SUB-COMMITTEE ON SHIP DESIGN AND EQUIPMENT

48th session

Agenda item 12

　

　

1 This document provides the proposal of modification to the draft alternative procedures in the draft performance standard of coating as contained in document DE 48/12. This document has been submitted in accordance with the provisions of paragraph 4.10.5 of the Guidelines on the organization and method of work of the MSC and MEPC and their subsidiary bodies (MSC/Circ.1099 - MEPC/Circ.405).

　

　

2 In the coating standard, in particular in the basic coating requirements in Table 1 of the draft performance standard of coating as contained in document DE 48/12, pre-construction primer shall be removed.

　

3 However, Japan believes that well quality-controlled primer has a capability of protecting the steel plate as a layer of protective coating and should not be removed. There is evidence of test results, which shows the effectiveness of zinc based primer for protection of steel plate against corrosion, as set out in the annex to this document.

　

　

4 The Sub-Committee is invited to consider the above comments together with the annex to this document, and take action as appropriate.

　

 The effectiveness of zinc based primer has been studied by experimental tests.

　

　

 This test is a "98-times paint film deterioration" accelerated test. This accelerated test method has been developed in the study for the maintenance management of oil storing ships. Infiltration of chloric ion (Cl) into coating corresponds to the deterioration of the coating. In this test, coating film deterioration is evaluated through electrochemical behaviour and the infiltration speed of Cl in the laboratory accelerated tests and in natural seawater dipping tests.

　

　

 Two types of test specimen were prepared, one with zinc primer + tar epoxy coating, another with sand blasted surface + tar epoxy coating.

 Test specimen plate: mild steel plate (150 mm long x 70 mm wide x 3.2mm thick).

 Pre-coating procedure: sand blast (remove the primer) or apply inorganic zinc primer (15μm)

 Thickness of dry coating film: 250μm

 Paint type: tar epoxy system

　

　

 Liquid for accelerated test: Sodium chloride 50g, acetic acid 10ml, and 30% hydrogen peroxide in water of one litre

 Temperature: 40℃

　

　

 After dipping, the surface of the coating of the test specimens was observed, and the adhesion strength, the electric resistance and the current density of coating film were measured.

　

 The following measurement results were obtained.

　

（1）Observation of deterioration of coatings

 After 30 days accelerated immersion tests (corresponding to eight years in service time of the storing ship), there was no difference in the observation results between the test specimens which zinc primer was removed and test specimens which zinc primer remained.

　

 After 45 days immersion tests, swelling of a coating film was observed similarly on both types of the test specimens.

　

（2）Adhesion

 As shown in Figure 1, there is no significant difference in the measurement of adhesion between the two types of the test specimen.

　

（3）AC coating resistance

 The relation between the immersion days and the AC coating resistance is shown in Figure 2.

 There is no significant difference between the two types of the test specimen.

　

（4）Current density

 The relation between the immersion days and the current density is shown in Figure 3.

 During 90 days (corresponded for 24 years in an actual operation of ships), the test specimen with the primer shows slightly superior to the test specimen which primer was removed.

　

　

 It is clear, by the test results, that the inorganic zinc primer does not influence deterioration of coating. Therefore, it is not necessary to remove the inorganic zinc primer by processing of groundwork which is specified in the draft performance standard of coating system in document DE 48/12.