corrosion of weldments

코렐테크놀로지㈜ 이선엽[email protected]

¡ What is Corrosion?¡ Some Aspects of Corrosion¡ Corrosion and Welding

¡ Corrosion is the degradation of materials (esp. metals) by its combination with a non-metal such as oxygen , sulfur etc.

Steel making

Corrosion

¡ Man sees ore in lowest energy state.

¡ Man transforms ore in useful object.

¡ Nature wants her dirt back.¡ Nature transforms the object

back to dirt.

Time

Energy

Ore

Rusted away

Left out in the rain

Smelted, heated and beated, forged, ground, drilled and machined

Useful life!

Gas explosion caused by charged soil (crater size: 15 m x 34 m) due to microbial corrosion of gas pipeline (Carlsbed, New Mexico, USA, Aug. 19, 2000)

Source: National Transportation Safety Board (USA) www.ntsb.gov

The leak caused by corrosion at this elbow started the fire that destroyed this refinery

7

Kansai Electric Power Co.

Highway BridgesGas and Liquid Transm. Pipelines

Waterways and PortsHazardous Materials Storage

Air PortsRailroads

Gas Distribution

Electrical UtilitiesTelecommunication

Motor VehiclesShips

AircraftRailroad Cars

Hazardous Materials TransportOil and Gas Expl. And Production

MiningPetroleum Refining

Chem., Petrochem., Parm.Pulp and Paper

AgriculturalFood Processing

ElectronicsHome Appliances

DefenseNuclear Waste Storage

$8.3$7.0

$0.3$7.0

--

$5.0

$6.9-

$23.4$2.7

$2.2$0.5$0.9$1.4

$3.7$1.7

$6.0$1.1

$2.1-

$1.5$20.0

$0.1

$0.1

Cost of Corrosion Per Analyzed Economic Sector, ($ x billion)

$10 $20 $30$0 $40

Drinking Water and Sewer System $36.0

Total 276 billion $ !!3.1% of GDP

Cost of corrosion (1998, USA)

Corrosion Cost and Preventive Strategies in the United States, G.H. Koch, M.P.H. Brongers, N.G. Thompson, Y.P. Virmani and J.H. Payer, FHWA-RD-01-156, Office of Infrastructure Research and Development Federal Highway Administration, Department of Transportation, 2003

Corrosion rate Classification References mm/yr mpy

<0.127 0.127~1.27

>1.27

<5 5~50 >50

Class A (resistant) Class B (acceptable) Class C (unavailable)

1

<0.11 <1.12 <3.37 >3.37

<4.5 <45

<13.5 >13.5

(g/m2/day) <2.4 favorable <24 acceptable <72 poor >72 unavailable

2

<0.05 <0.5

0.5~1.27 >1.27

<2 <20

20~50 >50

Favorable Acceptable

Usable unavailable

3 4

<0.45 <0.6

<18 <24

(g/m2/hr) <0.42 (304,316,317) <0.57 (347cast steel)

Sensitization test in concentrated

nitric acid

5 1. H.H.Uhlig, Corrosion Handbook, John Wiley & Sons Inc., New York (1969)

2. E.Rabald, Corrosion Guide, Elsevier, New York (1968)

3. Shell Development Co., Corrosion Data Survey (1960)

4. R.W.Swanby, Corrosion Charts (1962)

5. Du Pont Internal Report

¡ Corrosion is electrochemical§ Anode (Oxidizing – losing electrons) Electrode§ Cathode (Reducing – gaining electrons) Electrode§ Need “Short circuit” for electrons between terminals§ And need a medium for ion transport

¡ Electricity and chemicals are main drivers¡ Influenced by other factors

¡ Nearly all metallic corrosion processes are electrochemical, in nature, which involve transfer of electronic charge.§ Wollaston (circa 1815) regarded corrosion to be an electrochemical process.§ In 1824, Davy showed galvanic effect.

¡ Not just “chemical” but “electrochemical” process.§ Comprehensive understanding of corrosion mechanism§ Development of various test methods and preventive measures

Figure 2.6 – Electrochemical reactions occurring during the corrosion of zinc in air-free hydrochloric acid. Corrosion Basics: An Introduction, NACE, 1984, pg 28.

Attack of Zn in HClZn + 2HCl ZnCl2 + H2

This overall reaction can be split into:Anodic reaction : Zn Zn2+ + 2e-

Cathodic reaction: 2H+ + 2e- H2

Both reactions must occur simultaneously

and at the same rate!!!

No corrosion if one of these is missing:

anodecathodeelectrolyteelectrical connectionclosed circuit

No corrosion if one of these is missing:

anodecathodeelectrolyteelectrical connectionclosed circuit

anode cathode

electrolyte

electrical connection

¡ Corrosion rates are almost initially very high¡ Polarization – something to slow down reactions

§ Cathodic and anodic surface polarization § Film thickness of corrosion product§ Rate of hydrogen or oxygen diffusion to and from surfaces§ Rate of corrodant ion diffusion away

¡ Areas of reaction (anode to cathode)¡ Oxygen content (cathodic depolarizer)¡ Temperature – every 10°C = 2 x corrosion rate¡ Velocity effects – moving species to & fro

¡ P/S potential measurement§ Pipe to Soil potential§ Reference electrode: Sat. Cu/CuSO4 ref. electrode

¡ Potential measurement at test post (point)V

voltmeter

Ref.electrode

pipe

Test post (box)

¡ Electrochemical corrosion vs. Direct combination (oxidation)

¡ WET vs. DRY corrosion¡ It is often classified according to the forms of corrosion:

§ Uniform or localized corrosion§ Pitting, crevice corrosion, intergranular corrosion (IGC), SCC§ Mechanically induced corrosion: erosion, cavitation, fretting§ Hydrogen related problems: embrittlement, blistering, HIC

¡ Intergranular Corrosion (입계부식)¡ Galvanic & Concentration Cell¡ Crevice Corrosion (틈새부식)¡ Pitting Corrosion (공식)¡ Erosion-Corrosion¡ Environmentally Induced Cracking¡ Uniform Corrosion¡ Hydrogen damage¡ Dealloying (Dezincification)

Microstructure Environment Stress Geometry Time

¡ The even removal of metal.¡ Allows great planning

§ Monitoring§ Replacement and scheduling

¡ Unfortunately, rare in the real world.

What general corrosion might look like!

¡ Most common form of localized attack¡ Break down of protective scale¡ Localized attack in break¡ Pit sets up its own environment¡ Draws in chlorides and sulfates¡ Can form caps over pits¡ Low corrosion rates are deceitful

Pitting corrosion – small and large

Pit의형태, 크기, density 육안검사

39

(a) Narrow & dip (b) Elliptical(c) Wide and shallow (d) Subsurface(e) Undercutting (f) Shapes determined by microstructural orientation

¡ Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution, A: Ferric chloride pitting test§ Shell ES/247 Revision 2, 2003

¡ SS의공식발생에대한저항성비교목적¡ 6% FeCl3용액에시편침지 (강산화성, 강산성)

§ 22±2 or 50±2°C§ 72 hrs§ 가속시험§ 육안관찰/무게감량

40

Specimens of various stainless steels after G 48 testing at 80°C for 24 hours.


¡ 전기화학적으로일정주사속도로전위를상승시키면서 부동태 피막의 파괴로 전류밀도가 급격하게증가하는전위 (Epit)를구함§ Critical pitting potential (CPP)이라고도부름

¡ 반대방향으로 전위를 scan하면 repassivationpotential(Erp)을구할수있음§ Pitting이일어나지않는다면 Epit=Erp

§ Pitting이일어난다면 hysteresis loop를관찰할수있음.

a. E>Epit : Pits nucleation occurs.b. Erp<E<Epit : New pits can not be formed, but

the existing pits may propagatec. E<Erp : the metal remains passive (or pits repassivate)

log i ®

Epit

Erp

¬E ®

ASTM G61: Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys

42

Epit = A log [Cl-] + B

¡ SS의 Cr, Mo, N content 이국부부식에대한저항성을결정하는인자

¡ PRE § %Cr + 3.3 (%Mo + 0.5%W) + 16~30 %N § Superalloy개발의지표중의하나

43

44

Al-6X 904L

317L

PRE317L: 30.2904L: 35.3Al-6X: 40.7

-1000

-500

0

500

1000

1500

-10 -9 -8 -7 -6 -5 -4 -3 -2

log i, A /cm 2

E, m

V(S

CE) .

¡ Critical pitting temperature§ Pitting corrosion이발생하기시작하는임계온도§ CPT가낮을수록 pitting corrosion에대한저항성이낮음§ CPC (oC) = 2.5 %Cr + 7.6 %Mo + 31.9 %N – 41.0§ 0~85oC 사이에서 72h(C, 24h/E) 시험 (약 5oC씩온도증가시키며)

45

¡ 정전위법에의해서전류가급격히증가하는온도를구함§ 1M NaCl용액§ 700mVSCE

§ 1C/min§ 전류밀도가 100 μA/cm2이상 60초이상유지되는온도

46

1. Cooling coil2. Gas distributor3. Reference electrode4. Test specimen5. Counter electrode6. Temperature sensor7. Immersion heater8. Specimen holder with connection9. Reflux cooler

48

log i ®

Epit

Erp

¬E ®

50Simulating natural seawater

¡ Supplementary technical requirements for the supply of components in 6% Mo Austenitic, 22% Cr Duplex and 25% Cr Super Duplex Stainless Steel

¡ Requires§ Impact testing§ Hardness testing§ Microstructure examination and ferrite phase balance (not 6Mo)§ Pitting corrosion testing (additionally, stress corrosion cracking for 25Cr if specified)

Base Metal: ▪ ASTM G48 method A test required for each lot. Test temperature shall be 122°F (50°C) and the exposure

time 24 hours. Corrosion test specimens shall be at same location as those for tension tests. The test shall expose the external and internal surfaces in the as delivered condition (including pickling) and a cross section surface in full wall thickness.

Weld▪ Corrosion test shall be performed on one sample from each of the 3 and 6 o’clock welding locations in

accordance with ASTM G48 method A. A sample including the root shall be taken and be exposed to the solution. Test temperature shall be 104°F (40°C) and exposure time 24 hours.

§ No pitting at 20X magnification & weight loss shall be less than 4.0 g/m2.

52Procurement Spec. for MWP

SS NozzleCSTL Pipe

Soot blower metallographic sample

¡ Copper alloys§ Brasses with >30% zinc (bath sink tap screws)§ Copper nickel alloys (nickel removed)

¡ Cast iron (graphitization) ¡ Almost any alloy can have the problem¡ Two Theories

§ One element is “leached” from solution § Both elements corroded but more noble plates back.

Brass River Water Impellor suffering from dealloying and cavitation

Severe crevice attack as well as general

Crevice attack on titanium from fluorinated o-ring

Crevice corrosion of socket weld at gap formed between type 304L pipe and type 316L valve

¡ ASTM G78: Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments

¡ [0.5M FeCl3 + 0.05M NaCl] 용액중에최소 30일침지

¡ 304SS

¡ ASTM G48D/F§ MCA를이용하되원리는 CPT측정과같음.

¡ 전기화학적방법

82

83


86

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Figure 22.11 Alternative methods for joining two pieces of steel: (a) Fasteners may produce a concentration cell, (b) brazing or soldering may produce a composition cell, and (c) welding with a filler metal that matches the base metal may avoid the formation of galvanic cells (for Example 22.8)

¡ Very similar to crevice corrosion but a larger¡ Usually an unplanned occurrence

§ Tools left on floor§ River water silt buildup in bottoms

¡ Sometimes called poultice corrosion¡ Sometimes called oxygen concentration cell

장기간정체된오염수내의 SRB 활동

0 2 4 6 8 100.0

0.5

1.0

1.5

2.0

AlSiFe

O

CP

S

Fe

Fe

Coun

ts (A

rb. U

nit)

Energy (keV)

93

Fe-Cr alloy in boiling 50% H2SO4 with Fe2(SO4)3

PRACTICE TEST TEMP. TIME APPLICABILITY EVALUATION

A Oxalic acid etchScreening test Ambient 1.5 m Chromium carbide sensitization

Only (1.5A/cm2)Microscopic: classification of etch Structure (screening test)

BStreicher

Fe2(SO4)350% H2SO4

Boiling 120 h Chromium carbide Weight loss corrosion rate

CHuey 65% HNO3 Boiling 240 h Chromium carbide and σ phase Weight loss corrosion rate

DMod. Strauß

10% HNO33% HF 70°C 4 h Chronium carbide in 316, 316 L,

317 and 317 LCorrosion ratio compared to solution annealed specimen

EStrauß

6% CuSO410% H2SO4Metallic copper

Boiling 24 h Chromium carbide Examination for fissures after bending

FCuSO450% H2SO4Metallic copper

Boiling 120 h Chromium carbide in cast 316 and 316 L Weight loss corrosion rate

99

101Source: http://www.sandvik.com

¡ IGC 시험결과를 TTC의형태로표현하는경우가많음. (according to ASTM G28, Streicher, ferric sulfate+sulfuric acid)

102

105

Stress

Strain

SCC

No SCC

Transgranular chloride SCC in 316 stainless steel

Weld metal

Knife line attack

Intergranular caustic SCC in 304L stainless steel finned tube.

¡ Starts with an alternating stress state¡ Protective oxide breaks open¡ Corrosive species attack and form products¡ Next cycle repeats:

§ crack growth§ more corrosion product§ accelerated fatigue failure

¡ Seen in rotating shafts

Corrosion fatigue, cracks can be oriented the other direction depending on stress state of shaft.

¡ Mostly found at ▪ Pump impellor tips▪ Boat propellers▪ Constriction in fast fluids

¡ Caused by formation of low pressure bubble¡ Bubble is a vacuum¡ Collapse of bubble slams the metal

▪ Breaking protective oxide▪ Causing great mechanical damage

Piece of pump impellor with tip cavitation

Valve trim diffuser with cavitation

Centrifuge feed nozzle

¡ Can be from § Gaseous vapor (steam cuts on flanges)§ Liquid § Solids (Coal slurry)

¡ Removes the protective oxide layer faster than it can heal

Look for “comet tails”! Water was flowing from right to left in copper water pipe.

¡ Visual examination for leaks¡ Lab testing¡ Field testing (Corrosion racks with coupons)¡ Corrosion probes

§ ER (electrical resistance)§ LPR (linear polarization resistance)§ New technology

¡ Metals analysis in process fluids

¡ All kinds of materials and shapes§ Metals§ Plastics§ Fiber reinforced plastics§ Ceramics§ Elastomers§ Glass

¡ Homemade or “store bought” coupons¡ Welds ¡ Heat treatments

¡ Coupons¡ Lab testing at many temps but low pressure¡ Heat flux testing to simulate exchangers ¡ High pressure labs¡ Ingenious bench scale or pilot plant testing¡ Key question - What do you want to know?

Agitator blades as corrosion coupons

Weld wires as coupons

¡ Corrosion racks¡ Electrical resistance probes¡ Linear polarization resistance probes¡ New technology

§ Using electrical noise§ LPRs § ERs

¡ Metals analysis in solutions

Typical field corrosion rack for insertion through a nozzle.

V = I*R

R = ρ*l/A

Electrical resistance probe

• Gives instantaneous corrosion rates

• Only used in conductive solutions

• Based on the current flow between two or more electrodes

• Requires the surface to become passivated (or polarized) and current resistance is measured.

• Sometimes probe has a reference electrode as well.

Honeywell’s SmartCET® uses a sensor for background electrochemical noise to detect pitting along with LPR probe and a sophisticated computer program.

http://hpsweb.honeywell.com/NR/rdonlyres/8418C7B6-EBB9-4948-8441-C3803B06BA2E/44686/ChemEngJune07.pdf

Newer Technology

¡ Electron Microscopy§ Elemental analysis§ Surface features

¡ FTIR for identification¡ X-ray Diffraction¡ X-ray Fluorescence¡ Looking for clues by

§ Metals in fluids § Fluids in plastics§ Corrosion products

¡ Material selection, e.g., CS to CRA¡ Coatings & linings (organic, inorganic)¡ Cathodic & anodic protection¡ Corrosion inhibitor¡ Design Factors, etc.

¡ If an active-passive alloy is maintained in the passive region with the potentiostat, its corrosion rate will reduced to ipass. This technique is based on the formation of protective film on metals by externally applied anodic currents.

An inhibitor is a chemical substance that, when added in small concentration to an environment, effectively decreases the corrosion rate. A minimum conc. of inhibitor must be present to maintain the inhibiting surface film. Thus, good circulation and the absence of any stagnant areas are necessary to maintain the required level of inhibitor concentration.An addition of inhibitors reduces icorr by increasing the Tafel slope and/or by reducing the exchange current density.

A. Adsorption-type inhibitors Organic compounds which adsorb on the metal surface and suppress metal dissolution and reduction reactions. Typical of this class of inhibitors are the organic amines.

B. Hydrogen evolution poisonsThe susbstances such as As, Sb, P, and S retard hydrogen recombination reaction, thereby reducing corrosion rate of a metal in acid solutions.

Anodic inhibitor Cathodic inhibitor

E

log i

E

log i

C. ScavengersThese substances act by removing corrosive species from solution :Sodium sulfite : 2Na2SO3 + O2(sol.) ® 2Na2SO4Hydrazine : N2H4 + O2 ® N2 + 2H2O

D. Oxidizers or passivators- effective only in metals showing active/passive transition.: CrO4

2- , NO2-, MoO4

2- , WO42- , and ferric salt

¡ Oxidation§ Flux§ Inert environment

¡ Stress Build-up§ Choose appropriate material for welding rod to reduce mechanical

stress

¡ Weld Decay§ De-zincification§ Grain Boundary Chromium Depletion§ Knifeline Attack

¡ Weldment Corrosion

¡ Weldments have a high sensitivity to environmentally induced corrosion.

¡ Residual stresses can cause Stress Corrosion Cracking (SCC)¡ Stress risers and inclusions due to welding can increase the

sensitivity to Corrosion Fatigue Cracking (CFC)¡ Welding in the presence of water or organic molecules can trap

hydrogen in the weld, leading to Hydrogen Induced Cracking (HIC)¡ Misoriented welding geometry can cause crevice corrosion/pitting

corrosion.

¡ Susceptibility to environmentally induced corrosion is increased by improper welding technique.

¡ Defects include:§ Stress risers§ Inclusions§ Incomplete welding§ Improper filler metal or flux used

¡ Causes:§ Residual tensile stresses§ Stress risers caused by improper

welding

¡ Prevention:§ Use a metal that is resistant to

SCC in environment§ Specify post welding heat

treatment § Ensure smooth weld bead

¡ Possible when weld is subjected to variable loads

¡ Stress risers at the weld will increase the sensitivity to CFC

¡ Prevention:§ Smooth weld area§ Minimization of inclusions

¡ Hydrogen present in water or organic molecules is released at high temperatures

¡ Monatomic Hydrogen is infused in the weld pool¡ Hydrogen combines within metal and causes HIC

¡ Prevention:§ Store welding rods in dry environment§ Use low-hydrogen electrodes (non-organic binders and flux)§ Local heat treatment before & after welding

Arc WeldingAtmosphere

Surface contaminantsWelding rod

H2O

WELD

Heat breaks down Water.

H2O

HO2

WELD

Hydrogen causes crackingOxygen creates crevice

HO2

WELD

¡ Mechanism§ Environment

▪ High P, high T hydrogen environment▪ Petrochemical plants Hydrocarbon processing at 21 MPa, 540°C

▪ Stress level, exposure time, steel composition§ Mechanism

▪ Hydrogen reacts with carbides to form methane▪ Methane bubbles form at grain boundaries▪ Bubbles merge to create fissures

¡ Characteristics§ Symptoms

▪ Unexpected failure

§ Microstructure▪ Decarburization along grain boundaries▪ Fissuring along grain boundaries▪ Embedded methane bubbles

¡ Case Study§ Hydrogen attack at the ID weld

in a high pressure carbon steel boiler tube

¡ Case Study§ Cracking due to hydrogen attack

at the ID weld in a high pressure carbon steel boiler tube

¡ Transgranular Cleavage

¡ The filler metal is usually of a different composition than the base metal.

¡ Making the filler metal noble to the base metal will cathodically protect the weld.

¡ Oxidation and diffusion will lower the alloy. concentration in weld, so filler metal should also have higher amounts of alloying elements than the base metal.

¡ In the Unmixed Zone, non-equilibrium cooling can cause potential problems:§ Grain boundary segregation§ Primary phase structures§ Precipitation of alloying elements

¡ Prevention:§ Pre and post heating avoids unequilibrium cooling

¡ Stress Corrosion Cracking¡ Hydrogen Induced Cracking¡ Crevice corrosion¡ Intergranular Cracking

§ Sensitization from welding – heat & stress§ Carbide precipitation

¡ 세가지인자의결합작용§ 재료내부의인장응력§ 부식성환경 (Cl-, H2S, etc)§ 고온

¡ SCC환경예§ Acid chloride solution§ Seawater § Condensing steam from chloride waters§ H2S + chloride§ Polythionic acid (sensitized material)§ NaCl – H2O2

§ NaOH - H2S

162

Materials

CompositionHeat TreatmentMicrostructureSurface Condition

Corrosion

Environment

CompositionTemperatureElectrode PotentialFlow rate

Fatigue Corrosion-Fatigue

SCC

Stress, Strain

Service StressFit-up StressResidual StressStrain Rate

¡ Offshore Structure – Weld Decay¡ Solar Power Plant – Chloride Corrosion¡ Gas Pipeline – SCC¡ Pipes – Crevice Corrosion¡ Chloride Stress Corrosion Cracking

¡ Welded Steels¡ Stress from sea and

Corrosive Environment¡ Problems

§ Weld decay§ Weld cracking§ Pitting

¡ Painting Structure¡ Better choice of weld material¡ Cathodic Protection – Zinc bar¡ Better welding of steels¡ Better weld geometry

¡ Stress Corrosion Cracking§ Caused by tough environment§ Operation of load and stresses§ Stress are either mechanical or chemical

▪ Mechanical Stresses: scratches, rivets, residual stresses▪ Chemical Stresses: cracks, corrosion in the media

¡ Stress Corrosion Cracking in weld

▪ Stress cracks under operation.▪ Cracking is longitudinal along the weld

¡ Crevice Corrosion in Duplex Steel¡ Corrosion due to Heating¡ Internal oxide layer created Crevice

§ Caused pipe to corrode and burst

¡ Aggressive media increased corrosion

Crevice corrosion of socket weld at gap formed between type 304L pipe and type 316L valve

¡ Use an oxygen-free backing gas to prevent scale formation during welding

¡ Annealing¡ Use specific filler metals

§ Increase in Ni increases resistance to SCC, but also becomes brittle

§ Find stability between heat and nickel concentration

3-4 cm

8-10 cm

¡ Intergranular crack propagation in welded region¡ Stressed region¡ Unprotected region

to environment.Chlorine accumulation

¡ High chloride concentration found¡ If compared to other parts of the sample there is no

cracking¡ Chloride enhances

cracking mechanisms

177

Molten lead is held in thick steel pots during refining. In this case, the molten lead has attacked a weld in a steel plate and cracks have developed. Eventually, the cracks propagate through the steel, and molten lead leaks from the pot.

¡ Materials Selection¡ Weld Geometry

§ Effective welding mechanisms (heat input, cooling rate)§ Cathodic Protection

¡ Annealing¡ De-sensitizing stainless steel

¡ Metallurgical effects§ Preferential corrosion of HAZ

¡ Geometrical effects§ Stress concentration at weld toe§ Creation of crevice due to joint design

¡ Environmental effects§ Temperature, conductivity, etc.

¡ Metallurgical + Geometrical effects§ SCC

¡ More common in carbon and C-Mn steel than in higher-alloy steels

¡ Tramline corrosion¡ High conductive aqueous

solution¡ pH below 7 and 8

(acidic mine water)

¡ Seawater

¡ Higher tensile residual stresses allow corrosion to proceed slightly faster than in the less highly stressed steel.

¡ Galvanic Corrosion

¡ Occurs in ERW/HFI pipe¡ Attack of seam weld line in aqueous environment or when

exposed to the water phase in a mixed-phase system due to flow conditions or water dropout in low points.

¡ Unstable Inclusion (MnS) produced during the welding cycle.§ Galvanic corrosion (1 to 10 mm/y)§ Potential difference between 30 to 70mV, but high C/A area ratio

¡ Low S content, alloying element such as Cu, REM

¡ Highest corrosion rate – Shielded metal are electrodes using a basic coating

¡ In seawater, a weld made using a basic-flux-coated consumable has a higher corrosion rate.

¡ In boilers and other vessels producing water at high temperature together with free alkalis (usually NaOH)§ 50 – 80oC§ Residual stress§ Local evaporation of water

can lead to high conc. of NaOH

¡ PWHT§ Residual stress relieving§ Reducing HAZ attack

¡ Selection of appropriate materials or welding procedure§ When PWHT is impractical

¡ Alloying§ Base metal (grooving corrosion)§ Make weld more cathodic to the adjacent base metal

• How to Select an Optimum Filler Metal

Welding of stainless steel can cause sensitization and hence intergranular corrosionin heat affected zone (HAZ)

As a filler metal, Select more resistive one to corrosion than base metal. (i.e., With higher Ecorr)(Small cathode – large anode)

base metal 304L SS (18Cr-8Ni)

filler metal 308L SS (20Cr-11Ni)347 SS (18Cr-11Ni-Nb)

¡ The most widespread type is hydrogen induced SCC when H2S is present, particularly in acidic solutions (oil & gas industry)

¡ Sulfide Stress Cracking (SSC)¡ Risk of cracking increases with the max. hardness.

§ Max 22 HRC (~ 248 HV)§ Alloy steel need to be PWHTed at a temp. exceeding

620oC.

¡ Cycling of heating and cooling during welding results:§ Microosegragation§ Precipitation of secondary phases§ Formation of unmixed zones§ Recrystalization and grain growth in the weld heat affected zone (HAZ)§ Volatilization of alloying elements from the molten weld pool§ Contamination of the solidifying weld pool

¡ Corrosion resistance maintenance§ Balancing alloy compositions to inhibit certain precipitation reactions§ Shielding molten and hot metal surfaces from reactive gases in the weld

environment§ Removing chromium-enriched oxides and chromium-depleted base metal from

thermally discolored§ (heat tinted) surfaces§ Choosing the proper welding parameters

¡ Main problem§ Precipitation effect§ Chemical segregation

¡ Remedy§ Control of base metal metallurgy§ Control of welding practice§ Selection of proper filler metal

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™is a trademark used herein under license.

The peak temperature surrounding a stainless-steel weld and the sensitized structure produced when the weld slowly cools

¡ Preferential weld ferrite attack & less severe attack in the sensitized HAZ¡ Acid cleaning of 304SS and 316SS black liquor evaporators in the pulp and paper

industry with poorly inhibited HCl can lead to weld metal δ-ferrite attack.

Voids where ferriteHas been attacked.

¡ In alloy depleted regions of weld metal austenite ¡ Moderately oxidizing environments¡ Microsegragation or coring of weld metal dendrites

¡ Most likely:§ In autogenous (no filler) GTA welding§ In 4 to 6% Mo alloys§ When the recommended filler metal has the same composition as

the base metal§ When higher-heat-input welding leaves a coarse microstructure

with surface-lying dendrites. Such a microstructure is avoided by use of a suitably alloyed filler metal.

¡ CPT vs. %Mo in 10% FeCl3.

¡ Pitting of underalloyed (relative to the base metal) type 308L weld metal. The type 316L stainless steel base metal is unaffected

¡ Tap water environment

¡ Defects such as:§ Residual welding flux§ Microfissures, etc.

¡ Especially in chloride-containing environments

¡ Example§ Slags from the basic- coated

electrodes for out-of-position welding can be difficult to remov

Crevice corrosion under residual slag (S) in IN-135 weld metal after bleach plant exposure.

Microfissure corrosion on IN-135 weld metal on an alloy 904L test coupon after bleach plant exposure

Microfissure caused by thermal contraction stress during weld solidificationWhen P & S content are higher (>0.015%)High-heat-input welding

Weld spatter is most troublesome when it is loose or poorly adherent.

Weld decay and methods for its prevention. The four different panels were joined by welding and then exposed to a hot solution of HNO3/HF. Weld decay, such as that shown for the type 304 steel (bottom right), is prevented by reduction of the carbon content (type 304L, top left) or by stabilizing with titanium (type 321, bottom left) or niobium (type 347, top right)

Selective attack of a type 317L stainless steel weldment and chloride stress-corrosion cracking ofthe adjacent 317L base metal. The environment was a bleaching solution (7 g/L Cl2) at 70oC Chloride stress-corrosion cracking of type 304

stainless steel base metal and type 308 weld metalin an aqueous chloride environment at 95oC. Cracks are branching and intergranular

¡ When the caustic concentration exceeds approximately 25% and temperatures are above 100 °C

¡ Cracking occurs most often in the weld HAZ.

¡ 316L reactor vessel failed repeatedly by caustic SCC in 50% sodium hydroxide (NaOH) at 105 °C (220 °F).

¡ Failure was restricted to the weld HAZ adjacent to bracket attachment welds used to hold a steam coil.

¡ The stresses caused by the thermal expansion of the Nickel 200 steam coil at 1034 kPa (150 psig) aggravated the problem.

¡ It was recommended that the vessel be weld overlayed with nickel or that the existing vessel be scrapped and a replacement fabricated from Nickel 200. Caustic SCC in the HAZ of 316L SSNaOH

reactor vessel. Cracks are branching and intergranular

¡ Found at§ Cooling water system§ Aqueous waste treatment§ Groundwater left in new equipment or piping after hydrotest

¡ Characteristics§ Underdeposit corrosion (discrete mound)§ Subsurface cavities with minute pinhole penetration at the surface§ Natural, untreated water containing one or more culprit species of

microorganisms.

¡ MIC of butt weld in water tanks

¡ 304L or 316L to resist HNO3 organic acid and to maintain product purity

¡ Hydrotest with water containing 200 ppmchlrodie.

¡ No drain of piping after hydrotest

¡ Reddish brown deposit and corrosion (& leakage) after 1 to 4 months

¡ a tiny mouth at the surface and a thin shell of metal covering a bottle-shape pit that had consumed both weld and base metal.

Moundlike microbiological deposits along a weld seam in the bottom of a type 304L stainless steeltank after several months of exposure to well water at ambient temperature

Moundlike microbiological deposits along a weld seam in the bottom of a type 304L stainless steeltank after several months of exposure to well water at ambient temperature

Close-up of a wet deposit

Deposit was slimy and gelatinous

Radiograph of a pitted weld seam in a type 304L stainless steel tank bottom

Cross section through a pitted weld seam from a type 304L tank showing a typical subsurface cavity

¡ Well water: high count of § Iron bacteria (Gallionella)§ Iron-manganese bacteria (Siderocapsa)§ No SRB & SOB

¡ Corrosion products contain large amount of§ Fe, Mn, Cl

¡ Nearly all biodeposits and pits were found at the edges of, or very close to, weld seams

1. R. Fernandez, E. Rege and M. Beatty, Weldment Corrosion. University of Berkeley2. D.H. Lister and W.G. Cook, Reactor Chemistry and Corrosion, Department of Chemical Engineering,

University of New Brunswick.3. R.J. Sinko, Corrosion Basics, AIChE, Jan. 21, 2009, http://www.tnengineering.net.4. D.R. Askeland and P.P. Phule, 5. W.H. Weber, Guided Wave Ultrasonics (GWUT) – An Effective Screening Tool, 40th Annual SIEO/NACE

Winter Symposium 2005.6. N. Bailey, Weldability of Ferritic Steels, Abington Publishing, 1994.7. 김홍식등, 아연도강관부식의원인규명및분석, 울산대학교부설산업기술연구소,

1997.8. C.G. Arnold, Galvanic Corrosion of Weldment, CORROSION/80, NACE Internatial, 1980.9. P.R. Roberge, Handbook of Corrosion Engineering, McGraw-Hill, NY, 1999.10. T.G. Gooch and P.H.M. Hartt, Review of Welding Practices for Carbon Steel Deaerator Vessels, Paper

no. 303, CORROSION/86, NACE International.11. http://en.wikipedia.org/wiki/Deaerator12. M.A. Streicher, Theory and Application of Evaluation Tests for Detecting Susceptibility to Intergranular

Attack in Stainless Steels and Related Alloys—Problems and Opportunities, Intergranulur Corrosion of Stainless Alloys, STP 656, American Society for Testing and Materials, 1978, p 70.

Questions?

corrosion of weldments

Documents