Effect of organic additives on the corrosion resistance properties of electroless nickel deposits

The effects of two organic additives, 3-S isothiuronium propyl sulfonate (UPS) and thiourea (TU) on the properties of electroless nickel (EN) deposit were investigated. The properties of EN deposits were examined by electrochemical impedance spectroscopy (EIS) and nitric acid corrosion test in combination with scanning electron microscope, X-ray fluorescence spectrometer, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy analysis. The results show that TU and UPS had different influences on the morphology of EN deposits. The two additives decreased phosphorus content and increased sulfur content in the EN deposit. XRD studies indicated that addition TU or UPS could enhance the crystallization of EN deposits. EIS studies demonstrated that the EN coating with TU or UPS has a lower corrosion resistance. However, the nitric acid test indicated that the corrosion resistance of EN deposit could be improved by adding UPS to EN bath. A cause for understanding the increase of the nitric acid corrosion resistance with UPS was indicated based on the above experiments.