elementtothe epoxy; cracking of glass seals because of differential expansion
in changing temperature environments; and erosion of the epoxy or glass
because of high velocities, abrasive materials in the flowstream, or both.
Flush ER probes mounted on the bottom of the line have been shown to
provide good results in asour gas–gathering system.
Because the measurement elementi sp art of the primary pressures eal
and because it is designed to corrode, ER probes have ar educed resistance
to leaking after prolonged exposure. Once the measurement elementh as
corroded through, the internals of the probe body are exposed to the process
fluid. Although materials are chosen in part for theirs trength and lacko f
permeability,experience shows that process fluids will permeate throughout
the probe packing materials. For this reason, quality probes are constructed of
corrosion-resistantb ody materials and include an outboard pressures eal,
often consisting of ahermetic glass-sealed connector. Othersealing materials
are utilizedi ns pecial cases, especially where process fluids will attack
glass (e.g., hydrofluoric acid service), and secondary seals are also used
in some probes, particularly when highly dangerous or toxic fluidsa re to
be contained.
The reference/check elements are protected from the process that the
measurement elementisd irectly exposed to.
Because of thev eryl ow resistances involved,t hese changesc an
significantly affect the metal loss. Temperature changesi nt he process
will, therefore, affect the measure elementb efore the reference and check
elementr eadings. ER probes incorporates pecial design features to
minimize the thermal resistance of the materials, insulating the reference
and check elements from the process. It should also be notedt hat
cylindrical probes are inherently better able to react to temperature changes
because of location of the reference and check elements concentrically
inside the measure element.
11.1.13 Linear Polarization Resistance
LPR is an electrochemical technique that measures the DC current ( i meas )
through the metal–fluid interface that results from polarization of one or
two electrodes of the material under study by application of as mall
electrical potential. Because i meas is related to i corr by af actor based on the
anodica nd cathodic Tafel slopes that arer elatively constantf or ag iven
metal–fluid system, and because i corr is directly proportional to corrosion
rate, LPR techniques result in instantaneous corrosion rate readings. Thisis
as ignificant advantageo ver ER or couponm onitoring in that as eries of
readings, over ap eriod of time, is required to determine corrosion rate.
LPR measurements cannot, however,b em ade in nonconductive fluids or
fluids that contain compounds that coat the electrodes (e.g., crude oil). LPR
techniques are, therefore, used most commonly in industrial water systems.
Corrosion Monitoring
541
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547
in changing temperature environments; and erosion of the epoxy or glass
because of high velocities, abrasive materials in the flowstream, or both.
Flush ER probes mounted on the bottom of the line have been shown to
provide good results in asour gas–gathering system.
Because the measurement elementi sp art of the primary pressures eal
and because it is designed to corrode, ER probes have ar educed resistance
to leaking after prolonged exposure. Once the measurement elementh as
corroded through, the internals of the probe body are exposed to the process
fluid. Although materials are chosen in part for theirs trength and lacko f
permeability,experience shows that process fluids will permeate throughout
the probe packing materials. For this reason, quality probes are constructed of
corrosion-resistantb ody materials and include an outboard pressures eal,
often consisting of ahermetic glass-sealed connector. Othersealing materials
are utilizedi ns pecial cases, especially where process fluids will attack
glass (e.g., hydrofluoric acid service), and secondary seals are also used
in some probes, particularly when highly dangerous or toxic fluidsa re to
be contained.
The reference/check elements are protected from the process that the
measurement elementisd irectly exposed to.
Because of thev eryl ow resistances involved,t hese changesc an
significantly affect the metal loss. Temperature changesi nt he process
will, therefore, affect the measure elementb efore the reference and check
elementr eadings. ER probes incorporates pecial design features to
minimize the thermal resistance of the materials, insulating the reference
and check elements from the process. It should also be notedt hat
cylindrical probes are inherently better able to react to temperature changes
because of location of the reference and check elements concentrically
inside the measure element.
11.1.13 Linear Polarization Resistance
LPR is an electrochemical technique that measures the DC current ( i meas )
through the metal–fluid interface that results from polarization of one or
two electrodes of the material under study by application of as mall
electrical potential. Because i meas is related to i corr by af actor based on the
anodica nd cathodic Tafel slopes that arer elatively constantf or ag iven
metal–fluid system, and because i corr is directly proportional to corrosion
rate, LPR techniques result in instantaneous corrosion rate readings. Thisis
as ignificant advantageo ver ER or couponm onitoring in that as eries of
readings, over ap eriod of time, is required to determine corrosion rate.
LPR measurements cannot, however,b em ade in nonconductive fluids or
fluids that contain compounds that coat the electrodes (e.g., crude oil). LPR
techniques are, therefore, used most commonly in industrial water systems.
Corrosion Monitoring
541
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547