api 571 damage mechanism questions

7/22/2019 API 571 DAMAGE MECHANISM QUESTIONS

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Matthews Engineering Training Ltd

API 510 CERT

Slide 1

API 571:CONTENTS

Now we will look at the last group of damagemechanisms covered by API 571

These are fairly complicated

corrosion mechanisms,manyof them related to higher

temperature and oil/gas

industry/ refinery applications

4.2.3 Temper Embrittlement

4.2.7 Brittle Fracture

4.2.9 Thermal Fatigue

4.2.14 Erosion/Erosion-Corrosion

4.2.16 Mechanical Fatigue

4.3.2 Atmospheric Corrosion4.3.3 Corrosion Under Insulation (CUI)

4.3.4 Cooling Water Corrosion

4.3.5 Boiler Water Condensate Corrosion

4.4.2 Sulfidation

4.5.1 Chloride Stress Corrosion Cracking (Cl-SCC)

4.5.2 Corrosion Fatigue

4.5.3 Caustic SCC (Caustic Embrittlement)

5.1.2.3 Wet H2S Damage (Blistering/HIC/SOHIC/SCC)

5.1.3.1 High Temperature Hydrogen Attack (HTHA


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Slide 2

REMEMBER THE WAY THAT API 571

COVERS EACH OF THE MECHANISMS

Description/appearance

of the

damage mechanism

Critical

factors

AffectedequipmentPrevention/

mitigation

Inspection/

monitoring

Related

mechanisms


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Slide 3

SULFIDATION

This is a high temperature corrosion mechanism

Carbon and alloy steels

Sulphur compounds

High temperatures

(260 degC +)

+

+ =Sulfidation corrosion

WARNING:

This is a common closed-book exam topic


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Slide 4

SULFIDATION

API EXAMINATIONS NEARLY ALWAYS HAVE

QUESTIONS ABOUT SULFIDATION

What is it?


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Slide 5

SULFIDATION

The main problem is caused by H2S (formed by the

degradation of Sulphur compounds at high temperature)

Occurs in crude

plant,cokers,hydroprocessor units,firedheaters etc.anywhere where there

are high temperature sulphur streams

Sulfidation starts to degrade steels abve

about 500degF (260 degC)


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Slide 6

SULFIDATION

The susceptibility of steels to Sulfidation is shown inthe McConomy curves shown in API 571

Watch out for exam questions on this

As the temperature rises

above 500 degF,the

sulfidation corrosionrate goes up


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Slide 7

Higher Cr alloys(300-400 series stainless steels)

may be more resistant to sulfidation corrosion

MITIGATION

SULFIDATION


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Slide 8

STRESS CORROSION CRACKING

One of the most common corrosion

mechanisms

Prevalent in 300 series austenitic stainlesssteel and high chromium alloys

Where does the come from?

Often from residual stresses caused by welding

316 304

API terminology also calls it Environmentalassisted cracking


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Slide 9

STRESS CORROSION CRACKING

The stress exposes the grain

boundaries to corrosion

Temperature range above 60 degC

(140 degF) and pH>2


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Slide 10

SCC

Rule of thumb

In many cases ,SCC is due to

Chlorides attaching the stainless steelSalt


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Slide 11

UT will not show very small SCC cracks

Neitherwill RT

SCC CAN BE VERY DIFFICULT TODETECT.ALMOST IMPOSSIBLE IN ITS EARLY

STAGES


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Slide 12

CLASSIC SCC

BRANCHED CRACKS

Also known as

bifurcatedcracks


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Slide 13

SCC DETECTED BY PT

Surface abrasion may be

needed before PT in order to

show fine SCC cracks

Cracks would remain hidden

without surface abrasion


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Slide 14

CAUSTIC

EMBRITTLEMENTA specialist type of SCC caused by alkaline conditions

The worst offenders are :

Sodium Hydroxide (NaOH)

Caustic Potash (KOH)

Typically found in H2

S removal units and acid neutralisation units

Caustic attack in a heat

exchanger tubesheet


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Slide 15

CAUSTIC EMBRITTLEMENT

Cracks normally start from the surface of the material

Look at Fig 4-85 in API 571 showing

how temperature and NaOH

concentration affects the susceptibility

of Carbon steel to NaOH


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Slide 16

CORROSION FATIGUE

Cracks caused by a combination of:

Corrosion Cyclic loadings+

These cracks often initiate at pits or under deposits


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Slide 17Fatigue cracks in corroded area: initiated the failure

Secondary brittle fracture

CORROSION FATIGUE


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Slide 18

Unlike normal fatigue, there is no endurance limit

for corrosion-assisted fatigue

Stress

S

CyclesN

No endurance

limit

UTS

CORROSION FATIGUE

AN IMPORTANT POINT


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Slide 19

WET H2S DAMAGE

API 571 Sec 5.1.2.3 identifies 4 damage mechanisms

They affect carbon steels and low alloys steels in wet

H2S environments

Hydrogen

blistering

Hydrogen induced

cracking (HIC)

Stress Oriented

Hydrogen induced

cracking (SOHIC)

Sulfide

SCC


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Slide 20

WET H2S DAMAGE

The type of wet H2S damage that occurs is related tothese factors(see 570 Sec 5.1.2.3..3.)

pH

The H2S level presentTemperature

Hardness

Type of steel

PWHT (an important one)

The actual damage mechanism for all 4

categories is the permeation of the H2 into the

materials grain boundariesThis weakens the material and

causes failure


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Slide 21

WET H2S DAMAGE

Hydrogen blistering

The Hydrogen is liberated from corrosion

(not the process fluid)

It weakens the material structure causing a

blister (and eventual failure)


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Slide 22

WET H2S DAMAGE

HIC

Sometimes called stepwise cracking as

hydrogen causes cracks in the structure

The cracks weaken the structure and cause failure)

Can be worse

near a weld


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Slide 23

WET H2S DAMAGE

SOHIC

A type of HIC in which the cracks are made

worse by stress concentrations

Stress Oriented Hydrogen induced

cracking (SOHIC)

Occurs inHAZ at weld

toes


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Slide 24

WET H2S DAMAGE

Sulfide SCC

Essentially ..SCC made worse by thepresence of water and H2S

Can appear in areas of high hardness (e.g. in welds)

Weld preheat and PWHT

can help reduce the risk

(depending on the alloy)


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Slide 25

HIGH TEMPERATURE

HYDROGEN ATTACK(HTHA)

This is a specialist and complex corrosion mechanism

In simple terms:At high temperatures,H2 reacts

with the Carbon in the steel

forming Ch4 (Methane)

The resulting loss of Carbides

weakens the steel

Fissures start to form,and

propagate into cracks


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Slide 26

Thick-walled tube

failure

A neat rectangular section

is blown out without

any bulging

HTHA has attacked the grain

boundaries

HTHA TUBE FAILURE


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Slide 27

API 571 SUMMARY

In these 3 presentations we have looked at all of the

mechanisms in API 571 that are in the API 510 exam syllabus

Now finish off the module text

and try the test questions

NEXT STEP4.2.3 Temper Embrittlement4.2.7 Brittle Fracture4.2.9 Thermal Fatigue

4.2.14 Erosion/Erosion-Corrosion

4.2.16 Mechanical Fatigue

4.3.2 Atmospheric Corrosion4.3.3 Corrosion Under Insulation (CUI)

4.3.4 Cooling Water Corrosion

4.3.5 Boiler Water Condensate Corrosion

4.4.2 Sulfidation

4.5.1 Chloride Stress Corrosion Cracking (Cl-SCC)

4.5.2 Corrosion Fatigue4.5.3 Caustic SCC (Caustic Embrittlement)

5.1.2.3 Wet H2S Damage (Blistering/HIC/SOHIC/SCC)

5.1.3.1 High Temperature Hydrogen Attack (HTHA

api 571 damage mechanism questions

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