What are the common types of rust on ship hulls, and how can they be identified?

The common types of rust on ship hulls include seven kinds: layered dead rust, thick hard rust, nail rust, prickly rust, thin frothy rust, oxidative rust, and cottony rust. Its characteristics are as follows: (1) Layered rust mostly appears on both sides of the deck and above the sewage gutters, primarily due to the alternating effects of moisture and exposure to sunlight. It has a large area and a layered structure, with three to four layers in total; the outer layers are crisp while the inner layers are harder and more difficult to remove. (2) Full-hard rust appears mostly on the bottom of the ship; it results from corrosion caused by contact with seawater and marine fouling organisms, and it is hard in texture and difficult to remove. (3) Nail rust, commonly known as “pimple rust,” appears in needle-like form. It occurs mostly at the tip of the ship; the area affected is small, but the corrosion is deep. In severe cases, it can cause holes in the steel plates, and removal is rather difficult. (4) Miliaria rust: On the surface of steel plates and paint coats, there are small patches of rust scattered densely, resembling miliaria in shape. It is caused by the rust on the steel plate not being completely removed before painting. If not dealt with promptly, it will develop into nail rust. (5) Thin foam rust appears mostly on new steel plates and resembles orange peel in shape. It occurs due to the steel plates being stored outdoors for an extended period, causing the surface oxide scale to weather away. (6) Oxidized rust is commonly referred to as scale. Although rust oxidation provides protection for the steel sheet, over time, once this rust falls off, the steel sheet is exposed and begins to corrode. (7) Cotton rust: It has properties similar to cotton, attaching softly to the surface of steel plates. It mostly occurs in coal holds, due to the high humidity, poor ventilation, and lack of sunlight there.