Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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50 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 BLOG A: Te most critical points to cover to control this problem are: • Operative mix particle size distribu- tion (no or close to zero particles with 50 mesh or smaller) • Relationship between the hardness of the blasted piece and the blast media used • Te replenishing rate of new media into the operative mix—enough to maintain the loss and constant enough to avoid segregation • Te relationship of the air wash separator size and centrifugal blaster capacity • Air fow in the air wash separator and in the blast booth, removing all small particles and dust • Dust collector size—centrifugal blaster size, angle, velocity, cage size, and position • Hot spot analysis • Quality control practices, particle size analysis, vial test, calibration of electric motors, ISO 8502-3 backside contamination, and peak count It is not an easy problem to solve; the best approach is to get an expert who can look at your equipment and make recom- mendations for your operation. I have seen important differences with very minute changes. Inorganic zinc thickness Q: I am going to work as a paint contractor for quality control on a rig life-extension project. The client has given the following specif ication: • 50-µm inorganic zinc (IOZ) • 50-µm epoxy polyamide • 200-µm build coat However, the data sheet of this IOZ specif ies that it is a shop primer applied at 15-µm dry f ilm thickness (DFT). I wish to have your views on this specif ication, knowing that the tank is a very complex structure and the guarantee is f ive years. A: Something is amiss here. Nor- mally, IOZ shop primers are not intended to be applied in excess of 15 µm. Tey will mud crack in corners, welds, etc. when applied at excess thickness. Since this is a life-extension project, I have to assume there is a coating system already in place. If that system is an organic coating system, the IOZ will not adhere to the organic system. To apply the specif ied system would require abrasive blasting to bare metal. You might wish to ask for a conference with the owner's representative to clarif y that specif ication. A: Tis approach is fairly common. It sounds like they are ofering a preconstruction primer at a thickness consistent with a weldable primer. Ty pically this is spot-blasted on weld seams and abraded areas and swept over the remaining area. Tere will be very little zinc primer left after this process. Te primary function of the zinc is to preserve the blast profle. If there is no zinc left, it is of little concern. I would suggest a little more epoxy for the system if life expectancy is a concern. A thick- ness of 250 µm (10 mils) is fairly common for immersion, but there are systems available that go to 500 µm in two coats quite easily. A: Wet flm thickness (W FT) gauges correlated to actual DFT for the IOZ coating will reduce but not eliminate inaccuracies. Correlate wet to dry thick- ness for every IOZ batch and diferent manufacturers. Variations in batch consistency and coating design prevent accurate assumptions about wet to dry thickness ratios. A: Actually, it is the responsibility of the contractor to bring questions to the owner at the pre-bid meeting before he ofers a price. Once the contrac- tor has signed a contract to perform the work, he has accepted the contract and the included specifcation as written. If he fnds a problem after signing the contract, he should, of course, bring it to the owner's attention. Ten he has to negotiate a reasonable change order, but it might not be as favorable to the contractor by then. A: Measuring W FT of IOZ with a W FT gauge is not practical for two reasons: the extremely fast evapora- tion rate of the solvent and the fact that the zinc particles will not allow the outside legs of the gauge to seat properly on the steel surface. Te only way to fgure W FT is to calculate the amount of material necessary to cover the area to be painted and use that much material. But even that is a problem as the shrinkage rate of IOZ is not totally related to the solids by volume. Again, you have a heav y load of zinc particles that will not shrink. nace.org/store 1 (800) 797 6223 For more information and additional titles ORDER YOUR COPY TODAY Published 2012 A NACE Publication $120 .00 $90 .00 LIST PRICE NACE MEMBER PRICE Item No. 37596 Content includes: ■ Comparative tables of marine coatings by major global manufacturers ■ Requirements of work processes in new building shipyards ■ IMO PSPC requirements ■ Tips for anticipating coating failures and controlling fouling on marine vessels A practical approach to every major marine coating challenge by Louis D. Vincent, Ph.D. The Marine Coatings User's Handbook Continued f rom page 49

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