Structure

• Wasting
• Pitting
• Dezincification
• Intergranular corrosion and
cracking

Ways of reducing corrosion:
Eliminate the corrosive environment e. Elimination of Cl- ions by ion exchange from aqueous solutions.

Cathodic protection system: more reactive metallic element plating that reacts in preference to vehicle material

Improvements in internal structure - avoiding pools of stagnant water within the vehicle.

Plastics for the three sisters:

The three sisters are AUVs that are internally flooded, so the outer fairing does not have to provide structural support, or be a pressure vessel. Also, the Three Sisters will avoid high temperatures near the vent by swimming at a fairly constant vertical distance above the vents. Therefore we chose to use ABS, a plastic polymer for the outer fairing in these vehicles. Before we can consider its pros and cons, it is important to understand the fundamental properties of plastics.

There are two types of plastic :

Thermosetting: A substance that does not soften on heating
Thermoplastic: A substance that softens on heating

Plastics are molded into shape by a process called thermoforming, where a thermoplastic sheet is clamped onto a frame, heated in order to make soft and pliable, then made to conform to the shape of the mould by applying differential pressure. The pressure can be provided mechanically (e.g. matched mold forming) via air pressure.

ABS is thermoplastic, which makes it shock absorbent. Unfortunately, it can become deformed if it gets to high temperature regions. An advantage of ABS is that it has the same acoustic properties as water and so allows sonar signals to enter and leave the AUV without much distortion.

Below are some of the properties of ABS. Its chemical name, ABS, stands for Acrylonitrile Butadiene Styrene.

Physical Properties

Metallic Alloy for ROVs

Titanium is a bad choice from a corrosion point of view and at high temperatures near vent it would be toast from a structural point of view. Any titanium alloy used would be at it's limit structurally and not good from an environmental cracking point. The vent chemistry is quite noxious. The best options are a composite (metal layer type) of some kind where the environment-facing material is a high strength Ni-base alloy, one that might be used in a sour gas environment, but backed up by something lighter. One can bond Ni-Base materials to Ti.

Properties of Ni-base Corrosion Resistant Alloy (containing chromium, amongst other elements)

The Properties of Titanium Alloy

References