Copper is a well known alloying element that is used to improve the resistance to general corrosion of stainless steel as well as copper alloy. And Cu cation have the anti-fouling effect to inhibit adhesion of marine algae and shellfish to the surface of heat exchanger cooling pipe or outside wall of the ship. Therefore there are some anti-fouling methods using the copper such as anti-fouling paint mixed with copper oxide or MGPS(Marine Growth Preventing System) by using Cu cation dissolved to the sea water solution.
However, although a copper have the anti-fouling effect, it cannot be avoided that copper and copper alloy materials were inevitably corroded in sea water solution with dissolved oxygen or chloride ion when a heat exchanger was constructed with copper and copper alloy for anti-fouling. Therefore in order to get the synergic effect of the anti-fouling as well as corrosion resistance it is necessary to improve corrosion resistance of copper and copper alloy by some optimum methods.
In this study, the annealing heat treatment to promote the corrosion resistance in sea water solution was investigated with a electrochemical point of view.
Annealing heat treatment was carried out with parameters of heating temperature such as 100℃, 200℃, 700℃, 800℃, 900℃ and 1000℃ for 1 hr. The corrosion resistance was evaluated with electrochemical methods such as variation of corrosion potential, anodic and cathodic polarization curves, cyclic voltammogram, AC impedance, SEM photograph and vickers hardness measurement etc.
Chapter 3.1 shows the effect of annealing heat treatment to the corrosion resistance of pure copper.
The grain size of the surface at 700℃ annealing temperature was the smallest than that of other annealing temperatures, and the corrosion potential showed more positive potential than that of other annealing temperatures.
The galvanic current between Ti and Cu with annealing temperature at 700℃ was the largest value in case of non-flow condition, however in case of flow condition its value was the smallest than that of the other temperatures. Therefore in order to increase anti-fouling effect by Cu cation, in non-flow condition of sea water, the optimum annealing temperature was 700℃ for one hour, however in case of flow condition non heat treatment might be desirable.
In chapter 3.2, the corrosion resistance with annealing heat treatment was investigated about Cu-10%Ni alloy.
Vickers hardness value was decreased with increasing of heating temperature. However corrosion resistance showed the increasing tendency with increasing of heating temperature. Especially it was shown that corrosion resistance was clearly promoted by annealing temperature at 1000℃ more than other temperatures.
Annealing temperature at 200℃, vickers hardness increased than other temperatures and corrosion resistance was slightly improved compared to the no heat treatment. So in case of considering the mechanical property as well as corrosion resistance, it is considered that the desirable heating temperature may be 200℃ .
Chapter 3.3 was examined about Cu-30%Ni alloy in sea water solution with a electrochemical point of view.
Vickers hardness was decreased with increasing heating temperature the same as Cu-10%Ni alloy. However in annealing temperature at 200℃, vickers hardness contrary increased than both no heat treatment and other heating temperature. The improvement of corrosion resistance was also observed with annealing heat treatment the same as Cu-10%Ni alloy. Especially in case of Cu-30%Ni alloy, corrosion resistance was apparently improved in annealing heat treatment at 1000℃. Thus to improve the corrosion resistances in sea water as well as anti-fouling effect, the annealing heat treatment may be important for Cu-30%Ni alloy . Eventually, it is necessary to perform the annealing heat treatment for corrosion resistance improvement of Cu-Ni alloy material which is being used as a heat exchanger having the anti-fouling effect.