CORROSION SCIENCE SECTION CORROSION—Vol. 66, No. 6 065005-1 Submitted for publication November 3, 2008; in revised form, April 28, 2010. ‡ Corresponding author. E-mail: sbaxe@statoil.com. * Norwegian University of Science and Technology, NTNU 7491 Trondheim, Norway. Present address: Statoil ASA, Forusbeen 50, 4035 Stavanger. ** Norwegian University of Science and Technology, NTNU 7491 Trondheim, Norway. *** SINTEF Materials and Chemistry and Norwegian University of Sci- ence and Technology, NTNU 7465 Trondheim, Norway. Internal Stress in Polysiloxane Topcoats S.B. Axelsen, ‡, * R. Johnsen,** and O.Ø. Knudsen*** ABSTRACT This paper summarizes the results from laboratory investiga- tions on internal stress in protective coatings. Internal stress, together with a lack of paint film flexibility and adhesion, are expected to be the main reasons for cracking and flak- ing of protective coatings. Mechanical properties and adhesion are to be discussed in later papers. Stress buildup at vari- ous climatic conditions has been studied. Two commercially available three-coat systems with polysiloxane topcoat were included in the test program. Both the individual coats and the full systems were investigated. The polysiloxane topcoats showed low levels of internal stress at all test conditions, and it is considered unlikely that topcoat cracking, and subsequent flaking, will occur as a result of the stress levels measured in this work. KEY WORDS: aging, corrosion, internal stress, protective coatings INTRODUCTION Topcoat flaking has been considered to be, and still is, a significant problem on both onshore and off- shore structures in the Norwegian sector. Polysilox- anes (PSO), the predominant topcoat in the Norwegian market during the last few years, may seem to be par- ticularly vulnerable to this degradation mechanism, but other generic-type topcoats also exhibit a simi- lar behavior. Due to health aspects, polyurethane paints containing isocyanates were banned from the Norwegian offshore industry in the late 1990s, and polysiloxane paints became the preferred topcoat for atmospheric conditions. The number of coat layers was also reduced from three to two to lower the appli- cation cost and increase productivity (faster turn- around times). Failures of the two-coat PSO systems due to cracking and flaking were reported after a rela- tively short time in service; however, and in the 2004 revision of NORSOK M-501, three-coat systems again became a requirement. 1 Whether this is an adequate solution to the cracking/flaking problem is still sub- ject for discussion. According to a NORSOK task force established to investigate the reported problems, the extent of flaking/delamination of PSO in three-coat systems is reported as less extensive than for two-coat systems. 2 Internal stress, the subject of a number of studies within the last few decades, 3-17 has been suggested, together with paint film flexibility and intercoat adhe- sion or adhesion to the substrate, as the main param- eters for cracking and flaking. Recent studies by Knudsen, et al., quantified both the formation of internal stress and the mechanical properties of coat- ing systems relevant to the Norwegian offshore indus- try. 16-17 The present work is a continuation of the investigations made by Knudsen, et al., where the goal is to establish a better understanding of the impor- tance of internal stress, mechanical properties, and adhesion, with respect to susceptibility for topcoat ISSN 0010-9312 (print), 1938-159X (online) 10/000079/$5.00+$0.50/0 © 2010, NACE International