International Symposium Shipbuilding Technology (ISST), Osaka 2007

STUDY ON THE ALTERNATIVES TO THE SECONDARY SURFACE PREPARATION IN PROTECTIVE COATINGS

Naoki OSAWA, Osaka University, Japan Koichiro UMEMOTO, Kawasaki Shipbuilding Ltd., Japan Yukinori NAMBU, Universal Shipbuilding Ltd., Japan Tatsuya KURAMOTO, Mitsui Engineering & Shipbuilding Ltd., Japan

Anti-corrosive performance of a protective coat applied on blowholes dressed out by puttying is investigated. The protective performance of a top coat applied to a weld bead with blowholes which is dressed out by 100% solid epoxy polyamide putty is better than or equivalent to that for the case where blowholes are dressed out by repair- welding. Puttying by 100% solid epoxy polyamide putty is an effective alternative to repair welding for blowhole dressing.

INTRODUCTION Ship structures often come with complex geometric configurations, having large surface and highly stressed areas, such as corners, edges, and weld seams areas. Coating defects such as insufficient film thickness are often observed, resulting early coating failure and corrosion in these areas. It is well known that secondary surface preparation (SSP) is effective in preventing these coating defects. It is mandated to apply grinding weld beads contaminant and mechanical grinding of sharp edges by IMO PSPC. The SSP defined in IMO PSPC requires finishing weld beads in accordance with ISO 8501-3 grade P2. In this standard, it is required that 'surface pores shall be sufficiently open to allow penetration of paint or dressed out'. This requires that blowholes are removed or filled up. Usually, blowholes are dressed out by repair welding, but contingent works, such as surface re-preparation and removal of dust, are associated with this process. Blowholes can be dressed out by puttying up. If adequate protective coating performance is obtained when blowholes are dressed out by puttying, we can save the manpower substantially. In this paper, anti-corrosive performance of a protective coat applied to blowholes dressed out by puttying is investigated by adhesion measurements, immersion tests in NaCl solution, and salt spray cabinet tests. The effectiveness of puttying as an alternative to repair welding for blowhole dressing is discussed by comparing the protective performance for the cases with blowholes dressed out by puttying and that for the cases without blowholes.

EFFECTIVENESS OF BLOWHOLE DRESSING BY PUTTYING EXPERIMENTAL

T-weld joints shown in Figure 1 are used as the test specimens.

The sizes of main and attached plates (length, width and thickness) are 50 mm x 100 mm x 8 mm and 50 mm x 50 mm x 8 mm. The joints are made of mild steel. The total number of the specimens is 20. The leg length of the fillet weld ranges from 5 mm to 8mm, and the flank angle of the weld ranges from 135 deg. to 145 deg. For all specimens, the shape of the weld bead meets ISO 8501-3 P2 requirements except that there are blowholes in some specimens. There are blowholes in 16 of 20 specimens. The maximum diameter of blowhole is about 3mm. The specimens are blasted with steel grits to near white metal finish (According Standard ISO 8501-1, Quality Sa 2 ½). Figure 2 shows the examples of blowholes in the weld bead before and after blasting.

Blowholes are puttyed up using 100% solid epoxy polyamide putty (Chugoku Marine Paints, BUNDET PUTTY).

BONDET PUTTY – BASE 3,5 Kg BONDET PUTTY – HARDENER 1,5 Kg The drying time of this putty at 20°C is 2 hours for surface dry, and 6 hours for hard dry, and the drying shrinkage is negligible because the solid volume ratio (SVR) is 100%. This putty is applied to the blowholes so that the putty surface is flushed with the bead surface using spatulas. For the specimens with blowholes, the specimens are coated with anticorrosive paint after the blowholes are puttied up. The time between puttying and coating application, Tp is 0 hours (apply paint immediately after puttying), 2 hours (apply paint when the putty surface gets dried) and 6 hours (apply paint after the whole putty gets hardened). For the specimens without blowholes, the specimens are coated just after blasting. The applied paint systems are tar epoxy resin system and modified epoxy resin system. The top coat is applied by airless spray after the touch-up coat is applied to the weld bead with a brush. After the top coat is applied, the painted specimens are dried for more than 24 hours at room temperature (about 20 C). Dry film thickness ranges from 173 um to 442 um. Additional touch-up coat is not applied to the specimen's end faces. Figure 3 shows an example of the weld bead surface after the dressing by puttying, and Figure 4 shows an example of the weld bead surface after the application of the top coat.

The back faces of 4 of 16 specimens with blowholes are heated by LP gas flame so that the maximum back face temperature is above 600°C. This gas heating causes the reverse side bum damage of the top coat on the weld beads as shown in Figure 5. These bum damages are repaired by grinding and repainting.

Test conditions of blowhole dressing tests are identified by presence or absence of blowholes, putty drying time (Tp = 0, 2, or 6 hours), type of paint system (tar epoxy or modified epoxy), presence or absence of burn damage. The conditions are identified by the names described in Table 1. Four specimens for each test condition are prepared. Dry film thicknesses measured are summarized in Table 1.

Duplicate specimens of each test condition are subjected to the immersion test in NaCl solution for 300 days. Duplicates of these specimens are tested in a salt spray cabinet for 1000 hours. In these tests, the dimension and number of blisters as well as the presence of corrosion spots on the weld beads of every painted specimen are monitored. The immersion and spray tests are performed, and the blistering corrosion data are measured according to the procedures described in Sec. 2.1. In the rating of corrosion data, stain made by the rust which forms on the specimen's end faces is not treated as corrosion damage. After the immersion and spray tests, the adhesion data on the weld bead is measured by the knife-cut test described in Sec. 2.2. The knife-cut test is performed at the location where a blowhole is puttied-up for the specimens with blowholes (CASE Al, A2, A3, Bl. B2. B3, Dl and D2 in Table 1), and at a distance of 20mm from the specimen end for the specimens without blowholes (CASE A4 and B4 in Table 1).

EXPERIMENT AT RESULTS AND DISCUSSION Table 2 lists blistering corrosion data obtained for the welded joint specimens with / without blowholes subjected to the immersion tests. The results of the knife-cut tests are also shown in this table. The ratings for each test condition shown in this table are the mean values of the duplicate specimens. It is shown that all specimens show neither corrosion products nor coating defects on the weld beads during the periodic visual inspections. It is also shown that no signs of the adhesion loss are recognized for all specimens.

Table 3 lists blistering corrosion data for the welded joint specimens subjected to the salt spray cabinet tests.

The results of the knife-cut tests are also shown. The ratings for each test condition are the mean values of the duplicate specimens. It is shown that all specimens show neither corrosion products nor coating defects on the weld beads during the periodic visual inspections except that the rust which forms on the specimen's end faces stain the coating film on the weld beads. It is also shown that no signs of the adhesion loss are recognized for all specimens.

The above results can be summarized as follows:

1) Under the conditions chosen, the performance of the anticorrosive tar-epoxy or

modified-epoxy top coats applied to the weld bead, on which blowholes are dressed out using 100% solid epoxy / polyamide putty, is almost equivalent to that for the cases where there is no blowhole on the weld bead. 2) Under the conditions chosen, for the weld joint specimens with putty-upped blowholes, loss of the anticorrosive performance of the top coat coming from the shortening of the putty drying time is not recognized. 3) Under the conditions chosen, loss of the anticorrosive performance of the top coat applied to the weld bead with putty-upped blowholes is not recognized when reverse side burn damage occurs. It is considered that the protective performance of the top coat on the weld bead without blowholes is equivalent or better than that for the weld bead dressed out by repair welding. The above results suggest that the performance for the puttying case is better than or equivalent to that for the repair-welded case. Contingent works needed for the repair-welded cases, such as surface re-preparation and removal of dust, is unnecessary in the puttying cases.

It can be said that dressing by 100% solid epoxy polyamide putty is an effective alternative to repair welding. This document prepare from original documents from International Symposium Shipbuilding Technology (ISST), Osaka 2007:

Franjo Tomljenovic Painting Supervisor