Studi pengaruh elektrolit H2SO4, NaOH, H3PO4 dan H2C2O4 terhadap nilai kekerasan dan ketebalan lapisan oksida aluminium hasil anodizing untuk aplikasi piston = Study influence of electrolyte H2SO4, NaOH, H3PO4 and H2C2O4 compare with hardness value and alumunium oxide layer thickness result of anodizing for piston application