Pitting Corrosion in ships

Introduction

In general, ships’ structural damages and deterioration imply deficiencies caused by:

1. Excessive corrosion
2. Design faults
3. Material defects or bad workmanship
4. Navigation in extreme weather conditions
5. Loading and unloading operations, water ballast exchange at sea
6. Wear and tear
7. Contact (with quay side, ice, touching underwater objects, etc.) but not as a direct consequence of accidents such as collisions, groundings and fire/explosions.

Deficiencies are normally recognized as:

1. Material wastage
2. Fractures
3. Deformations

Material wastage is a common form of deficiency on ship structure which is divided to the following four categories:

1. General corrosion appears as a non-protective, friable rust which can occur uniformly on hold or tank internal surfaces that are uncoated. The rust scale continually breaks off, exposing fresh metal to corrosive attack. Thickness loss cannot usually be judged visually until excessive loss has occurred. Failure to remove mill scale during construction of the ship can accelerate corrosion experienced in service. Severe general corrosion in all types of ships, usually characterized by heavy scale accumulation, can lead to extensive steel renewals.
2. Grooving corrosion is often found in or beside welds, especially in the heat affected zone. The corrosion is caused by the galvanic current generated from the difference of the metallographic structure between the heat affected zone and base metal. Coating of the welds is generally less effective compared to other areas due to roughness of the surface which exacerbates the corrosion. Grooving corrosion may lead to stress concentrations and further accelerate the corrosion process. Grooving corrosion may be found in the base material where coating has been scratched or the metal itself has been mechanically damaged.
3. Pitting corrosion is often found in the bottom plating or in horizontal surfaces, such as face plates, in ballast tanks and is normally initiated due to local breakdown of coating. Once pitting corrosion starts, it is exacerbated by the galvanic current between the pit and other metal.
4. Erosion which is caused by the wearing effect of flowing liquid and abrasion which is caused by mechanical actions may also be responsible for material wastage.

Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. Corrosion products often cover the pits. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. Pitting corrosion, which, for example, is almost a common denominator of all types of localized corrosion attack, may assume different shapes. 

Types of Pitting Corrosion:

Any damage to or excessive wastage of the following structures is to be promptly and thoroughly repaired:

1. Side shell frames, their end attachments and adjacent shell plating
2. Deck structure and deck plating between hatches
3. Watertight bulkheads
4. Hatch covers and coamings

In general, where part of the structure has deteriorated to the permissible minimum thickness, then the affected area is to be cropped and renewed. Double plates must not be used for the compensation of wasted plate.

Measurement of Pitting

If pitting intensity is higher than 15% in an area, then thickness measurements are to be taken to check the extent of the pitting corrosion. The 15% is based upon pitting or grooving on only one side of the plate.

In cases where pitting is evident as defined above (exceeding 15 %) then an area of 300 mm diameter or more (or, where it is impracticable define the area on the frame flange or the side shell, hopper tank plating or topside tank plating attached to the side frame, an equivalent rectangular area), at the most pitted part of the frame, is to be cleaned to bare metal, and the thickness measured in way of the five deepest pits within the cleaned area. The least thickness measured in way of any of these pits is to be taken as the thickness to be recorded. To measure the remaining thickness of pits or grooving the normal ultrasonic transducer (generally 10 mm diameter) will not suffice. A miniature transducer (3 to 5 mm diameter) must be used. Alternatively the gauging firm must use a pit gauge to measure the depth of the pits and grooving and calculate the remaining thickness.