1、 Guidance Notes on Cathodic Protection of Ships GUIDANCE NOTES ON CATHODIC PROTECTION OF SHIPS DECEMBER 2017 American Bureau of Shipping Incorporated by Act of Legislature of the State of New York 1862 2017 American Bureau of Shipping. All rights reserved. ABS Plaza 16855 Northchase Drive Houston, T
2、X 77060 USA Foreword Foreword Protective coatings are the most efficient way to protect ship steel structures from corrosion. However, cathodic protection, often in conjunction with protective coatings, is also used to protect immersed parts of bare steel surfaces (including coating damaged areas) f
3、rom corrosion. This includes the external hull surface and the internal surfaces of tanks, such as ballast tanks. Cathodic protection (CP) can be Impressed Current Cathodic Protection, Galvanic Anode Cathodic Protection or a combination of both. Cathodic protection controls corrosion by supplying di
4、rect current to the immersed surface of the structure, thus making the structure a cathode of a cell. The external hull of a ship is exposed to different waters with differing chemistries, which have a profound influence on the cathodic protection. These Guidance Notes on Cathodic Protection of Ship
5、s are developed to provide guidelines for ship CP design, installation, and maintenance. It is a common practice for a ship to have cathodic protection systems installed during its new construction. These Guidance Notes become effective on the first day of the month of publication. Users are advised
6、 to check periodically on the ABS website www.eagle.org to verify that this version of these Guidance Notes is the most current. We welcome your feedback. Comments or suggestions can be sent electronically by email to rsdeagle.org. Terms of Use The information presented herein is intended solely to
7、assist the reader in the methodologies and/or techniques discussed. These Guidance Notes do not and cannot replace the analysis and/or advice of a qualified professional. It is the responsibility of the reader to perform their own assessment and obtain professional advice. Information contained here
8、in is considered to be pertinent at the time of publication, but may be invalidated as a result of subsequent legislations, regulations, standards, methods, and/or more updated information and the reader assumes full responsibility for compliance. This publication may not be copied or redistributed
9、in part or in whole without prior written consent from ABS. ii ABSGUIDANCE NOTES ON CATHODIC PROTECTION OF SHIPS .2017 Table of Contents GUIDANCE NOTES ON CATHODIC PROTECTION OF SHIPS CONTENTS SECTION 1 General 1 1 Scope 1 2 Materials 1 3 Personnel 1 4 Normative References 2 5 Terms and Definitions
10、. 2 SECTION 2 Design Criteria and Recommendations . 3 1 General . 3 2 Design Life of Cathodic Protection Systems . 3 3 Cathodic Protection Potential . 3 3.1 Cathodic Protection Potential Criteria 3 3.2 Detrimental Effects from Cathodic Protection 4 3.3 Potential Measurements 5 3.4 Reference Electrod
11、e 6 3.5 Factors Affecting CP Potential . 7 4 Design Current 8 4.1 General 8 4.2 Structure Subdivision and Surface Area Calculations . 8 4.3 Design Current Density for Bare Steel 10 4.4 Design Current Density for Coated Steel. 10 4.5 Current Demand 11 5 Circuit Resistance . 12 6 Anode Resistance Calc
12、ulations 13 6.1 For Slender Anodes Mounted at Least 0.3 m (11.8 in.) Offset from the Structure Steel Surface . 13 6.2 Long Flush Mounted Anodes on the Structure Steel Surface where Length 4 Width 14 6.3 Short Flat Plate Mounted Flush on the Structure Steel Surface where Length 690 MPa or hardness 35
13、0 HV) 0.80 0.83 to 0.95 (1)Austenitic stainless steel for aerobic and anaerobic conditions NPRE = % Cr + 3.3% (Mo + 0.5W) + 16% N 0.30 for NPRE 40 (2)1.10 0.60 NPRE 690 MPa (100 ksi) or hardness 350 HV, it has been the practice to use potentials in the range of 0.80 V to 0.95 V (Ag/AgCl/seawater ref
14、erence electrode). For high-strength steels susceptible to hydrogen-induced stress cracking (HISC), the maximum negative potential should be more positive (less negative) than 0.83 V (Ag/AgCl/seawater reference electrode). ABSGUIDANCE NOTES ON CATHODIC PROTECTION OF SHIPS .2017 4 Section 2 Design Cr
15、iteria and Recommendations 3.2.4 Austenitic Stainless Steels and Nickel-Based Alloys Austenitic stainless steels and nickel-based alloys in the solution annealed condition are generally considered immune to HISC for all practical applications. Moderate cold work would not induce HISC sensitivity of
16、these materials except for UNS S30200 (AISI 302) and UNS S30400 (AISI 304) stainless steels. The same applies for welding or hot forming according to an appropriate procedure. For certain nickel-based alloys, precipitation hardening may induce increased sensitivity to HISC. For precipitation hardene
17、d austenitic stainless steels, the susceptibility is low and a hardness of maximum 300 HV may be considered a reasonably safe limit, while component materials with a hardness higher than 350 HV should generally not receive CP. In the intermediate hardness range (i.e., 300 to 350 HV), precautions sho
18、uld be taken during design to avoid local yielding and/or to specify a qualified coating system for eliminating hydrogen absorption by CP. The qualified coating system should resist CP disbonding during service. 3.2.5 Ferritic and Ferritic-Pearlitic Steels (Stainless Steels) Ferritic and ferritic-pe
19、arlitic structural steels with specified minimum yield strength (SMYS) up to 500 MPa (72 ksi) have proven in practice to have compatibility with marine CP systems. However, laboratory testing has demonstrated that steels which have passed their yield point have susceptibility to HISC. It is recommen
20、ded that all welds should have 350 HV of hardness as an absolute upper limit. Within the range of 300 to 350 HV hardness, precautions should be applied during design to avoid local yielding and/or to specify a qualified coating system for eliminating hydrogen absorption by CP. Again, the qualified c
21、oating system should resist CP disbonding. 3.2.6 Martensitic Carbon, Low-alloy and Stainless Steels For martensitic carbon, low-alloy, and stainless steels, failures of CP-induced HISC have occurred in materials with a yield stress of 690 MPa (100 ksi) and a hardness of 350 HV. It is widely recogniz
22、ed that untempered martensite is prone to HISC. Welds susceptible to martensite formation should receive post-welding heat treatment (PWHT) so as to reduce heat-affected zone (HAZ) hardness and residual stresses from welding. The same recommendations for hardness limits and design consideration for
23、ferritic steels above apply. 3.2.7 Ferritic-austenitic (“Duplex”) Stainless Steels Ferritic-austenitic (“duplex”) stainless steels should be regarded as potentially susceptible to HISC, independent of SMYS typically 400 to 550 MPa (58 to 80 ksi) or specified maximum hardness. Welding may cause incre
24、ased HISC susceptibility in the weld metal and in the HAZ adjacent to the fusion line. This is related to an increased ferrite content rather than hardness. Qualification of welding should therefore prove that the maximum ferrite content in the weld metal and the inner HAZ (about 0.1 mm (0.004 in.)
25、wide) can be efficiently controlled; contents of maximum 60 to 70% are typically specified. Forgings are more prone to HISC than wrought materials due to the coarse microstructure allowing HISC to propagate preferentially in the ferrite phase. Cold bent pipes with small diameters uncoated and with m
26、echanical connections (i.e., no welding) have a proven record for CP compatibility when used as production control piping for subsea installations. Design precautions should include avoiding local plastic yielding and use of qualified coating systems. 3.2.8 Copper- and Aluminum-Based Alloys Copper-
27、and aluminum-based alloys are generally considered immune to HISC, regardless of fabrication modes. 3.3 Potential Measurements The protection criteria and effectiveness of cathodic protection systems should be confirmed by direct measurement of the structure potential. However, visual observations o
28、f progressive coating deterioration and/or corrosion are indicators of possible inadequate protection. Steel plate thickness gauging can also indicate deficiencies in corrosion protection. Potential measurements should be made with the reference electrode and a high impedance (minimum 10 M). The ref
29、erence electrode should be located in the seawater as close as practicable to the ships hull to minimize voltage drops. The following should be considered when measuring the hull potentials: ABSGUIDANCE NOTES ON CATHODIC PROTECTION OF SHIPS .2017 5 Section 2 Design Criteria and Recommendations i) Po
30、tential measurement techniques can be referred to EN 13509 “Cathodic Protection Measurement Techniques”. ii) In areas of greatest shielding and/or in brackish waters, particular attention should be given in evaluating the protective level of ship hulls. iii) Changes in water resistivity from causes
31、such as freshwater flow from a river or temperature variation can affect voltage level. iv) In impressed current systems under conditions involving high-resistivity water or high current density, the voltage drop may be excessive. Instant-off potential measurements can provide useful information by
32、eliminating voltage drop in the water by switching off the direct current (DC). v) In conventionally designed galvanic anode protection systems, current-off readings are not possible. However, the included voltage drop is generally not significant in ordinary seawater if the reference electrode is p
33、laced close to the structure. The voltage drop may become significant in brackish waters. In such cases, it may be necessary to use interruptible coupons or other IR correction techniques (see NACE SP0169) to determine the true potential of the metal surface. When permanent measurement electrodes ar
34、e installed, they should be at locations representative of the most negative and the most positive potentials on the external hull surface. While the exact electrode locations are known, the information obtained from these electrodes is limited to the adjacent structure surfaces. Although this limit
35、ation holds true for any potential measurement, this method can provide a reproducible basis for comparing potentials at different times. The accuracy of permanent electrodes should be periodically checked against another electrode. Dual reference electrodes that combine zinc and Ag/AgCl/seawater re
36、ferences into a single, permanently-installed unit also help to detect/reduce malfunctions. In the case of impressed current systems, reference electrodes should be fitted to the structure at suitable locations in order to automatically control the output of the anodes and maintain the polarization
37、of critical areas within the set limits. If this measurement circuit remains permanently connected, care should be taken that it does not draw excessive current (including when idle) from the reference electrode, which may become polarized and give false indications. The use of portable coupons and
38、electrical resistance probes can be considered for commissioning to allow readings to be made in areas where there is no permanent reference electrode installation. They can be particularly useful when establishing the effectiveness of the dielectric shields. In addition to reference electrodes, som
39、e structures are equipped with permanent monitors to measure current density and current output from representative galvanic anodes. These devices are particularly useful when assessing new structure designs or new environments in which precise CP design criteria are not available. These devices typ
40、ically use calibrated shunts to arrive at the current output or current density value. Signals are usually transmitted topside using hard-wired connections. 3.4 Reference Electrode The polarized potential of the steel surface in its environment (such as seawater) is measured relative to the potentia
41、l of a reference electrode. Consequently, no equipment item is of more fundamental importance to the cathodic protection (CP) system than the reference electrode. The most commonly-used reference electrode in marine environments is the solid junction (SJ) silver/silver chloride (SSC) electrode in di
42、rect contact with seawater. However, the potentials of a SJ Ag/AgCl/seawater electrode can be affected by the chloride concentration and temperature of the seawater in which it is immersed. It is also affected by light (photo-sensitive) and contaminants in its environments. These are most likely to
43、affect reference electrodes which operate in a variety of environments and permanent electrodes on ships sailing in different waters (seawater, brackish, and fresh water). The reference potential will vary as the logarithm of the concentration of active species, which is especially significant in dr
44、y electrodes. This effect is minimized by the use of wet electrodes. 6 ABSGUIDANCE NOTES ON CATHODIC PROTECTION OF SHIPS .2017 Section 2 Design Criteria and Recommendations Temperature has direct and indirect effects on the reference electrode. The direct effect is a linear variation of potential wi
45、th temperature provided with temperature coefficient mV/C (mV/F). The indirect effect is a function of the increased quantity of salt able to be dissolved at higher temperatures. These temperature effects are large enough to produce a significant error in potential measurements if left uncompensated
46、. The effect of temperature on reference electrode potential can be calculated by the following equation: Et= ESHE at 25C+ kt(T 25C) Et= ESHE at 77F+ kt(T 77F) where Et= reference potential at temperature, T ESHE at 25C= reference potential at 25C (see Section 2, Table 2) ESHE at 77F= reference pote
47、ntial at 77F (see Section 2, Table 2) kt= temperature coefficient, in mV/C (mV/F) T = actual temperature, in C (F) Other standard reference electrodes may be substituted for the Ag/AgCl (seawater) with their protection potential equivalent to 800 mV referred to an Ag/AgCl (seawater): Saturated coppe
48、r/copper sulfate reference electrode (CSE saturated CuSO4): 860 V (or more negative) for protection. This electrode is not stable for long-term immersion service. High-purity zinc electrode TABLE 2 Common Reference Electrodes and Their Potentials and Temperature Coefficients Electrode Potential Rela
49、tive to SHE at 25C (77F) mV Protection Potential Reading at 25C (77F) mV Temperature Coefficient ktmV/C (mV/F) Typical Usage CSE (Cu/CuSO4/Saturated CuSO4) +316 860 +0.90 (+0.5) Soil, fresh water SSC (SJ Ag/AgCl/0.6M (3.5%) NaCl) +256 800 0.33 (0.18) Seawater, brackish SSC (LJ Ag/AgCl/0.5M KCl) +256 800 - - SSC (LJ Ag/AgCl/KCl saturated) +199 743 0.70 (0.39) - SCE (Saturated Calomel Electrode) +0.244 788 0.70 (0.39) Water, laboratory Steel protection potential 544 - - - ZRE (Zinc Reference Electrode) 780
