Weld-Corrosion

Spectrum of practical heat intensities used for fusion welding (ASM

Handbook)

• Tungsten Inert Gas (TIG ) Welding

• Metal Inert Gas (MIG) Welding
• Submerged Arc Welding (SAW)
• Laser
• Electron Beam welding

1
Formation of regions having different
microstructures during welding

Weld is a composite having different

properties in different zones

IGC attack at weldments

2
 Metallurgical factors
• Microsegregation
• Precipitation of Secondary Phases
• Formation of Un-mixed Zones
• Re-crytallization & Grain growth in Weld Heat
Affected Zones
• Contamination of the solidifying weld pool

Welding Practice to minimize

corrosion
• Material & Welding Consumable Selection
• Surface preparation
• Welding Design
• Welding Practice
• Weld Surface Finishing
• Surface Coating

3
Effect of weld speed on the structure of fusion zone of Al

HS a

Low speed

LS 100 m

Curved columnar grains point

towards weld direction at low speed.

Various regions in HAZ formed during welding

4
 9

Intermittent or skip welds like those shown below constitute

poor design in corrosive service because of the inherent
crevice (Landrum 1989)

5
 Factors Influencing corrosion of
• Incomplete Weld
weldments
Penetration & Fusion
• Porosity
• Crevices
• High Residual Stresses
• Improper Choice of
Filler Metal
• Final Surface Finish
Material : Stainless Steel (AISI 316); System : Hot tape
water system; Part Pipe sections; Phenomenon : Crevice
corrosion; (Corrosion Atlas, E. D. D. DURING)

Residual stress distribution

20
various BWR stainless steel
weld HAZs

15
% strain

10

0
0 5 10 15 20 25 30 35
Distance from weld fusion line (mm)

6
 Factors Influencing corrosion
of weldments
• Weldment design
• Welding Practice
• Welding Sequence
• Moisture Contamination
• Oxide Film & Scale
• Weld Slag & Spatter

Welding Practice to minimize

corrosion
➢Preheat & Interpass Temp.
➢Post Weld Heat Treatment
➢Passivation Treatment
➢Crevice Formation
➢Hydrogen Sources

7
 A – Preferential attack of base metal adjacent to weld bead
(Magnification: 50X)
B - Cross section of stainless steel weld showing crevice corrosion
along a site of incomplete fusion (Magnification: 15X)

A B

Solutionized (a) and Sensitized (b) SS

a b

100 m
100 m

8
 A schematic of microstructural features found in stainless
steels (Sedriks 1996)

Effect of Carbon Content on Carbide Precipitation

9
 Weld decay(sensitization) in austenitic stainless steel and methods for it’s
prevention .Panel of four different AISI 300-series stainless steels were joined
by welding and exposed to hot HNO3 and HF solution.The weld decay evident in
the type 304 panel was prevented in the other panels by reduction in carbon
content (type 304 L) or by addition of carbon stabilizing elements(Ti in 321, and
Nb in 347

10
Variation of the critical pitting temperature (determined in ferric chloride
solution) with the molybdenum content of various stainless steel
(Sedriks1996)

Influence of heat input on corrosion of welded S31803 steel in

ferric chloride. [Gooch.,1991 ]

11
 Effect of ferrite-austenite balance on pitting resistance of Fe-22Cr-
5.5Ni-3.0Mo-0.12N gas tungsten arc stainless steel welds [Gooch.,1991]

Time-temperature-transformation (TTT) curve for alloy 2205 (UNS

31803) showing the effect of alloying element on precipitation reactions.
These phases can negatively affect the corrosion resistance and the
ductility of the material and are most serious threats to the successful
applications of duplex grades. [Charles,1991]

12
 Plot of pitting temperature versus oxygen content of backing gas for Fe-
22Cr-5.5Ni-3.0Mo-0.15N and Fe-23Cr-4Ni-0.1N duplex stainless steel
tested in 3.5 % NaCl and 01% NaCl solutions, respectively, both at anodic
potential of 300mv [Odegard etal.,1990]

Microstructural features of 904L SS weld clad shown for various

N additions. The variations in the microstructure of the equiaxed
zone close to the centre of the weld clad are shown. a) 0.03 wt% N,
b) 0.05 wt% N, c) 0.19 wt% N and d) 0.25 wt% N.

VS Raja, Aqueous Corrosion Lab,

IITB; Electrochemical Techniques

13
Typical cyclic polarization curves of weld
clad under various N contents in 3.5%
NaCl.

VS Raja, Aqueous Corrosion Lab,

IITB; Electrochemical Techniques

14
15
 Post weld clean-up of SS
Contamination Prevention during welding
Weld spatter: protective film is penetrated and tiny
crevices are created within the weakened film.
Repair welds: Ground weld spatter, spatter prevention
paste.
Heat tint: pickling by immersion; Pickling paste.
Electro polishing.Glass bead blasting, use clean
beads; over roughen is detrimental.
Slag : Careful wire brushing; C-steel and (coated
electrodes) . 400 grade steels contaminate the surface
so the weld must be pickled after wire brushing.

16
 Cleaning after welding (See ASTM A380)
A. Degrease in solvent or caustic
B. Descale (remove heat tint,slig residue)
• Mechanical descale (avoid iron)
▪ Grind
▪ Sand
▪ Grit Blast
▪ Power brush
C. Chemical descale (pickling)
• 10-25% HNO3 + 2%HF
• Pickle pastes containing HNO3 + HF
• 10% H2SO4 followed by 10% HNO3

D. Electrochemical(electropolishing)(not in ASTM A380)

• 50% H3PO4 (85%concentration)-
• 50%H2SO4 (concentrated)
• 12 volts DC power, 3000 A m-2
• stainless as anode, copper cathode

EFFECT OF Pickling

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Application of paste

PICKLING IN BATH

18
Pickling by spray

PICKLING

19
 Effect of passivation
Flash Attack (right) due to Alkali-Acid-Alkali
contaminated passivation Treatment (Right normal
solution: So, clean the passivation)
surface

Steels

NACE SP0472-2010
Methods and Controls to Prevent In-
service Environmental Cracking of
Carbon Steels Weldments in
Corrosive Petroleum Refining
Environments

20
Cracking found in the Absorber/deethanizer in a FCCU light ends unit.
Cracks in the base metal adjacent to a repair weld (nital etch, 6X)

Surface breaking crack. Micro hardness values are shown in Brinell

units next to knoop 500g indentations. The crack is a typical sulfide stress
crack terminating in soft base metal (nital etch, 30X)

21
 Hardness profile across welds in ASTM A302 tuning forks

Various regions in HAZ formed during welding

22
 Graville diagram showing susceptibility of steels to hydrogen assisted cold
cracking relative to carbon content and carbon equivalent (CE), where CE=
%C + (%Mn + %Si)/6 + (%Ni + %Cu)/15 + (%Cr + %Mo + %V)/5.
Susceptibility to cold cracking progressively increases as steels migrate from
zone 1 to zone 2 to zone 3.

Susceptibility to cold cracking

Effect of hydrogen associated with submerged arc welds on the lower critical
stress and the time needed for HIC. Flux as received, 4 ppm; flux baked 2
ppm; flux moistened 8 ppm. Data are based on 30Kh2N2M Soviet Steel with
a yield strength of 930 MPa (135 ksi)

23
 Effect of baking time and temperature on
moisture in E-7018 covered electrodes

Moisture absorption in covered electrode coatings when exposed to moist,

humid environments. The amount absorbed depends on the electrode and
increases with time. Temperature 21+/- 30C (70 +/- 50F) ; relative humidity
65+/- 5%

24
 Effect of cooling rate on strength of HSLA steel weld metal. Tensile
yield strength of HSLA steel weld metal is affected by the cooling rate
at which it transforms. Data are based on AX-140 GMAW and
E11018 SMAW filler metals

Need for Preheating Welds

• C-steels have high thermal conductivity
• Hence cooling rates at HAZ will be very
high
• This leads to
– Distortion and residual stresses in weldment
– martensite formation in HAZ

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 Criteria for Preheating

• The phase transition is based in alloying

elements and hence preheating requirements
are based on the latter
• This is given as Carbon Equivalent (CE)
CE = %C + (1/6) %Mn + (1/15)%Ni + (1/4)%[Mo +
Cr] + (1/13)% Cu
• Criteria
– CE < 0.45% Optional Preheating
– CE > 0.45% and < 0.60% 93-370 °C
– CE > 0.60% 200-370 °C

What Is Preheating?

• Heating the base metal along the weld joint

to a predetermined minimum temperature
immediately before starting the weld.
• Heating by Oxy fuel flame or electric
resistant coil
• Heating from opposite side of welding
wherever possible

52

26
How does Preheating Eliminate Crack?

• Preheating promotes slow cooling of weld and

HAZ
• Slow cooling softens or mininise hardening of
weld and HAZ of CS & LAS
• Soft material is not prone to crack even in
restrained condition

53

Effect of preheat on HIC susceptibility. Preheat increases critical

stress needed to cause HIC, LCS, lower critical stress

27
 Post-Weld Heat Treatment

• Objectives are:
– Stress relief
– Dimensional stability
– Resistance to SCC
– Occasionally improved toughness and
mechanical properties
– Highly hardenable steels, such as 4130 forms
martensite

Types of Post-Weld Heat Treatment

• Subcritical Stress Relieving

• Normalizing
• Quench and Tempering

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 Subcritical Stress Relieving
• 480-670 °C, 1h/25.4 mm thick
• No phase transformation, if No
Martensite is present
• Eliminates SCC (ASTM 516,
Grade 70 steel)
• If Martensite is present, then
tempering occurs

https://www.process-heating.com/articles/93401-industrial-
furnaces-designed-for-tempering-stress-relieving

29
 Normalizing

• Eliminates coarse grain structure of HAZ

• ESW weld (2286 kJ/in ) high heat input
and hence leads to coarse grain structure
• Expensive Operation than subcritical
annealing

Quench and Tempering

• Reserved only for weld deposits on heat

treatable steels such as
– 4130, 4140, 4340 and H11
– Other steels for high strength and high
hardness

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 Intermediate Stress relieving

• Mostly for Cr-Mo steels that are prone to

cold cracking

Problems of Cr-Mo steels

• Cr-Mo steels require both preheat and PWHT
• 2.25Cr-1Mo is usually quench and tempered to
produce the best combination of strength (70-100 ksi)
and toughness
• PWHT must be controlled so that the strength does not
tend to become lower than the required value
• If Cr content exceeds 1.75%, preheat is normally held
until an intermediate PWHT is performed.
• Stress relive cracking (SRC) might occur if sulfides are
not controlled and by adding Ce and Zr this can be
controlled.
• SRC = %Cr + 3.3%Mo + 8.1%V-2, for SRC > 0
cracking is predicted

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 Problems of Cr-Mo steels In Weld
Overlay
• PWHT can cause sigma phase embrittlement of Type
347 SS weld overlays, secondary hardening of the base
metal and even the cracking of internal attachment
welds to the shell
• Sigma and ferrite phases are susceptible to HE.
• The detrimental effect of sigma phase can be avoided
by keeping ferrite content of overlay to be lower than
10%. Minimum of 2% ferrite is needed to avoid weld
cracking
• The above can be controlled by adjusting Cr and Ni
equivalents
• Secondary hardening resulting from the precipitation
of a phase during PWHT is minimized by limiting Cu
to 0.2% in 2.25Cr-1Mo steel

Important Terminologies used in Critical

Welding Operation

• Preheating
• Post Heating or Dehydrogenation
• Intermediate Stress relieving
• Inter pass Temperature
• Post Weld Heat Treatment

64

32
Acceptable hardness values for steels

33
34
 Low amperage, high speed Dirty, greasy, or contaminated surface Hydrogen
electrodes
High TSN, Low heat input No preheat
High plate thickness

Hydrogen contamination
Fast cooling of weld metal

Poor joint Hydrogen diffusion to

Design and fit-up weld metal

Susceptible microstructure Hydrogen trapped in weld metal

High stress

Causes

Cold cracking of weld metal (HE)

•Micro fissures
•Microcracks
•Weld-metal cracks
Remedies

Non-susceptible Post heat Hydrogen- Free welding

microstructure Stress relief

Slow cooling

Interpass
Preheat High heat input
Temperature Clean, wire Brush, Use low-
control and/or Grind surface Hydrogen electrodes
High amperage,
Low speed

What does one do for stainless

steels?

35
 Weld overlay intergranular SCC mitigation technique

Illustration of heating and cooling process for induction

heating stress improvement

36