Section 1 Corrosion philosophy

1.1.1 Determination of the extent of corrosion is one of the fundamental requirements for survey. For the purposes of this Chapter, corrosion can, in order of importance:

1. Affect global hull strength.

2. Affect watertight integrity.

3. Affect other structurally significant structure.

4. Have no structural significance.

1.1.2 Assessing the importance of the affected structure will assist in determining the course of action (eg. full or temporary repair, imposition of conditions of class or operational restrictions) and the consequences of not undertaking repairs.

1.1.3 The terms substantial or excessive corrosion are used to describe corroded structure and these have a specific meaning as defined in the regulations Vol 1, Pt 1, Ch 3, 1.5 Definitions of the Rules and Regulations for the Classification of Naval Ships (hereinafter referred to as the Rules for Naval Ships). The acceptable limits of corrosion are defined in Ch 11 Thickness Measurement.

1.1.4 Locations of substantial corrosion, which need to be examined periodically, but do not constitute a Condition of Class, as defined below but should be included in the Hull Memoranda.

1.2.1 Examples of corrosion that has an effect on the global strength of the ship could be:

1. A general but consistent diminution of the hull plating around the girth such that the hull section modulus is no longer adequate to withstand the extreme sea loads that may be encountered (see Ch 11 Thickness Measurement).

2. A general corrosion of one or more ship side frames such that the ring strength of the side shell is no longer adequate to withstand wave pressures.

3. Localised but highly aligned areas of corrosion such as grooving or chain pitting that could initiate cracking.

1.2.2 Where such corrosion is considered excessive, repairs are mandatory.

1.3.1 Examples of corrosion that has an effect on the watertight integrity could be:

1. Localised pitting resulting in a breach of watertight integrity (and, where applicable, the gas tight citadel boundary).

2. Highly aligned areas of corrosion such as grooving or chain pitting at the lower hull/bulkhead connection.

3. Excessive main watertight bulkhead thinning which could lead to structural failure under damaged heads of water

1.3.2 Such corrosion may compromise the relevant stability and watertight integrity certification; where so compromised, the extent of the corrosion is to be brought to the attention of the relevant Naval Authority.

1.3.3 Where the structural boundary has been or is likely to be breached there may be a risk to ship safety from water damage of critical equipment or systems.

1.3.4 Where such corrosion is considered excessive, repairs are mandatory and may only be deferred (as a Condition of Class) subject to the agreement of both the Surveyor and the Naval Authority responsible for stability and watertight integrity certification.

1.4.1 Examples of corrosion that affects significant structure other than that defined in Ch 10, 1.2 Corrosion with global hull strength implications and Ch 10, 1.3 Corrosion with implications for watertight integrity could be:

1. Localised pitting, that if allowed to continue unchecked, would result in tank boundary leakage.

2. Localised diminution of the hull plating which does not immediately affect the hull section modulus to any significant extent.

3. Heavily corroded deck grillage.

4. A locally corroded frame web in a piece of secondary structure in a wet space.

1.4.2 In this case it is important to recognise that the subsequent failure mechanism resulting from further degradation may lead to mechanism that could cause primary hull failure (eg. a tank leaking a corrosive fluid could result in accelerated corrosion of the outer bottom, or a heavily corroded stiffener web could lead to a buckling failure of the associated plating).

1.4.3 Where such corrosion is considered excessive, repairs should always be made but may, subject to the agreement of the Surveyor, be deferred as a condition of Class. The due date of the condition of class will be dependant on the severity of the corrosion, likely corrosion rate and opportunity for repair.

1.5.1 On many occasions corrosion will have no significant strength implications but will need to be repaired (eg. to eliminate superficial rusting for personnel hygiene reasons, a need to maintain the cleanliness of fuel and other stored liquids, or cosmetic reasons). In these cases the Navy or relevant Naval Authority is to be advised.

1.5.2 Where such corrosion exceeds the general limits given in Ch 11 Thickness Measurement and Ch 12 Pitting, the decision to repair the wastage or coating is to be left to the Navy or Naval Authority.