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7/29/2019 Lecture40laboratory Experiments in Corrosion
1/9
Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
1
Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture - 40
Laboratory Experiments in Corrosion Engineering II
Keywords: Polarization Experiments, Pitting Potentials, Microbial Corrosion.
A.Electrochemical tests in a given environmentPolarization curves and Tafel plots for generalized corrosion.
Polarization resistance measurements.
Corrosion potential, pitting and repassivation potential.
Galvanic coupling effect on polarization curves.
Electrochemical impedance (to study behavior of coating, passivation).
Anodic polarization for establishing active-passive behavior of metals and alloys in
different environmentsAnodic protection.
Impressed current cathodic protection.
In order to establish Tafel constants, corrosion potential, corrosion current and exchange
currents, extrapolated regions of anodic and cathodic curves have to be used.
Two types of approach
a. Wide range of overpotentials with reference to rest potential (for example, -150mV to
+150mV), to facilitate determination of Tafel slopes through extrapolation to the
corrosion potential.
b. Narrow range of overpotentials (+20 mV to -20mV), facilitating determination of linear
polarization resistance (slow scan rate).
7/29/2019 Lecture40laboratory Experiments in Corrosion
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Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Description of cell and experimental arrangements are given in Fig 40.1 and 40.2.
Fig. 40.1 Typical electrolytic cell with various electrodes for polarization measurements
Fig. 40.2 Automatic polarization measurement
7/29/2019 Lecture40laboratory Experiments in Corrosion
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Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
B.Measurement of pitting potentials:Cyclic polarization techniques can be used to evaluate pitting tendency of an active-passive
metal or alloy (Fig. 40.3). A potential scan starting from Ecorr in the anodic direction is applied
till significant current increase occurs. The final potential is negative with reference to
repassivation potential. The potential where the loop closes on the reverse scan is the protection
potential, which can also be estimated by extrapolating the reverse scan to zero current. Pitting
potential (Epit) corresponds to the potential at which current increases sharply. The larger the
loop, the higher the tendency for pitting. Pitting shows up as an increasing anodic current before
transpassive corrosion or evolution of oxygen.
Fig. 40.3 Cyclic polarization to determine pitting and protection potentials.
New pits can initiate only above pitting potential, and not between Epit and Eprot. No hysteresis is
exhibited by an alloy which is resistant to pitting. There will be potential and current
distributions around pits.
7/29/2019 Lecture40laboratory Experiments in Corrosion
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Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
C.Experiments for evaluation of sensitization in stainless steels.Study chromium depletion and precipitation as carbides at the grain boundaries.
Oxalic acid test: Polished specimen is anodically etched at 1A/cm
2
for a minute in 10% oxalicacid at room temperature. Examine the specimen under the microscope to reveal step, dual or
ditch structures.
D.Determination of effect of alloy chemistry on passivation parameters:For development of corrosion resistant alloys with reference to active passive behavior, the
following key parameters need to be optimized.
Epp
Primary passive potential
icritCritical anodic passivating current density.
1. Establish Anodic polarization curves for iron , nickel and chromium in 1N H2SO4. Comment
on the passivity curves with respect to passivity potential range, Epp and icrit.
2. Establish the effect of chromium (0 30%) in stainless steels on Epp and icrit in 1N H2SO4.
Plot your results with respect to
(a)Epp Vs percent chromium
(b)icrit Vs percent chromium
3. Determine pitting potentials for 18-8 stainless steel in different chloride concentrations.
E.Corrosion testing for metals and alloysThe following systems for corrosion testing are available, each covering different method of
corrosion evaluation
Humidity test chamberSalt sprayTemperature and humidity control.
Test set-up for alternate immersion testing of metals and alloys in 3.5% NaCl solution for stress
corrosion testing.
7/29/2019 Lecture40laboratory Experiments in Corrosion
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Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Bimetallic corrosion testing in specific liquids under humidity control current and potential
difference recorded.
F. Laboratory techniques for studying amenability to MIC due to biofilms:Evaluation of biofilms from deposit samples collected from various locations
organic
and inorganic content of biofilms.
Carbohydrate and protein analysis (spectrophotometer)
Presence of aerobes and anaerobes in the deposits.
Redox potential measurements in liquid samples.
Corrosion potential measurements Biofilm growth on metal surface influences anodic
and cathodic reactions Shifting of corrosion potentials in positive or negative directions
to be monitored.
Examples: Stainless steels in aerated seawater
Mild steel in anaerobic seawater.
Distinguish between aerobic and anaerobic corrosion.
Polarization experiments in the presence and absence of biofilms on metals in the
presence and absence of microorganisms.
G. Monitoring and characterization procedures for different bacteria involved in MIC are listedin Table 40.1. Microbiological aspects of MIC microbes are illustrated in lectures 24 27.
Various strains of different bacterial species can be procured from culture banks and
characterized as per recommended procedures.
7/29/2019 Lecture40laboratory Experiments in Corrosion
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Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Table 40.1 Testing and Analyses of different bacteria relevant to MIC,
Type of Bacteria Monitoring and characterization
1. Acid producing Production of inorganic and organic acids
attack on various metals and alloys.
2. Denitrifying Ammonia production attack on copper
alloys.
3. Iron-related Ferrous oxidation and ferric-reduction-
plugging of water and oil pipelines- tubercle
formation.
4. Slime-forming Slime-sludge characterization.
5. Sulfate-reducing Sulfide production (H2S) FeS production -
corrosion of metal surfaces.
Common media used for routine isolation of bacteria and fungi
Filamentous fungi - Potato Dextrose Agar
Aerobic and anaerobic bacteria - Nutrient Agar
Pseudomonas Sp - Select media from literature
Sulfate Reducing Bacteria (SRB) - Postgate media
7/29/2019 Lecture40laboratory Experiments in Corrosion
7/9
Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
H.Microbially influenced corrosion of aluminium alloysChoose desired aluminium alloys and make suitable specimens. Use naturally collected sea
water and fresh water samples from identified locations. A biofilm growth chamber (under
conditions of both stagnant and flowing liquid) can be constructed to expose the metallic
specimens for different periods of time.
Monitor biofilm growth by removing specimen frequently and characterize the biofilm with
respect to thickness, microbial assay, chemical and metallurgical analysis, surface roughness and
morphology. Isolate important bacterial species from the biofilm and carry out steady-state
potential and polarization measurements in the presence and absence of isolated bacteria.
I. Biofouling and MIC of stainless steels in sea water.Experiments similar to the previous one for aluminium alloys.
J. Microbial diversity of pipelines and establishment of MICLocate a pipeline transporting water and petroleum products.
From the pipeline, collect aseptically, samples of water, oil and corrosion products (debris).
a) Visual, physical and chemical characterization of the water, oil and debris samples for
colour, chemical composition, pH.
b) Isolation and enumeration of different types of microorganisms through standard
microbiological procedures characterization of isolated organisms with respect to
Autotrophs, Heterotrophs.
Bacteria, fungi
Aerobe, anaerobe
Iron oxidising, Manganese oxidising.
c) Based on the microbial assay and characterization and failure analysis of the pipeline
samples, predict nature of MIC (Microbiallyinfluenced corrosion)
7/29/2019 Lecture40laboratory Experiments in Corrosion
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Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
K.Examination of biocorrosion of concrete in the laboratory simulating sewerconditions
Samples of sewer pipes collected from sewerage processing stations can also be used.
Corrosion testing coupons
Fresh coupons from new sewer pipes.
Coupons prepared from corroded concrete slabs from sewer treatment plants.
Corrosion chambers for exposing the coupons to bacterial activity under simulated conditions
can be used.
Monitor conditions with respect to pH changes, H2S generation, temperature and humidity.
Growth of Anaerobic Sulfate Reducing Bactria (SRB), aerobic sulfur oxidizers such as
Acidithiobaullus can be monitored and their role on concrete corrosion assessed.
L.Bacterial kinetics of sulfur oxidation ofAcidithiobacillusthiooxidansand itsinfluence on concrete corrosion.
Experimental strategy:
Bacterial growth in recommended media.
Growth curve with respect to cell number, pH and sulfate concentration as a function of
time.
Establish bacterial growth kinetics.
Concrete corrosion tests in aqueous media at bacterial acidic pH under different
conditions of temperature, metal-ion concentrations, and types of reinforcement steels.
7/29/2019 Lecture40laboratory Experiments in Corrosion
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Lecture 40: Laboratory Experiments in Corrosion Engineering II NPTEL Web Course
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
M.Corrosion testing for medical implantsCompatibility
Tissue response
Dissolution rates
Toxicity
In vivo corrosion
How susceptible is the implant metal to corrosion?
Effect of corrosion on body response
Rest potential : Measurements over extended periods of time to predict metal dissolution.
Cyclic potentiodynamic polarization: Corrosion susceptibility of small implant devices.
Galvanic corrosion: Coupled and uncoupled leach rates.
Fretting: Fretting corrosion in moving body parts.
Various metal and alloy samples representative of implant materials can be shaped into
electrodes and tested in body fluids and simulated electrolytes.