Coatings and Corrosion

The inside story

Malcolm Latarche
Malcolm Latarche

18 December 2018

The inside story

Regulations governing coatings on ships do not extend to all of the various types of coating and areas of the ship that are coated. The most regulated is of course the hull and the anti-foulings used to limit marine growth. Beyond this, only three areas are subject to IMO regulations and these are places where corrosion and degradation of the ship’s structure frequently occur. Because of limited access to these areas in normal operation, rules have been put in place to address the specific concerns.

Ballast tanks and bulker voids

Every shipowner should know that ballast tanks have historically proven a troublesome part of the ship to maintain and that any failure in their structure can have catastrophic results.

Any movement of the ship can start a scouring process inside the tank by the sand and sediments taken in during ballasting. Cleaning and coating of tanks was most often carried out more to meet cost and time limits rather than to a high standard. Inspection of ballast tanks during construction, repair and in service was also often a cursory process and the standard of training sometimes such that difficult to spot deficiencies were overlooked.

As work at the IMO on the 2004 Ballast Water Management Convention was coming to a close, attention there and in IACS turned to addressing the issues of ballast tank coatings.

In 2006 at MSC 82, the IMO adopted Resolution MSC.215(82) Performance standard for protective coatings of dedicated seawater ballast tanks on all new ships and of double-side skin spaces of bulk carriers, which was made mandatory by way of amendments to SOLAS regulations II-1/3-2, also adopted at the session. The resolution title is generally referred to in abbreviated form as PSPC.

The amendments subsequently entered force in 2008 and applied to newbuild contracts from that date. As from July 2012, most vessels delivered are covered by the new standards but existing ships built before 2008 and those contracted before then but commenced later are not covered by the regulation.

In practice, shipbuilders are responsible for implementing the PSPC requirements during new construction. Before construction begins, the yard, class society and coatings sub-contractors agree surface preparation, coating process and inspection procedures. The details are entered into the ship’s Technical Coating File along with full details and MSDs of the products used.

Cargo tanks

Four years after the PSPC for ballast tanks was adopted by the IMO, a similar regulation was adopted to cover the cargo tanks of crude oil tankers. It would appear that the move was necessary due to the move from single-hull to double-hull crude oil tankers. The phenomenon of accelerated corrosion in cargo oil tanks had begun to be investigated in the mid to late 1990s when double-hull tankers became common.

A 1997 report by the Oil Companies International Marine Forum (OCIMF) suggested that in addition to the more conventional corrosion mechanisms, a possible contributory cause of accelerated corrosion has been microbial attack from bacteria in the cargo oil. It would appear that, as crude oil is often loaded at temperatures higher than ambient air and sea temperatures, during the loaded passage the temperature of the cargo tank structure is being maintained at higher levels than normal due to the insulating effect of the double hull spaces.

SOLAS Regulation II-1/3-11, which entered into force on 1 January 2012, on corrosion protection of cargo oil tanks of crude oil tankers, requires cargo oil tanks to be protected against corrosion and makes IMO Resolution MSC.288(87) Performance Standard for Protective Coatings for Cargo Oil Tanks of Crude Oil Tankers mandatory.

The cargo holds of dry cargo vessels and bulkers in particular are other areas that are prone to damage and can become starting points for corrosion. It should be standard practice to inspect the conditions of holds on a regular basis.

In many instances, charter parties will call for the holds to be clean and dry and free from scale and corrosion and inspections will take place before loading. However, this is not a universal requirement for all cargoes. Coal and ores are particularly notorious for causing damage to hold coatings and coal can have a high sulphur content that can initiate corrosion. Regular inspections should pay particular attention to areas where cargo may lodge and especially to any ladders and their securing to the hold.