Monitor or monitoring system to continuously detect or measure flaws, thickness or corrosion in production line, plant, production or field applications.
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Instruments or monitors capable of measuring the corrosion rate of a material. Corrosion rate is the rate of weight loss of metal, generally expressed in mils per year (mpy) that occurs as the conversion to corrosion product proceeds.
Instruments or monitors used to detect corrosion activity or a system with a high condition or factor that will promote corrosion. The system could be a metal or a combination of different metals in an aqueous solution or chemical.
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Flaws or features within a mechanical system provide specific acoustic or vibrational responses. If a break, deformation, or other failure occurs in a piece of metal or material, sensitive acoustic emission (AE) sensors can detect the high frequency elastic wave bursts given off during the event. Continuous acoustic emissions are associated with dislocation movement and the resulting strain or deformation. Bursts or short pulses of acoustic emissions signals are produced by micro-yielding, twinning and crack formation. Melting, phase transformation, thermal stresses, fiber breakage and fiber-matrix debonding in composites also result in acoustic emissions.
Monitoring acoustic emissions can also access the location and severity. This NDT technique is particularly useful in determining the structural adequacy of tanks and pressure vessels. AE is also used for the detection of faults or leakage in pressure vessels, tanks, and piping systems. Welds and stress corrosion cracking can be monitored on-line with AE techniques.
A variety of coating flaws exist such as pin point discontinuities, openings or holidays; thin spots; bubbles, blisters, inclusions, streaks, waviness and chicken tracks.
Eddy current, penetrating radar, and other electromagnetic techniques are used to detect or measure flaws, bond or weld integrity, thickness, electrical conductivity, and detect the presence of rebar or metals. Eddy current is the most widely applied electromagnetic NDT technique. The eddy current method is also useful in sorting alloys and verifying heat treatment. Eddy current testing uses an electromagnet to induce an eddy current in a conductive sample. The response of the material to the induced current is sensed. Since the probe does not have to contact the work surface, eddy current testing is useful on rough surfaces or surfaces with wet films or coatings.
EIS is a powerful and accurate method for measuring corrosion rates. The corrosion rate at the monitored interface is proportional to the charge transfer resistance or polarization resistance. The actual relationship requires modeling of the interface for useful interpretation of the results. An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. To make an EIS measurement, a small amplitude signal, usually a voltage between 5 to 50 mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000 Hz. The EIS instrument records the real (resistance) and imaginary (capacitance) components of the impedance response of the system.
The electrical resistance technique is widely applied in monitoring material loss occurring in the interior of plants and pipelines. This technique operates by measuring the change in electrical resistance of a metallic element immersed in a product media relative to a reference element sealed within the probe body. Since temperature changes affect the resistance of both the exposed and protected element equally, measuring the resistance ratio minimizes the influence of changes in the ambient temperature. If the corrosion occurring in the vessel under study is roughly uniform, a change in resistance is proportional to an increment of corrosion. Although universally applicable, the ER method is uniquely suited to corrosive environments having either poor or non-continuous electrolytes such as vapors, gases, soils, "wet" hydrocarbons, and non-aqueous liquids. An ER monitoring system consists of an instrument usually with data logging functions connected to a probe. The instrument may be permanently installed to provide continuous information, or may be portable to gather periodic data from a number of locations. The probe is equipped with a sensing element having a composition and material processing history similar to that of the process equipment of interest.
Electrochemical noise (EN) is a useful, sensitive, and nonintrusive technique for corrosion monitoring. Fluctuations of potential or current of a corroding metallic specimen are monitored to gage and understand the corrosion process. Electrochemical noise is used to investigate localized corrosion processes such as pitting or stress corrosion cracking (SCC), exfoliation, and erosion-corrosion in either laboratory or diverse and complex industrial environments. During localized corrosion, film formation, passivation breakdown or pit propagation processes generate the electrochemical noise that is observed. The most traditional way to analyze electrochemical noise data has been to transform time records in the frequency domain in order to obtain power spectra with FFT methods.
Electromagnetic acoustic resonance (EMAR) uses a non-contact electromagnetic transducer to impart a signal for resonant acoustic analysis. The non-contact EM induced signal produces a pure resonant response. Mechanical vibration from surrounding machinery does not influence the ultrasonic signals in the materials or part being inspected. Localized or wide acoustic field can be generated depending what features need to be isolated and measured. Small surface or subsurface flaws or anomalies can be isolated. The entire part can be resonated for elastic stiffness and structural integrity assessments.
A galvanostat automatically maintains an electrode at a constant current or a controlled set or scanned current value with respect to a suitable reference electrode. A potentiostat is an instrument for automatically maintaining an electrode in an electrolyte at a constant potential or or a controlled set or scanned potentials values with respect to a suitable reference electrode. Galvanostats or potentiostats are typically used in laboratory settings to provide conditions for the evaluation of corrosion systems.
Hydrogen probes detect and measure the hydrogen given off in a corrosion process. The presence or level of hydrogen provides an indication or measurement of corrosion.
Inductive resistance probes are similar to ER probes. The weight loss in the sensor element is detected by measuring changes in the inductive resistance of a coil, located inside the element. The inductive measurement technique provides greatly improved sensitivity and earlier detection of corrosion rate changes compared to conventional electrical resistance probes. Inductive resistance probes require temperature compensation, similar to ER probes. Like ER probes, the sensors can be used in a broad range of environments such as low conductivity and nonaqueous environments, where electrochemical techniques are generally unsuitable.
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 in almost all types of water-based, corrosive environments. The measurement of polarization resistance has very similar requirements to the measurement of full polarization curves.
The method uses a series of electrodes, a voltmeter, an ammeter, and a current source. The direction of current flow is reversed in the test causing the test electrode to become the anode with an accelerated corrosion rate. The current level is increased to maintain a constant polarization potential voltage. The corrosion rate is directly proportional to the current level [Corrosion rate = ((K) (I)]. The corrosion rate information is not as accurate as corrosion coupon data. 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.
Optical-based instruments using methods such as laser shearography, magneto-optical, holographic interferometry to detect flaws, residual stress or measure thickness.
Equipment using penetrating X-rays or gamma rays to capture images of the internal structure or a part or finished product. The density and composition of the internal features will alter the intensity or density of these features in the X-ray image. Densitometers are used to quantify the density variations in the X-ray image. Penetrameters or other X-ray opaque gage references are located with the part during imaging for sizing of internal cracks, pores, defects or other features.
Ultrasonic (UT) inspection techniques are used to detect surface and subsurface flaws or to measure thickness. Beams of high frequency acoustic energy are introduced into the material and subsequently retrieved. Distance calculations are based on the speed of sound through the material being evaluated. The most widely used of all UT techniques is the pulse-echo technique. Flaws are detected and sizes estimated by comparing the amplitude of a reflected echo from an interface (flaw or back surface) with that of a reference interface of known size.
In zero resistance ammetry (ZRA), a macrocell current is measured between two corrosion sensor elements. The technique is useful in measuring the magnitude of galvanic current flow between different materials or different microstructures of the same material. Preferential weld corrosion has been measured with ZRA. For reinforcing steel (rebar) in concrete, this principle has been adopted in the ASTM G102 test procedure and also the method is used in a rebar corrosion monitoring system.
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Higher temperatures can activate or accelerate chemical reactions. Temperature often changes the rate at which a chemical solution attacks or corrodes a metal or material.
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pH is the negative logarithm of the hydrogen ion activity in solution. The pH value of a substance is directly related to the ratio of the hydrogen ion [H+] and the hydroxyl ion [OH-] concentrations. Corrosion is often dependent on pH.
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The corrosion rate, often measured in mils per year (mpy), is the amount of weight loss of metal over a unit time that occurs as the conversion to corrosion product proceeds.
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