By Yuri Kuzmin and Valery Troschenko from TsNII Prometheus, the central scientific research institute of construction materials, in joint authorship with Alexander Aminov, Anatoly Mikheev and Anatoly Grephenshtein from the Bogoslovsky Aluminum Smelter.
    Since the 1970s electrochemical and cathodic systems have been widely spread to protect shells of vessels and off-shore facilities against sea water corrosion.
    The key principle of this protection method is based on a physical phenomenon when corrosion of metal is completely stopped under the effect of electric current. As it is well known, the surface of any metal is non-homogenous galvanically that is the main reason for the metal to start corroding when put into electrolyte solutions like sea water, soil and most process medium.
    In the process of corrosion only those areas of the metal surface are destroyed, which have most electronegative potentials (acting as anodes). It is these areas from which corrosion current runs into the surrounding medium while electropositive areas of the metal surface (i.e. cathodes), where electric current enters the metal from the surrounding medium, remain intact. When applied the electrochemical corrosion protection system makes the protected metal structure a single nondestructive cathode, while anodes are either plates of a more electronegative metal (protectors), which are connected to the metal structure being protected, or plates of an inert material (e.g. graphite, magnetite, ferroselite, platinized titanium, etc.), which are connected to the positive terminal of a direct current source. In the first case the electrochemical protection method is called the galvanic protector, in the second case it is called the cathodic protection.
    The galvanic protector method is the simplest and most reliable corrosion protection which needs no maintenance in the course of operation. When applied the galvanic protector and the protected metal structure form a galvanic pair which generates electric current.
    Galvanic protectors are usually made of specially developed alloys based on metals, which feature a more negative electrolytic potential. These metals include magnesium, aluminum and zinc. In the course of operation protectors are worn out (dissolve anodically) thus protecting the main metal structure. That is why they call them «sacrificial anodes». Electrochemical corrosion protection by means of galvanic protectors is the only existing method providing effective protection against local corrosion while preventing the corrosive attack from further development. It means that this protection method can be successfully applied both for operating objects and those under construction. Unlike passive protection methods e.g. paint coating electrochemical protection permits control over effectiveness of the protection in the course of operation.
    Numerous scientific researches and many years of practical experience show that under potentials lower than 800 mV, if measured against the silver-chloride electrode, which serves a reference electrode for the purpose of control over electrochemical protection of metal parts against sea water corrosion, all types of sea water corrosion completely cease for welded structures of carbon steel, stainless and low-alloyed steel.
    Electrochemical corrosion protection by means of galvanic protectors is usually applied in combination with paint-and-lacquer coating. Such a combination of passive and active corrosion protection permits lower consumption and longer service life of the galvanic protectors while providing an equal distribution of protective current across the surface of a metal structure being protected. That is more important, this method makes it possible to offset defects in the pain coating, which are inevitable while installation, transportation and operation as well as due to natural aging.
    Protective current passes from protectors to the very areas of the metal surface, where the paint coating has been damaged, and thus prevents the stripped metal from corrosion. It should be pointed out that under cathodic polarization in sea water stripped areas of a metal become centers of cathodic deposition of calcium and magnesium salts, with the deposited salt acting as an additional coating layer. As a sort of remedy to «cure wounds» inflicted on the metal this salt deposition renders higher surface resistivity to a composite coating film formed thereby.
    Electrochemical protection can prevent corrosion of construction steel materials without using any paint coatings at all though it will call for higher consumption of galvanic protectors. Therefore, for off-shore objects with a long service life of 30-40 years electrochemical corrosion protection is designed in combination with paint coatings assuming that these coatings will be then replaced with the cathodically deposited salt.
    One of the Russian leaders in the field of designing various means and systems for electrochemical corrosion protection of both cathodic and galvanic protector types, including development of galvanic protector alloys, is TsNII Prometheus, a St. Petersburg based scientific research and design institute of construction materials. Today, galvanic protectors of aluminum based alloys developed by TsNII Prometheus co-jointly with the Bogoslovsky Aluminum Smelter currently incorporated into the SUAL Group have been intensively used for shipbuilding purposes and marine applications.
    The developers have chosen aluminum as a basis for galvanic protector alloys for a number of reasons. As it is well known, any aluminum surface is covered with a chemically stable oxide layer, which makes it impossible to use pure aluminum as a galvanic protector material. To reduce the passivating effect of the oxide layer aluminum is doped with different alloying additives. The most effective activating additives are zinc, indium, gallium and tin. By adding different combinations and amounts of the alloying additives one can increase an electronegative potential of the final alloy. Alloying additives should be doped in amounts which would neither disturb homogeneousness or grain fineness of the alloy structure nor bring inclusions of other phases. The latter is a crucial factor which is mostly responsible for efficiency of the protector` s performance. When doped into an aluminum alloy such cathodic components as iron, copper and silicon may form with the aluminum matrix cathodic phases like FeAl3 (or Fe2Àl3) in the form of fragile needles thus affecting electrochemical performance of the galvanic protector alloy. If iron content exceeds 0.5 % the aluminum alloy, when cooled, feature two constituents, which can be distinguished in its structure, namely: primary -crystals and eutecticum (-crystals + FeAl3). The FeAl3 phase, which is cathodic by its electrochemical properties in relation to the solid solution, shifts the working electrochemical potential of the alloy towards more positive values thus hindering the electric current exchange process and reducing the effective utilization ratio due to higher current of self–dissolving under the influence of newly formed microgalvanic pairs. When doped into an aluminum alloy, copper and silicon inclusions produce though not so strong yet similar effect.
    Results of scientific researches have shown that galvanic protector alloys feature the best electrochemical performance when based on pure primary aluminum of not lower than grade # A85 doped with 0.15% of alloying additives, namely: > 99.85 Al; 0.08 Fe; 0.01 Cu; 0.06 Si; 0.02 Zn and 0.008 Ti.
    The TsNII Prometheus scientific research institute has developed technical standards for galvanic protectors and galvanic protector based electrochemical corrosion protection systems while the Bogoslovsky Aluminum Smelter has adjusted the process of casting galvanic protectors of all standard dimensions and arranged their commercial production.
    Owing to simplicity of arrangement and high corrosion protection performance, galvanic protector based anticorrosion systems have been widely applied. In 1980-1990 an overwhelming majority of sea vessels were equipped with anticorrosion systems based on galvanic protectors made of aluminum alloys. During that period annual production of such protectors at the Bogoslovsky Aluminum Smelter totaled 3,000 tonnes per year. In the 1990s when Russia suffered a drop in industrial activities consumption of such protectors for marine applications got significantly reduced.
    A loss of this market has been partly offset by a growing demand for galvanic protectors in the oil and gas industry. Under contracts with oil and gas production companies from Western Siberia the TsNII Prometheus institute developed an electrochemical anticorrosion system to protect inside surface of settling tanks by using galvanic protectors of the APZ grade aluminum based alloy. A decade of successful operation has proved high effectiveness of the system. The tanks equipped with the protectors have needed no maintenance while those operating under the same conditions without the protectors have been either already been written off or called for major repair.
    Though shipyards have recently started placing more orders for galvanic protectors it looks like ship owners and companies involved in building and operating off-shore and marine objects (e.g. berths, piling walls, off-shore pipelines and other marine facilities) have not yet done justice to this simple, more effective and affordable means for preventing sea water corrosion.
    At the same time the Bogoslovsky Aluminum Smelter has kept its production of all types of aluminum alloy galvanic protectors and in cooperation with the TsNII Prometheus scientific research institute has been ready to consult on application of galvanic protectors to prevent corrosion of steel structures operating in sea water or in a similar highly corrosive environment.
    Table # 1
    Contents (in percentage) of key components and alloying additives i.e. Fe, Cu and Si in aluminum based galvanic protector alloys.
    Elements AP2 AP3 AP4Í
    Zn – 4.0–6.0 4–5
    Zr – 0.001–0.1 0.01–0.1
    Sn – – 0.01–0.1
    Mn, In 0.01–0.2 Mn – 0.01–0.06 In
    Al-based
    Fe 0.10 (max) 0.10 (max) 0.10 (max)
    Cu 0,01 (max) 0,01 (max) 0.01 (max)
    Si 0.10 (max) 0.10 (max) 0.10 (max)
    Table # 2
    Electrochemical performances of aluminium based alloys for galvanic protectors
    Alloy Grade
    Negative electric potential measured versus the reference silver-chloride electrode in mV.
    Specific estimated current exchange ratio, in Amp.*h/kg.
    Effective utilization factor,
    In %, not less than
    Chemical composition
    Stationary working
    AP2 890 840 2940 80
    AP3 1060 940 2880 80
    AP4Í 1140 1090 2880 85 1 

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