1、 Item No. 24234 NACE International Publication 05107 This Technical Committee Report has been prepared by NACE International Task Group 321* on Corrosion Probes: Cathodic Protection Effectiveness and Soil Corrosiveness Report on Corrosion Probes in Soil or Concrete August 2007, NACE International Th
2、is NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, processes, or procedure
3、s not included in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against lia
4、bility for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefuln
5、ess of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability
6、in relation to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this report. Users of this NACE report are als
7、o responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this report. CAUTIONARY NOTICE: The
8、user is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE reports may receive current information
9、 on all NACE International publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281/228-6200).Foreword Metal weight loss coupons have been used in the past to determine the effectiveness of cathodic protection (CP) for buried st
10、ructures. More recently, electrical resistance (ER) corrosion measurement probes, typically used for internal corrosion monitoring, have been adapted for use in soil-side applications. ER probes respond with an increase in resistance as a function of metal loss in the cross section of the probe elem
11、ent and provide a measure of the cumulative metal loss in the environment. They are commonly referred to as soil corrosion probes (SCP) when used in soil-side applications. SCP are often used to determine the effectiveness of corrosion protection applied to a variety of structures such as buried pip
12、elines, underground and aboveground storage tanks, and reinforced concrete. The body of this report mainly addresses applications for ER SCP attached to buried pipelines. This report also includes information on linear polarization resistance (LPR) probes used for assessing soil corrosiveness. The u
13、se of corrosion probes for reinforced concrete structures is covered in Appendix A of this report. This report is intended for use by corrosion control personnel responsible for monitoring the condition of buried structures or those in contact with an electrolyte. This report was prepared by Task Gr
14、oup (TG) 321 Corrosion Probes: Cathodic Protection Effectiveness and Soil Corrosiveness. TG 321 is administered by Specific Technology Group (STG) 05 on Cathodic/Anodic Protection and is sponsored by STG 35 on Pipelines, Tanks, and Well Casings. This report is issued by NACE under the auspices of ST
15、G 05. _ *Chair Naeem Khan, Saudi Aramco, Dhahran, Saudi Arabia. NACE International 2 Section 1: General Structure-to-soil potential measurements to determine the effectiveness of CP is sometimes prone to error due to unstable reference cell contact or high soil IR drops. In certain soil environments
16、, effective CP is achieved even at potentials less negative than the applicable criteria (on or off potentials, 100-mV polarization, etc.) because of the low corrosiveness of the soil and its chemistry. CP potential coupons are used to assess the adequacy of CP on pipelines through coupon potential
17、measure-ments.1,2,3 These measurements include coupon off and depolarization potential measurements used for alternative criteria mentioned in NACE SP0169,4 such as 850-mV off or 100-mV cathodic polarization. CP potential coupons are also used in situations in which it is not possible to interrupt m
18、ultiple CP sources or on structures with direct connected galvanic anodes. As in the case of pipeline internal corrosion rate measurements, metal weight loss coupons have been used for soil-side applications in the past to determine the effectiveness of CP. More recently, ER corrosion measurement pr
19、obes,5,6 typically used for internal corrosion monitoring, have been manufactured specifically for use in soil-side applications. ER probes may be a better indicator of CP effectiveness in areas affected by dynamic stray traction current, telluric current-affected pipelines, and also in areas of alt
20、ernating current (AC) induction and AC-induced corrosion (ACIC). LPR-type probes measure corrosion rates of metals, but only without CP. The fundamental theory of the validity of LPR requires that the measurements be made at the free-corroding potential of the metal. The measurement technique is inv
21、alidated as soon as CP current is applied. In addition, because these probes make electrochemical measurements, they typically need a sufficiently conductive medium in which to make the measurements. Section 2: Definitions AC-Induced Corrosion (ACIC): Corrosion usually resulting from AC voltages ind
22、uced onto the pipeline where the pipeline route is parallel with or crosses high-voltage powerlines, or electrified railways. Corrosion Potential (Ecorr): The potential of a corroding surface in an electrolyte relative to a reference electrode under open-circuit conditions (also known as rest potent
23、ial, open-circuit potential, or freely corroding potential). Corrosion Rate: The rate at which corrosion proceeds. Coupon: A metal specimen made of similar material as the structure under investigation. Coupon-to-Electrolyte Potential: The potential difference between the surface of a buried or subm
24、erged coupon and the electrolyte that is measured with reference to an electrode in contact with the electrolyte (for buried coupons, typically measured with the electrode in an access tube that restricts contact with the electrolyte to the vicinity of the coupon). CP Potential Coupon: The coupon co
25、nnected to the structure being protected by CP. Depolarized Potential: The steady-state potential that the CP potential coupon reaches some time after disconnecting from the structure. Electrical Resistance (ER) Instrument: An electrical resistance instrument that measures ER probe resistances and c
26、onverts the measurements to metal loss. ER Probe Element: Metallic part of an ER probe whose electrical resistance is to be measured for the corrosion rate estimation. ER Soil Corrosion Probe: An ER corrosion measurement probe that has been adapted for use in soil-side applications. ER probes measur
27、e a change in the resistance as a function of metal loss in the cross section of a probe, and indicate the cumulative metal loss of the sample. The resistance is measured relative to a reference element, to compensate for resistance changes due to temperature. The probe elements are generally made o
28、f the same alloy as the pipework, vessel, or structure to be monitored. Free Corrosion Potential: See Corrosion Potential. Galvanic Anode: A metal that provides sacrificial protection to another metal that is more noble when electrically coupled in an electrolyte. This type of anode is the electron
29、source in one type of CP. Holiday: A discontinuity in a protective coating that exposes unprotected surface to the environment. NACE technical committee reports are intended to convey technical information or state-of-the-art knowledge regarding corrosion. In many cases, they discuss specific applic
30、ations of corrosion mitigation technology, whether considered successful or not. Statements used to convey this information are factual and are provided to the reader as input and guidance for consideration when applying this technology in the future. However, these statements are not intended to be
31、 recommendations for general application of this technology, and must not be construed as such. NACE International 3 Impressed Current: An electric current supplied by a device employing a power source that is external to the electrode system. (An example is direct current for CP.) Instant-Disconnec
32、t Potential: The coupon-to-electrolyte potential made immediately after disconnecting the coupon from the structure. Instant-Off Potential: The polarized half-cell potential of an electrode taken without perceptible delay after the CP current is stopped, which closely approximates the potential with
33、out IR drop (i.e., the polarized potential) when the current was on. IR Drop: The voltage across a resistance in accordance with Ohms law. Linear Polarization Resistance (LPR) Probe: Determines the corrosion rate on its metal electrode or electrodes by measuring the polarization resistance under the
34、 application of a small applied potential, typically 10 to 20 mV, above (more positive) the free-corroding potential of the electrode or electrodes. Polarization resistance is defined as the slope (dE/di) at the free-corroding potential of a potential E against current density i curve. By applying o
35、nly a small potential, the linear polarization resistance slope E/i is close to the true polarization resistance dE/di. Note that these assumptions and measurements are not valid under applied CP potentials. The probe elements are generally made of the same alloy as the pipework, vessel, or structur
36、e to be monitored. Native Potential: See Corrosion Potential. Polarization: The change from the open-circuit potential as a result of current across the electrode/electrolyte interface. Polarized Potential: The potential across the structure/electrolyte interface that is the sum of the corrosion pot
37、ential and the cathodic polarization. Reference Electrode: An electrode whose open-circuit potential is constant under similar conditions of measurement, which is used for measuring the relative potentials of other electrodes. Reference Tube: See Soil Access Tube. Shielding: Preventing or diverting
38、CP current from its natural path. Soil Access Tube: A tube that is nonconductive and impermeable to moisture (polyvinyl chloride PVC, polyethylene, polycarbonate) used in conjunction with a soil corrosion probe or CP potential coupon, and filled with electrolyte. Also known as a reference tube. Soil
39、 Corrosion Probe (SCP): See ER Soil Corrosion Probe or LPR Probe. Structure-to-Electrolyte Potential: The potential dif-ference between the surface of a buried or submerged metallic structure and the electrolyte that is measured with reference to an electrode in contact with the electrolyte. Section
40、 3: Types of Corrosion Probes SCP fall into two categories: (a) ER-type metal loss probes; and (b) LPR-type electrochemical corrosion rate probes. The ER-type probes are the most widely used because they measure metal loss with or without CP applied. ER Probes Several considerations relate to the ap
41、plicability of an ER probe: (a) The probe element material; (b) The size of the probe element or electrode; (c) Contact with the soil; (d) Installation; and (e) Probe life. Probe Element Material The probe element material normally matches that of the pipeline as closely as practicable, e.g., carbon
42、 steel or ductile iron.7 An exact match of the probe element to the carbon steel grade of the pipe or its strength is not typical, because the corrosion characteristics under CP generally vary negligibly with carbon content or strength of the steel. Size of the Probe Element The size of the probe el
43、ement is designed in a similar fashion to CP potential coupons,1 i.e., they are suitably sized metal samples simulating a holiday in the pipeline coating. In the case of ER probes, they are connected to the CP circuit at the test location, thereby providing them with a similar level of CP as the pip
44、eline. In addition, a second probe that is not connected to the CP system is often used to monitor the unprotected corrosion rate that would occur if the CP were not present or if the CP system failed. For the sizing of the ER probe element, the same principles apply as for CP potential coupons, con
45、sidering that the NACE International 4 probe is also often used as a CP potential coupon. Thus, the size of the probe is generally representative of the possible coating defect sizes of the pipeline under investigation. The probe is typically as representative of the structure as possible. As the pr
46、obe is located in the same soil environment as the protected structure, it is assumed that it replicates the effectiveness of the CP applied to the structure. A larger probe element is often used when the environment is non-homogenous in order to obtain a larger statistical result. If AC induction/i
47、nterference is an issue, the probe size approximates the expected defect size, so that any size effect is the same on the probe as on the defect it is simulating. Contact with the Soil The ER probe is typically installed near the pipe or structure in the same soil/electrolyte or environment as the p
48、ipe or structure, and the entire surface of the probe element typically maintains contact with the soil or environment. Loss of contact due to shrinkage, freezing, or drying out of the soil results in loss of the CP protection, as it also does on the pipe. Installation The ER probe is installed clos
49、e to the pipe or structure in the same environment/electrolyte as the pipe or structure and the same distance from the anode, so that the probe sees the same potential field as the structure. This is typically not a problem on a pipeline where the anode is some distance from the pipeline. It is much more critical in situations in which the anode is relatively close to the pipeline or structure, such as in double-skinned tanks with an anode grid, or inside water tanks where the potential gradients between the anode and the structure are steep because
