Process for Forming Anticorrosive Coating
Anticorrosive coating is formed cheaply and in a short period, which facilitates its general application to marine steel structures. A steel caisson 1 is used as a cathode, an undersea member 3 arranged in seawater and opposed to the steel caisson 1 is used as anode. Direct current is passed between the electrodes so that anticorrosive coating 8 is formed on the steel caisson 1 through electrolytic reaction of the seawater, thereby attaining anticorrosion. The electric current is passed between the electrodes such that coating 7 with magnesium hydrate as dominant constituent is formed on the steel caisson 1 to have a predetermined thickness; then, supply of the electric current is stopped. Thus, anticorrosive coating 8 is formed through compositional substitution effect of substituting calcium carbonate for the magnesium hydrate in the presence of seawater.
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The present invention relates to a process for forming anticorrosive coating and more specifically to a process for forming anticorrosive coating on a marine steel structure in a short period.
BACKGROUND ARTA process for anticorrosion of a marine steel structure has been proposed in which the steel structure is used as a cathode, an anode being arranged in seawater to be opposed to the steel structure. Direct current is passed between the electrodes to form coating (anticorrosive coating) on the steel structure through electrolytic reaction of the seawater, thereby attaining anticorrosion of the steel structure.
Thus, Reference 1 discloses that a steel member constituting a surface of a marine steel structure is used as a cathode, an anode being arranged in seawater to be opposed to the steel member. Direct current is passed between the electrodes to remove rust and the like scales on the surface of the steel structure. Then, direct current is passed between the electrodes to deposit electrodeposit, which has electrolytic reaction product of the seawater as dominant constituent, on the surface and any corroded pores of the steel structure, thereby forming anticorrosive coating.
[Reference 1] JP10-313728A SUMMARY OF THE INVENTION Problems to be Solved by the InventionThe coating formed as mentioned above on the marine steel structure through electrolytic reaction of the seawater has calcium carbonate CaCO3 and magnesium hydrate Mg(OH)2 as main components. As is known in the art, it is calcium carbonate formed hard in hardness that exhibits anticorrosive effect. Therefore, in order to make a marine steel structure anticorrosive, anticorrosive coating having calcium carbonate as dominating component must be formed on the steel structure.
As disclosed also in Reference 1, relationship between coating composition on a marine steel structure and current density due to electric current applied between electrodes is as shown in
So, it is disclosed in Reference 1 that current density during electrodeposition is selected within a range of 0.2 to 2 A/m2 (1 A/m2 on an average) so as to form anticorrosive coating with calcium carbonate as dominant constituent and that anticorrosive coating in the form of hard electrodeposition coating and with thickness of about 5 mm or more is formed on the marine steel structure.
However, it is known that a long period of about 10 months or more is required for formation of anticorrosive coating with calcium carbonate as dominant constituent and having thickness of for example 5 mm or more by a low current density of for example 1 A/m2 as disclosed in Reference 1.
Thus, the conventional process for forming anticorrosive coating as disclosed in Reference 1 requires long construction period and long-term management and is costly due to increased electric power consumption, so that the process has not been practically applied except special cases such as bridge piers at sites in deep water or in violent tidal current.
The invention was made in view of the above and has its object to provide a process for forming anticorrosive coating which can be conducted cheaply in a short period, thereby easily leading to general application to marine steel structures.
Means or Measures for Solving the ProblemsThe invention is directed to a process for forming anticorrosive coating on a marine steel structure wherein the steel structure is used as a cathode, an anode being arranged in seawater to be opposed to said steel structure, direct current being passed between the electrodes, anticorrosive coating being formed on the steel structure through electrolytic reaction of the seawater, thereby attaining anticorrosion of the marine steel structure, characterized by passing the electric current between the electrodes so as to have current density to form coating having magnesium hydrate as dominant constituent on said marine steel structure, thereby forming the coating with a predetermined thickness, then stopping supply of the electric current to thereby provide anticorrosive coating through compositional substitution effect which occurs in the presence of the seawater to substitute calcium carbonate for the magnesium hydrate.
It is preferable in the above-mentioned invention that the electric current is passed between the electrodes so as to attain current density of the marine steel structure in a range of 3 to 10 A/m2.
Effects of the InventionAccording to a process for forming anticorrosive coating of the invention, electric current is passed between electrodes so as to keep high current density of a marine steel structure, so that coating with magnesium hydrate as dominant constituent is formed on the steel structure in a short period. Then, supply of the electric current is stopped to utilize a compositional substitution effect which occurs in the presence of the seawater to substitute calcium carbonate for the magnesium hydrate, thereby forming anticorrosive coating. As a result, the invention has an effect that it can form good anticorrosive coating with calcium carbonate as dominant constituent in by far a shorter period than ever before.
Such formation of anticorrosive coating in a short period leads to reduction in construction period and easiness in management as well as reduction in cost due to reduced electric power consumption. Therefore, the invention has an effect that it can be easily applicable to any kind of marine steel structures unlike the conventional process with limited applicability to special sites.
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1 steel caisson (marine steel structure) (cathode)
2 DC power supply
3 undersea member (anode)
7 coating
8 anticorrosive coating
BEST MODE FOR CARRYING OUT THE INVENTIONAn embodiment of the invention will be described in conjunction with the attached drawings.
EMBODIMENT 1The DC power supply 2 is connected at its minus (−) side to the steel caisson 1 so as to use the steel caisson 1 as a cathode and is connected at its plus (+) side to the undersea member 3 so as to use the undersea member 3 as an anode. Such construction is made to each of the plural steel caissons 1.
A predetermined constant current is passed between the electrodes, i.e., between the steel caisson 1 and the undersea member 3 by the DC power supply 2, so that deposited coating is formed on the steel caisson 1 through electrolytic reaction of the seawater.
Moreover, as shown in
Mode of operation of the invention will be described in conjunction with experimental example.
Using the equipment shown in
It turned out that the above-mentioned tendencies in
Then, the inventors took notice of
However, with the current density being set to be as high as 3 to 10 A/m2 as mentioned above, the coating formed on the steel caisson 1 has magnesium hydrate as dominant constituent (for example, 95% of magnesium hydrate), failing to be anticorrosive coating.
Thus, the inventors made researches so as to change the coating with magnesium hydrate as dominant constituent and formed in a short period as mentioned above into anticorrosive coating with calcium carbonate as dominant constituent and found out that compositional substitution effect occurs in seawater to substitute calcium carbonate for the magnesium hydrate, thereby utilizing such effect to attain formation of anticorrosive coating.
The inventors used experimental equipment similar to that shown in
In the experimental equipment, cathode substrate in opposed relationship to the steel caisson 1 is made from SS (stainless steel) 400 and the anode member corresponding to the undersea member 3 is made from Mg, using natural seawater with temperature of 25° C. with the current applying condition of current density being 3 A/m2. Thus, coating test for 30 hours was conducted.
As shown in
Then, tests were conducted to ascertain the compositional substitution effect by the coating 7 with magnesium hydrate as dominant constituent as mentioned above.
More specifically, after the coating 7 with magnesium hydrate as dominant constituent as mentioned above was formed on the cathode substrate 6, supply of electric current by the DC power supply was stopped. Thereafter, the coating was kept submerged in seawater; coating composition ratio of magnesium hydrate and calcium carbonate and coating amounts in g/cm2 and in mmol thereof were detected at 7, 14 and 21 days after the start of submerging (the stop of current supply).
According to
With the lapse of submerged period, there was no variation in coating amount on the mol basis as shown in
More specifically, it is regarded that the following reaction formula (1) occurs to bring about the reaction formula (2), leading to the reaction formula (3).
Mg(OH)2→Mg2++2OH− (1)
Ca2++H2CO3+2OH−→CaCO3+2H2O (2)
Mg(OH)2+Ca2++H2CO3→Mg2++CaCO3+2H2O (3)
Thus, the coating 7 with magnesium hydrate as dominant constituent as shown in
As can be seen from
In the experimental tests shown in the above, the electric current applying condition was the current density of 3 A/m2. That is, the experiments were conducted with lower current density since the coating tends to fall off in the case of the experimental equipment with no flows of seawater. However, it turned out that in actual natural seawater with flows, coating can be formed well with no falling-off even if the process is conducted with the current density of as high as 3 to 10 A/m2.
Thus, by contrast with the conventional process having a period of 10 months or more in formation of anticorrosive coating, anticorrosive coating according to the invention can be formed in a very short period on the order of one month or one month and a half. This facilitates the process management and brings about reduction in electricity consumption, resulting in decrease in cost. Therefore, with no limitation to the special sites unlike the conventional process, the invention can be easily applied to any kinds of marine steel structures.
When a process for forming anticorrosive coating according to the invention is applied to an existing marine steel structure and where any extraneous matter such as rust on a surface of the marine steel structure is required to be removed beforehand, various countermeasures may be employed such as application of direct current between electrodes for removal of rust and the like, removal through injection of high-pressure water or removal through manpower.
It is to be understood that the invention is not limited to the above embodiment and that various changes and modifications may be made without departing from the scope of the invention. For example, the description of the embodiment has been made with respect to steel caisson as an example; however, the invention may be similarly applicable for anticorrosion of various marine steel structures such as steel sheet pile or steel bridge pier.
Claims
1. A process for forming anticorrosive coating on a marine steel structure wherein the steel structure is used as a cathode, an anode being arranged in seawater to be opposed to said steel structure, direct current being passed between the electrodes, anticorrosive coating being formed on the steel structure through electrolytic reaction of the seawater, thereby attaining anticorrosion of the marine steel structure, characterized by passing the electric current between the electrodes so as to have current density to form coating having magnesium hydrate as dominant constituent on said marine steel structure, thereby forming the coating with a predetermined thickness, then stopping supply of the electric current to thereby provide anticorrosive coating through compositional substitution effect which occurs in the presence of the seawater to substitute calcium carbonate for the magnesium hydrate.
2. A process for forming anticorrosive coating as claimed in claim 1, characterized in that the electric current is passed between the electrodes so as to attain current density of the marine steel structure in a range of 3 to 10 A/m2.
Type: Application
Filed: Apr 28, 2005
Publication Date: Feb 7, 2008
Applicant: Ishikawajima-Harima Heavy Industries Co., Ltd. (Tokyo)
Inventors: Kenichi Akamine (Tokyo), Yasunobu Suzuki (Tokyo)
Application Number: 11/547,475
International Classification: C23F 15/00 (20060101); C25D 9/10 (20060101); E02D 31/06 (20060101);