677:
465:(CP) works by introducing another metal (the galvanic anode) with a much more anodic surface, so that all the current will flow from the introduced anode and the metal to be protected becomes cathodic in comparison to the anode. This effectively stops the oxidation reactions on the metal surface by transferring them to the galvanic anode, which will be sacrificed in favour of the structure under protection. More simply put, this takes advantage of the relatively low stability of magnesium, aluminum or zinc metals; they dissolve instead of iron because their
814:
48:
515:) and is more suitable for areas where the electrolyte (soil or water) resistivity is higher. This is usually on-shore pipelines and other buried structures, although it is also used on boats in fresh water and in water heaters. In some cases, the negative potential of magnesium can be a disadvantage: if the potential of the protected metal becomes too negative, reduction of water or solvated protons may evolve hydrogen atoms on the cathode surface, for instance according to
40:
482:
645:(the oxide layer formed shields from further oxidation); if this happens, current may cease to flow and the anode stops working. Zinc has a relatively low driving voltage, which means in higher-resistivity soils or water it may not be able to provide sufficient current. However, in some circumstances — where there is a risk of
688:
The primary calculation is how much anode material will be required to protect the structure for the required time. Too little material may provide protection for a while, but need to be replaced regularly. Too much material would provide protection at an unnecessary cost. The mass in kg is given by
637:
Zinc and aluminium are generally used in salt water, where the resistivity is generally lower and magnesium dissolves relatively quickly by reaction with water under hydrogen evolution (self-corrosion). Typical uses are for the hulls of ships and boats, offshore pipelines and production platforms, in
739:
The arrangement of the anodes is then planned so as to provide an even distribution of current over the whole structure. For example, if a particular design shows that a pipeline 10 kilometres (6.2 mi) long needs 10 anodes, then approximately one anode per kilometre would be more effective than
821:
As the anode materials used are generally more costly than iron, using this method to protect ferrous metal structures may not appear to be particularly cost effective. However, consideration should also be given to the costs incurred to repair a corroded hull or to replace a steel pipeline or tank
735:
Once the required mass of material is known, the particular type of anode is chosen. Differently shaped anodes will have a different resistance to earth, which governs how much current can be produced, so the resistance of the anode is calculated to ensure that sufficient current will be available.
652:
Aluminium anodes have several advantages, such as a lighter weight, and much higher capacity than zinc. However, their electrochemical behavior is not considered as reliable as zinc, and greater care must be taken in how they are used. Aluminium anodes will passivate where chloride concentration is
457:
As the metal continues to corrode, the local potentials on the surface of the metal will change and the anodic and cathodic areas will change and move. As a result, in ferrous metals, a general covering of rust is formed over the whole surface, which will eventually consume all the metal. This is
726:
Anode capacity is an indication of how much material is consumed as current flows over time. The value for zinc in seawater is 780 Ah/kg but aluminium is 2000 Ah/kg, which reflects the lower atomic mass of aluminium and means that, in theory, aluminium can produce much more current per
840:
The high pressure die-casting process for sacrificial anodes is widespread. It is a fully automated machine process. In order for the manufacturing process to run reliably and in a repeatable manner, a modification of the processed sacrificial anode alloy is required. Alternatively, the gravity
722:
The utilisation factor (UF) of the anode is a constant value, depending on the shape of the anode and how it is attached, which signifies how much of the anode can be consumed before it ceases to be effective. A value of 0.8 indicates that 80% of the anode can be consumed, before it should be
472:
For this protection to work there must be an electron pathway between the anode and the metal to be protected (e.g., a wire or direct contact) and an ion pathway between both the oxidizing agent (e.g., oxygen and water or moist soil) and the anode, and the oxidizing agent and the metal to be
449:
As corrosion takes place, oxidation and reduction reactions occur and electrochemical cells are formed on the surface of the metal so that some areas will become anodic (oxidation) and some cathodic (reduction). Electrons flow from the anodic areas into the electrolyte as the metal corrodes.
667:
Since the operation of a galvanic anode relies on the difference in electropotential between the anode and the cathode, practically any metal can be used to protect some other, providing there is a sufficient difference in potential. For example, iron anodes can be used to protect copper.
731:
The amount of current required corresponds directly to the surface area of the metal exposed to the soil or water, so the application of a coating drastically reduces the mass of anode material required. The better the coating, the less anode material is needed.
841:
casting process is used for the production of the sacrificial anodes. This process is performed manually or partially automated. The alloy does not have to be adapted to the manufacturing process, but is designed for 100% optimum corrosion protection.
684:
The design of a galvanic anode CP system should consider many factors, including the type of structure, the resistivity of the electrolyte (soil or water) it will operate in, the type of coating and the service life.
473:
protected, thus forming a closed circuit; therefore simply bolting a piece of active metal such as zinc to a less active metal, such as mild steel, in air (a poor ionic conductor) will not furnish any protection.
431:
297:
612:
825:
However, there is a limit to the cost effectiveness of a galvanic system. On larger structures, such as long pipelines, so many anodes may be needed that it would be more cost-effective to install
104:), where electrons leave the metal (and the metal dissolves, i.e. actual loss of metal results) and reduction, where the electrons are used to convert oxygen and water to hydroxide ions (equation
179:
86:) than the metal of the structure. The difference in potential between the two metals means that the galvanic anode corrodes, in effect being "sacrificed" in order to protect the structure.
945:
723:
replaced. A long slender stand off anode (installed on legs to keep the anode away from the structure) has a UF value of 0.9, whereas the UF of a short, flush mounted anode is 0.8.
634:
or to disbonding of the coating. Where this is a concern, zinc anodes may be used. An aluminum-zinc-tin alloy called KA90 is commonly used in marine and water heater applications.
450:
Conversely, as electrons flow from the electrolyte to the cathodic areas, the rate of corrosion is reduced. (The flow of electrons is in the opposite direction of the flow of
949:
736:
If the resistance of the anode is too high, either a differently shaped or sized anode is chosen, or a greater quantity of anodes must be used.
664:
spark may be generated, so its use is restricted in tanks where there may be explosive atmospheres and there is a risk of the anode falling.
826:
327:
837:
The basic method is to produce sacrificial anodes through a casting process. However, two casting methods can be distinguished.
202:
523:
988:
de Rincon, O.; Sanchez, M.; Salas, O.; Romero, G.; Palacios, C.; Basile, J.; Suarez, J.; de Romero, M.; Zamora, R. (2010),
638:
salt-water-cooled marine engines, on small boat propellers and rudders, and for the internal surface of storage tanks.
727:
weight than zinc before being depleted and this is one of the factors to consider when choosing a particular material.
1078:
1068:
1058:
508:. They are all available as blocks, rods, plates or extruded ribbon. Each material has advantages and disadvantages.
118:
17:
798:
Where large arrays are used, wiring is needed due to high current flow and need to keep resistance losses low.
1104:
760:
Lower voltages and current mean that risk of causing stray current interference on other structures is low.
641:
Zinc is considered a reliable material, but is not suitable for use at higher temperatures, as it tends to
676:
1084:
989:
795:
Where D.C. power is available, electrical energy can be obtained more cheaply than by galvanic anodes.
1109:
804:
To retain effectiveness, the anodes must be inspected and/or replaced as part of normal maintenance.
458:
rather a simplified view of the corrosion process, because it can occur in several different forms.
642:
466:
785:
Often requires that the protected structure be electrically isolated from other structures and
646:
631:
990:"Comparative behavior of sacrificial anodes based on Mg, Zn, and Al alloys in brackish water"
1053:
A. W. Peabody, Peabody's
Control of Pipeline Corrosion, 2nd ed., 2001, NACE International.
994:
Comparative
Behavior of Sacrificial Anodes Based on Mg, Zn, and Al Alloys in Brackish Water
792:
Anodes are heavy and will increase water resistance on moving structures or pipe interiors.
786:
469:
is weaker compared to iron, which is bonded strongly via its partially filled d-orbitals.
8:
769:
Once installed, testing the system components is relatively simple for trained personnel.
462:
83:
79:
64:
813:
801:
Anodes must be carefully placed to avoid interfering with water flow into the propeller.
680:
Galvanic anode on a submarine. It is the light stripe on the casing near the tailplanes.
649:, for example — this lower voltage is advantageous, as overprotection is avoided.
94:
In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism (a
850:
31:
701:
Mass = (Current
Required x Design Life x 8760) ÷ (Utilisation Factor x Anode Capacity)
1074:
1073:
Baeckmann, Schwenck, Prinz. Handbook of
Cathodic Corrosion Protection, 3rd ed. 1997.
1064:
1054:
782:
Lower driving voltage means the anodes may not work in high-resistivity environments.
654:
316:
779:
Current capacity limited by anode mass and self consumption at low current density.
451:
946:"80251 KA90 Aluminum Alloy Anodes in Hot and Cold Seawater and Brine Environments"
98:). During corrosion of iron or steel there are two reactions, oxidation (equation
512:
1038:
95:
1098:
660:
One disadvantage of aluminium is that if it strikes a rusty surface, a large
47:
51:
The bright rectangular objects on these ship components are galvanic anodes.
315:
In most environments, the hydroxide ions and ferrous ions combine to form
1083:
Det Norske
Veritas Recommended Practice for Cathodic Protection Design
822:
because their structural integrity has been compromised by corrosion.
497:
68:
39:
763:
Require less frequent monitoring than impressed current CP systems.
661:
511:
Magnesium has the most negative electropotential of the three (see
501:
817:
A galvanic anode (sacrificial anode) on a yacht's propeller shaft
75:
67:
system used to protect buried or submerged metal structures from
426:{\displaystyle {\ce {Fe^2+(aq) + 2OH- (aq) -> Fe(OH)2(s)}}}
489:
486:
481:
987:
505:
542:
408:
292:{\displaystyle {\ce {O2 + 2 H2O + 4e- -> 4 OH- (aq)}}}
237:
217:
1037:
Quality aspects in the production of sacrificial anodes
607:{\displaystyle {\ce {2 H2O + 2e- -> 2H + 2OH- (aq)}}}
492:, showing a newly blacked hull and new magnesium anodes.
43:
Detailed view of a galvanic anode on the hull of a ship
1063:
Shreir L. L. et al., Corrosion Vol. 2, 3rd ed., 1994,
74:
They are made from a metal alloy with a more "active"
526:
496:
There are three main metals used as galvanic anodes:
330:
205:
121:
319:, which eventually becomes the familiar brown rust:
832:
740:putting all 10 anodes at one end or in the centre.
719:The design life is in years (1 year = 8760 hours).
606:
425:
291:
173:
1096:
174:{\displaystyle {\ce {Fe -> Fe^2+(aq) + 2e-}}}
743:
1039:https://opferanode24.de/en/interesting-facts/
981:
581:
571:
554:
531:
362:
266:
249:
226:
159:
931:
929:
812:
675:
671:
480:
46:
38:
1031:
14:
1097:
1024:
1022:
1020:
1018:
972:
766:Relatively low risk of overprotection.
63:, is the main component of a galvanic
926:
827:impressed current cathodic protection
808:
27:Main component of cathodic protection
863:
697:
517:
321:
196:
112:
1015:
948:. ASM International. Archived from
754:No external power sources required.
24:
476:
25:
1121:
1090:
969:Baeckmann, Schwenck, Prinz. p.185
833:Production of sacrificial anodes
773:
1006:
963:
938:
917:
908:
899:
890:
881:
872:
599:
593:
565:
418:
412:
398:
392:
384:
380:
374:
352:
346:
284:
278:
260:
149:
143:
126:
13:
1:
1047:
748:
744:Advantages and disadvantages
7:
844:
757:Relatively easy to install.
708:
691:
620:
461:Prevention of corrosion by
439:
305:
187:
106:
100:
10:
1126:
29:
89:
856:
30:Not to be confused with
818:
681:
647:hydrogen embrittlement
632:hydrogen embrittlement
608:
493:
427:
293:
175:
52:
44:
816:
679:
672:Design considerations
609:
484:
428:
294:
176:
50:
42:
1105:Corrosion prevention
524:
328:
203:
119:
544:
463:cathodic protection
410:
239:
219:
84:oxidation potential
80:reduction potential
65:cathodic protection
996:, NACE, p. 15
851:Galvanic corrosion
819:
809:Cost effectiveness
682:
604:
532:
494:
485:A steel wide-beam
423:
390:
289:
227:
207:
171:
53:
45:
32:Bleeder (resistor)
716:
715:
655:parts per million
628:
627:
598:
585:
574:
558:
547:
535:
447:
446:
417:
397:
389:
379:
366:
351:
335:
317:ferrous hydroxide
313:
312:
283:
270:
253:
242:
230:
210:
195:
194:
163:
148:
132:
125:
61:sacrificial anode
18:Sacrificial anode
16:(Redirected from
1117:
1110:Electrochemistry
1085:DNV RP-B401-2005
1041:
1035:
1029:
1028:DNV RP-B401-2005
1026:
1013:
1010:
1004:
1003:
1002:
1001:
985:
979:
976:
970:
967:
961:
960:
958:
957:
942:
936:
933:
924:
921:
915:
912:
906:
903:
897:
894:
888:
885:
879:
876:
870:
867:
710:
698:
622:
613:
611:
610:
605:
603:
602:
596:
591:
590:
583:
572:
564:
563:
556:
545:
543:
540:
533:
518:
452:electric current
441:
432:
430:
429:
424:
422:
421:
415:
409:
406:
401:
395:
387:
383:
377:
372:
371:
364:
355:
349:
344:
343:
333:
322:
307:
298:
296:
295:
290:
288:
287:
281:
276:
275:
268:
259:
258:
251:
240:
238:
235:
228:
218:
215:
208:
197:
189:
180:
178:
177:
172:
170:
169:
168:
161:
152:
146:
141:
140:
130:
123:
113:
82:/ more positive
21:
1125:
1124:
1120:
1119:
1118:
1116:
1115:
1114:
1095:
1094:
1093:
1050:
1045:
1044:
1036:
1032:
1027:
1016:
1011:
1007:
999:
997:
986:
982:
977:
973:
968:
964:
955:
953:
944:
943:
939:
934:
927:
922:
918:
913:
909:
904:
900:
895:
891:
886:
882:
877:
873:
868:
864:
859:
847:
835:
811:
776:
751:
746:
674:
592:
586:
582:
559:
555:
541:
536:
527:
525:
522:
521:
513:galvanic series
479:
477:Anode materials
411:
407:
402:
391:
373:
367:
363:
345:
336:
332:
331:
329:
326:
325:
277:
271:
267:
254:
250:
236:
231:
216:
211:
206:
204:
201:
200:
164:
160:
142:
133:
129:
122:
120:
117:
116:
92:
78:(more negative
35:
28:
23:
22:
15:
12:
11:
5:
1123:
1113:
1112:
1107:
1092:
1091:External links
1089:
1088:
1087:
1081:
1071:
1061:
1049:
1046:
1043:
1042:
1030:
1014:
1005:
980:
971:
962:
937:
925:
916:
907:
898:
889:
880:
871:
861:
860:
858:
855:
854:
853:
846:
843:
834:
831:
810:
807:
806:
805:
802:
799:
796:
793:
790:
783:
780:
775:
772:
771:
770:
767:
764:
761:
758:
755:
750:
747:
745:
742:
729:
728:
724:
720:
714:
713:
704:
702:
673:
670:
626:
625:
616:
614:
601:
595:
589:
580:
577:
570:
567:
562:
553:
550:
539:
530:
478:
475:
445:
444:
435:
433:
420:
414:
405:
400:
394:
386:
382:
376:
370:
361:
358:
354:
348:
342:
339:
311:
310:
301:
299:
286:
280:
274:
265:
262:
257:
248:
245:
234:
225:
222:
214:
193:
192:
183:
181:
167:
158:
155:
151:
145:
139:
136:
128:
96:redox reaction
91:
88:
57:galvanic anode
26:
9:
6:
4:
3:
2:
1122:
1111:
1108:
1106:
1103:
1102:
1100:
1086:
1082:
1080:
1079:0-88415-056-9
1076:
1072:
1070:
1069:0-7506-1077-8
1066:
1062:
1060:
1059:1-57590-092-0
1056:
1052:
1051:
1040:
1034:
1025:
1023:
1021:
1019:
1009:
995:
991:
984:
975:
966:
952:on 2022-01-04
951:
947:
941:
935:Schreir 10:44
932:
930:
920:
911:
902:
896:Peabody p. 21
893:
884:
875:
866:
862:
852:
849:
848:
842:
838:
830:
828:
823:
815:
803:
800:
797:
794:
791:
788:
784:
781:
778:
777:
774:Disadvantages
768:
765:
762:
759:
756:
753:
752:
741:
737:
733:
725:
721:
718:
717:
712:
705:
703:
700:
699:
696:
694:
693:
686:
678:
669:
665:
663:
658:
656:
650:
648:
644:
639:
635:
633:
624:
617:
615:
587:
578:
575:
568:
560:
551:
548:
537:
528:
520:
519:
516:
514:
509:
507:
503:
499:
491:
488:
483:
474:
470:
468:
464:
459:
455:
453:
443:
436:
434:
403:
368:
359:
356:
340:
337:
324:
323:
320:
318:
309:
302:
300:
272:
263:
255:
246:
243:
232:
223:
220:
212:
199:
198:
191:
184:
182:
165:
156:
153:
137:
134:
115:
114:
111:
109:
108:
103:
102:
97:
87:
85:
81:
77:
72:
70:
66:
62:
58:
49:
41:
37:
33:
19:
1033:
1012:Shreir 10:12
1008:
998:, retrieved
993:
983:
978:Shreir 10:43
974:
965:
954:. Retrieved
950:the original
940:
923:Peabody p.37
919:
914:Shrier 10:29
910:
901:
892:
883:
874:
865:
839:
836:
824:
820:
738:
734:
730:
706:
690:
687:
683:
666:
659:
653:below 1,446
651:
640:
636:
629:
618:
510:
495:
471:
460:
456:
448:
437:
314:
303:
185:
105:
99:
93:
73:
60:
56:
54:
36:
878:Peabody p.2
869:Shrier 10:4
630:leading to
1099:Categories
1048:References
1000:2013-09-05
956:2022-01-04
905:Shrier 1:2
887:Shrier 3:4
749:Advantages
689:equation (
643:passivate
588:−
566:⟶
561:−
498:magnesium
385:⟶
369:−
273:−
261:⟶
256:−
166:−
127:⟶
69:corrosion
845:See also
662:thermite
502:aluminum
467:bonding
76:voltage
1077:
1067:
1057:
787:ground
90:Theory
857:Notes
490:barge
487:canal
59:, or
1075:ISBN
1065:ISBN
1055:ISBN
506:zinc
504:and
695:).
454:.)
110:):
1101::
1017:^
992:,
928:^
829:.
657:.
597:aq
584:OH
500:,
396:OH
388:Fe
378:aq
365:OH
350:aq
334:Fe
282:aq
269:OH
147:aq
131:Fe
124:Fe
71:.
55:A
959:.
789:.
711:)
709:5
707:(
692:5
623:)
621:4
619:(
600:)
594:(
579:2
576:+
573:H
569:2
557:e
552:2
549:+
546:O
538:2
534:H
529:2
442:)
440:3
438:(
419:)
416:s
413:(
404:2
399:)
393:(
381:)
375:(
360:2
357:+
353:)
347:(
341:+
338:2
308:)
306:2
304:(
285:)
279:(
264:4
252:e
247:4
244:+
241:O
233:2
229:H
224:2
221:+
213:2
209:O
190:)
188:1
186:(
162:e
157:2
154:+
150:)
144:(
138:+
135:2
107:2
101:1
34:.
20:)
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
_-_dry_dock_Pearl_Harbor_(2).jpg)