| Because of the special environment and structures, corrosion is always a severe problem for submarines and is one of the main factors impeding normal performance of submarines, especially in Southern Sea where the environment is very harsh with high temperature, high salt content and high humidity. Corrosion rates of the body structures, equipments, tubes and joints for submarines are much higher than surface ships. The severe corrosion increases the maintaining work, influences normal performance and decreases the lifetime of the vehicles.In recent years due to the ocurrence of the submarines with noise elimenation tiles and the increase of training strength, new requirements are proposed for anti-corrosion coatings on submarines such as longer performance life, better protection property, more resistant to pressured sea water and simple coating and maintenance technology. Therefore, based on the special application environments and operation conditions, it is necessary to develop the studies on failure mechanism of coatings, coating system design, evaluation methods and standards, testing methods and coating and maintenance technologies, in order to direct the development, design, coating and maintenance of submarine coatings, and resolve the long time corrosion protection for submarines.Currently, there are few studies on the performance and protection mechanism of anti-corrosion coatings under pressured deep sea. In this paper, by simulating the pressured state for submarine in deep sea, the performances of several coatings which are currently applied in navy were studied with the methods of simulating testing, electrochemical impedance spectroscopy, X-rays photoelectron spectroscopy, infrared spectroscopy and scanning electron microscope. The performance behaviors and failure mechanisms of the coatings under different environments such as different sea water pressure were analyzed. The main results are summarized below.(1) The protection of inorganic zinc-rich coating to the substance mainly come from the cathodic protection in the early stage and the barrier effect by the depositions of corrosion products in the coating. As immersion time prolonged, the corrosion reaction of zinc powders in coating is controlled by the diffusion process of corrosive species or corrosion products. However, above protection effects gradually dissappear in a few monthes. Meanwhile, because the polarity difference of the functional groups between the resins in the sealing coating and zinc-rich coating, the applied force between molecules is small, leading to decreased adhesion strength. When the corrosive species enter in the coatings, the sealing layer will strip off and the system may failure quickly.(2) The salt spray testing resistances of inorganic and organic zinc-rich coating systems are very close, but obvious differences exist in the cathodic stripping resistance and corrosion resistance in sea water. Under the salt spray condition, because the liquid mediums in the transmission channels are not uniform and even non-continuous, the transmission of corrosion products of zinc powders to the surface is more difficult than in sea water immersion. In addition, the stronger effect of zinc powders as cathodic anodes in zinc-rich inorganic coating results in more corrosion products if zinc to remain in the transmission channels. This effect leads to similar barrier effects for inorganic and organic zinc-rich coatings. However, under the condition of continuous testing medium, the above effect is not exist and the corrosion resistances of the two coatings are different. According to the EIS results, the corrosion resistance of organic zinc-rich coating system is obviously higher than that of inorganic-zinc-rich coating system.The experimental results show that the inorganic zinc-rich coating and the mating sealing coating system which are currently applied on submarines are not very reasonable. Inorganic zinc-rich coating system is not suitable for flowing sea water environment and complicated potential environment due to multi-materials and cathodic protection.(3) EIS and FTIR measurements showed that, compared with immersion in sea water under constant pressure, in sea water under3.5MPa the permeation rate and quantity of water in coating increases. As the result, the protection property of coating decreases more quickly and the performance life of coating shortens. According to the failure morphologies, in sea water under ordinary pressure and higher pressure the failure mechanisms of coating are different. The influences of hydrostatic pressure on coating failure:①Water permeated into coating may enter the molecular chains of epoxy resin and form hydrogen bond or weak chemical bonds, forming so called "bond water" which will accelerate deterioration of the coatings.②Hydrostatic pressure promotes diffusion of NaCl solution into the coating, leading to decreased coating resistance and protection.③Hydrostatic pressure results in early electrochemical reactions at coating/metal boundary and facilitates the corrosion reactions.④Under higher pressure the coating is more easily to be stripped and the stripped area increases in contrast to the situation under constant pressure, which accelerates failure of the coatings.(4) Failure behavior of epoxy anti-corrosion coating under alternate pressures was studied with EIS. The results show: ①The impedance of coating shows periodic variations;②Higher pressure is beneficial for diffusion of electrolyte into the coating, hence the barrier effect of coating decreases more quickly.③Higher alternate pressure promotes diffusion of NaCl solution into the coating, leading to decreased coating resistance and protection.④The electrolyte solution is more easy to permeate to the metal/coating boundary and the boundary corrosion reactions are promoted.⑤Water absorption of coating is increased under higher pressure condition.⑥The coating adherence gradually decreases with immersion time extends. As the alternate pressure increases, the variation in the adherence property of epoxy anti-corrosion coating is relatively small.(5) The requirements for submarine coatings in pressured sea water are proposed:Under the conditions of higher pressure sea water, alternate dry/wet environments and alternate pressures, coatings must have good resistances to sea water permeation, sea water corrosion, salt spray corrosion and good mechanical properties, as well as good property to resist cathodic stripping and are convenient to be applied. Based on basic coating properties, anti-corrosion properties under ordinary pressure and high pressure sea water, practical sea environment conditions and the applying and maintaining natures, a system of methods to evaluate anti-corrosion coatings for submarines is established.(6) The main research directions for further study on performance of coatings for submarines in sea water under high pressure are suggested. By systematic testing and the studies on the developing tends of properties, types and varieties of coatings, it is proposed that high solid epoxy resin coatings should be developed as main submarine coatings in pressured sea water. |
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