CORROSION AND

ITS PROTECTION IN OIL & GAS PRODUCTION

CORROSION IN OIL FILED : INTERNAL AND EXTERNAL THREATS

INTERNAL THREATS

WELL TREATMENT INFLUENCED

WATER CARRY OVER

UNDERDOSING DEMULSIFIER

INJECTION PUMP with LOW CAPACITY

UNDERDOSING CORROSION INHIBITOR

WATER SETTLE OUT

CORROSION CAUSES

5

Typical E&P process conditions

• Temperature– Typical E&P process

temperatures range from -100ºC to >200ºC

– Corrosion rates increase with temperature

• Pressure – Pressure: up to 10,000psi– Increase partial pressure

of dissolved gases • Flowrate & flow regime

– High-flow: erosion and corrosion-erosion.

– Low-flow or stagnant conditions promote bacteria

6

Internal corrosion

Hydrocarbon phase • Not normally corrosive

at temperatures experienced in production systems

• Corrosivity depends on extent and distribution of the aqueous and hydrocarbon phases.

Aqueous phase • Responsible for corrosion• Corrosion exacerbated

by acid gases & organic acids

• CO2, H2S and O2 are the most aggressive species

• Chlorides increase corrosion

• Generally,– ‘no water, no

corrosion’

7

Internal (process-side) damage mechanisms

• H2S

• CO2

• Solids & velocity effects• Chlorides – pitting, stress corrosion cracking• Oxygen (crevice / under deposit / differential

aeration)• Galvanic corrosion• Preferential weld corrosion (PWC)• Microbially induced corrosion (MIC)• Liquid metal embrittlement (LME)• Chemicals

TYPICAL REACTIONS

9

There is no species more corrosive on a concentration basis than oxygen!

Corroded seawater injection tubing

Dissolved gas - effect on corrosion

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8

Corr

osio

n R

ate

of

Carb

on

Ste

el

O2

CO2

H2S

Dissolved Gas Concentration in Water Phase, ppm

0 1 2 3 4 5 6 7 8 0 100 200 300 400 500 600 700 800

0 50 100 150 200 250 300 350 400

O2

H2SCO2

10

H2S CORROSION

11

H2S corrosion – metal loss

– Formation of a thin protective FeS surface film often means general corrosion rates are low on steels

– Main risk is localised pitting corrosion where film is damaged

– Pitting will be galvanically driven

12

Wet H2S corrosion

• H2S is soluble in water

– Produces a weak acid and lowers the pHH2S H+ + SH-

– At low concentrations, H2S helps form protective FeS film

– Main risk is localised pitting corrosion which can be rapid

• H2S also poisons combination of atomic hydrogen into molecular hydrogen

H+ + e- HH + H H2X

Atomic hydrogen -

dangerous to steels!!

13

Cracking in sour service

HHH

H

H

Higher Strength Steels YS > 500 MPa Low Strength Steels YS < 550 MPa

Applied Stress No Applied Stress

H2

H2

H2 H+

S2-Fe2+

H

H

FeS Film

Metal Matrix

14

Sulphide stress cracking (SSC)

Key parameters:• pH and pH2S

– Domain diagrams for carbon steel

• Material hardness– High strength steels and areas

of high hardness susceptible.• Temperature

– Maximum susceptibility at low temperatures for carbon steels (15-25°C), higher for CRAs (5-70°C).

• Stress– Cracking promoted by high

stress levels e.g. residual welding

HAZ WELD HAZ

Hardness readings

15

Protection against SSC

• Avoid wetness• Minimise hardness

– Guidance on limits in ISO 15156

• Optimise microstructure and minimise residual stresses

Upgrade to CRAs• Martensitic and duplex

stainless steels have limited resistance

• H2S limits for duplex and super-duplex steels are complex– Function of temperature,

pH, chlorides, pH2S

• Nickel-base alloys such as 625 and 825 have high resistance

• Testing: NACE TM0177

16

ISO 15156 SSC zones for carbon steel

0.0034bara 0.05psia

Service Domain

Max hardness (parent metal,

HAZ, weld metal)

0 No requirements

1 300HV

2 280HV

3 250HV root 275HV cap

17

SSC limits for selected CRAs

Alloy pH2S limit (bara)

13% Cr martensitic 0.008

22% Cr duplex 0.10

25% Cr super-duplex 0.25

Alloy 825 No limit

Alloy 625 No limit

18

HIC / SWC / blistering

• Laminar cracking in plane of inclusions or blistering (HIC).

• Transverse cracking between laminar cracks on different planes (SWC).

Step-wise cracking Blistering of CS plate

Hydrogen

blisters

19

Avoiding HIC / SWC

• Avoid plate steels (rolled)– otherwise qualify by HIC

test• Control impurities e.g. S, P• Uniform microstructure• Use internal coatings

– isolate steel from process fluid

• Testing: NACE TM0284

Banded

Uniform

20

ISO 15156 (NACE MR0175)• ISO 15156 combination of

– NACE MR0175 and NACE testing requirements TM0177 & TM0284

– European Federation of Corrosion Guidelines No.16 & 17

• Part 1: General principles for selecting crack-resistant materials

• Part 2: Cracking resistant carbon & low-alloy steels & cast iron

• Part 3: Cracking resistant corrosion resistant alloys (CRAs)

• Covers all cracking mechanisms

• Goes beyond application of the 0.05 psia pH2S threshold for sour service

• It is the equipment user’s responsibility to select suitable materials

• HIC/SWC of flat rolled carbon steel products for environments containing even trace amounts of H2S to be evaluated

• BP ETP: GP 06-20 Materials for Sour Service

21

Designing for H2S service

• Materials requirements– Reference ISO 15156 and GP 06-20

– pH2S and pH

– Temperature– Chlorides– Hardness limits

• Welding QA/QC (HIC)– Maintain hardness limits

• HIC testing for plate products

22

CO2 CORROSION

23

CO2 - containing environments

• CO2 always present in produced fluids– Corrosive to carbon

steel when water present

– Most CRAs have good resistance to CO2

corrosion.

MechanismCO2 + H2O H2CO3

H2CO3 + e- HCO3- + H

2H H2

Fe Fe2+ + 2e-

Fe + H2O + CO2 FeCO3 + H2

24

Types of CO2 damage

Mesa corrosion

Localised weld corrosion

Flow-assisted-corrosion (CO2)

General & pitting corrosion

25

CO2 corrosion in a production flowline

• 6” CS production flowline (Magnus, 1983)

• 25mm thick, 90bar, 30°C, 2%CO2

• Heavily pitted pipe wall and welds (not necessarily uniform corrosion)

• Didn’t fail – removed due to crevice corrosion of hub sealing faces

26

Factors in CO2 corrosion

•

Main factors

–

pCO2, temperature, velocity, pH

- CO2 prediction model

Temperature, (ºC) pCO2 (bar) Carbon steel corrosion rate

(mm/yr)

130 0.6 7

75 0.6 6

149 30 >50

For an ideal gas mixture, the partial pressure is the pressure exerted by one component if it

alone occupied the volume. Total pressure is the sum of the partial

pressures of each gas component in the mixture

27

Effect of sand on CO2 corrosion

• Produced sand can affect inhibitor efficiency– Inhibitor adsorption loss

• Sand (and other solid) deposits give increased risk of localised corrosion;– Prevent access of corrosion inhibitor to the metal– Provide locations for bacteria proliferation– Galvanic effects (area under deposit at more negative

potential than area immediately adjacent to deposit)– Formation of concentration cells/gradients

28

Mitigation of CO2 corrosion

• Internal CO2 corrosion of carbon steel needs to be managed

– Usually mitigate by chemical inhibitors– Simple geometries only (mainly pipelines)

• Assume inhibitor availability (90-95%)– Inhibited corrosion rate of 0.1mm/year– Remaining time at full predicted corrosion rate– Apply a corrosion allowance for the design life– If calculated corrosion allowance >8mm use CRAs

29

CO2 corrosion inhibition

• Filming type• Retention time• Continuous injection • Adsorption onto clean

surfaces

Clean steel

30

CO2 + H2S corrosion – metal loss

• H2S corrosion (CO2/H2S < 20)

– Initial corrosion rate high– Protective FeS film quickly slows down corrosion to low

level– The corrosion rate is much less than the Cassandra

prediction

CO2/H2S > 500 CO2 dominates

500 > CO2/H2S > 20 mixed CO2/H2S

20 > CO2/H2S > 0.05 H2S dominates

H2S + CO2 materials selection guide

Carbon/low alloy steels

Duplex SS

Nickel-based alloys

Partial pressure H2S (bar)

Pa

rtial p

ressu

re C

O2 (b

ar) 13% Cr SS

32

EROSION & EROSION-CORROSION

33

Flow regimes

Liquid

Gas

Bubble (bubbly) flow

Stratified flow

GasLiquidAnnular flow

Churn flow

Gas

Liquid

GasLiq

uid

Plug flow

Wave (wavy) flow

Liquid

Gas

Slug flow

Mist (spray) flow

• Various multi-phase flow regimes possible;

− erosion characteristics

− distribution of phases

− carrier phase for solids

• Flow regimes with particles in the gas show higher erosion rates than those with particles in the liquid phase.

34

Erosion & erosion-corrosion

• Erosion

– Caused by high velocity impact & cutting action of liquid and/or solid particles

– Erosion failures can be rapid

• Erosion-corrosion

– Occurs in environments that are both erosive and corrosive.

– Erosion and corrosion can be independent or synergistic.

Erosion of tungsten carbide choke trim

35

Typical vulnerable areas for erosion

• Areas wherever flow is restricted or disturbed– T-pieces, bends, chokes, valves,

weld beads• Areas exposed to excessive flow

rates• Sand washing

– Washing infrequently allowing sand to accumulate

– High pressure drop during washing of separators

• Sea water systems– High flow areas in water

injection / cooling systems

Trinidad

Algeria (duplex)

36

Erosion in piping

• Sand accumulation– Build up of sand in a test

separator

• Pressure drop– Large pressure drop across

sand drain pipework during washing

• Rapid failure – Occurred within 2 minutes

of opening the drain

Erosion at bend

37

Erosion in a vessel• Sand allowed to accumulate in

separator – Wash nozzles embedded in sand

• PCV not working properly– High pressure / flowrate– Nozzle not erosion-resistant– Erosion of wash nozzle– Spray changed to a jet causing

erosion of shell• Local changes to operating

procedures not communicated– Frequency of sand washing– Risk not captured or assessed in

RBI

Water spray

Water jet

38

Erosion of sandwash nozzle

Progressive nozzle

damage

39

Erosion-corrosion

• Occurs in environments that can be erosive and corrosive.

• Erosion and corrosion can either be:– independent of each other;

• wastage equals sum of individual wastage rates

– synergistic;• wastage rate > sum of individual rates• localised protective film breakdown at

bends, elbows areas of turbulence

40

Impingement

• Water speed or local turbulence damages or removes protective film

• 90-10 Cu-Ni susceptible to internal erosion-corrosion (impingement) at velocities >3.5ms-1

• Water-swept pits (horse-shoe shaped)

41

Cavitation

• Occurs at high fluid velocities• Formation & collapse of vapour

bubbles in liquid flow on metal surface.

• No solids required • Typical locations

– Pump impellers (rapid change in pressure which damages films)

– Stirrers, hydraulic propellers

• Use erosion resistant materials– Stellite, tungsten carbide

42

CORROSION IN SEAWATER

43

Raw seawater• Composition of raw seawater varies around the world

– Temperature, pH, salinity, dissolved oxygen, marine life

• Very corrosive to unprotected carbon steel, other materials susceptible to pitting and crevice corrosion

• Select seawater resistant materials– Super-duplex grades, 6Mo, CuNi, titanium

• Consider galvanic corrosion– Most seawater resistant grades of stainless steel and

Ni-Cr-Mo alloys are compatible with each other in seawater.

• Seawater can cause SCC of 300-series, duplex grades and 6Mo

44

Pitting resistance of stainless steels

• Pitting Resistance Equivalent Number (PREw)

• Formula for comparing relative pitting resistance

• Applicable to stainless steels & Ni-Cr-Fe alloys

• Typically PREw ≥40 required for exposure to raw sea water <30ºC

• Alternatively, use titanium or GRE

Alloy PREw

13Cr 13

316ss 23

Alloy 825 28

22Cr duplex 33

25Cr super-duplex

40

Alloy 625 46

PREw = %Cr + 3.3x (%Mo + 0.5%W) + 16%N

45

Internal & external pitting

• Section of 3” 316L pipe fitting• Failed due to internal corrosion (pinhole leak)• Poor hydrotest practice - seawater left within spool

Internal pitting

46

Failure of a seawater pump cooling coil……

• 316 SS coil, raw seawater service, hypochlorite added• Shellside: lube oil up to 50°C• Tubeside: seawater inlet ~6°C, return ~18°C• Failed due to localised internal pitting

– 316 SS has low PREw• Material upgrade required

Internal surface of coil

External surface of coil

Indication on coil

47

Oxygen - concentration cells

• Crevice corrosion – O2 is consumed in the crevice and

becomes the anode– pH decreases in the crevice

increasing attack• Differential aeration cells

– Air/water interfaces with attack below the water line e.g. splash zone

– Pipelines in soils containing different amounts of oxygen

• Under deposit corrosion– Deposits of scale, sand or sludge– Produces differential concentration– SRBs thrive - H2S pitting

Crevice corrosion

under baffle

48

Galvanic corrosion

• Three conditions are required for galvanic corrosion;– A conducting electrolyte (typically seawater).– Two different metals in contact with the electrolyte.– An electrical connection between the two metals.

• Relative positions within the electrochemical series (for given electrolyte) provides driving potential and affects rate.

• Corrosion of base metal (anode) stimulated by contact with noble metal (cathode).

• Relative area of anode and cathode can significantly affect corrosion rate.

• Higher conductivity increases corrosion e.g. presence of salts

49

Galvanic corrosion – firewater piping

• Firewater – CuNi / super duplex stainless steel connections.

• 4”CuNi pipe with a 550mm isolation spool (i.e. 5x OD)

• Leaks experienced on CuNi spools at welds

• Same problems with CuNi / 6Mo

50

Galvanic corrosion - seal rings

• ETAP platform• Techlok joints in a

firewater piping system– Piping: super-duplex– Seal rings: 17-4PH

51

• Brass tubesheet in seawater service– Brass is Cu-Zn alloy– Cu is more noble than Zn– Zn dissolves

preferentially leaving Cu behind

• Result– Loss of strength– Difficult to seal

• Remedy– Add arsenic to the brass

Dealloying of brass

52

Mitigation of galvanic corrosion

• Avoid dissimilar materials in seawater system designs– MoC for later changes

• Avoid small anode/large cathode

• Avoid graphite gaskets & seals

• Avoid connecting carbon steel to titanium alloys– Galvanic corrosion or

hydrogen charging of titanium may occur

• Electrical isolation between different alloy classes

• Install distance spools, separation of at least 20x pipe diameters

– Solid non-conducting spool e.g. GRP

– Line the noble metal internally with an electrically non-conducting material e.g. rubber

• Apply a non-conducting internal coating on the more noble material. Extend coating for 20 pipe diameters.

53

Example : CuNi-Super duplex

Apply a non-conducting internal coating on the more noble material.

Distance spool: solid, non-conducting material e.g. GRP

Distance spool: noble metal internally lined with an electrically non-conducting material such as rubber

54

Cathodic protection (CP) – what is it?

• By connecting an external anode to the component to be protected and passing a dc current, it becomes cathodic and does not corrode.– External anode may be a galvanic (sacrificial) anode, the

current is the result of the potential difference between the two metals

– External anode may be an impressed current anode, current is supplied from an external dc power source.

• CP is mostly applied to coated, immersed and buried structures– The coating is the primary protection, acting as a barrier

between the metal and the environment– CP protects steel at coating defects

• Coating + CP is most practical and economic protection system.– Primary principle in GP 06-31

55

Cathodic protection – how does it work?

ANODIC

MagnesiumZinc

AluminiumIron (steel)

CopperStainless steels

TitaniumGraphite

CATHODIC

Corrosion of steel by copper

plating

Cathodic protection of steel by zinc

plating

• CP works by making the component to be protected the cathode in an electrolytic cell

• When two metals are connected in an electrolyte, electrons flow from the anode to the cathode due difference in the electrical potential

56

Galvanic (sacrificial) CP• Aluminium anodes: require alloy

additions to become active e.g. Zn + In, high efficiency (>90%).– Typically used in seawater applications.

• Zinc anodes: ambient applications only. Alloyed with Al or Cd to improve efficiency.– Typically used on coated pipelines in

seawater• Magnesium anodes: large driving

potential, alloyed with e.g. Al or Zn to reduce rapid activation, limited efficiency (50-60%)– Used in soils and other high-resistance

environments (risk of over-protection/rapid consumption in seawater).

Sacrificial anodes, new and wasted

(therefore working!)

57

Applications of internal CP

• Anodes in shell & tube seawater cooler water boxes

• Oil storage tanks (in water bottom)

• Water tanks

•Stainless steel piping systems in warm/hot chlorinated seawater.

−To avoid high anode consumption rates, resistor controlled CP (RCP) systems should be considered.

−E.g. RCP + 25Cr super duplex piping instead of titanium or other higher-alloy CRA.

−Used on Greater Plutonio

58

Chloride stress corrosion cracking (SCC)

• Susceptibility varies considerably (no absolutes);– Material grade, strength,

residual stress, chlorides, oxygen and temperature

• 300-series austenitic stainless steels susceptible to at temps >50°C

• Highly-alloyed austenitic and duplex SS have improved resistance

• Nickel-base alloys with Ni ≥ 42% are highly resistant, e.g. 825

59

Chloride SCC (22Cr duplex vessel drain)

• 22Cr duplex drain ex-production separator

− heat-traced to 60°C (vessel temp up to 105°C)

• Internal chloride SCC (cracking in parent metal, HAZ and weld metal)

• Contributory factors:

− Susceptible material

− Local stress concentration (weld toe and lack of support)

− Environment (elevated temperature, chlorides).

60

Water injection systems (deaerated)

Oxygen:

• Trace amounts corrosive to carbon steel. As a guide:

– <20ppb O2 maintains general corrosion rates <0.25mm/yr

– Stricter limits often applied e.g. <10ppb if 13Cr completions

Microbial-induced Corrosion, MIC

• SRB require anaerobic conditions

– deaerated water

– conditions within and under biofilms

• SRB use sulphate in water in their metabolisms to generate H2S

Fluid Velocity:

• Areas of high fluid velocity or turbulence and O2

– O2 from poor deaeration or air ingress

– susceptible areas include pump discharge piping, bends tees and reducers.

61

Mitigation & monitoring

• Deaeration and supplementary O2 scavenging

– Monitor O2 concentrations on-line (orbisphere) or colorimetric analysis

– Maintain oxygen scavenger residual to mop-up oxygen spikes.

• Chlorination u/s of deaerator, biocide applied into or d/s of deaerator

• Effective biociding based on;

– Type, frequency, dosage, duration

• Bacterial monitoring (sidestreams, scrapings or bioprobes)

• Corrosion monitoring

Leaking deaerator

Seawater injection tubing

62

Preferential weld corrosion (PWC)

• The selective corrosion of weld zones (WM/HAZ)• Relevant factors include;

– Electrochemical properties of the materials and any corrosion cell forming around the weld joint

– Water phase liquid film thickness and conductivity– Temperature and tendency to form protective scale– Corrosion inhibitor effectiveness, (film formation,

composition)– Weld joint metallurgy– Flow pattern and flow induced shear stress

• PWC rate of attack can be high, up to 12mm/yr observed

63

Preferential weld corrosion (1%Ni)

Water Injection:• 1% Ni-containing welds

beneficial for avoiding PWC in WI systems.

• Weld cathodic to parent metal, protected by large area of parent metal.

Wet hydrocarbon service:• Lower conductivity, no benefit of

selecting ‘cathodic’ weld metal• Reliant on intrinsic corrosion

resistance of the weld metal• Require corrosion inhibitor for

protection (test against WM and PM)• Attack of weld metal promoted by

under-dosing of inhibitor (WM needs more inhibitor than PM)

Welds exposed to hydrocarbon service

64

Lomond drains - PWC

• TEG contactor scrubber drain pipework (hydrocarbon)

• Carbon steel parent metal• ~2%Ni deposited in weld

metal• Groove along 6 o’clock

position• Accelerated corrosion at the

weld• Large number of isolations,

extensive inspection and repair

65

MIC & DEADLEG CORROSION

66

Microbially induced corrosion (MIC)

• Anaerobic environments often support development of biofilms.

• Sulphate reducing bacteria (SRB) thrive in anaerobic conditions

• SRB biofilms generate H2S

• FeS corrosion product cathodic to bare steel, increasing corrosion rate.

• MIC of carbon steel usually localized pitting under biofilm.

• Corrosion rates of 5-10 mm/yr seen

• CRAs also susceptible

67

Bacterial growth factors• pH

MIC growth in pH 5-9.5 range

• Temperature SRB can grow in temps of

5-100°C. Optimum temp <45ºC.

• Sulphates– Necessary for SRB

activity.– Growth restricted if <10

ppm

• Carbon source SRB growth restricted if

organic carbon (volatile fatty acids) not available (<20ppm)

• NitrogenImportant but at levels

which are difficult to detect

• Flow– Highest corrosion rates in

stagnant conditions.– Biofilms unstable at high

flows.

68

Deadlegs – types & locations

• A deadleg is a section of pipework or vessel which contains hydrocarbon fluids and/or water under– stagnant conditions (permanent or intermittent)– or where there is no measurable flow.

• Permanent or physical deadlegs (long term stagnation by design)

• Operational deadlegs (stagnant for operational reasons)

• Unprotected mothballed items (plus those temporarily out of service)

69

Examples of deadlegs

70

Deadlegs – assessment factors

• Consequence of failure• Location of pipework• Nutrients replenished by regularly opening /closing

valves?• Is draining of pipework possible?• Is removal of deadleg possible?• Presence of SRBs, deposits, biocide?• Material of construction• Wall thickness• Fluid type (aqueous phase, sulphates, nutrients, oxygen

ingress)• Temperature• Stagnant – permanent/intermittent• Prior history of corrosion

71

Example of deadleg corrosion

• Crude oil recycle cooler bypass• Scale-inhibited seawater left in line after leak test (of u/s

valve)• Severe corrosion rate at and around pinhole.• Fortunately, a leak of water not crude.• Two week shutdown

Pin Hole leaksReleasing waterPin Hole leaksReleasing water

72

Root causes

110mm

80mm

Area of internal corrosion reading from 3.5 mm tapering out to average of 10.7mm

Area of internal corrosion 4.2 mm tapering out to average wall thickness of 10.0 mm

VIEW LOOKING WEST

Photo 1

North

30mm

250 mm

Corroded area approx 80mm x 110mm.

• Failure to identify the bypass line as an operational deadleg

• No deadleg register

• Failure to recognise introduction of new corrosion hazard

• No mitigation measures.

73

Mitigation & inspection

• Flush system of deposits and treat with biocide, nitrate

• Out of service items – Biocide treat or mothball

procedure• Use treated water

– Hydrotest & washing• Profile radiography or UT

scanning– low points, bottom of vertical

sections etc.• Lowest parts of vessel bridle

together with any associated level gauges.

74

OTHER CORROSION MECHANISMS

75

Corrosion due to chemicals• Chemicals can be corrosive • Carbon steel OK for non-corrosive

chemical piping, e.g. methanol• Corrosive chemicals (e.g.

concentrated solutions of inhibitors and biocides) require CRAs – vendor will specify– 316 SS is typical

• Notable exceptions:– Hypochlorite: very corrosive, titanium

or GRP piping required– Avoid titanium alloys in dry methanol

service due SCCSCC of a titanium seal exposed to pure methanol instead of 5% water content

76

Corrosion due to chemicals

• Carbon steel open drain pipework.

• Seepage of scale inhibitor (passing valve)

• Scale inhibitor pH <2.

• Chemical entered drains, not flushed

77

Injection point issues

• Inadequate mixing – corrosion• Intermittent use

– switch off when not flowing• Areas affected

– Impingement / turbulent areas– Bends and low points

• Use quill/other mixer– Upgrade material– Thicker schedule

• Valve arrangement– Make self-draining– Enable quill removal

Main Flow

Injected Fluid

Impingement

78

High temperature corrosion

• Environments less common in E&P– Flare tips, fired heaters, boilers

• Oxidation– Oxidation significant >530°C– Oxidation rate varies with temp,

gas composition and alloy Cr content

• Firetubes: usually CS, but Cr-Mo alloys needed for high temps

• Flare tips: 310 SS, alloy 800H • Other high temperature mechanisms

– sulphidation (H2S and SO2)

– carburizing, metal dusting, hot salt– thermal fatigue and creep

79

Amine stress corrosion cracking

• Material: carbon/low-alloy steels• Environment: aqueous amine

systems• Cracking due to residual stresses

at/next to non-PWHT’d weldments– Cracking develops parallel to the

weld• Mitigation:

– PWHT all CS welds including repair and internal/external attachment welds.

– Use solid/clad stainless steel• 304 SS or 316 SS

Intergranular cracking

Amine piping welds require

PWHT to avoid SCC

80

Corrosion in glycol system• Glycol usually regarded as benign• Corrosion in glycol regeneration

systems usually due to;– Acid gases absorbed by rich

glycol or– Organic acids from oxidation of

glycol and thermal decomposition products

• Condensation of low pH water giving carbonic acid attack.

• Risk recognised in design– On-skid: CRA piping & clad

vessels– However, off-skid piping mix of

regular CS and LTCS

81

Corrosion fatigue

• Combined action of cyclic tensile stress and a corrosive environment

• Fatigue is caused by cyclic stressing below the yield stress– Cracks start at stress raisers– Can occur due to vibration e.g.

smallbore nozzles & with heavy valve attachments

• Presence of corrosive environment exacerbates the problem– Can lead to pitting, which acts

as stress concentrators

82

Example of corrosion fatigue

• 2” A106 GrB carbon steel piping

• Wet gas service, 1.2%CO2 and 160ppm H2S

• Operating @ 120°C and 70bar• Elbow exposed to vibration (used in

a gas compression train)• Crack located at 12 o'clock position• Crack initiated internally

83

EXTERNAL CORROSION – SURFACE FACILITIES

84

External corrosion

• External corrosion of unprotected steel surfaces• External corrosion of coated surfaces• Corrosion under insulation (CUI)• Corrosion under fireproofing (CUF)• Pitting & crevice Corrosion• Environmental cracking

85

Where does it occur?• Bare steel surfaces• At locations of coating breakdown• Under deposits such as dirt, adhesive tape or nameplates• Mating faces between pipe/pipe support saddles & clamps• Isolated equipment not maintained or adequately mothballed• Water sources include:

– sea spray and green water (FPSO or semi-sub)– rain– deluge water– leaking process water– condensation– downwind of cooling towers.

86

What does it look like?

• Damage can be extensive or localised.• Corrosion can be general attack, pitting or cracking.• Seen as flaking, cracking, and blistering of coating

with corrosion of the substrate.

87

Appearance

• Carbon/low alloy steels usually covered in compact scale/thick scab

• Stainless steels have light stains on the surface possibly with stained water droplets and / or salts.

• Corroding copper alloys covered in blue/green corrosion products.

88

Piping, supports & clamps

89

Not just carbon steel

• 25Cr super-duplex (PREN ≥40)• Seawater service• 12 months exposure in tropical

climate• External corrosion along welds• Poor quality fabrication

90

Corrosion of bolts and fasteners

• Bolted joints– Onshore and offshore: exposed to frequent

wetting • Low alloy bolts

– General or localised corrosion– Galvanic corrosion in stainless steel flanges

• CRA bolts susceptible to pitting and/or SCC • Crevice corrosion under bolt heads and nuts• Hydrogen embrittlement possible• Fatigue

91

Corrosion of bolts and fasteners

General corrosion Galvanic corrosion

Crevice corrosion Stress corrosion cracking

92

Flanged connections• Corrosion

– General surface corrosion– Galvanic corrosion

• e.g. 316 SS / carbon steel• Use of graphite gaskets

• Potential problems– Failure of flanged connection

due to corroded fasteners– Joint leak

• Corrective actions– Change gasket/fastener

materials – Replace graphite gaskets

with non-asbestos or rubber material

93

Corroded fasteners (seawater service)

Location of graphite gaskets

94

Structures / valves

• Valves– Valve handles– Chain-wheels– Valve body

• Structures– Stairways and walkways– Gratings, ladders,

handrails– Cable trays and unistruts

• Threaded plugs– Valve bodies, xmas trees,

piping– Dissimilar metals

95

Coating damage and breakdown

• Deterioration of coating with time– All paints let water through - continuously wet areas will fail

• Poor original surface preparation / paint application• Mechanical damage

– Small area of damage can lead to major corrosion

96

External cathodic protection

• Types of structures with external CP– Buried pipelines / structures /

piping / tanks– Floors of above-ground

storage tanks– Submerged jetty structures

• Factors affecting corrosion– Extent of wetness– Oxygen – depends on depth– Resistivity of soil & presence

of salts– Equipment temperature

97

Impressed current CP

• Adjustable dc source

– Negative terminal connected to the steel structure

– Positive terminal connected to the anodes

• Typically used on larger structures where galvanic anodes cannot economically deliver enough current.

98

Corrosion under insulation (CUI) and Corrosion under

fireproofing (CUF) • CUI

– Water seeps into insulation and becomes trapped, results in wetting and corrosion of the metal

– Carbon steel corrodes in the presence of water due to the availability of oxygen.

• CUF– Same mechanism except

water gets behind the fireproofing.

99

Insulation

• Typical insulation types;– Process– Personnel protection

(PP)– Winterisation– Acoustic

• Challenge the need– Remove unnecessary

insulation– Replace PP with

cages‘Lobster-back’

joint

Mitred joint

Pre-formed bends

100

CUI incident

• 4” gas compression recycle line

• Operating pressure, 35bar– 3 bar pressure surge

• Temperature: 50ºC• 6.02mm nominal WT• Rockwool insulation• Extensive corrosion –

rupture• Unusual, burst rather than

leaked

101

CUI gas leak• 2” fuel gas piping outside

edge of platform - exposed• CS, heat-traced, Rockwool• Operating @ 5bar, 45°C,

5.4mm NWT• Failed during plant start-up• External corrosion scale, CUI

• Focus on internal corrosion

• Previous survey found defect in an adjacent line.

• Failed line in survey but not failed area.

– Features selected from onshore not site survey

102

piping CUI

• 4” CS hydrocarbon line

• 55°C, inlet to PSV (153 bar)

• Thermally-sprayed aluminium (TSA)

• CUI found, radiographed – ok to refurbish.

• Found during needle-gunning (paint removal)

• Max pit depth 10mm

• Insulation permanently removed

103

CUI on pressure vessel

• CS offshore vessel

• Operating at 85°C and 11 bar

• PFP coating (passive fire protection)

• Extensive corrosion scabbing on both sides of vessel.

• Scaling runs in two horizontal distinct lines along each side.

• Scaling directly above lower seam of insulation

– location of water retention.

400x300x30mm

400x100x25mm

104

External pitting & crevice corrosion

• Stainless steels in marine environments (chlorides, O2)

– 316L stainless steel commonly used for instrument tubing

– Particularly susceptible at supports and fittings.

• Primary mitigation is materials selection (higher PREw)

– Tungum, 6Mo, super-duplex

• Alternative mitigation methods (coating, cleaning), not easy or practical.

105

Instrument tubing (316 SS and super-duplex)

316 SS tubing super-duplex tubing

316 SS (pitting/crevice corrosion) super-duplex (no pitting)

106

Crevice corrosion under clamps/supports

• Pitting and crevice corrosion of 316ss piping– Clamps– Plastic retaining

blocks

107

External chloride stress corrosion cracking

• Mechanism same as internal chloride SCC however:• Numerous variables influence susceptibility therefore

guidance differs– Material, stress, chlorides, oxygen and temperature– No absolute guidance available, seek expert advice

Chloride SCC is characterised by trans-

granular crack paths

108

External stress corrosion cracking

• UK HSE:– Coat 22Cr duplex >80°C

• NORSOK M-001 SCC temp limits:– 22Cr duplex >100°C– 25Cr super-duplex >110°C

• Recent testing has shown failures at 80°C– now recommend 70°C as limit

• Reliant on external coatings to act as barrier (isolate from environment)

• Beware solar heating - can raise external temperature above threshold limits!– SCC failure of 316L