الفهرس 
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Abstract
In order to achieve consistent high quality coatings to meet the more demanding performance requirements of today’s applications, there is a need to put more effort into improved control of the coating process. Coatings may fail because of poor adhesion to the surface, porosity, oxidation, and mechanical damage.
Questions that need to be answered include: “What is a good coating? How can we characterize it? And what the optimum surface roughness of steel sheets?” These questions lead us to check the adhesion, and the roughness of the substrate and the roughness of the coating layer.
So this thesis investigates the coated surface performance of steel sheets by study the effect of surface roughness on the coat quality and examines the properties of the precoated steel sheets from the point of view of corrosion resistance and formability. Surface roughness of steel sheets (37 H2) obviously changed by using sandpaper with different grade. One group of specimens was kept unpolished (as reference sample) and for the other groups the surface roughness was measured with (Taylsurf 4) system in conjunction with SRPC program to calculate the surface roughness parameters. Three different types of powder coatings were deposited by electrostatic coating process. After coatings process, the surface roughness were measured again. The sample sheets were cut into (L 40 mm х W 15 mm) for corrosion test, and bond strength tests. Two chemical acids were used as a corrosive media (sulfuric acid (H2SO4) and hydraulic acid (HCl)) concentrated at 4M to investigate the corrosion resistance of studied coatings. The measured data were tabulated and plotted using spreadsheets. Finally the formability of the coated steel sheets was determined by forming three different shapes (S, O, U-Shapes). The quality of the coatings (adhesive strength of the layer, etc.), help to determine forming results when processing precoated steel sheets. At the same time, surface behavior of the coatings (degree of gloss, cracks…etc.) had to be evaluated in order to predict a change in coat quality under forming process.
The important results can be summarized as the following.
1. The quality of the coating is strongly affected by the roughness of the substrate.
2. Coatings improve the surface roughness of steel sheets.
3. The investigated coatings can be used as decorative colored coatings due to its color stability in corrosive environment.
4. The coatings have maximum bond strength and an unparalleled resistance to peeling, unlike conventional coatings.
5. Coatings on smoother surface exhibit a better corrosion resistance compared with coated rough surfaces.
6. The coatings, keep its surface gloss at the interior and exterior surface after forming process.
7. For all coatings the unpolished surface exhibit an unqualified result.