The effect of variation concentration cristobalite silica from natural silica sand to hydrophobicity on steel plate surface

In this research, pure silica powder has been synthesized from the sand of the BangkaBelitung Islands. Natural sand is extracted with permanent magnets and immersed with HCl to obtain pure silica, followed by coprecipitation and calcination processes at a temperature of 900 ̊C for 10 hours. The final result of the synthesis process is pure silica powder in the cristobalite phase (83.03% wt) and the tridymite phase (16.97% wt). The synthesized silica powder is used as a modification on topcoat of steel to increase the hydrophobicity of steel plate surface so that it can reduce the rate of corrosion. Steel plate has been painted by using the brush painting method and consists of three layers, namely the primary layer, midcoat and topcoat. The variation in this research is the concentration of silica powder on the topcoat of steel such as 0% wt (sample 1), 3% wt (sample 2), 6% wt (sample 3), 9% wt (sample 4), and 12% wt. (sample 5). These variations have an effect on the surface geometry of the steel plate, namely the surface gets rougher as the concentration of silica powder is mixed. The hydrophobicity of the steel plate can be seen from the measurement Water Contact Angle (WCA). The WCA using fresh water in sample first until five’th are 75,828 ̊, 90 ̊, 91,397 ̊ 96,520 ̊, and 104 ̊, respectively. While the WCA using seawater in sample first, second and fourth are 80.618 ̊, 102 ̊, and 104.56 ̊, respectively.