The document summarizes corrosion of weldments. It discusses the microstructure of weldments and the distinct regions that form. It then covers the various forms of weld corrosion including galvanic, pitting, crevice, intergranular, stress corrosion, and hydrogen cracking. Factors that influence weld corrosion like material selection and welding parameters are presented. Testing methods for weld corrosion like linear polarization resistance and corrosion potential measurements are briefly described.

1. CORROSION OF WELDMENTS
By:
Sumeet Sharma
M.Tech- Material Enigneering
E-Mail: sumeetsharma005@gmail.com
DEPARTMENT OF METALLURGICAL AND MATERIALS ENGINEERING
Visvesvaraya National Institute of Technology, Nagpur

2. What we are going to see??
I. Microstructure of weldments
II. Forms of Weld Corrosion
III. Factors Influencing Corrosion of Weldments
IV. Welding Practice to Minimize Corrosion
V. Corrosion Testing

3.  WHAT IS CORROSION??
Corrosion is a chemical or electrochemical reaction
between a metal and its environment,resulting in
deterioration of the properties and/or appearance of
metal.
 Welded joints may in some case more susceptible to
corrosion and may not in other case as compared to
metal being joined.
 Failuredueto corrosion mayoccur inspite of the fact that
the proper base metal and filler metal have been
selected, industry codes and standards have been
followed, and welds have been deposited that possess
full weld penetration and have proper shape and
contour.
 Corrosion in welds is more severe as it leads to
catastrophic failure
Fig 1 : failure due corrosion of welded region in valve pipe of oil
and gas plant

4. Weld Microstructures
Fig : Schematic showing the regions of heterogeneous weld
Fig : Concentration profile of chromium-nickel across the weld fusion
boundary region of type 304SS

5.  A weldment consist of a transition from base metal through
HAZ to solidified weld metal involving five mirostructurally
distinct regions:
I. Fusion zone
II. Unmixed region
III. Partially melted region
IV. HAZ
V. Unaffected Base metal
 Unmixed region is a part of Fusion zone , and partially melted
region is a part of HAZ
 It is not necessary that all these zones will be present in a
welded region.for e.g. Autogenous welds do not have Unmixed
region.
FUSION ZONE
 Result of melting which fuses the base metal and filler metal to
produce a zone with a composition that is most often different
from that of the base metal.
 Compositional difference produces a galvanic couple, which can
influence the corrosion process in the vicinity of the weld.
 The fusion zone itself offers a microscopic galvanic effect due
to microstructural segregation resulting from solidification.
UNMIXED REGION
 Fusion zone has a thin region adjacent to the fusion line, known
as the unmixed (chilled) region, where the base metal is melted
and then quickly solidified to produce composition similar to
the base metal.
HEAT AFFECTED ZONE (HAZ)
 HAZ is the portion of the weld joint which has experienced peak
temperatures high enough to produce solid-state
microstructural changes but too low to cause any melting.

6.  Each position has its own microstructural features and
corrosion susceptibility.
 On fine scale microstructural gradients are present.
PARTIALLY MELTED REGION
 Usually one or two grains into the HAZ relative to the fusion
line.
 Characterized by grain boundary liquation, which may result in
liquation cracking.
 These cracks are potential initiation sites for hydrogen-
promoted underbead cracking in high-strength steel.
 Unaffected Base Metal is that part of the workpiece that has
not undergone any metallurgical change.

7. FORMS OF WELD CORROSION
Weldments can experience all the classical forms of corrosion
but most general forms are:
I. Galvaniccorrosion
II. Pitting corrosion
III. Crevice corrosion
IV. Intergranularcorrosion
V. Stress corrosion
VI. Hydrogen cracking
VII. Microbiologicallyinfluencedcorrosion
GALVANIC CORROSION
 Occurs when two different metals in contact are
exposed to a conductive solution.
 Use of filler metal of composition different from base
metal may produce an electrochemicalpotential
difference that results in flow of current leading to
corrosion of anodic areas.
 For majority of Aluminiumalloysweld metal and HAZ
more noblecompared to base metal shown in fig(a)&(b)
for a saltwater environment.
 However in certain Al alloys,narrow anodic region
formed in HAZ which is more prone to localisedattack.
Alloys7005 and 7039 are particularlysusceptible to this
problem.fig(c)

8. Fig : Effect of welding heat on microstructure, hardness, and corrosion potential of
three aluminum alloy welded assemblies. (a) Alloy 5456-H321 base metal with
alloy 5556 filler. (b) Alloy 2219-T87 base metal with alloy 2319 filler (c) Alloy 7039-
T651 base metal with alloy 5183 filler

9.  Plain carbon steel weldmentscan also exhibit galvanic
attack. For example, the E6013 welding electrode is
known to be highlyanodic to A285 base metal in a
seawater environment.
 Selection of suitable filler material is thus very
important.
Weld Decay of Stainless Steel
 During welding of stainless steels, localsensitized zones
(i.e., regions susceptible to corrosion) often develop.
 Sensitizationis due to the formation of chromium
carbide along grain boundaries,resulting in depletionof
chromium in the region adjacent to the grain boundary.
 This chromium depletionproduces very localized
galvaniccells.
 If this depletiondrops the chromium content below the
necessary 12 wt% that is required to maintaina
protective passive film, the region will become
sensitized to corrosion, resulting in intergranularattack.
This type of corrosion most often occurs in the HAZ.
 Intergranularcorrosion causes a loss of metal in a region
that parallelsthe weld deposit.Thiscorrosion behavioris
called weld decay.
Fig: Depleted regions adjacent to precipitates. These regions cause an electrochemical
potential difference (E) that promote localised microstructural attack

10. Fig : Intergranular corrosion (weld decay) of stainless steel weldments
Figure shows susceptibilityto sensitization as a function of
temperature, time, and carbon content. If the cooling rate is
sufficiently great (curve A in Fig.) the cooling curve will not
intersect the given C-shaped curve for chromium carbide and
the stainless steel will
not be sensitized.
Fig : Time-temperature-sensitization curves for type 304 stainless steel in a
mixture of CuSO4 and HSO4 containing copper

11.  By decreasing the cooling rate, the cooling curve (curve
B) eventuallyintersects the C-shape nucleationcurve,
indicatingthat sensitization may occur. At very low
cooling rates, the formation of chromium carbide occurs
at higher temperature and allowsfor more nucleation
and growth, resulting in a more extensive chromium-
depleted region.
The control of stainlesssteel sensitizationmay be achieved
by using:
 A postweld high-temperature annealand quench to
redissolve the chromium at grain boundaries,and
hinder chromium carbide formation on cooling.
 A low-carbon grade of stainless steel (e.g.,304L or 316L)
to avoid carbide formation.
 A stabilized grade of stainless steel containingtitanium
(alloy 321) or niobium (alloy327), which preferentially
form carbides and leave chromium in solution.(Thereis
the possibilityof knife-lineattack in stabilized grades of
stainless steel.)
 A high-chromium alloy (e.g., alloy310)
PITTING
 Form of localizedattack caused by a breakdownin the
thin passive oxide film that protects material from the
corrosion process.
 Pits are commonly the result of a concentrationcell
established by a variationin solutioncomposition that is
in contact with the alloymaterial. Such compositional
variationsresult when the solution at a surface

12. irregularity is different from that of the bulk solution
composition.
 Once a pit has formed, it acts as an anode supported by
relativelylarge cathodicregions.
 Pitting has a delay time prior to nucleationand growth,
and nucleationis very site-selective and microstructure-
dependent.
 Pits are often initiatedat specific microstructural
features in the weld deposit .
 Pitting occurs when the material/solutioncombination
achieves a potentialthat exceeds a critical value, known
as the pitting potential.
 Pits developmore readily in metallurgically
heterogeneousmaterials.
Fig : Pitting corrosion at the welded region

13. CREVICE CORROSION
 It is a form of local attack caused by concentrationcell
effects similarto pitting within a crevice (gap).
 Crevices in weld occurs as:surface porosity or
cracking,slag or weld scale deposits,inadequatejoint
penetration,undercut,anddesign defects.
 Crevice corrosion does not occur in all metals or their
combinations
 Inside the crevice, anodicreaction is also self-
accelerating (autocatalytic)in nature. The processes
inside the crevice is essentiallythe same as the
propagationof pitting
 SS and Al are often susceptible to this.
STRESS-CORROSION CRACKING
 Combinationof corrosive media, susceptible
microstructure, and tensile stress leads to cracking
 Welds are often loadedin tension (due to residual
stress) to a level approachingthe yield strength of the
base metal.
 A weld, with its variousheterogeneous microstructural
features, thus becomes an excellent candidatefor SCC.
 Cracking is often characterized by crack branching and
usuallyhas a delay time prior to crack initiation,with
initiationoccurring at corrosion pits.
 Increasing the ferrite content in stainless steel weld
metal reduces SCC susceptibility.Approximately 50 vol%
ferrite gives optimum SCC resistance
 Welding parameters influencethe amount and
distributionof residual stress, because the extent of the
stressed region and the amount of distortion are directly

14. proportionalto the size of the weld deposit;this deposit
is directly related to the heat input. The thermal
experience of welding is often very localized, resulting in
strains that can cause distortionand residual stress.
These residual stresses can be important in the initiation
and propagationof environmentallyassisted cracking.
 The use of small weld deposits reduces the stress and
thus reduces the susceptibilityof environmentally
enhancedcracking.
 Postweld heat treatment can reduce SCC by
redistributing the localizedload and by reducing the
magnitude of the residual tensile stress available
to induce corrosion cracking
 The combination of a low carbon content and a
carefully specified nitrogen addition have been
reported to improve resistance to pitting
corrosion, SCC, and intergranular corrosion in the
aswelded condition. The low carbon content helps
avoid sensitization, while the addition of nitrogen
slows the precipitation kinetics associated with the
segregation of chromium and molybdenum during
the welding process.

15. Fig : Schematic view of stress corrosion cracking and SCC conditons
HYDROGEN-INDUCED COLD CRACKING
 Cold cracking is the term used for cracks that occur after
the weld has solidifiedand cooled;it occurs in either the
HAZ or the weld metal of low-alloyand other
hardenablesteels.
 They are often referred to as restraint cracks. may occur
several hours, days, or weeks after the
 weld has cooled;consequently,the term delayed
cracking is also used.
 On the basis of location,cracks are often described as
toe cracking, root cracking, or underbeadcracking
For cold cracks to occur in steels, three principal factors
must be present:
I. atomic hydrogen,
II. HAZ or portion of the weld metal that is susceptible to
hydrogen embrittlement, and
III. a high tensile stress resulting from restraint
In steels, cracking in the base metal is often attributed to
high carbon, alloy,or sulfur content. Control of this cracking
requires the use of low-hydrogen electrodes, high preheat,
sufficient interpass temperature, and greater penetration
through the use of higher currents and larger electrodes.
The susceptibilityof the microstructure to cold cracking
relates to the solubilityof hydrogen and the possibilityof
supersaturation.

16. Fig : Causes and cures of hydrogen-induced cold cracking in weld metal

17. MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)
 Phenomenon in which microorganisms play a role in the
corrosion of metals. This role may be to initiateor
accelerate the corrosion process.
 For example, water and some organic media may
contain certain microorganisms that can produce a
biofilm when exposed to a metal surface.
 The resulting nonuniform coverage may lead to a
concentrationcell and eventuallyinitiatecorrosion
 In addition,the metabolicmetabolic process of the
microorganism can produce a localizedacid
environment that changes the corrosion behaviorof the
exposed metal by, for example, altering anodic and
cathodic reactions, destroying protective films, or
creating corrosive deposits.
 In austenitic stainless steel weldments, the effects of
MIC are usually observed as pitting on or adjacentto
welds.
 Welding design and plant operationcan minimize MIC
attack, mainlyby preventing an acceptableenvironment
for microorganisms.
Welding Practice to Minimize Corrosion
 Material and Welding Consumable Selection.
 Surface Preparation A properly selected cleaning
process can reduce defects that are often sites for
corrosive attack in aggressive environments.
 Welding design should promote deposits that have
relativelyflat beads with low profiles and have minimal
slag entrapment. A poor design can generate crevices.

18.  Welding Practice Complete penetration is preferred to
avoid underbeadgaps. Slag should be removed after
each pass with a power grinder or power chippingtool.
If the welding method uses flux, the geometry of the
joint must permit thorough flux removal, because many
flux residues are hydrophilicandcorrosive.
 Weld Surface Finishing Maximum corrosion resistance
usuallydemands a smooth uniformly oxidized surface
that is free from foreign particles and irregularities
 Surface Coating When a variationin compositionacross
the weld metal can cause localizedattack, it may be
desirable to use protective coatings.
 Postweld Heat Treatment. A postweld heat treatment
can be an effective way to reduce corrosion
susceptibility.This improved corrosion resistance is
accomplishedthrough a reduction in residual stress
gradients that influence SCC growth. Postweld heat
treatment can assist in the transport of hydrogen from
the weldment and reduce susceptibilityto hydrogen
cracking. The treatment can also reduce compositional
gradients (i.e., microsegregation) and corresponding
microgalvaniccells.
 Preheat and Interpass Temperature. The selection and
use of proper preheat treatment and interpass
temperature may prevent hydrogen cracking in carbon
and low-alloysteel.
 Passivation Treatment. A passivationtreatment may
increase the corrosion resistance of stainless steel
welded components.
 Removing Sources of Hydrogen

19.  Avoidance of FormingCrevices. Slag that is still
adhering to the weld deposit and defects such as lack of
penetrationand microfissures can result in crevices that
can promote a localizedconcentrationcell, resulting in
crevice corrosion. Proper selection of welding
consumables, proper welding practice, and thorough
slag removal can alleviatethis form of corrosion
damage.
Testing of weld corrosion
Linear Polarization Resistance (LPR):
The techniqueof LPR provides an estimate of the corrosion
rate based on the Stern-Geary equation.
A small voltage (or polarizationpotential)is appliedto an
electrode in solution. The current needed to maintaina
specific voltage shift (typically10 mV) is directly related to
the corrosion on the surface of the electrode in the solution.
By measuring the current, a corrosion rate can be derived.
For an activationcontrolledreaction:
Rp is resistance to polarization (Ω·cm2)
Α (cm2
) is the exposed area
A linear relationship exists between the polarisation of a metal in
contact with a corrodent and the current induced to flow by the
application of the polarisation.

20. The proportionality factor, B, is a function of the anodic and cathodic
Tafel slopes, which depend on temperature and the chemical species
and concentration in the electrolyte. The advantage of the LPR
technique is that the measurement of corrosion rate is made
instantaneously.
The limitations to this approach include:
• A requirement that the electrolyte has a sufficiently high
conductivity
• The vessel wall or pipe must be penetrated
• The use of a stable reference electrode
Corrosion Potential
 Relatively simple concept, and the underlying principle is widely
used for monitoring the corrosion state of reinforcing steel in
concrete and buried pipelines under cathodic protection.
 This approach can also be valuable in cases where an alloy
could show both active and passive corrosion behavior in a
given process stream.
 Corrosion potential measurements can indicate the transition
from passive to active corrosion .
 The success of corrosion potential measurements depends on
the long-term stability of a reference electrode.
 temperature, pressure, electrolyte composition, pH, and other
variables can limit the applications of these electrodes for
corrosion- monitoring service.
Several factors influence the corrosion resistance of welded
joints. These factors should be included in the test report when

21. recording the results obtained in the corrosion tests of welded
joints. These include:
1. Chemical composition and structure of bas and weld metal;
2. Welding process and procedure, especially shielding;
3. Dimensions of the welded test plate and the corrosion
specimens removed from the plate;
4. Composition, phase, and temperature of the corrosive
media; and
5. Details of any galvanic or impressed current cathodic and
anodic electrical system used.
Weight Loss Test
The most common method of evaluating corrosion resistance is
to measure the weight lost during exposure to the corrodent.
Method:
I. A sample is prepared and carefully weighed.
II. The surface area that will be exposed to the corrodent is
measured after masking off support locations or other
areas that would not be exposed directly. Masking should
leave a known volume so that the effective weight can be
adjusted accordingly.
III. The sample is then subjected to the corrodent for a
measured amount of time under either standardized
conditions or actual conditions, such as immersed in the
ocean.
IV. The weight loss is converted to an average corrosion rate
using the following formula:
R = average corrosion rate in depth of attack per unit time, mils/day
(mm/day);
K = constant;
w = weight lost by the specimen during the test, ounce (oz) [gram (g)];
A = total surface area of the test specimen, in2 (mm2);
D = density of the specimen material, oz/in3 (g/mm3); and
t = duration of exposure, seconds (s).