6.1 Process considerations for repairs
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC.1/Circular.1330 – Guidelines for Maintenance and Repair of Protective Coatings – (11 June 2009) - Annex – Guidelines for Maintenance and Repair of Protective Coatings - 6 Coating Repairs - 6.1 Process considerations for repairs

6.1 Process considerations for repairs

  6.1.1 Major considerations are:

  • .1 safety;

  • .2 salt contamination;

  • .3 rust scale;

  • .4 pitting corrosion;

  • .5 temperature;

  • .6 condensation;

  • .7 ventilation;

  • .8 dehumidification;

  • .9 compatibility of coating systems;

  • .10 design/surface area; and

  • .11 cathodic protections.

  6.1.2 Safety. Refer to the Recommendations for entering enclosed spaces aboard ships (resolution A.864(20)). It is an absolute requirement that all of the ship's safety and tank entry procedures and policies are adhered to. When a ship is out of service, in a yard repair, local regulations apply covering safety. The yard is responsible for their implementation.

  6.1.3 Salt contamination will cause accelerated deterioration of the coating if not removed prior to coating application. A recommended procedure to reduce salt contamination is to remove loose rust scale followed by good fresh water rinsing, at elevated temperatures and high pressure, if possible. Test the salt content after washing and before coating using standard ISO 8502-9 or other equivalent method and re-wash if necessary until the salt level is less than or equal to 80 mg/m2 of total soluble salts, calculated as sodium chloride or as recommended by the coating manufacturer. This should be the starting point in any surface preparation process in ballast tanks onboard ships. In case of major repair or full recoating, any deviation should be agreed between the parties concerned and noted in the CTF.

  6.1.4 Rust scale that is not removed prior to coating application will cause early failure. Loose top-scale is easy to remove, however the inner (black) hard scale is much more adherent. When over-coated it will soon detach between the steel and the scale and come off, typically with the coating adhering very well to the outside of it. If the hard scale cannot be removed, the service life expectancy of the treatment is 1 to 2 years regardless of the coating used.

  6.1.5 Pitting corrosion is a major problem on board ships on plates that have been exposed to seawater for some time. If it has been accepted that the pits need not be welded up in order to prevent further accelerated damage, a coating should be applied. Soluble salts will be present within the pits and it is essential that these are removed otherwise corrosion will soon start inside over-coated pits, affecting the service life. Various methods of salt removal from pits have been proposed, e.g., water-jetting followed by blast cleaning possibly also exposure to high humidity and repeating of water-jetting. Whichever methods are chosen, any residues from the washing processes should be removed otherwise the soluble salt will precipitate out of the water on drying.

  6.1.6 When Microbiologically Influenced Corrosion (MIC) is involved the pits are of a much wider nature, typically “shiny” clean inside with sharp edges to unaffected surrounding steel and often with a foul smell, like rotten egg, being evident when breaking up the scale cap. An MIC attack can proceed very deep, very fast.

  6.1.7 Temperature is a critical parameter to consider. When repairs are carried out in a shipyard, proper temperature control can more readily be achieved in the areas requiring coating.

  6.1.8 Condensation is always a risk. It is an absolute necessity that the contractors have a good understanding about relative humidity and its relation to substrate temperature and dew point. To paint over a surface that is at or below the dew point, or that will be at or below the dew point while the coating is wet, will not perform. Ideally the temperature should be at least 3°C above the dew point.

  6.1.9 Ventilation is a vital factor. This is one item that clearly supports both the quality of the application and the safety of the operation. Arrange the ventilation that it extracts from the lowest and furthest corners to ensure the fast and efficient removal of dangerous solvents. The use of solvent free coating systems does not mean that ventilation is not required!

  6.1.10 Dehumidification is the best insurance for good productivity and performance. There are two different types, i.e. desiccant and refrigeration. Both work well, the desiccant type being ideal in moderate and cold climates, and the refrigeration type in warmer climates. The use of dehumidifiers prevents condensation by lowering the dew point, ensures proper cure of the coating, reduces flash-back rusting, prevents grit blasting from “turning” and assists productivity.

  6.1.11 Compatibility of coating systems is of utmost importance for a good end result. Unless the original coating system is totally removed, a coating system compatible to the original system should be used in accordance with the paint manufacturer recommendations. The coating system requires a Statement of Compliance or Type Approval Certificate according to the Performance standard for protective coatings for dedicated seawater ballast tanks in all types of ships and double-side skin spaces of bulk carriers (resolution MSC.215(82)). Demonstration of compatibility should not require separate approval of the combined coating system consisting of the old coating and new coating.

  6.1.12 Stripe coating/design/surface areas should be differentiated with respect to coating application as degree of access varies. Edges, corners, weld seams and other areas that are difficult to coat need special treatment. “Stripe coating” is used to produce a satisfactory coating and to obtain specified Dry Film Thickness (DFT) on such areas. Stripe coats should be applied as a coherent film showing good film formation and no visible defects, such as pores or de-wetted areas. The application method employed should ensure that all areas which cannot be adequately coated by spray application are properly stripe coated. Stripe coats should be applied by brush or roller. Roller to be used for scallops, ratholes, etc., only.

  6.1.13 It is recommended to apply a stripe coat before or after each main coat. This should be done using a colour that contrasts with each main coat, as this makes it easier to see that the stripe coat is satisfactory.

  6.1.14 Cathodic protection is one commonly used anti-corrosion method in ballast tanks. Since the electric potential of certain anodes may damage the coating in their vicinity, it is recommended that the impact of electric potential on coating be considered in the area where cathodic protection system is applied.

Parent topic: 6 Coating Repairs
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