IS NOT THE CORROSIONRATE IN ACTUAL SERVICE.

THE RESULT IS A RELATIVEMEASURE

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 NOCRACKS. CRACKS FROM EDGE ,WRINKLES AND ORANGE PEEL EFFECT ARE DISREGARDED.

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 hoursd = 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

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)

PRACTICE EEVALUATION

FAILED MARGINAL( DOUBTFUL )

PASSED

STEP DUAL

DITCH

As Received

After exposure

After bending

PARENT METAL

100 x

100 x

100 x

HAZ

Before exposure

After exposure

WELD

Before exposure

After exposure

PASSED

FAILED