The resistance of seawater between the propeller and the zinc sacrifice anode of FRP Ship has been calculated with special interest to the corrosion protection of the propeller. The seawater resistance was found to be the controlling factor of designing the arrangement of the zinc anodes for the corrosion protection. In the calculation, the propeller was transformed to a equivalent ellipsoid using the conformal mapping. Then, the mathematical expression was deduced for the resistance between the equivalent ellipsoid and an anode with infinite or finite area.
R_l=ρ·a(h-a)/4πb²h(Ω), for infinite area,
R_l=f·ρa(h-a)/4πb²h(Ω), for finite area.
where,
a: major axis of equivalent ellipsoid ×1/2 (cm)
b: minor
h: distance between equivalent ellipsoids (cm)
ρ: resistivity of seawater (Ωcm)
f: resistance coefficient relative to area coefficient of zinc anode
The resistance calculated from the equation was found to be in good agreement with that measured on a FRP ship.

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For the purpose of development of aluminum alloy anodes for cathodic protection of stainless steel in chloride solution at elevated temperature, galvanic interaction between pure aluminum, Al-Mn alloys, Al-Mg alloys, Al-Ca alloys, and stainless steel (SUS 304) has been studied in aerated 0.5N NaCl solution at 80°C by measuring the weight loss and monitoring the galvanic current with a zero impedance ammeter in 140 hour tests. The values of the average galvanic current density agree well with the increase of dissolution rate due to galvanic coupling. Galvanic corrosion of pure aluminum and aluminum alloy anodes coupled to stainless steel decreased in the order Al-5%Mg>Al-5%Ca>Al-1%Mg>Al-5%Mn>Al-1%Mn>99.98Al>Al-1%Ca. Dissolution rates and galvanic corrosion potentials of Al-1%Mn, 99.98%Al and Al-1% Ca showed relatively high performance as sacrificed anodes for cathodic protection of stainless steel in chloride solution at elevated temperature. Boehmite was produced by the reaction between aluminum and water in 0.5N NaCl at 80°C, and scanning electron microscopic observations revealed that the boehmite film consisted of a continuous thin film and numerous fine needle crystals.

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Nitrided titanium composed of multilayers of different nitrogen contents was easily corroded in hydrofluoric acid, although it showed high corrosion resistance in hydrochloric and sulfuric acids. Corrosion potential of every layer were very low in hydrofluoric acid in comparison with those in hydrochloric acid. This can be atributed to the extreme decreasing of simple Ti ions in the solution, because of formation of a complex ion in hydrofluoric acid. The existence of the complex ion was proved by the experiment using an anion exchange resin substituted with F ion. It was also verified by coulometry that a part of the complex ion consisted of lower valency Ti than Ti (IV), which was assumed to be Ti (III).

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Repeated measurement of pitting potentials has been accomplished for 304 and 316 stainless steels in 3.5% NaCl solution deaerated with nitrogen gas at 35°C, using a potentio-kinetic method (20mV/min). Chemical etching using 30% H₂SO₄ for 10min at 35°C or mechanical polishing with 2/0 emery paper is used for surface preparation. It is clarified that measured values are described as a normal distribution, which is easily demonstrated by plotting measured values arranged in ascending order and corresponding probability in a normal probability paper. Although a well controlled condition is used, fairly large standard deviation, or coefficient of variation, is found, the order of which is comparable to that of fracture strength of solid material. Application of “the weakest link model” which is originally developed in fracture phenomena to pitting corrosion is suggested.

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