Cathodic protection, in the form of Impressed Current Cathodic Protection (ICCP), is used to reduce the rate of deterioration of the underwater hull. From a coatings perspective, cathodic protection primarily takes the form of dielectric shields applied to the underwater hull. For detailed information regarding cathodic protection, see the Preservation Resources – Cathodic Protection page.
ICCP system anodes applied to the hull are surrounded by thick shielding material (often referred to as “capastic coating” or “capastic epoxy”) consisting of a high-solids epoxy with high dielectric strength. This shielding presents shorting of the anode current to the hull near the anode and aids in wider current distribution to the hull. This shielding material is followed by layers of the same anti-corrosive intermediate coat and anti-fouling topcoat used on the surrounding hull.
In lieu of a prime coat, dielectric shield materials are applied directly to the cleaned steel hull surrounding the anode. Dielectric shield materials are available in trowelable and sprayable forms. These coatings are intended to be faired out from the anode over a distance of at least 10 inches in order to minimize hull turbulence.
For trowelable dielectric shields, NAVSEA Standard Item (NSI) 009-32 requires the use of one coat of Evoqua Water Technologies’ Capastic, Part No. 35524 (W3T 106410), applied to 100 mils in the inner shield area extending from the anode to three feet out, and to 22 mils in the outer shield area extending from the inner shield to six feet out from the anode.
For sprayable dielectric shields, NSI 009-32 requires one coat of International Interline 624 Buff or Sherwin-Williams Novaplate UHS primer out to 7 feet from the anode, followed by one coat of Interline 624 or Novaplate UHS out to 6.5 feet from the anode, and another coat of the same out to 6 feet from the anode.
Following application of the sprayed or trowelable dielectric shield, one coat of the same anti-corrosive paint used on the surrounding hull is applied. The topcoat also consists of the same anti-fouling paint used on the surrounding hull.
To ensure proper coating adhesion, special attention should be paid to the surface preparation of the underwater hull to which dielectric shields are to be applied. NSI 009-32 requires a surface that meets the NACE 1/SSPC-SP 5, White Metal Blast standard.
NSI 009-32 is the governing document for preservation requirements. Permitted coatings are specified in Table 1. All dielectric shield coating system preservation efforts must be accomplished in accordance with NSI 009-32.
Ship’s force is not permitted to make repairs to the underwater hull. As such, maintenance of dielectric shields must be performed by repair maintenance activity. Naval Ships Technical Manual: Chapter 631 (NSTM 631), Preservation of Ships in Service – General provides requirements, instructions, and guidance for surface preparation and coating application.
New construction ships are painted in accordance with the ship build specification for that class of ship.