Substrate, Surface Preparation, and Priming

Substrate. In marine construction the substrate is generally hot-rolled mild steel, with some high-tensile steel in highly stressed areas (apertures in container ships), and aluminum where there are weight penalties (e.g., topsides of passenger liners, ferries, and naval vessels). 

Surface Preparation. Steel hot rolled at 800-900 °C acquires a tenacious oxide layer (mill scale) that is cathodic with respect to the steel to the extent of about 300 mV. In the presence of an electrolyte (seawater containing 3.5% salts, mainly sodium chloride) the steel would corrode and pit and roughen severely. The first process in new construction and refurbishment is therefore the complete removal of mill scale. A small amount of very light-gauge steel is prepared by acid pickling, but most steel for ship and off-shore construction is centrifugally or pneumatically blasted with steel shot that can be recycled or with expendable abrasive grit. Freedom from scale and soluble salt contamination are the main requirements, texture and profile are less important [11.13], 

Priming. In new construction the freshly blasted steel surface is highly reactive and requires corrosion protection during construction. It immediately passes through an 

1) Speed of Drying. The primer is applied to the substrate on a horizontal moving eonveyor the plate must dry quickly to allow handling and stacking within a few minutes. 

2) Speed and Quality of Welding. The primed plates are cut and welded by a number of techniques (plasma, submerged arc, inert gas, manual metal arc, laser). The primers must not interfere with the speed and quality of welding. The principal defects of the previous generation of materials was their high organic content, which yielded porous welds. 

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