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ANALYSIS REVIEW


manufacturer. With no time to spare, and pressure from the engineer to get back to sea, the painters do their best, and the inspectors do their best to check it. How well they do their jobs has a major effect on the durability of the coating. The most important measurement is dry film thickness (DFT). Together with class


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of blast, DFT is considered to be most important factor determining durability of the coating. Coating specifiers, applicators, and inspectors are all concerned with DFT measurements. But the method of measurement is significant and the specifiers’ choice of method may be taken without full understanding of the differences. 300,000 square metres of ballast tanks may have to be sprayed, and then


passed by the inspector. Differences in the detail of measurement methodology can make a huge difference in time, cost, and ultimately in protection against corrosion. Inspection is also a large job: the time required simply for an inspector to check the DFT on those 300,000 square metres might be, according to the letter of the different standards, from 8 hours to 83 hours. DFT measurements are considered important enough to be a required part of


a ship’s Coating Technical File, which under SOLAS regulations must be kept at all times with any ship over 150m in length. The File contains specifications of the coating system applied to dedicated seawater ballast tanks, a record of coating work and detailed criteria including job specifications, inspection, maintenance and repair. Three different standards are in use: IMO guidelines, ISO standards, and the US


SSPC. They all exist for the same reason but the devil, as always, is in the detail. The measurement standards have different demands, and individual strengths and weaknesses. At one level, understanding these could make the difference between an inspection being passed or failed. But more importantly which system, if any, will provide the best result and be most practical in use? And are any of them fully fit for purpose? Rob Francis, a metallurgist and corrosion and coatings specialist with over 35


years experience in the industry, recently presented the results of his research and analysis into the current state of Standards at a recent Marine Coatings conference at the Royal Institute of Naval Architects.


MOST IMPORTANT MEASUREMENT Francis’ paper begins: “Dry Film Thickness is probably the single most important measurement made during the inspection or quality control of protective coating application. Even the most basic protective coating specification will inevitably require the DFT to be measured.” The introduction of IMO resolution MSC 215(88) on Performance Standards for Protective Coatings has drawn attention to the importance of DFT measurement in achieving the desired durability of protective coatings in marine applications. “This document, along with DFT standards from ISO in Europe and SSPC in the USA, provide the coating specifier, applicator and inspector with information on measuring techniques, acceptability of readings, inspection plans and other important aspects of DFT measurement”. Total system thickness will have to be measured and recorded to show that


the specified system will meet the desired durability. Usually the DFT requirement will state a minimum thickness, and only in special circumstances will a maximum tolerated thickness also be specified.


MEASUREMENT GAUGES A paint inspector wouldn’t leave home today without a modern electronic DFT gauge, a pocket wonder unrecognisable from older equipment. Modern gauges rapidly measure the thickness of a coating to a tolerance of two or three microns. 60 years ago the first mechanical gauges, using permanent magnets, came into use. Though comparatively crude by today’s standards even they were a revolution, replacing purely visual inspections.


Francis offers the following manufacturers claims of a common model available today: • Can take more than 60 readings per minute, •


Is accurate to ± 1 to 3% ( ± 2.5 microns),


• Can provide the user with the average thickness, number of readings taken, standard deviation, coefficient of variation and minimum and maximum readings,


APRIL-JUNE 2013 PCE 25


n a damp November morning at Southampton, a cruise ship arrives for its scheduled drydock. November is the slack season for cruise ships, but time is still money. The schedule includes recoating the ballast tanks and hull, but conditions are far from the ideal envisioned by the paint


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