Mechanical and corrosion behavior of amorphous and crystalline electroless Ni-W-P coatings

Two types of electroless Ni-W-P coatings: nanocrystalline with low P and amorphous with higher P content are investigated. Scanning probe microscopy is applied to study their morphology. Textured nanocrystalline coatings consist of coarse pyramids built of nanometer thick lamellas. The surface morphology of amorphous coatings is much finer and uniform. Nanohardness of all coatings depends on W content. Microhardness is increasing during the heat treatment up to 350 A degrees C due to nickel phosphide precipitation affected by tungsten also. The wear resistance of nanocrystalline Ni-W-P coatings is much higher than that of amorphous in spite of the similar tungsten content in both. Lower corrosion resistance of amorphous Ni-W-P coatings is found by weight loss method during long-term immersion in 5 % NaCl. Electrochemical tests by potentiodynamic polarization curves in two model corrosion media-solutions of 0.5 M H2SO4 and 5 % NaCl-are performed. The corrosion of bi-layered Ni-W-P/Ni-P and Ni-W-P/Ni-Cu-P deposits on mild steel is also investigated. The results prove that an electroless Ni-W-P coating on mild steel extremely improves its mechanical and corrosion behavior. It is demonstrated that in addition to deposit's structure and composition, the distribution and chemical state of alloy ingredients are also responsible for its properties.