Materials Performance Supplements

Corrosion Prevention and Control for Tanks 2017

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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23 TANK CORROSION CONTROL SUPPLEMENT TO MP MATERIALS PERFORMANCE MAY 2017 Corrosion Basics Tank Lining Epoxies Some of the most critical uses of pro- tective coatings involve service conditions that require the use of coatings as linings. This article focuses on the characteristics and use of epoxies, including phenolics. Tank Lining Epoxies These products are normally variations of polyamine, epoxy phenolic, and novolac epoxies. The curing agents are limited to those with very high cross-linking capabil- ities in order to obtain the highest density of the applied and cured epoxy. Polyamide epoxy tank linings are generally limited to water tanks. Traditional epoxy tank lining systems typically call for three coats at 3 to 6 mils (75 to 150 μm) each. The volume solids of these products range from the low 40s to the mid-60s. They have a long track record of successful service mainly because of the high resin vs. pigment content, and the lack of pinholes and holidays due to the overlap- ping of three separate coats and the ease of atomization and application. Pot life was usually fairly long and so were the recoat windows; therefore, intercoat delamina- tion was not often seen. Proper ventilation and curing temperatures are critical factors when applying coatings because the resid- ual solvents in each coat must be evacuated in order to avoid blistering. Newer epoxy tank lining systems tend to be higher in volume solids (70 to 90%), and are applied at thicker dry film thick- nesses (DFTs) (6 to 8 mils [150 to 200 μm]) and fewer coats. Although this reduces the tendency for solvent entrapment, it does require better equipment and greater atten- tion to application techniques to avoid pin- holes, holidays, and blisters. Pot life tends to be quite short, and recoat windows are also short. Some of these require inline heaters to atomize properly due to the heavy-bodied thixotropy necessary to get the higher film build per coat. Some of the newest and best tank lining epoxies are 100% volume solids, thus elim- inating the problem of solvent entrapment. However, few of these can be sprayed with normal airless spray equipment. The mate- rial is so viscous, and pot life is so short, that plural-component heated airless units are required. Unfortunately, very few appli- cators have received adequate training with plural-component airless spray equipment. They do not know how to adjust them prop- erly, they do not have proper inline or trace heating setups, and most important, they do not clean and maintain this equipment properly. The result is that the quality of the application is often unsatisfactory. Unless the inspector is on the job and watching carefully, the applicator is likely to use thin- ners to obtain proper atomization, thus introducing the possibility of solvent blis- tering. Tank Lining Epoxy Phenolics Linings for chemical tanks are more likely to use either epoxy phenolics or pure phenolics than polyamine epoxies. Epoxy novolacs are sometimes included in the description of epoxy phenolics because novolacs are high molecular weight epoxy phenolics. Regardless of whether the tank lining is an epoxy phenolic, novolac, or phenolic, the surface preparation should meet the requirements of NACE No. 1/SSPC-SP 5/ISO Sa 3.0 "White Metal" with an anchor profile of 1.5 to 3.0 mils (38 to 75 μm). The epoxy phenolics and the novolacs are normally applied in two to three coats of 4 to 6 mils (100 to 150 μm) each. The straight pheno- lics are usually applied in three to five coats of 1 to 2 mils (25 to 50 μm) each. All these require heat curing to reach their maximum resistance to a broad range of chemicals. The epoxy phenolics and novolacs are nor- mally cured after the entire system has been applied, holiday detected, and repaired. The thin-film straight phenolics do not cure without heat. Each coat has to be partially cured with low-temperature heat for a few hours to achieve a dry hardness sufficient to walk on, but not so hard that the next coat will not adhere. This is done by checking the color of the applied film. Prior to heating, the film will be a buff or cream color. After partial curing, the color changes to tan. Each coat must be partially cured in the same manner. Once the entire system has been partially cured, holiday detected, and repaired, the final cure is a bake at higher temperatures for a specific amount of time as required on the manu- facturer's application instructions. The tim- ing is critical; too little time or temperature and the chemical stored in the tank may soften the film; too much time or tempera- ture and the phenolic is overcured to the point of embrittlement or charring. This article is adapted from The Pro- tective Coating User's Handbook, 3rd. ed., Louis D. Vincent (Houston, TX: NACE Inter- national, 2016).

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