Auxiliary device, a marine surface vessel, and a method for corrosion protection in a marine construction

#The present invention relates to an auxiliary device, a marine surface vessel, and a method for corrosion protection in a marine construction, such as a marine surface vessel or a marine structure, the marine construction comprising a plurality of metal elements and at least one reference electrode, the metal elements and the reference electrode being at least partly immerged in an electrolyte in the form of water, in which the marine construction is at least partially immerged, the metal elements comprising an anode and a metal part, the anode being provided for corrosion protection of the metal part by the provision of an electrical protection current through an electrical circuit comprising the anode, the metal part and the electrolyte, the method comprising measuring an electric potential of the metal part with the reference electrode as a ground reference.

#In marine constructions, such as marine vessels and marine structures; a known way to protect an immersed metal part against galvanic corrosion is to provide a sacrificial anode, made of very pure zinc, magnesium, cast iron or an alloy of aluminum, which is directly fastened to, or electrically connected via a cable to the immersed metal part. In such a system, herein referred to as a passive corrosion protection system, or simply a passive system, the sacrificial anode will waste away, preventing damage to the immersed metal part. An alternative to passive arrangements with sacrificial anodes is impressed current cathodic protection (ICCP) systems. In such a system, one or more active anodes are immersed and connected along with the immersed metal part to a common DC electrical power source. ICCP system active anodes usually have tubular or solid rod shapes, or are provided as continuous ribbons, and can include materials such as titanium, platinum, high silicon cast iron, graphite, mixed metal oxide, and niobium. Regardless whether the corrosion protection system is a passive system or of the ICCP type, a circuit is provided by an electrical current through the water serving as an electrolyte, and a surface polarization at the interface between the metal part and the water is created, serving to protect the metal part against corrosion.

#In a passive system, as well as in an ICCP system, for a good corrosion protection, the electrical current between the anode and the metal part should be such that the surface polarization of the metal part is kept close to a desired value, or a desired interval, which depends on the material of the metal part. If said current is too low, the protection is too low, and the metal part may corrode at an undesired rate. For some metals, e.g. aluminum, damages can occur also if the current is too high. In general, too high of a current between the anode and the metal part gives calcium oxide precipitation which can stimulate excessive growth of sea weeds and sea animals on the metal part. Such growth is a particularly large concern in the case of recreational boats, which often stay docked giving opportunities for the growth.

#Below, the invention will be described in detail with reference to the drawings, in which

#FIG. 1 shows a schematic cross-sectional side view of a boat,

#FIG. 2 is a schematic perspective view of a back wall of an external unit for a corrosion protection system of the boat in FIG. 1,

#FIG. 1 shows a schematic cross-sectional side view of a boat 1 with a corrosion protection system in turn comprising an impressed current cathodic protection (ICCP) system. The boat 1 is provided with an engine 2 connected to a drive M, with propellers for the propulsion of the boat. In this example, the drive M, immerged in the water W, constitutes a metal part to be protected by the corrosion protection system. In FIG. 1, the drive M is schematically presented as a drive manufactured and marketed by Volvo Penta as an IPS (Inboard Performance System) drive, but the invention is of course applicable to boats with any kind of drive, for example a stern drive or a traditional propeller and rudder combination. Also, the invention is applicable to boats with any number of engines and drives.

#Reference is also made to FIG. 2. The corrosion protection system comprises an external unit 3, mounted on a transom of the boat 1. FIG. 2 shows a back wall 3 a of the external unit 3, more specifically a side of the back wall 3 a facing inwards in the external unit 3. The external unit 3 is adapted to be immerged in the water, and comprises an active anode A, which is provided in the form of a MMO (mixed metal oxide) coated titanium rod mounted on the back wall 3 a. More generally, the anode A can be provided as one or more platinum coated titanium rods, or MMO (mixed metal oxide) coated anodes. Alternative shapes for the active anode A include tubular shapes and shapes as continuous ribbons, and alternative materials include high silicon cast iron, graphite and niobium. The external unit 3 also comprises three reference electrodes R1, R2, R3, herein referred to as a first, second and third reference electrode R1, R2, R3. The reference electrodes are supplied in the form of solid rods made of silver coated with silver chloride. Alternatively, the above mentioned core/silver/silver chloride reference electrodes can be provided.

#It should be noted that in alternative embodiments, the active anode A and the reference electrodes R1, R2, R3 can be provided in separate units.