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UNIVERSITY OF GAZIANTEP FACULTY OF ENGINEERING CIVIL DEPARTMENT
CE-547
Corrosion of Plain &Reinforced concrete
Report #4 About :
(Test Methods for measurement and monitoring of corrosion )
Submitted to:
Doç.Dr. Mehmet GESOĞLU
Prepared by:
Chalak Ahmed Mohammed [email protected]
2014 45056
Date : 26.03. 2015
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List of contents :
What is Corrosion Monitoring?
The Need for Corrosion Monitoring
Methods of measurements
*Open circuit potential (half-cell) MEASUREMENTS
* Surface Potential (SP) Measurements
* concrete resistivity - Resistivity Meter
*Lnear Polarization Resistance (LPR)
Conclusions
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What is Corrosion Monitoring?
The field of corrosion measurement, control, and prevention covers a very broad spectrum of technical activities. Within the sphere of corrosion control and prevention, there are technical options such as cathodic and anodic protection, materials selection, chemical dosing and the application of internal and external coatings. Corrosion measurement employs a variety of techniques to determine how corrosive the environment is and at what rate metal loss is being experienced. Corrosion measurement is the quantitative method by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. A wide variety of corrosion measurement techniques exists, including:
Non Destructive Testing Analytical Chemistry
Analytical Chemistry
• Ultrasonic testing • Radiography • Thermography • Eddy current/magnetic flux • Intelligent pigs
• pH measurement • Dissolved gas (O2, CO2, H2S) • Metal ion count (Fe2+, Fe3+) • Microbiological analysis
Operational Data Fluid Electrochemistry
• pH • Flow rate (velocity) • Pressure • Temperature
• Potential measurement • Potentiostatic measurements • Potentiodynamic measurements • A.C. impedance
Corrosion Monitoring
• Weight loss coupons • Electrical resistance • Linear polarization • Hydrogen penetration • Galvanic current
Some corrosion measurement techniques can be used on-line, constantly exposed to the process stream, while others provide off-line measurement, such as that determined in a laboratory analysis. Some techniques give a direct measure of metal loss or corrosion rate, while others are used to infer that a corrosive environment may exist. Corrosion monitoring is the practice of measuring the corrosivity of process stream conditions by the use of "probes" which are inserted into the process stream and which are continuously exposed to the process stream condition. Corrosion monitoring "probes" can be mechanical, electrical, or electrochemical devices. Corrosion monitoring techniques alone provide direct and online measurement of metal
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loss/corrosion rate in industrial process systems. Typically, a corrosion measurement, inspection and maintenance program used in any industrial facility will incorporate the measurement elements provided by the four combinations of on-line/offline, direct/indirect measurements.
Corrosion Monitoring Direct, On-line Non Destructive Testing Direct, Off-line Analytical Chemistry Indirect, Off-line Operational Data Indirect, On-line
In a well controlled and coordinated program, data from each source will be used to draw meaningful conclusions about the operational corrosion rates with the process system and how these are most effectively minimized.
The Need for Corrosion Monitoring
The rate of corrosion dictates how long any process plant can be usefully and safely operated. The measurement of corrosion and the action to remedy high corrosion rates permits the most cost effective plant operation to be achieved while reducing the life-cycle costs associated with the operation. Corrosion monitoring techniques can help in several ways:
1. by providing an early warning that damaging process conditions exist which may result in a corrosion-induced failure.
2. by studying the correlation of changes in process parameters and their effect on system corrosivity.
3. by diagnosing a particular corrosion problem, identifying its cause and the rate controlling parameters, such as pressure, temperature, pH, flow rate, etc.
4. by evaluating the effectiveness of a corrosion control/prevention technique such as chemical inhibition and the determination of optimal applications.
5. by providing management information relating to the maintenance requirements and ongoing condition of plant.
Methods of measurements:
Many of the strategic reinforced and prestressed concrete structures have
started showing signs of distress with in a short period usually the condition
of the structures is monitored by visual inspection and remedial measures are
resorted to only when the condition becomes very serious by
way to heavy rusting of steel reinforcements followed by cracking and
spalling on concrete.
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It is desirable to, monitor the condition of such strategic structures right from
the construction stage by carrying out periodic corrosion surveys and
maintaining a record of data. For measurement of the corrosion rate of
reinforcing steel in concrete, many electrochemical and non-destructive
techniques are available for monitoring corrosion of steel in concrete
structures.
Open circuit potential (half-cell)
MEASUREMENTS
The principle involved in this method
is appearance of an electrical
potential between the reinforcing
steel and a reference electrode named
half-cell. The half-cell consists of a
metal rod immersed in a solution of
its own ions (Fig. 1).
The role of the half-cell is to insure constant reference potential. The metal rod is
connected with reinforcement steel by a voltmeter, and the ion solution is
connected to the pore water via moist porous plug. Measuring method is based on
many measurements of potential and correlation of measured potentials with
observed corrosion rate at reinforcement. Table 2 presents criteria according to
ASTM C-876 standard for cooper-cooper sulphate electrode, and also for calomel
and silver-silver chloride. The main application of this method is in situ.
Cu/CuSO4 Calomel (SCE) Ag/AgCl Interpretation
E>-200mV E>-126mV E>-119mV Greater than 90% probability that no
corrosion is occurring
-200mV < E < -
350mV
-126mV < E < -
276mV
-119mV < E < -
269mV Corrosion activity is uncertain
E<-350mV E<-276mV E<-269mV Greater than 90% probability that no
corrosion is occurring
Table 2: Interpretation of corrosion potential measurements
Fig 1: Principle of the half-cell method
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Hand held equipment The half-cell is moved across the concrete surface to be investigated, and the
electrode potentials are measured at many points. The measured potential is drawn
as equipotential lines to identify the corrosion areas . Extra devices are constructed
to accelerate measuring.
Surface Potential (SP) Measurements
This technique measures the potential drop on the surface of the concrete, at
different locations; based on the fact that an electric current flows between the cathodic
and anodic sites through the concrete during corrosion process the degree of corrosion
process is estimated. In this method two reference electrodes are used and they are
applied to the surface of the reinforced concrete member. No connection to the steel bar
is required. One reference electrode is held at a fixed location while the other, called the
movable reference electrode, is moved along the reinforced concrete member. The
greater the potential difference between anodic and cathodic areas greater is the
probability of corrosion .
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concrete resistivity - Resistivity Meter Surface resistivity measurement provides extremely useful information about the state
of a concrete structure. Not only has it been proven to be directly linked to the
likelihood of corrosion and the corrosion rate, recent studies have shown that there is a
direct correlation between resistivity and chloride diffusion rate.
ResiPod is a fully integrated 4-point Wenner probe, designed to perform
concreteresistivity measurement in a completely non-destructive test. It is the most
accurate instrument available, extremely fast and stable and packaged in a robust,
waterproof housing designed to operate in a demanding site environment.
Applications
Resistivity is one of the key controlling factors once corrosion begins. Corrosion
requires a flow of ions between anodic and cathodic sites on the reinforcement of
concrete; the resistivity controls the rate of this flow of ions and therefore directly
controls the rate of concrete corrosion. What this means is that by measuring concrete
resistivity it is possible to determine if a structure that is corroding and will deteriorate
quickly or if the corrosion rate is slow and can be addressed in a more conservative
manner. This can be a very useful test in combination with Half-Cell potential
measurement to locate hot spots for corrosion likelihood.
T
he Surface Resistivity (SR) test is a much quicker and easier test for estimating
concrete permeability. The traditional testing method, Rapid Chloride Penetration was
laboratory based and very labour intensive, taking 4 days of preparation and testing
including cutting, epoxying, desiccation and testing. A study initiated in 2002 by the
Florida Department of transport has determined a strong correlation between Rapid
Chloride Penetration [RCP] measurements and resistivity measurements (see image
below). This discovery now means permeability testing can now be done in 10
minutes instead of 10 days using the same cylinders or cores used to determine
compressive strength.
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The versatility of the method can be seen in these example applications:
Estimation of the likelihood of corrosion
Indication of corrosion rate
Correlation to chloride permeability
On site assessment of curing efficiency
Determination of zonal requirements for cathodic protection systems
Identification of wet and dry areas in a concrete structure
Indication of variations in the water/cement ratios within a concrete structure
Identification of areas within a structure most susceptible to chloride penetration
Correlation to water permeability of rock Principle of Operation
Operating on the principle of the Wenner probe, the ResiPod is designed as a
resistivity meter to measure the electrical resistivity of concrete or rock. A current is
applied to the two outer probes, and the potential difference is measured between the
two inner probes. The current is carried by ions in the pore liquid. The calculated
resistivity depends on the spacing of the probes.
Resistivity ρ= 2πaV/l [kΩcm]
When taking measurements the unit automatically changes its measurement methods
to suit the subjects. Normally it will attempt to drive a full 200uA current through the
concrete, if this is not possible due to a high resistance it will drive 50 uA and if that is
not possible due to a very high resistance the device will instead drive a voltage across
the outside probes and measure the induced current to obtain a resistivity estimate.
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This allows the Resipod to measure much higher resistivity’s than a basic Wenner
Probe.
Resipod Models and Probe Spacing
Wider probe spacings provide a more consistent reading when measuring on an
inhomogeneous material like concrete. However, if the spacing is too wide, there is
more danger of the measurement being affected by the reinforcement steel. The
industry standard 50 mm probe spacing has long been seen as a good compromise.
The 38mm model is designed specifically to comply with the AASHTO standard
(under development) for “Surface Resistivity Indication of Concrete’s Ability to
Resist Chloride Ion Penetration”.
The contact extension cable kit can also be used to attain variable spacing for the
contacts. Spacing from 20-85mm can be achieved using the kit, which is bought as an
optional accessory.
Features
Despite being extremely simple to use, ResiPod provides a variety of features that are
unique in a concrete surface resistivity instrument.
Fully integrated surface resistivity instrument
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Wide measuring range (0 to ca. 1000 kΩcm)
Fast and accurate delivery of measuring results
Highest resolution available for a surface resistivity instrument
Meets the AASHTO standard (38mm spacing) (standard under development)
Current flow indication and poor contact indication
Hold, save and delete function, with onboard memory
USB connection and dedicated PC software
Designed to float (waterproof according to IPX7)
Allows variable probe spacing to be set
Allows replacement of standard tips with accessories
ResipodLink Software
The collected measurement values can then be analysed comfortably with the ResiPod
Link PC tool.
System requirements: Windows XP, Windows Vista, Windows 7, USB-Connector.
An internet connection is necessary for soft- and firmware (using PqUpgrade) updates
if available.
Bulk Resistivity Module
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The ResiPod can now be upgraded to take bulk resistivity of 102 x 204 mm cylinders
using the Bulk Resistivity Module. Bulk Resistivity measurements are taken in such a
way that the whole of the cylinder informs the result and are able to be correlated to
surface resistivity measurements. The module includes 2 metal and sponge contacts
and a convenient stand for the ResiPod.
Lnear Polarization Resistance (LPR)
Polarization resistance is particularly useful as a method to rapidly identify corrosion
upsets and initiate remedial action, thereby prolonging plant life and minimizing
unscheduled downtime. The technique is utilized to maximum effect, when installed as a
continuous monitoring system. This technique has been used successfully for over thirty
years, in almost all types of water-based, corrosive environments. Some of the more
common applications are:
Cooling water systems
Secondary recovery system
Potable water treatment and distribution systems
Amine sweetening
Waste water treatment systems
Pickling and mineral extraction processes
Pulp and paper manufacturing
Hydrocarbon production with free water
The measurement of polarization resistance has very similar requirements to the
measurement of full polarization curves. There are essentially four different methods of
making the measurement according to whether the current or the potential is controlled
and whether the current (or potential) is swept smoothly from one value to another, or
simply switched between two values. In addition the measurement may be made
between two nominally identical electrodes (a two-electrode system), or a conventional
three-electrode system (working, reference and counter) may be used.
The principle of LPR measurements is explained in more details here, with a view of
some of the fundamental pitfalls of the technique.
With this widely used technique in corrosion monitoring, the polarization resistance of a
material is defined as the slope of the potential-current density (DE/Di) curve at the free
corrosion potential, yielding the polarization resistance Rp that can be related
(forreactions under activation control) to the corrosion current by the Stern-Geary
equation:
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where:
Rp is the polarization resistance
icorr the corrosion current
The proportionality constant , B, for a particular system can he determined
empirically (calibrated from separate weight loss measurements) or, as shown
by Stern and Geary, can be calculated from baand bc, the slopes of the anodic and
cathodic Tafel
The Tafel slopes themselves can be evaluated experimentally using
real polarization plots. The corrosion currents estimated using these
techniques can be converted into penetration rates
using Faraday's law or a generic conversion chart.
The study of uniform corrosion or studies assuming corrosion uniformity are probably the
most widespread application of electrochemical measurements both in the laboratory and
in the field. The widespread use of these electrochemical techniques does not mean that
they are without complications. Both linear polarization and Tafel extrapolation need
special precautions for their results to be valid. The main complications or obstacles in
performing polarization measurements can be summarized in the following categories:
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Conclusions
• A number of electrochemical rebar corrosion measurement
techniques available presently are reviewed. Each technique is
reviewed to possess with certain advantages and limitations.
• To obtain maximum information about the corrosion state of rebar
in a particular structure, a combination of measuring techniques is
recommended.
• Although the electrochemical corrosion measurements are usually
qualitative and also semi quantitative, significant benefits can be
derived from them.
• The development of durable, embeddable sensors and inexpensive
microprocessor control and communications, have encouraged the
development of corrosion monitoring systems for new and existing
reinforced concrete structures.
• The deployment of sensor systems, such as those described, to assess
cover concrete and steel performance forms the important component
of an integrated monitoring system.
• Monitoring systems can consist of sensors to measure the corrosion
rate and concrete condition.
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Regards...
