Monitoring corrosion coupons

Where a solid corrosion product is formed, meaningful results are only obtained after a conditioning period for a new measured element. Even so, the conditions under which the scale is laid down may not be the same as that for the original equipment. This objection applies equally to coupons or spools, and points to one of the basic objections of using anything other than the plant itself to monitor corrosion rates. 

When implementing an in-service cleaning procedure, use should be made of monitoring equipment, such as corrosion coupons and test racks. 

Corrosion monitoring, including corrosion coupons and possibly more specialist techniques, such as portable or permanently installed corrosion rate measuring equipment. 

For small systems, the use of corrosion coupons plus a hand-held LPRM or ERM monitor is probably the most satisfactory. 

Fig. 10.2 Various monitoring devices (1 to r) corrosion/fouling/ biofouling mesh coupons, LPRM monitor, corrosion rack, deposit monitor, membrane filter on poured plate, and dip-slide... 

Portable deposit/corrosion monitors are typically housed in an enclosure of perhaps 30 in. H x 20 in. W x 15 in. D. Components include inlet flow controller, strainer, adjustable electric heater, (outer) see-through glass housing, (inner) heated specimen tube or block, hot/cold temperature readout, corrosion rack, plus thermal overload, low-flow cut-off, and other safety devices. The specimen tubes or blocks are available in different metals (as are the corrosion coupons) and can usually be replaced in a matter of minutes. Unlike test heat exchangers, the cooling water in this type of monitor flows on the shell side of the specimen tube. 

CORROSION, COUPONS - Pre-weighed metal strips installed into fluid systems for the purpose of monitoring metal losses. 

The techniques used for monitoring corrosion in a process plant are (i) visual inspection (ii) weight loss coupons (iii) electrical resistance probes (iv) measurement of corrosion potential (v) linear polarization measurement (vi) hydrogen probes (vii) thickness measurement and crack detection (viii) visual inspection (ix) sentinel holes. 

The pitting rate equation discussed in this chapter, although not accurate, gives a reasonable evaluation of the aggressiveness of the water quahty in fuel storage basins. It has been used to monitor the basin water cleanup activities at SRS, and improvements in water chemistry have been verified by corrosion coupon tests. Additional work is needed to improve this correlation. 

This report documents the work performed in the IAEA Co-ordinated Research Project (CRP) on Corrosion of Research Reactor Aluminium Clad Spent Fuel in Water. The project consisted of the exposure of standard racks of corrosion coupons in the spent fuel pools of the participating research reactor laboratories and the evaluation of the coupons after predetermined exposure times, along with periodic monitoring of the storage water. The project was overseen by a supervisory group consisting of experts in the field, who also contributed a state of the art review that is included in this report. 

Low pH (acidic) may be the cause of corrosion and aggravates, so corrosion coupon is essential to monitor the type of corrosion and simultaneously can suggest the right material (chemical inhibitor) to resist it. 

The use of corrosion coupons is included in the test procedure as a means for monitoring corrosive activity within a given chamber. A minimum of six corrosion coupons, made of 25.4 X 50.8 mm AISI 1006-1010 steel, are used. The coupons are cleaned and weighed in accordance with the procedure described in J2334 and mounted in a nonmetallic rack. One coupon from each end of the rack is removed, cleaned, and reweighed every 20 cycles. At the present writing, no target mass loss numbers for each cycle or end-of-test mass loss have been published. 

Thin layer activation coupons have been used to continuously monitor corrosion rates in continuous digesters, and to verify the effectiveness of anodic protection systems [180], The surface of a thin layer activation coupon is irradiated to a shallow depth and monitoring is performed with a Geiger counter 6x>m outside the digester wall. Subtracting for effects of half-life decay, the corrosion rate can be estimated from the decreased activity of the coupon. 

Procedures for monitoring corrosion of pipelines and related surface facilities using retrievable specimens are described in NACE Standard Practice for Preparation and Installation of Corrosion Coupons and Interpretation of Test Data in Oilfield Operations (RP0775). One of the main concerns in the use of such specimens is the problem of location, since corrosion is generally associated with a separate water phase. In some cases, special dropout pots are used to trap a water phase to provide a severe location for specimens. Electric resistance probes can be used in place of specimens. In locations where there is a continuous water phase and fouUng is not a problem, polarization type probes can be used [74,75]. 

Fouling and deposit monitors are used primarily to track the buildup of scale or deposits on heat transfer surfaces. The heat transfer tube can also be used as a tubular corrosion coupon to monitor corrosion under heat transfer conditions. 

Once the site has been exposed, corrosion monitoring devices (e.g., coupons, electrical resistance probes) can be installed to identify and monitor corrosivity in the pipeline. Inline inspection results may also provide information to assess the downstream conditions of the pipe. Once regions most susceptible to corrosion are free of damage, it is then considered that pipeline integrity to a large extent is assured. 

Intrusive probes and coupons for monitoring corrosion and erosion... 

Successful inhibitor tests require suitable corrosion measurement and analysis techniques that are able to correctly record and interpret corrosion rate data. Many testing and monitoring techniques that were developed initially for the diagnosis and prediction of corrosion have been successful employed in laboratory and field corrosion inhibitor testing and research. These techniques include the use of corrosion coupons, solution analysis, electrical resistance probe, polarization resistance, electrochemical impedance spectroscopy and many other physical, electrical and electrochemical methods. 

Corrosion coupons (intrusive). In what is perhaps the simplest form of corrosion monitoring, small specimens are exposed to an environment for a specific period of time and subsequently removed for weight loss measurement and more detailed examination. Even though the principle is very simple, there are numerous potential pitfalls, which can be avoided by following the recommendations of a comprehensive ASTM guide (ASTM G4 standard). 

Corrosion coupons. Corrosion coupons connected to cathodically protected structures are finding increasing application for performance monitoring of the CP system. Essentially these coupons, installed uncoated, represent a defect simulation on the pipeline under controlled conditions. These coupons can be connected to the pipeline via a test post outlet, facilitating a number of measurements such as potential and current flow. 

Properly installed coupons are excellent for monitoring corrosion. They are not very successful in pipeline programs because they need to be installed in places that are generally not easily accessible. 

Results obtained on field probes can be correlated with chemical analysis data, residual inhibitor data, and corrosion coupon monitoring data. 

Choices of alternative materials. Corrosion probes are carefully chosen to be as close as possible to the alloy composition, heat treatment, and stress condition of the material that is being monitored. Care must be taken to ensure that the environment at the probe matches the service environment. Choices of other alloys or heat treatments and other conditions must be made by comparison. Laboratory testing or coupon testing in the process stream can be used to examine alternatives to the current material, but the probes and the monitors can only provide information about the conditions which are present during the test exposure and cannot extrapolate beyond those conditions. 

If changes have been made to the process (e.g. if incoming water quality cannot be maintained or other uncertainties arise concerning the corrosion behavior of the construction materials) it is possible to incorporate coupons or probes of the material into the plant and monitor their corrosion behavior. This approach may be used to assist in the materials selection process for a replacement plant. Small coupons (typically, 25 x 50 mm) of any material may be suspended in the process stream and removed at intervals for weight loss determination and visual inspection for localized corrosion. Electrical resistance probes comprise short strands for the appropriate material electrically isolated from the item of plant. An electrical connection from each end of the probe is fed out of the plant to a control box. The box senses the electrical resistance of the probe. The probe s resistance rises as its cross-sectional area is lost through corrosion. 

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