tailieunhanh - Corrosion Control Through Organic Coatings Part 6

Tham khảo tài liệu 'corrosion control through organic coatings part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 6 Weathering and Aging of Paint This chapter presents a brief overview of the major mechanisms that cause aging and subsequent failure of organic coatings. Even the best organic coatings properly applied to compatible substrates eventually age when exposed to weather losing their ability to protect the metal. In real-life environments the aging process that leads to coating failure can generally be described as follows 1. Weakening of the coating by significant amounts of bond breakage within the polymer matrix. Such bond breakage may be caused chemically . through hydrolysis reactions oxidation or free-radical reactions or mechanically . through freeze-thaw cycling which leads to alternating tensile and compressive stresses in the coating . 2. Overall barrier properties may be decreased as bonds are broken in the polymeric backbone in other words as transportation of water oxygen and ions through the coating increases. The polymeric network may be plasticized by absorbed water which softens it and makes it more vulnerable to mechanical damages. The coating may begin to lose small water-soluble components causing further damage. Flaws such as microcracks develop or if preexisting are enlarged in the coating. 3. Even more transportation of water oxygen and ions through the coating. 4. Deterioration of coating-metal adhesion at this interface. 5. Development of an aqueous phase at the coating metal interface. 6. Activation of the metal surface for the anodic and cathodic reactions. 7. Corrosion and delamination of the coating. Many factors can contribute in various degrees to coating degradation such as Ultraviolet UV radiation Water and moisture uptake Elevated temperatures Chemical damage . from pollutants Thermal changes Molecular and singlet oxygen Ozone Abrasion or other mechanical stresses The major weathering stresses that cause degradation of organic coatings are the first four in the list above UV radiation moisture heat and chemical damage. And 99