Edge and Weld Preparation for an Effective Anti-corrosion Coating
Experience has shown that edges and welds are generally the first areas to show corrosion and coating breakdown, particularly in ballast and cargo tanks. This is due to a number of inter-related processes including surface preparation, coating application, deflection, and shear and buckling stresses on the edges and welds and so on. The quality of the surface preparation will play a major role in determining the service lifetime of the coating. Ring stiffeners around openings may reduce this effect.
The process of welding generally produces some type of slag on the weld itself, together with spatter (small droplets of the parent or weld material) and fume (smoke). Submerged Arc Welding (SAW) does not generally produce welding fume as the arc is covered by a slag blanket. Slag may have to be removed manually. Residual slag on a surface can result in early failure of the paint in service.
Removal of the weld spatter is essential as this material will cause an irregular surface and will result in poor coverage by the paint. Spatter is adherent in the welding process and must be removed by chipping or other mechanical means. Weld fume must also be removed as it is loosely adherent to the steel and depending upon the type of weld consumable used, the fume may contain water soluble species. If weld fume is over-coated, the paint may blister and/or peel from the steel in service. Blisters can also form where the shop primer is damaged due to the welding of stiffeners on the other side of the plate. This is often referred to as “burn through” and it is typical of the results in service when the burn through is not adequately cleaned or removed prior to application of anti-corrosion coating. The position of welds can present difficulties for the cleaning and surface preparation process, particularly when the weld forms part of a complex structure.
In many cases, only an abrasive or water blasting process will provide an efficient cleaning of the weld, however this may not be practical in some new building shipyards or for on board maintenance of vessels in service. Welds must be prepared efficiently so that the possibility of creating voids under the anti-corrosion coating is eliminated. Porosity can also occur in welds and this may not become visible until the weld has been blasted clean.
After application as the anti-corrosion coating is still liquid, there is a tendency for many anti-corrosion coatings to pull back from sharp edges leaving a very thin layer of paint which can quickly breakdown in service. Grinding profiles into the edges of cut outs, drainage holes, etc., as shown in Figure 4-4, greatly improves the adhesion and coverage of the anti-corrosion coating around the edge. Rounded edge preparation will generally provide the most effective service performance from the anti-corrosion coating. Three passes of the grinding disc over the cut edge will give the next best preparation, then two passes. Even one pass of the grinding tool will give a better surface for painting than no preparation. The addition of a stripe coating to the edges is also beneficial in providing long term protection.
Good surface preparation around cut edges is also very important. Cutting fume is an additional source of contamination at edges and this must be removed for good coating adhesion. Ring stiffening around manholes may prove beneficial for the longevity of the anti-corrosion coating around the opening.
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