tailieunhanh - Corrosion Control Through Organic Coatings Part 9

Tham khảo tài liệu 'corrosion control through organic coatings part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Corrosion Testing Background and Theoretical Considerations 115 correctly mimic the mass transport phenomena that occur in the field. There is a limit to how much we can scale down the duration of a temperature-humidity cycle in order to fit more cycles in a 24-hour period. Beyond that limit the mass transport occurring in the test no longer mirrors that seen in the field. Temperature salt load relative humidity RH . The balance of these factors helps to determine the size of the active corrosion cell. If that is not to scale in the accelerated test the results can diverge greatly from that seen in actual field service. Strom and Strom 1 have described instances of this imbalance in which high salt loads combined with low temperatures led to an off-scale cell. Type of pollutant RH. Salts such as sodium chloride NaCl and calcium chloride CaCl2 are hygroscopic but liquefy at different RHs. NaCl liquefies at 76 RH and CaCl2 at 35 to 40 RH depending on temperature . At an intermediate RH for example 50 RH the type of salt used can determine whether or not a thin film of moisture forms on the sample surface due to hygroscopic salts. Various polymers and therefore coating types react differently to a change in one or more of these weathering stresses. Therefore in order to predict the service life of a coating in a particular application it is necessary to know not only the environment average time of wetness amounts of airborne contaminants UV exposure and so on but also how these weathering stresses affect the particular polymer 2 . UV Exposure UV exposure is extremely important in the aging and degradation of organic coatings. As the polymeric backbone of a coating is slowly broken down by UV light the coating s barrier properties can be expected to worsen. However UV exposure s importance in anticorrosion paints is strictly limited. This is because a coating can be protected from UV exposure simply by painting over it with another paint that does not transmit .