Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
  • C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
  • C23F13/06—Constructional parts, or assemblies of cathodic-protection apparatus
  • C23F13/08—Electrodes specially adapted for inhibiting corrosion by cathodic protection; Manufacture thereof; Conducting electric current thereto
  • C23F13/12—Electrodes characterised by the material
  • C23F13/14—Material for sacrificial anodes
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C18/00—Alloys based on zinc
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C20/00—Alloys based on cadmium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/10—Alloys based on aluminium with zinc as the next major constituent

Definitions

• Examples of these structures are all dockside facilities other than wood, barges, tugs, metal sea-walls, sheet piling, cables, chains, etc., and every boat that has any exposed metalright from the smallest to the very largest.
• Protective Coatings In this category fall paints, mastics, bitumens, rubber and rubber-like plastics, chemical surface treatments, metal coatings sprayed or electrolytically deposited, vitreous enamels and in enclosed containers, the use of inhibitors.
• the protective current can be supplied from any source of direct current such as a battery, a rectifier with suitable controls on an alternating current line or by the use of suitable metals connected to the structure. These metals have higher values of negative voltages or anode potentials than the metals being protected. The result is that these anodes corrode instead of the structural metals. In effect the system becomes a short circuited battery with electrons flowing from the anode, through the metal-to-metal or conductive connection to the cathode and thence into the liquid to discharge hydrogen ions. The electro-chemistry of this reaction is treated more fully in text books on corrosion.
• the ideal anode installation should use inexpensive materials, should be easy to install, should have a reasonably long life and should be easily replaced.
• Zinc the historical material, has proven quite eifective. It is inexpensive. However, it has had a disadvantage in fouling or as pointed out above, in forming an impervious coating over itself that limits or prevents its action, or that the metal disintegrated rapidly without pro ducing any protective current. Recently it has been found that these actions are caused by impurities in the metal.
• Aluminum should be effective, but as pointed out it tends to polarize and become inactive. However, researchers have recently found that if it is alloyed with about 5% of zinc. The anode potential is raised slightly and this polarizing tendency disappears. But in use this alloy operates at only about 53% efiioiency, and fairly large volumes of gases and precipitation products are formed. The volume and nature of these deposits could be objectionable in enclosed containers. No sparking danger has been found. The theoretical energy available is about 1370 ampere-hours per pound, but there are other factors that affect the rate of consumption. At lower current outputs the aluminum anode has a lower efliciency and hence a lesser amount of the current going for protection and a larger amount being consumed in self destruction. At maximum current delivery, with the alloy operating at 53% efficiency, the cost factor based on a fabricated cost of $0.47 per pound is 1540 ampere-hours per dollar.
• impurities in the metal form cathode areas of such a driving potential (voltage difference) that a high rate of self destruction occurs.
• the magnesium alloys operate at about 50%, and at lower output values the efiiciency approaches zero.
• the second reason is that the voltage is so great that the cathode cannot accumulate a protective calcareous deposit. The current continues to flow causing a continuous increase in the deposits on the surface and causes them to spall or flake off in chips.
• the cost factor for the magnesium anodes at a fabricated cost of $0.45 per pound is 1111 ampere-hours per dollar.
• the corrosion rate of the alloy by itself is unusually low. Laboratory testing indicates a rate of about 0.18 mdd. (milligrams per square decimeter per day). But some experimental data have shown an even lesser value. One sample of about 250 gm. and an estimated surface area of about one square decimeter has been exposed to sea water but not attached to a cathode for 120 days. It shows no visible evidence of corrosion.
• the cathode shows no corrosion whatsoever. When it is dried it has a light gray color. Originally it would dry white, but the oil cycles have darkened the coating. The anode is free from any fouling deposits, the cathode has a hard adherent coating estimated at about 0.08 to 0.13 mm. thick. It has been experimentally ruptured in small areas to determine its thickness and characteristics. The coating was observed to reform within 6 or 7 days of the sea water cycle. Considering the total weight loss, the anode life projects to over years and when the final 124 days are taken by themselves, even longer. Thus it is possible with this material to install anodes when the ship is built and have them last with complete protection for its entire useful life. By contrast, magnesium type anodes now in use require almost twice as much weight but are depleted to the replacement point within 2 or 3 years.
• This anode material with its nearly 50% of aluminum and its exceptionally high efficiency has more amperehours capacity and a lower cost factor. Its energy potential at 98% efliciency is about 885 ampere-hours, and
• This alloy composition approximates the ideal anode composition. Its voltage or anode potential is ample to polarize the cathode, but not great enough to blast paint, millscale or calcareous deposits from the surfaces. Its self corrosion rate is very low, and thus its efficiency high. It is inexpensive, has a high energy producing potential and is thus inexpensive to install since minimum weight is required. It has long service life. Its gassing is minimal since there is little or no wasted current.
• the calcareous deposit formed in sea water types of media is unusual in that it is thin, hard and quite adherent, and that its polarizing effect on the cathode is so nearly complete. The reaction products of the anode itself are soft and porous and do not interfere with its anode functions.
• the precipitation products evolved in sea Water types of media are low in volume and extremely fluffy.” They form as fine fiocculent flake-like agglomerations that gradually settle toward the bottom, if there is no movement to keep them dispersed. It is easily handled by all kinds of pumps; it does not tend to cake or cause any trouble in the bottoms of tanks.
• the current generated by the anode polarizes adjacent surfaces first and ranges outward spherically (generally the metallic resistance of the circuit is insignificant when compared with the electrolytic resistance of the corroding medium).
• the current flows farther and farther until either all of the cathode surface is coated or until the total flow of current through the coating just balances the ability of the anode to produce. It is known that the corroded condition of the cathode surface affects this balance. Thus if the surface has been corroded prior to the installation of anodes and if there are adherent corrosion products remaining, more polarizing current is required.
• Method of protecting a body of iron, steel or other metals against corrosion in the presence of a water solution containing at least one ion selected from the group consisting of Cl, SO SO CO and Br which comprises attaching thereto in metal-to-metal contact a body of an alloy having a composition within the ranges

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Prevention Of Electric Corrosion (AREA)

Priority Applications (3)

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Cited By (7)

Citations (9)

• 0
  • NL NL263598D patent/NL263598A/xx unknown
• 1960
  • 1960-04-13 US US21875A patent/US3137642A/en not_active Expired - Lifetime
• 1961
  • 1961-04-12 DE DES73443A patent/DE1167036B/de active Pending

Patent Citations (9)

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