Monitoring field corrosion

It is common in corrosion laboratories and in field corrosion monitoring probes to immerse two vertical rods parallel to one another in an electrolyte. In the lab, one of the rods consists of a high-density graphite counterelectrode while the other is a working electrode. A reference electrode may be placed in between the two rods. In the field, polarization resistance or electrochemical noise measurements are often made between two nominally identical rods that both consist of the material of interest. The primary current distribution is nonuniform with respect to circumferential position about each electrode when the distance between the two rods is small in comparison to the radius of the rod, Fig. 10a (16). Again, the value of Ra varies from where the rods face each other to where they... 

The EN technique differs in many ways from other electrochemical techniques used in corrosion. One important difference is that ENA does not require that the sensing element be polarized in order to generate a signal. However, it is also possible to measure current noise under an applied potential, or measure potential noise under an applied current. The potential and current between freely corroding electrodes (in many cases < 1 pV and < 1 nA) are measured with sensitive instrumentation. A measurement frequency of 1 Hz is usually appropriate to provide meaningful data. For simultaneous measurement of electrochemical potential and current noise, a three-electrode sensor is required. In field corrosion monitoring, the threesensor elements are usually made of the same material. 

Whilst the measurement of electrochemical noise is relatively straightforward, the data analysis can be complex and inconclusive. Even if ENA was first applied in field corrosion monitoring in the late 1960s, an understanding of the method of analysis is still evolving, partly because the technique has been used to look at several types of corrosion. The relationships between potential and current noise are inherently complex to analyze quantitatively because the naturally... 

Table 8.12 Summary of Devices for Field Corrosion Monitoring in Power Plants [17]...

Use of Corrosion Probes The major use of corrosion monitoring probes is to measure the corrosion rate in the plant or the field. In addition to corrosion-rate measurements, corrosion probes can be used to detect process upsets that may change the corrosion resistance of the equipment of interest. This is usually equally as important a measurement as corrosion rate since a change in the process conditions can lead to dramatic changes in the corrosion rate. 

Heat Flux Tests Removable tube test heat exchangers find an ideal use in the field for monitoring heat flux (corrosion) conditions, NACE TM0286-94 (similar to laboratory test. Fig. 28-4, page 28-12). 

Strommen, R. and Roland, A., Corrosion Surveillance of Submarine Pipelines by Electric Field Strength Monitoring , Materials Performance, 20 No. 10, 47 (1981)... 

A considerable catalyst to the corrosion monitoring market has been expansion in the production of oil and gas, not only in the usual oil areas (US and the Middle East), but also the offshore developments in Europe. In addition to the usual uncertainty of the onset or progress of internal corrosion in the operation of plant, the oil industry has to face the considerable problem concerning prediction of field corrosivity and the possibility of the producing field becoming corrosive or more corrosive as depletion progresses. These factors have considerable influence on the installation of corrosion monitoring as oil and gas production is the major user of such equipment. 

Oil and Gas Production This sector is a major user of corrosion monitoring equipment, in particular for offshore fields where ramifications of corrosion and consequent maintenance are far more serious and costly compared with onshore production. Carbon steel is used for approximately 70-80 70 of production facilities. The development of a field is assessed on a defined corrosion risk which may not be correct, leading to serious corrosion. In addition, a reservoir may become more corrosive as the field is extracted owing to (a) increased water content, and (b) eventual souring of the field (hydrogen sulphide production). 

Vennett, R. M., Corrosion Monitoring in Oil Field Operations Using a Vacuum Hydrogen Probe , Materials Performance, 16, 8, 31-41 (1977)... 

Comeau, B. D., and Marsden, C. J., Unexpected Field Corrosion Leads To New Monitoring with Revised Predictive Model , Oil and Gas J., 45-48, 1 June (1987)... 

ENA was recently used for remote on-line corrosion monitoring of carbon steel electrodes in a test loop of a surge water tank at a gas storage field. An experimental design and system for remote ENA and collection of electrochemical impedance spectroscopy (EIS) data (Fig. 13) have been presented elsewhere. In the gas storage field, noise measurements were compared with electrode weight loss measurements. Noise resistance (R ) was defined as... 

It can be concluded that selection of proper material or lining for seawater application is important and depends on the criticality of the system, availability of standby system/equipment and availability of isolation devices for maintenance. For process piping in seawater application the success of proper lining or coating material depends on the suitability for in-situ/field application and the proper corrosion monitoring plan. 

We need more information on the performance of inhibitors, particularly well controlled field trials, and long-term corrosion monitoring. 

The buildup of soluble corrosion products can be used to monitor corrosion kinetics. This method has been used extensively in oil field corrosion inhibitor testing, particularly in sweet (CO2) systems with only small amounts of HjS present [29]. Iron analysis in the laboratory is most rapidly done on the bench with the Hach method (Phenantridine) [.30], although a host of other wet chemical methods are applicable. Instrumental methods include Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Argon Plasma Spectroscopy (ICAP). While both these methods are well suited for high volume routine analysis, care must be taken that the samples are not contaminated by even traces of hydrocarbons. This includes soluble hydrocarbons such as methanol, chelating compounds such as EDTA, or scale inhibition products. Also used in the laboratory is Ion Chromatography (IC). This latter method is even more sensitive to sample composition and not recommended on a routine basis. 

Continuous, on-line corrosion monitoring can be used to operate FGD systems at optimal efficiency while avoiding high corrosion rates. Conventional corrosion monitors are designed for use in bulk liquids and do not function satisfactorily in the thin condensate films that form on the surfaces of FGD ducts. Recent EPRI research has made use of an advanced electrochemical method of corrosion surveillance developed by the Corrosion and Protection Centre Industrial Services (CAPCIS) in England [30]. This method allows on-line monitoring of corrosion activity in either thin condensate films or bulk liquids. EPRI introduced the CAPCIS system to the U.S. electric utility industry in 1985, sponsoring several field demonstrations. 

The field of corrosion monitoring is advancing rapidly and several trends are noteworthy ... 

To obtain more information than simple voltage or current noise, and to make measurements convenient for field applications and the corrosion monitoring of plants, the measurement of a quantity denoted as noise resistance / n was proposed some time ago (Eden et al., 1991,1992 Mansfeld and Xiao, 1993 Xiao and Mansfeld, 1994) and has recently increased in interest (Bertocci andHuet, 1995 Bertocci et al., 1997 a, b Schauer et al., 1998). 

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