Galvanic Couples:

A galvanic couple is a combination of an anode material and a cathode material. The autophagous battery pack is being made out of a galvanic couples. The greatest potential comes from the largest difference between the anode and the cathode. The electron flow is produced when the materials in contact with the electrolyte (sea water) corrode. The potential can be maximized by two ways:
1.) The voltage difference between the two materials from table 1.
2.) The exposed size of the cathode compared to the exposed size of the anode.

Below is a table of materials and their voltage range of alloy vs. reference electrode. The table begins with anodes and ends with cathodes.

Table 1. Galvanic Series in Flowing Seawater
Source: http://www.ocean.udel.edu/seagrant/publications/images/corrosion.pdf

The ideal theoretical galvanic couple would be magnesium and graphite. But material properties and availability have to be considered.

Material Properties and Availability:

The properties of each material used in the galvanic couple had to be researched to determine if it was applicable for the design. The materials researched were the ones that provided the maximum potential difference based off the information gathered from the table shown above.

Magnesium was the top choice for the anode material. Magnesium is an alkaline earth metal. Magnesium is a strong, light weight material but is extremely flammable.

The second anode selection for the anode material was Zinc. Zinc was determined to be unavailable in its pure form.

Aluminum was the third choice for the anode material. Aluminum is a light weight material with good strength and durability physical properties. Aluminum provides a lower potential difference since it is lower on Table 1; however it is low in cost and is easily available.

The top cathode material choice would be Graphite. This material however is brittle and is not machinable with the available equipment. Therefore, graphite would not be a practical material choice for the vehicle design.

Platinum, Hastelloy C-276, and Inconel 625 were the next three possible materials for the cathode. However the availability and cost for these materials were insufficient for this design.

Titanium was the next possible material choice for the cathode. Titanium is lightweight, durable, and machinable. It is high in cost; however its mechanical properties and high potential difference made it a possible selection.

Another cathode material had to be selected to be tested against the Titanium. The 300 series steel was the next best selection. Stainless Steel 304 was chosen based on its durability, machinability, and its availability.

In conclusion the materials tested for this project were: Magnesium and Aluminum as anode materials, and Stainless Steel and Titanium as cathode materials.

Theoretical Calculations:

The following calculations were used from the theoretical voltage values in Table 1.

Magnesium (1.615) + Titanium (.005) = Potential of 1.61 V
Magnesium (1.615) + Stainless Steel (.07) = Potential of 1.545 V
Aluminum (0.8) + Titanium (.005) = Potential of 0.795 V
Aluminum (0.8) + Stainless Steel (.07) = Potential of 0.73 V

The calculated theoretical voltage outputs concluded that the best galvanic couple would be magnesium and titanium. This combination would give a potential of 1.61 V.

The second way to maximize the potential produced from the galvanic couple is altering the cathode-to-anode ratio. The larger the ratio the more severe the corrosion will be. The design of the materials will have to find a median to maximize the potential with an acceptable corrosion rate to ensure its durability.

Material Surface Area:

The following design analysis refers to the maximization of the surface area of the anode and cathode. This will provide long life for the autophagous battery because of its dependence on the rate of corrosion of the anode; once the surface of the anode is completely corroded the battery will stop working. This analysis is done with respect to the maximum area that can be obtained within an enclosed space of 4 in^2. Each one of the designs will be enclosed in this area and therefore analyzed for that particular size.

Design 1:
This design is just a single plate of the material as shown below.

Figure 1. Material Design 1: Plate

This layout the total surface area is 8 in^2; counting both sides and assuming a thin plate where the thickness is negligible.

Design 2:
This design is similar to the first one but some areas are removed and a thickness of 1in is added as shown below.

Figure 2. Material Design 2: X

For this layout in Figure 2 the total surface area is around 27 in^2; counting both sides of each edge.

Design 3:
This design has the same characteristic of the second one but a different shape is used, a polygon, the same thickness is added as shown below.

Figure 3. Material Design 3: Polygon

For this layout the total surface area is around 12 in^2; counting both sides of each edge.

All these layouts can be optimized but it machinability can be compromised on the process. It was concluded that design one in Figure 1 would be the best selection since it produces the least amount of drag and exposes a considerable surface area.